초록 ▼

A focus detection apparatus has a pair of photoelectric element arrays, a plurality of filters having different MTF characteristics, and an operation unit. A filter is selected from the plurality of filters in accordance with an object or a focusing state. The selected filter filters the data from t

A focus detection apparatus has a pair of photoelectric element arrays, a plurality of filters having different MTF characteristics, and an operation unit. A filter is selected from the plurality of filters in accordance with an object or a focusing state. The selected filter filters the data from the photoelectric element arrays, and the operation unit calculates a focus detection signal based on the filtered data.

대표청구항 ▼

1. A focus detection apparatus for detecting a focusing state of an imaging optical system comprising: (1) optical means for forming a first image and a second image of an object with light passing through said imaging optical system, relative displacement between said first and second images var

1. A focus detection apparatus for detecting a focusing state of an imaging optical system comprising: (1) optical means for forming a first image and a second image of an object with light passing through said imaging optical system, relative displacement between said first and second images varying in response to said focusing state of said imaging optical system, (2) generating means for generating a series of first electric signals and a series of second electric signals each of which represents an illumination distribution pattern of said first image and an illumination distribution pattern of said second image, respectively, said first and second electric signals being image data with respect to a spatial pitch P, (3) memory means connected to said generating means for storing said first and second electric signals, (4) operation means connected to said memory means for filtering said first and second signals stored in said memory means with MTF characteristics suppressing a spatial DC component to generate third and fourth electric signals, for calculating, from said third and fourth electric signals, a first relative displacement amount between said first and second images, and for calculating, from said first and second electric signals stored in said memory means and not subjected to any filtering process inclusive of the filtering of said operation means, a second relative displacement amount between said first and second images. 2. A focus detection apparatus according to claim 1 wherein said generating means includes first and second photoelectric element arrays, each of said arrays including a plurality of photoelectric elements arranged at said pitch P, and wherein said MTF characteristics have a peak of MTF value located near a spatial frequency 1/4P, the MTF value gradually decreasing as spatial frequencies depart from said spatial frequency 1/4P. 3. An apparatus comprising: (1) an imaging optical system for forming an image of an object; (2) array means including a plurality of photoelectric elements disposed to receive light passed through said imaging optical system, said array means producing a series of primary data associated with photoelectric outputs of said photoelectric elements; (3) first filter means for filtering said primary data to generate a series o f secondary data; (4) second filter means for filtering said secondary data to generate a series of tertiary data; and (5) operation means for operating upon said tertiary data to generate an output which represents a focusing state of said imaging optical system, and wherein said array means includes first and second photoelectric element arrays, said first array including a plurality of photoelectric elements for receiving light passing through a portion of said imaging optical system to generate a series of photoelectric outputs serving as a part of said primary data, said second array including a plurality of photoelectric elements for receiving light passing through another portion of said imaging optical system to generate another series of photoelectric outputs serving as another part of said primary data. 4. An apparatus according to claim 3, wherein said operation means further operates upon said secondary data to generate a further output which represents a focusing state of said imaging optical system. 5. An apparatus according to claim 3, further comprising sampling means for sampling said secondary data at sampling pitch P=nPo, where n is a natural number and Po is a spatial pitch of said primary data, and wherein said second filter means filters said sampled secondary data from said sampling means. 6. An apparatus according to claim 5, wherein said first filter means has MTF characteristics suppressing the spatial frequency components above Nyquist frequency determined by said sampling pitch P. 7. An apparatus according to claim 6, wherein said second filter means has MTF characteristics suppressing a spatial D.C. component. 8. An apparatus according to claim 7, wherein said second filter means has MTF characteristics mainly extracting spatial frequency components near a half of said Nyquist frequency. 