Barnack help

Barnack provides an interactive platform for learning about the intrinsic camera parameters and their effect on the depth of field, the hyperfocal distance, field of view, et cetera.

It is named after Oscar Barnack (1879-1936) who designed the first camera using the 24x36 mm format for German microscope manufacturer Leitz in 1913. This camera was coined the "UR-Leica" and is today widely recognised as the precursor to all 35 mm film cameras.

Glossary

This part enumerates terms within optical theory. Knowledge of these is required for detailed interpretation of the results presented by Barnack. The section also details a few options specific to the Barnack user interface.

The spatial relationships between the central terms are given in the sketch below.

• 2-pixel coc
  Sthe circle of confusion so that it corresponds to twice the pixel pitch. Hence, this coc criterion will provide depth of field estimates, which will roughly correspond to the maximum detail extractable from the selected camera (assuming that it utilises a Bayer-type colour filter array).
  
  
• 35 mm coc reference
  The circle of confusion (coc) in Barnack is specified in terms of a corresponding circle of confusion for a 35 mm film camera. The actual circle of confusion is then calculated for the specific camera and shown in mm (in the right column). For digital cameras the circle of confusion is also displayed in pixels. See also the section "2-pixel coc" above.
  
  
• 35 mm equivalent focal length
  The focal length required on a 35 mm film camera, or a full-frame digital camera, to obtain a field of view that corresponds to the current field of view.
  
  
• Aperture
  The focal length divided by the effective aperture.
  
  
• Aperture stop
  The diaphragm mechanism that controls the amount of light transmitted in an optical system.
  
  
• Blur circle
  The diameter of the disc that covers the area that a point, at the image center, is defocused over. Hence, the blur circle for a perfect lens depicting a subject at the focus distance would be zero.
  
  
• Circle of confusion (coc)
  The amount of blurring that one would still perceive as "acceptably" sharp. Hence, the coc is equal to the diameter of an "acceptable" blur circle located at the focal plane. Choosing a circle of confusion is entirely dependent on the application. Often the coc is chosen based on a limiting factor, e.g. visual acuity or pixel size. A typical coc for a 35 mm film camera is 0.030 mm.
  
  
• Depth of field (dof)
  The distance around a subject that will be perceived as "acceptably" sharp (for a given coc). It is the sum of the near depth of field and far depth of field. Formally, this is the depth range where the blur circle is equal to, or smaller, than the chosen circle of confusion.
  
  
• Depth of focus
  The distance around the focal plane that will produce an "acceptably" sharp image (for a given coc).
  
  
• Diffraction spot diameter
  The diffraction spot diameter measures how much a light beam is (Fraunhofer) diffracted by a circular aperture. Barnack calculates the diameter of the Airy pattern for light at the selected wavelength where it contains approximately 84 percent of the total light energy. Diffraction constitutes the upper limit for optical performance. Hence, the circle of confusion should not be smaller than the diffraction spot. Not even for the fantasy construct of an ideal, aberration-free lens.
  
  
• Effective aperture
  The diameter of the physical aperture stop (the diaphragm) at the entrance pupil. This may be different from the actual physical aperture.
  
  
• Entrance pupil
  The virtual image of the aperture stop that can be seen when looking into the front element of a lens. The center of the diaphragm opening seen in this image constitutes the center of perspective.
  
  
• Far depth of field
  The distance behind the subject being depicted in the image as "acceptably" sharp (for a given coc).
  
  
• Far focus limit
  The distance to the end of the far depth of field.
  
  
• Field of view
  The angle the observation takes place under, e.g. observations using a rectangular focal plane can (redundantly) be described by a horizontal, vertical and diagonal field of view.
  
  
• Focal length
  The distance from the second principal point, to the focal plane for subjects positioned at infinity. Think of the distance from a magnifying glass to a piece of paper that the sun is about to burn a hole in. For a thin lens, the second principal point is identical to the lens position.
  
  
• Focal plane
  The plane behind a lens system where a focused image is formed, i.e. where the film or the digital sensor reside.
  
  
• Focus distance
  The subject distance that will be depicted in perfect focus on the focal plane. This distance is measured from the subject to the entrance pupil.
  
