Abstract

A focus-sensor module for large-format photographic cameras has been developed that permits the measurement of defocus at any location of interest in the image field. The focus sensor employs passive triangulation through a split imaging aperture. The main difference between commercial autofocus modules with fixed-measurement positions and the new module is that the imaging aperture is subdivided into more than two fields to compensate for the unknown location of the defocus measurement.

At f/5.6 the focus sensor shows a maximum resolution in defocus of approximately 0.1 mm at the image side at levels of illuminance in the recording plane ≥0.01 lx.

© 1995 Optical Society of America

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References

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  1. H.-C. Koch, C. Koch, K. Gfeller, “Apparatus for a photographic camera with lens and film carriers which can be reset in relation to one another,” U.S. patent4,564,277 (14January1986).
  2. N. Goldberg, “Inside autofocus: how the magic works,” Pop. Photogr. 63, No. 2, 77–83 (1982).
  3. The Centre Suisse d’Electronique et de Microtechnique SA, is located in Maladiere, Rue 71, CH-2000 Neuchâtel, Switzerland.
  4. M. T. Gale, G. K. Lang, J. M. Raynor, H. Schütz, “Fabrication of micro-optical components by laser beam writing in photoresist,” in Micro-Optics II, A. V. Scheggi, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 1506, 65–70 (1991).
  5. G. Wyszecki, W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd ed. (Wiley, New York, 1982), Chap. 4.
  6. Ref. 5, Chap. 3.
  7. K. Engelhardt, P. Seitz, “Optimum color filters for CCD digital cameras,” Appl. Opt. 32, 3015–3023 (1993).
    [CrossRef] [PubMed]
  8. N. Tanaka, T. Ohmi, Y. Nakamura, “A novel bipolar imaging device with self-noise-reduction capability,” IEEE Trans. Electron Devices 36, 31–38 (1989).
    [CrossRef]

1993

1989

N. Tanaka, T. Ohmi, Y. Nakamura, “A novel bipolar imaging device with self-noise-reduction capability,” IEEE Trans. Electron Devices 36, 31–38 (1989).
[CrossRef]

1982

N. Goldberg, “Inside autofocus: how the magic works,” Pop. Photogr. 63, No. 2, 77–83 (1982).

Engelhardt, K.

Gale, M. T.

M. T. Gale, G. K. Lang, J. M. Raynor, H. Schütz, “Fabrication of micro-optical components by laser beam writing in photoresist,” in Micro-Optics II, A. V. Scheggi, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 1506, 65–70 (1991).

Gfeller, K.

H.-C. Koch, C. Koch, K. Gfeller, “Apparatus for a photographic camera with lens and film carriers which can be reset in relation to one another,” U.S. patent4,564,277 (14January1986).

Goldberg, N.

N. Goldberg, “Inside autofocus: how the magic works,” Pop. Photogr. 63, No. 2, 77–83 (1982).

Koch, C.

H.-C. Koch, C. Koch, K. Gfeller, “Apparatus for a photographic camera with lens and film carriers which can be reset in relation to one another,” U.S. patent4,564,277 (14January1986).

Koch, H.-C.

H.-C. Koch, C. Koch, K. Gfeller, “Apparatus for a photographic camera with lens and film carriers which can be reset in relation to one another,” U.S. patent4,564,277 (14January1986).

Lang, G. K.

M. T. Gale, G. K. Lang, J. M. Raynor, H. Schütz, “Fabrication of micro-optical components by laser beam writing in photoresist,” in Micro-Optics II, A. V. Scheggi, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 1506, 65–70 (1991).

Nakamura, Y.

N. Tanaka, T. Ohmi, Y. Nakamura, “A novel bipolar imaging device with self-noise-reduction capability,” IEEE Trans. Electron Devices 36, 31–38 (1989).
[CrossRef]

Ohmi, T.

N. Tanaka, T. Ohmi, Y. Nakamura, “A novel bipolar imaging device with self-noise-reduction capability,” IEEE Trans. Electron Devices 36, 31–38 (1989).
[CrossRef]

Raynor, J. M.

M. T. Gale, G. K. Lang, J. M. Raynor, H. Schütz, “Fabrication of micro-optical components by laser beam writing in photoresist,” in Micro-Optics II, A. V. Scheggi, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 1506, 65–70 (1991).

Schütz, H.

M. T. Gale, G. K. Lang, J. M. Raynor, H. Schütz, “Fabrication of micro-optical components by laser beam writing in photoresist,” in Micro-Optics II, A. V. Scheggi, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 1506, 65–70 (1991).