9. An apparatus comprising: (1) an imaging optical system for forming an image of an object; (2) array means including a plurality of photoelectric elements disposed to receive light passed through said imaging optical system, said array means producing a series of primary data associated with photoelectric outputs of said photoelectric elements; (3) first filter means for filtering said primary data to generate a series of secondary data; (4) second filter means for filtering said secondary data to generate a series of tertiary data; and (5) operation means for operating upon said tertiary data to generate an output which represents a focusing state of said imaging optical system, and wherein said second filter means includes a plurality of filters which have different MTF characteristics, said filters being alternatively selected to filter said secondary data. 10. A focus detection apparatus for detecting a focusing state of an imaging optical system comprising: (1) first and second photoelectric element arrays, each of said arrays including a plurality of photoelectric elements arranged at a pitch Po; (2) optical means for forming first a image of an object on said first array and forming a second image of said object on said second array, said first array generating a series of first signals which represent said first image, said second array generating a series of second signals which represent said second image; (3) first filter means for filtering said first and second signals, said first filter means having MTF characteristics with a peak of MTF value being located near a spatial frequency 1/4Po, the MTF value gradually decreasing as spatial frequencies depart from said spatial frequency 1/4Po; (4) second filter means for filtering said first and second signals, said second filter means having MTF characteristics with MTF values being greater than those of said first filter means in a frequency region lower than about said frequency 1/4Po; (5) operation means for generating. based on first and second signals filtered by said first filter means, a first output which represents a focusing state of said imaging optical system and for generating, based on first and second signals filtered by said second filter means, a second output which represents said focusing state. 11. A focus detection apparatus according to claim 10, further comprising: selection means for alternatively selecting said first output and said second output, and means responsive to one of said first and second outputs selected by said selection means for effecting at least one of display operation of said focusing state and driving operation for driving said imaging optical system to an infocus position. 12. An apparatus according to claim 11, wherein said selection means selects said second output when a displacement amount between said first and second images is above a predetermined value and said first output when said displacement amount is below said predetermined value. 13. An apparatus according to claim 10, wherein said second filter means has MTF characteristics sufficiently extracting a D.C. component and suppressing frequency components near said spatial frequency 1/4Po. 14. An apparatus according to claim 10, wherein said second filter means has MTF characteristics with the peak of MTF value being located between frequency zero and frequency l/4Po. 15. A focus detection apparatus for detecting a focusing state of an imaging optical system comprising: (1) optical means for forming a first image and a second image of an object with light passing through said imaging optical system, relative displacement between said first and second image varying in response to said focusing state of said imaging optical system; (2) generating means for generating a series of first electric signals and a series of second electric signals each of which represents an illumination distribution pattern of said first image and an illumination distribution pattern of said second image, respectively, said first and second electric signals being image data with respect to a spatial pitch P; (3) first filter means for filtering said first and second electric signals, said first filtering means having MTF characteristics with a peak of MTF value being located near a spatial frequency 1/4P; (4) second filter means for filtering said first and second electric signals, said second filtering means having MTF characteristics substantially suppressing spatial frequency components higher than said spatial frequency 1/4P; and (5) producing means for producing a first output representing said focusing state in accordance with said first and second electric signals filtered by said first filter means, and for producing a second output representing said focusing state in accordance with said first and second electric signals filtered by said second filter means. 16. An apparatus according to claim 15, further comprising third filter means for filtering said first and second electric signals, said third filter means having MTF characteristics with a peak of MTF value being located near a spatial frequency 1/8P, and wherein said producing means further producing a third output representing said focusing state in accordance with said first and second electric signals filtered by said third filter means. 17. An apparatus according to claim 15, further comprising: sampling means for sampling said first and second electric signals filtered by said second filter means at a sampling pitch 2P; discriminating means for discriminating whether said output of said producing means is available or not; and means for causing said producing means to produce said first output in accordance with said first and second electric signals filtered by said first filter means, for causing said discriminating means to discriminate whether said first output is available or not, and for further causing said producing means to produce said second output in accordance with said first and second electric signals filtered by said second filter means and sampled by said sampling means when said discriminating means discriminates said first output to be unavailable. 