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Hyperfocal distance: a CoC for the 21st century
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PostPosted: Fri Mar 23, 2012 9:52 am    Post subject: Hyperfocal distance: a CoC for the 21st century Reply with quote
Traditionally, a circle of confusion of 0.03mm is used in depth of field charts or calculators.
The DoF scale engraved on manual lenses, also uses this value.

I exchanged a few emails with Zeiss.
I was not getting the expected results from my 21mm Distagon (ZE version).
I was focusing it at 2m and with f/8 or f/9 I thought I would get sharp items at infinity.
However, this was far from true.
Zeiss disagreed with the lens calculator I use.
But the interesting part was their comment regarding the proper value for the CoC.

For pixel peeping of 21MP photos, we need increased precision.

Here is an extract of our correspondence:

Nikos wrote:

The original reason for investigating the issue were scenes like No 3:
I focus at 2m and take a shot at f/8, expecting adequate depth of field
and good sharpness in the whole frame. I think you will agree that the
result is far below the expectations for a legendary lens like the 21mm Distagon.

Carl Zeiss wrote:

When a 21mm lens is focused at 2m and f/8, the DOF goes up to 34m (with a circle of confusion of 0.03mm), but reaches not infinity. If a more critical circle of confusion (0.015mm) is used which is much more realistic for the criteria when looking at a 21MP image at 100%, the depth of field is even shallower.
So it is not possible to get a sharp image at infinity when the lens was focused to 2m.


PostPosted: Fri Mar 23, 2012 12:30 pm    Post subject: Re: Hyperfocal distance: a CoC for the 21st century Reply with quote
Nikos wrote:
Traditionally, a circle of confusion of 0.03mm is used in depth of field charts or calculators.


Traditionally the size was defined as 1/1500 of the diagonal of the (intended) film frame. The idea was that when photos are printed for viewing, larger prints will be viewed from farther away, so the size of the smallest detail distinguishable by the human eye is roughly the same for any size of print. Therefore, the circle of confusion can be defined as a fraction of the film frame size: larger frames need to be enlarged less to reach a given print size, so they can use a larger circle of confusion. This means, however, that lenses adapted to a smaller format (e.g., FF lens to APS-C) will have incorrect DoF scales even if this (fairly loose) definition is used.

(Meanwhile Zeiss reputedly did not use 1/1500 but 1/1730 or something like that. 1/1500 of the full frame rounds to about 0.03, though.)

This formula could also be used on digital cameras if people would be content to view images at realistic sizes (e.g., filling their computer screen or as prints). However, pixel peeping necessitates a much more stringent definition for the acceptable CoC, as you noticed, since 100% crop is basically equivalent to viewing a large print with a magnifying glass.

Nikos wrote:

For pixel peeping of 21MP photos, we need increased precision.


Yes, but there are some problems with calculating the circle of confusion for pixel perfect DoF:

1) Almost all digital cameras use colour filter arrays over the sensor so it's not exactly clear whether one should set the CoC threshold based on the sensor's pixel pitch directly or multiply it with something like 1.5-2×. Personally I think 1.5× makes most theoretical sense, but in practice it varies.

2) Modern sensors have such “small pixels” that diffraction will often limit the maximum achievable resolution below that of the sensor's pixel pitch. (This is a good thing.) Most DoF calculators that use “pixel perfect” CoC size do not take this into account and give non-sensical values; if the photographer is willing to accept the diffraction limit at, say, f/11 as the maximum overall resolution, then surely their definition for being in focus cannot be more strict (or nothing would be in focus).

3) Pixel perfect DoF calculations give such shallow DoF on current sensors that they seldom make sense. For example, hyperfocal distance is almost always so far off that it's not possible to distinguish between that and infinity focus (unless, of course, one is stopped down past the diffraction limit and takes this into account)…

Last edited by Arkku on Fri Apr 25, 2014 10:38 am; edited 2 times in total


PostPosted: Fri Mar 23, 2012 10:24 pm    Post subject: Reply with quote
If you have an Iphone there are two very good DOF apps both from the same company. One is TrueDof and the other is OptimumCS.

The TrueDof allows you to set the final print width and also factors in diffraction. You also chose the LP/mm desired in final print then you watch the sliders to find an optimum f/stop. Very easy to use and I've found it to work nicely . Of course you have to input the sensor size as well which t accepts up to 6x9 medium format. Optimum CS is similar but more for landscape use. Quicker to find hyperfocus but a little less involved. Cheap way out of doing the math in the field.
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