Seitz, P.

Stiles, W. S.

G. Wyszecki, W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd ed. (Wiley, New York, 1982), Chap. 4.

Tanaka, N.

N. Tanaka, T. Ohmi, Y. Nakamura, “A novel bipolar imaging device with self-noise-reduction capability,” IEEE Trans. Electron Devices 36, 31–38 (1989).
[CrossRef]

Wyszecki, G.

G. Wyszecki, W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd ed. (Wiley, New York, 1982), Chap. 4.

Appl. Opt.

IEEE Trans. Electron Devices

N. Tanaka, T. Ohmi, Y. Nakamura, “A novel bipolar imaging device with self-noise-reduction capability,” IEEE Trans. Electron Devices 36, 31–38 (1989).
[CrossRef]

Pop. Photogr.

N. Goldberg, “Inside autofocus: how the magic works,” Pop. Photogr. 63, No. 2, 77–83 (1982).

Other

The Centre Suisse d’Electronique et de Microtechnique SA, is located in Maladiere, Rue 71, CH-2000 Neuchâtel, Switzerland.

M. T. Gale, G. K. Lang, J. M. Raynor, H. Schütz, “Fabrication of micro-optical components by laser beam writing in photoresist,” in Micro-Optics II, A. V. Scheggi, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 1506, 65–70 (1991).

G. Wyszecki, W. S. Stiles, Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd ed. (Wiley, New York, 1982), Chap. 4.

Ref. 5, Chap. 3.

H.-C. Koch, C. Koch, K. Gfeller, “Apparatus for a photographic camera with lens and film carriers which can be reset in relation to one another,” U.S. patent4,564,277 (14January1986).

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Figures (7)

Fig. 1
Fig. 1

Schematic diagrams of camera focus analyzers: R, right; L, left. (a) Camera with a focus analyzer imaging a point source. The numbers 1, 2, and 3 represent positions of the image plane. (b) Focus analyzer of a modern autofocus SLR camera. The analyzer consists of a microlens array and a photodetector array. Each microlens images the aperture of the camera lens onto a pair of photodetectors and thus splits the aperture into two halves.

Fig. 2
Fig. 2

Schematic diagram of the optical setup for focus sensing through passive triangulation: A microlens (lenslet) array in the image plane projects the exit pupil of the imaging (camera) lens onto a linear detector (photodiode) array. In the special configuration shown, four subimages, A, B, C, and D, are acquired through different parts of the imaging aperture.

Fig. 3
Fig. 3

Passive triangulation: Defocus dz causes a separation dx of the points where the chief rays (b and c, solid-line paths) through sections B and C of the imaging aperture (shaded area) hit the image plane. O, object; O′, image.

Fig. 4
Fig. 4

Spectral responses of the focus sensor (thin solid curve), which consists of the photodiode array and a 1-mm-thick BG38 infrared cut-off filter under the illumination of an equal-energy spectrum S E ; the focus sensor under halogen–tungsten illumination at a color temperature of 3200 K (thick solid curve); the focus sensor under CIE average daylight D65 (short-dashed curve); and the spectral sensitivity of the human eye (long-dashed curve), which is the standard Vλ curve.

Fig. 5
Fig. 5

Image signal acquired with the prototype focus sensor under illumination from a halogen–tungsten point source at 1 m distant from the focus sensor. The good contrast and periodicity indicate the good focusing quality of the diffractive–refractive microlens array.

Fig. 6
Fig. 6

Experimental focusing sequences of the same image structure at 2.5-lx (solid curve) and 0.008-lx (dashed curve) levels of illuminance in the image plane. The recording plane was moved manually along the optical axis with an almost constant step of approximately 1 mm, and the defocus values measured by the focus sensor were recorded.

Fig. 7
Fig. 7

Experimental focusing sequence acquired at 0.5-lx illumination in the image plane and with a focusing step width of 0.2 mm. When the measure of defocus (filled squares) is close to zero, the symmetry of the correlation coefficients (triangles) is at a maximum and the measure of structural content (open squares) is at a maximum. The combination of all three measures can be useful for the detection of error and for the detection of optimum focus with increased sensitivity.

Equations (2)

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c A B ( m ) = i = M N - M A i B i + m ,             i = 0 , , N ,
S = exp [ - m = 0 M 1 m c A B ( m ) - c A B ( - m ) c A B ( m ) + c A B ( - m ) ] .

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