18. A focus detection apparatus for detecting a focusing state of an imaging optical system comprising (1) optical means for forming a first image and a second image of an object with light passing through said imaging optical system, relative displacement between said first and second images varying in response to said focusing state of said imaging optical system; (2) generating means for generating a series of first electric signals and a series of second electric signals each of which represents an illumination distribution pattern of said first image and an illumination distribution pattern of said second image, respectively, said first and second electric signals being image data with respect to a spatial pitch P; (3) filter means for filtering said first and second electric signals, said filter means having MTF characteristics suppressing a spatial D.C. component; (4) producing means for producing a first focus detection signal representing said focusing state in accordance with said first and second electric signals filtered by said filter means, and for producing a second focus detection signal in accordance with said first and second electric signals not filtered by said filter means; (5) discriminating means for discriminating whether said imaging optical system is close to an in-focus position or not in accordance with said focus detection signal; and (6) causing means for causing said producing means to produce said second focus detection signal in accordance with said first and second electric signals not filtered by said filter means, and for causing said producing means to produce said first focus detection signal in accordance with said first and second electric signals filtered by said filter means when said discriminating means discriminates said imaging optical system to be close to said in-focus position. 19. An apparatus according to claim 18, wherein said filter means has MTF characteristics sufficiently extracting spatial frequency components near a spatial frequency 4P. 20. An apparatus according to claim 19, further comprising additional filter means for filtering said first and second electric signals, and having MTF characteristics different from MTF characteristics of said filter means, and wherein said causing means causes said producing means to produce a third focus detection signal in accordance with said first and second electric signals filtered by said additional filter means. 21. A focus detection apparatus for detecting a focusing state of an imaging optical system comprising: (1) optical means for forming a first image and a second image of an object with light passing through said imaging optical system. relative displacement between said first and second images varying in response to said focusing state of said imaging optical system; (2) generating means for generating a series of first electric signals and a series of second electric signals each of which represents an illumination distribution pattern of said first image and an illumination distribution pattern of said second image, respectively, said first and second electric signals being image data with respect to a spatial pitch P; (3) filter means for filtering said first and second electric signals, said filter means having MTF characteristics suppressing a spatial D.C. component; (4) displacement amount detecting means for detecting a relative displacement amount between said first and second images in accordance with said first and second electric signals filtered by said filter means and in accordance with said first and second electric signals not filtered by said filter means, respectively; (5) setting means for setting maximum displacement amount detectable for said displacement amount detecting means; and (6) selecting means for selecting first or second mode, in said first mode said setting means setting said maximum displacement amount relatively small and said displacement amount detecting means detecting said relative displacement amount in accordance with said first and second electric signals filtered by said filter means, and in said second mode said setting means setting said maximum displacement amount relatively large and said displacement amount detecting means detecting said relative displacement amount in accordance with said first and second electric signals not filtered by said filter means, said selecting means selecting said second mode from said first mode when in said first mode said displacement amount detecting means is unable to detect said relative displacement amount within said detectable maximum displacement amount. 22. An apparatus according to claim 21, wherein said filter means has MTF characteristics sufficiently extracting spatial frequency components near a spatial frequency 1/4P. 23. An apparatus according to claim 21, further comprising: a discriminating means for discriminating whether said relative displacement amount detected by said displacement amount detecting means is reliable or not, and wherein said selecting means selects said second mode from said first mode when in said first mode said relative displacement amount is detected within said maximum displacement amount by said displacement amount detecting means and said discriminating means discriminates said relative displacement amount to be unreliable. 24. An apparatus according to claim 22, further comprising: an additional filter means for filtering said first and second electric signals and having MTF characteristics different from MTF characteristics of said filter means, and wherein said displacement amount detecting means, in said second mode, detects said relative displacement amount in accordance with said first and second electric signals filtered by said additional filter means. 