  
• Hyperfocal distance (hfd)
  The focus distance that maximises the depth of field for a given circle of confusion. At hfd the range: [hfd/2;infinity] will be in focus. Thus, if the hyperfocal distance is 5 m, then everything from 2.5 m to infinity will be in focus. The hyperfocal distance is also visualized in the graph "Blur Circle vs. Subject Distance" when the circle of confusion (the horizontal green line) remain less than the asymptotic size of the blur circle (red graph).
  
  
• Image plane
  The plane covered in front of the lens by the focal plane at the focus distance.
  
  
• Magnification
  The reproduction ratio of a given image plane on the focal plane.
  
  
• Near depth of field
  The distance in front of the subject being depicted in the image as "acceptably" sharp (for a given coc).
  
  
• Near focus limit
  The distance to the beginning of the near depth of field.
  
  
• Recomposition focus circle (rfc)
  The area that a central target can be moved within and remain in focus. This assumes that: i) the recomposition is carried out as a rotation around the entrance pupil of the lens, ii) the focus distance is accurate, and iii) that the object focus field is flat. Notice that these conditions may not be satisfied. The (rarely visible) red and green circles show the bounds of the rfc for an auto focus tolerance of one depth of field (dof).
  
  
• Wavelength
  The wavelength of the light used in the diffraction spot calculation and all MTF estimates. This should typically be left at the default value of 550 nm, as this corresponds to a shade of green close to the point of maximal sensitivity in the human vision system (555 nm). Blue and red are located around 450 and 650 nm, respectively.
  

Limitations

All depth of field estimates assume that the the optical system is fully described by the thin-lens equation: 1/f = 1/c + 1/s, where f, c and s denote the focal length, focal plane distance and focus distance, respectively.

Implicitly, this means that the following assumptions are made:

• Symmetrical lens design / pupil magnification is unity / the first principal point coincides with the entrance pupil (in fact, both principal points, both nodal points, and entrance and exit pupils coincide).
  
• No lens aberrations (spherical aberration, astigmatism, coma, distortion, field curvature, chromatic aberration).
  
• Perfect circular aperture.
  
• No diffraction effects.

Strictly speaking, this means that the depth of field estimates are guidelines, rather than hard facts. That said, they can portray reality quite accurately. Particularly for lens designs that are not extremely wide-angle or tele-photo oriented. Secondly, their accuracy is by far sufficient for illustrating the effects on DOF that focal length, aperture, subject distance, sensor size, et cetera will have.

Calculating more accurate estimates would require detailed information regarding the specific lens design, which is typically not available.

Tips

• Camera type, focal length, aperture, sensitivity, shutter speed, and, for some lenses, focus distance, can all be read from JPEG images. This provides a handy alternative to set the values using the sliders. Use one of the following methods to load an image.
  
  
  1. Click the "Open Image..." button.
  2. Drag and drop the image onto the Barnack window.
  3. Use "Open With..." in Windows Explorer and select Barnack.
  4. Add Barnack as an external editor in your favourite image browser.
     
     
• A loaded image can be sent to the default image viewer by double-clicking on it in the "Image Plane" section.

Frequently asked questions

• Q: The calculations carried out by Barnack do not match my favourite DOF calculator. What's up with that?
  
  A: Some calculators introduce round-off errors, e.g. in the focal length multiplier, or have inaccurate sensor sizes. Such simplifications are typically very fair, but they complicate comparison. Another source of discrepancy is the common assumption, that the focus distance is much greater than the focal length. In that case, it can be justified to use a set of somewhat simpler, albeit slightly inaccurate, DOF equations.
  
  
• Q: The displayed magnification does not match the one specified by the manufacturer at the closest focusing distance.
  
  A: The closest focusing distance specified by the manufacturer is typically measured from the focal plane. The focus distance set in Barnack is the distance from the entrance pupil which is somewhat closer to the subject. This is also denoted the subject distance. Hence, for a 100 mm life-size macro lens with a closest focusing distance of 0.31 m, a distance from the entrance pupil to the subject of 0.20 m would result in a magnification of 1:1.

Recommended further reading

• Science of photography
  
• Understanding Depth of Field
  
• Depth of field
  
• DOFMaster - Depth of Field Articles
  
• Depth of field and your digital camera

Acknowledgements

I thank the following members of the Digital Photography Review forums for their very helpful elaborations, comments and suggestions (in no particular order): Douglas A. Kerr, Detlev Rackow, Leon Wittwer, and David (DRG). I also sincerely thank all the good people contributing to Wikipedia and other online resources with their write-ups shedding light at optical theory.