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.
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
The spatial relationships between the central terms are given in
the sketch below.
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
The focal length divided by the effective aperture.
The diaphragm mechanism that controls the amount of light transmitted in an
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.
The diameter of the physical aperture stop (the diaphragm) at the
entrance pupil. This may be different from the actual physical aperture.
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.
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.
The plane behind a lens system where a focused image is formed, i.e.
where the film or the digital sensor reside.
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).
The plane covered in front of the lens by the focal plane at the focus
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).
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.
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.
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.
Click the "Open Image..." button.
Drag and drop the image onto the Barnack window.
Use "Open With..." in Windows Explorer and select Barnack.
Add Barnack as an external editor in your favourite image
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
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.
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