25. A focus detection apparatus for detecting a focusing state of an imaging optical system comprisiing: (1) Optical means for forming a first image and a second image of an object with light passing through said imaging optical system, relative displacement between said first and second images varying in response to said focusing state of said imaging optical system; (2) generating means for generating a series of first electric signals and a series of second electric signals each of which represents an illumination distribution pattern of said first image and an illumination distribution pattern of said second image, respectively, said first and second electric signals being image data with respect to a spatial pitch P; (3) first and second filter means for filtering said first and second electric signals, said first and second filter means both having MTF characteristics suppressing a D.C. component, a peak frequency corresponding to a peak of MTF value of said first filter means being higher than that of said second filter means; (4) displacement amount detecting means for detecting a relative displacement amount between said first and second images in accordance with said first and second, electric signals filtered by said first filter means and for detecting said amount in accordance with said first and second electric signals filtered by said second filter means, respectively; (5) causing means for causing said displacement amount detecting means to detect said relative displacement amount in accordance with said first and second electric signals filtered by said first filter means, and then causing said displacement amount detecting means to detect said relative displacement amount in accordance with said first and second electric signals filtered by said second filter means. 26. An apparatus according to claim 25, further comprising: means for inhibiting detection of said relative displacement amount in accordance with said first and second electric signals filtered by said second filter means when said relative displacement amount is detected by said displacement amount detecting means in accordance with said first and second electric signals filtered by said first filter means. 27. An apparatus according to claim 26, wherein said peak frequencies of said first and second filter means are about 1/4P and 1/8P, respectively, where P is a spatial pitch of said first and second electric signals used for detection of said relative displacement amount by said displacement amount detecting means. 28. A focus detection apparatus for detecting a focusing state of an imaging optical system comprising: (1) first and second photoelectric element arrays, each of said arrays including a plurality of photoelectric elements arranged at a pitch Po; (2) optical means for forming a first image of an object on said first array and forming a second image of said object on said second array, said first array generating a series of first signals which represent said first image, said second array generating a series of second signals which represent said second image; (3) first filter means for filtering said first and second signals, said first filter means having MTF characteristics with spatial frequency components above frequency 1/2mPo being sufficiently suppressed; (4) first sampling means for sampling at sampling pitch mPo, said first and second signals filtered by said first filter means, (5) second filter means for filtering said first and second signals, said second filter means having MTF characterisitcs with spatial frequency components above frequency 1/2nPo being sufficiently suppressed; (6) second sampling means for sampling, at a sampling pitch nPo, said first and second signals filtered by said second filter means, where "n" is an integer equal to 2 or more, "m" being an integer more than "n"; (7) operation means for generating, based on first and second signals sampled by said first sampling means, a first output which represents a focusing state of said imaging optical system and for generating, based on first and second signals sampled by said second sampling means, a second output which represents said focusing state. 29. An apparatus according to claim 28, further comprising: selection means for alternatively selecting sa first output and said second output, and means responsive to one of said first and second outputs selected by said selection means for effecting at least one of display operation of said focusing state and driving operation for driving said imaging optical system to an in-focus position. 30. An apparatus according to claim 29, wherein said selection means selects said first output when a displacement amount between said first and second image is above a predetermined value and said second output when said displacement amount is below said predetermined value. 31. An apparatus according to claim 28, wherein said second filter means has MTF characteristics with a peak of MTF value being located near a frequency 1/4nPo. 32. An apparatus comprising: (1) an imaging optical system; (2) optical means for forming a first image and a second image of an object with light passing through said imaging optical system; (3) generating means for generating a series of first electric signals and a series of second electric signals each of which represents an illumination distribution pattern of said first image and an illumination distribution pattern of said second image, respectively, said first and second electric signals being image data with respect to a spatial pitch P; (4) filter means for filtering said first and second electric signals, said filter means suppressing spatial frequency components higher than a frequency 1/2nP, where n is an integer greater than or equal to 2; (5) sampling means for sampling, at a sampling pitch nP, said first and second electric signals filtered by said filter means; and (6) producing means for producing a first output which represents a relative displacement amount between said first and second images in accordance with said first and second electric signals with said spatial pitch P, and for producing a second output which represents said relative displacement amount in accordance with said first and second electric signals sampled by said sampling means. 33. An apparatus according to claim 32, further comprising selecting means for alternatively selecting said first or second output by manual operation. 34. A focus detection apparatus for detecting a focusing state of an imaging optical system comprising: (1) Optical means for forming a first image and second image of an object with light passing through said imaging optical system, relative displacement between said first and second images varying in response to said focusing state of said imaging optical system; (2) generating means for generating a series of first electric signals and a series of second electric signals each of which represents an illumination distribution pattern of said first image and an illumination distribution pattern of said second image, respectively, said first and second electric signals being image data with respect to a spatial pitch P; (3) first filter means for filtering said first and second electric signals so as to substantially suppress spatial frequency components higher than a frequency 1/4P; (4) first sampling means for sampling, at a sampling pitch 2P, said first and second electric signals filtered by said first filter means; (5) second filter means for filtering said first and second electric signals sampled by said first sampling means so as to suppress high spatial frequency components of said first and second electric signals; (6) second sampling means for sampling, at a sampling pitch 4P, said first and second electric signals filtered by said second filter means; and (7) producing means for producing an output in accordance with said first and second electric signals sampled by said first sampling means, and for producing an output in accordance with said first and second electric signals sampled by said second sampling means, each of said outputs representing said focusing state. 35. An apparatus comprising: (1) a light-receiving array including a plurality of light-receiving portions disposed at a pitch Po, each of of said light-receiving portions producing an electric output associated with an intensity of incident light thereto; (2) an imaging optical system forming an image of an object on said light-receiving array; (3) filter means for multiplying each of said electric outputs of more than 4 of said light-receiving elements adjacent to each other by a predetermined weighting coefficient, for adding said weighted electric outputs, and for producing said added output at every pitch nPo, where n is a natural number, said filter means having MTF characteristics in which MTF values are sufficiently small at least for a spatial frequency band ranging from 3/8Po to 5/8Po and gradually increase as the spatial frequency decreases from a spatial frequency 3/8Po to a predetermined spatial frequency where the MTF value is sufficiently large. 36. An apparatus according to claim 35, wherein said spatial frequency band ranges from about 1/4Po to about 3/4Po. 37. An apparatus according to calim 35, wherein n is an integer greater than or equal to 2, said spatial frequency band ranges from 3/4nPo to (1/Po-3/4nPo), and said predetermined spatial frequency is less than 1/4nPo. 38. An apparatus comprising: (1) an optical system for forming a first image and a second image of an object; (2) generating means including a plurality of light-receiving portions disposed at a pitch Po and for generating a series of first electric signals and a series of second electric signals each being associated with said first image and second image, respectively, said first and second electric signals being image data with respect to a spatial pitch P; and (3) filter means for receiving and filtering said first and second electric signals, MTF characteristics synthesized from MTF characteristics of said filter means and MTF characteristics determined by configurations of said light-receiving portions satisfying 0.1Ap<Ao<0.8Ap, where Ao is a value of said synthesized MTF at zero frequency and Ap is a maximum value of said synthesized MTF. 39. An apparatus according to claim 38, wherein said synthesized MTF characteristics satisfies 0.2Ap<Ao<0.7Ap. 40. An apparatus according to claim 38, wherein said spatial pitch P is equal to Po. 41. An apparatus according to claim 38, wherein said filter means multiplies said plural electric signals by three non-zero weighting coefficients W 1, W 2 and W 3 and then adds said weighted electric signals, weighting coefficients W 1 and W 3 being of the same sign and substantially equal to each other, and W 2 having an opposite sign to W 1, said weighting coefficients W 1, W 2 and W 3 are multiplied to said electric signals at every pitch 2P. 42. An apparatus comprising: (1) an imaging optical system for forming an image of an object; (2) means for obtaining a series of electric signals a 1, a 2, a 3, . . . from said image; (3) first and second weighting/adding filter means for receiving and filtering said series of electric signals a 1, a 2, a 3, . . . , said first filter means performing a weighting/adding operation ##EQU## on said received electric signals a 1, a 2, a 3, . . . , thereby producing outputs A 1 1, said second filter means performing a weighting/adding operation ##EQU## on said received electric signals a 1, a 2, a 3, . . . ,thereby producing outputs A 2 i, a progression of weighting coefficients W 1, W 2, . . . , W m for said first filter means being different from a progression of weighting coefficients M 1, M 2, . . . , M s for said second filter means; and (4) operating means for multiplying said output A 1 i of said first filter means and said output A 2 i of said second filter means by a first coefficient and a second coefficient, respectively, and then adding these multiplied outputs. 43. An apparatus according to claim 42, further comprising modifying means for modifying said first and second coefficients. 44. An apparatus comprising: (1) an optical means for forming a first image and a second image of an object; (2) generating means for generating a series of first electric signals a 1, a 2, a 3 . . . and a series of second electric signals b 1, b 2, b 3 . . . , each being associated with said first image and said second image, respectively, (3) weighting/adding filter means for receiving and filtering said first and second electric signals, said filter means performing weighting/adding operations ##EQU## on said received first and second electric signals, respectively, weighting coefficients W 1, W 2, . . . , W m being selected in such a way that at least three of weighting coefficients W 1, W 2, . . . , W m are not zero, and a summation of weighting coefficients W 1, W 2, . . . , W m is substantially equal to zero; and (4) producing means for producing, in accordance with said first and second electric signals filtered by said filter means, an output associated with a relative displacement amount between said first and second image. 45. An apparatus according to claim 44, wherein both of said first and second electric signals have a pitch P, number of non-zero weighting coefficients three, the designated by W k, W k+l , W k+l +t , respectively, k, l and t being natural numbers, W k and W k+l+t being of the same sign, W k+l being of an opposite sign to W k, l and t for said three weighting coefficients W k, W k+l , W k+l +t are determined so as to satisfy l=n and l+t=2n, n being a natural number. 46. An apparatus according to claim 45, wherein n=2, W k is equal to W k+l +t, and an absolute value of W k is half of that of W k+l . 47. An apparatus according to claim 44, wherein said filter means has MTF characteristics in which a spatial frequency f corresponding to a peak of MTF value satisfies 0<f≤l/4P, and the MTF value for a spatial frequency 3/8P is below a half of said peak of MTF value. 48. A focus detection apparatus comprising: (1) an imaging optical system; (2) optical means for forming a first image and a second image of an object, a relative displacement between said first and second images varying in accordance with a focusing state of said imaging optical system; (3) generating means for generating a series of first signals representing an illumination distribution pattern of said first image and a series of second signals representing an illumination distribution pattern of said second image, said first and second signals being image data with respect to a spatial pitch P; (4) correlating means for receiving said first and second signals and computing a correlation amount between patterns of said first and second signals every time when said first and second signals are relatively shifted to each other by a predetermined amount, said predetermined amount being an integer in a unit of said spatial pitch P; (5) interpolating means for interpolating from said plural correlation amounts obtained by said correlating means local minimums of said correlation amounts and shift amounts which give said local minimums; and (6) means for comparing said local minimums with a predetermined value, and, in accordance with said interpolated shift amounts which give said local minimums exceeding said predetermined value, for effecting at least one of driving operation of said imaging optical system to an in-focus position and display operation of said focusing state of said imaging optical system. 49. A focus detection apparatus according to claim 48, wherein said local minimums are normalized so as not to depend on said illumination distribution patterns of said first and second images, and said predetermined value is a constant. 50. A focus detection apparatus according to claim 48, further comprising causing means for causing said interpolating means to interpolate said shift amounts only when said shift amounts give said local minimums exceeding said predetermined value. 51. A detection apparatus for detecting a relative displacement amount between a first pattern and a second pattern to each other which are substantially the same to each other comprising: (1) generating means for generating a series of first electric signals A 1, A 2, . . . , A N representing said first pattern with respect to a spatial pitch P and a series of second electric signals B 1, B 2 . . . , B M representing said second pattern with respect to said spatial pitch P; (2) correlating means for receiving said first and second electric signals and for computing a set of correlation amounts between said first and second patterns C(L-1), C(L) and C(L+1) by ##EQU##C(L)=Σ|Ai-Bi+L| everytime when said first and second electric signals are relatively shifted by predetermined values L-1, L and L+1, and then repeating said computation of said set of correlation amounts with shift number L being varied, said predetermined values being integers in a unit of said spatial pitch P; and (3) shift amount operation means for computing a shift amount Lm from said set of correlation amounts obtained by said correlating means by ##EQU## E being a greater one of C(L+1)-C(L) and C(L-1)-C(L). 52. A detection apparatus according to claim 51, further comprising: producing means for producing an extremal output proportional to are extremal Cext obtained from said plural correlation amount by ##EQU##Cext=C(L)-0.5|C(L-1)-C(L+1)|; and discriminating means for discriminating whether said extremal output exceeds a preselected value or not, and then causing said shift amount operation means to compute said shift amount which gives said extremal output exceeding said preselected value. 53. A detection apparatus according to claim 52, wherein said extremal outputs are formed by normalizing said extremals so as not to depend on types of said first and second patterns, and said preselected value is a constant. 54. A detection apparatus according to claim 53, wherein said extremal output is equal to said extremal divided by E. 55. A detection apparatus according to claim 51, wherein said correlating means includes memory means storing at least one of the correlation arounts of said set, and said stored correlation amounts in said memory means are used as at least one of said sets of correlation amounts to be newly computed on varying said shift number L. 56. A detecting apparatus according to claim 52, wherein said correlating means computes said set of correlation amounts C(L-1), C(L) and C(L+1) for L=O, and then computes said sets of correlation amounts when said shift number L is varied successively as L=±1,±2, . . . , said producing means produces said extremal outputs in accordance with said sets of correlation amounts C(L-1), C(L) and C(L+1) transferred from said correlation means, and said discriminating means causes computating operations by said correlation means to terminate on detecting that said extremal output exceeds said preselected value. 57. A focus detection apparatus for detecting a focusing state of an imaging optical system comprising: (1) optical means for forming a first image and a second image of an object with light passing through said imaging optical system, relative displacement between said first and second images varying in response to said focusing state of said imaging optical system; (2) generating means for generating a series of first electric signals and a series of second electric signals each of which represents an illumination distribution pattern of said first image and an illumination distribution pattern of said second image, respectively, said first and second electric signals being image data with respect to a spatial pitch P; (3) filter means for filtering said first and second electric signals, said filter means having MTF characteristics suppressing a spatial DC component; (4) displacement amount detecting means for detecting a first relative displacement amount between said first and second images in accordance with said first and second electric signals filtered by said filter means, and for detecting a second relative displacement amount between said first and second images in accordance with said first and second electric signals not filtered by said filter means; (5) selecting means for selecting a first or a second mode, in said first mode said displacement amount detecting means detecting said first relative displacement amount in accordance with said first and second electric signals filtered by said filter means, in said second mode said displacement amount detecting means detecting said second relative displacement amount in accordance with said first and second electric signals not filtered by said filter means, said selecting means selecting said second mode from said first mode when in said first mode said displacement amount detecting means is unable to detect said first relative displacement amount. 58. A focus detection apparatus for detecting a focusing state of an imaging optical system comprising: (1) optical means for forming a first image and a second image of an object with light passing through said imaging optical system, relative displacement between said first and second images varying in response to said focusing state of said imaging optical system; (2) generating means for generating a series of first electric signals and a series of second electric signals each of which represents an illumination distribution pattern of said first image and an illumination distribution pattern of said second image, respectively, said first and second electric signals being image data with respect to a spatial pitch P; (3) filter means for filtering said first and second electric signals, said filter means having MTF characteristics suppressing a spatial DC component; (4) producing means for producing a first focus detection signal representing said focusing state in accordance with said first and second electric signals filtered by said filter means, and for producing a second focus detection signal in accordance with said first and second electric signals not filtered by said filter means; (5) causing means for causing said producing means to produce said first focus detection signal in accordance with said first and second electric signals filtered by said filter means, and for causing said producing means to produce said second focus detection signal in accordance with said first and second electric signals not filtered by said filter means when said producing means is unable to produce said first focus detection signal.