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05-17-2011, 01:59 PM   #1
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Circle of confusion

Hi:

What size CoC are you using for the 645D, to determine depth of field?

Thanks,

Paul

05-17-2011, 03:50 PM   #2
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I'm not. I just let the aperture do it.

But seriously, are you doing this for the film 645 and 67 lenses? Simply find the difference in crop factor between the two formats and square that. That will show the number of stops compensation and then use the lens scale--although it will not show maximum aperture. So for example, the 645D sensor format is about half the 67 format--factor of 2. Squared is 4--two stops. If you set your 67 lens to f/11, use the f/5.6 depth of field scale marks on the lens barrel.

If not using that method, the pixel pitch is a good starting point for the CoC (but I think I might be able to make a case for slightly higher). But then you should test. Unfortunately, a 100% monitor display cannot be used to determine if your numbers are "right." You will need to print the image and use standard viewing distance.

But too be honest, DoF preview and looking at an image in playback is much easier...

Last edited by Yamanobori; 05-17-2011 at 04:06 PM.
05-18-2011, 05:57 AM   #3
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After thinking about this for a bit, if I were using 645 and 67 film lenses on the 645D, as far as using the lens DoF scales, I would compensate the 645 lenses by one stop and 67 lenses by two.

So, if my aperture is set to f/11; with a 645 lens, I would use the f/8 scale; with a 67 lens, I would the f/5.6 scale.

If you are trying to calculate DoF for macro photography, then start with the pixel pitch as CoC.
05-20-2011, 01:39 PM   #4
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Just to add a bit to what's already been said, Pentax medium format lenses have traditionally been too optimistic when it comes to their DOF scales. Some trial and error testing needs to be done for each lens because they are all off by differing degrees. Some 6x7 lenses are off by a half stop on the scale while others are off by a full stop or more. I have put hyperfocal marks on my two zooms but only after some experimentation. When stopping down for DOF purposes, the once fuzzy areas of the frame become sharper but it is a subjective opinion as to when this sharpness is adequate. Different shooters will have different opinions and settings.

There is always a tradeoff between DOF and sharpness. We can only rarely shoot at the lenses' diffraction limited stop. On one side of that stop we have aberrations and the other we have diffraction.

05-21-2011, 09:10 PM   #5
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QuoteOriginally posted by desertscape Quote
Just to add a bit to what's already been said, Pentax medium format lenses have traditionally been too optimistic when it comes to their DOF scales
mamiya dof scales are worse, the DOF scales are only there to give you a ballpark figure. the 645D didn't exist when they were in production.

personally I use a COC of 0.045mm as a place to start.
05-24-2011, 10:19 PM   #6
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I calculate the Circle of Confusion for the 645D to be 0.0318mm based on the "Zeiss" formula, which it turns out has nothing to do with Zeiss. But, it has worked for me for many years and produces numbers that is slightly less agressive, thereby assuring better results.
05-24-2011, 10:28 PM   #7
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Paul, I put together an excel spreadsheet to do format comarisons and calculate image characteristics. You can download it in this thread: https://www.pentaxforums.com/forums/general-photography/55197-camera-lens-math-easy.html
05-24-2011, 10:34 PM   #8
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The formula is :
Sensor Diagonal /1730. It's that simple.
Therefore, 55mm/1730 = 0.0318mm

In comparison, the CoC for the Pentax 645 Film camera with 41.5 x 56mm flim dimensions is 0.0403mm.

Since the new 28mm only goes down to f/22, the best you'll do (hyperfocally speaking) is 21.5" as the nearest subject distance. That's good for most people, but not me. The 33-55mm lens at 33mm on the film camera yields 16.7" as the near distance at f/32, which is the f-stop that I frequently shoot at given my application. (Diffraction shmfraction. I'd rather have some softness due to diffraction than blurriness due to no depth of field.)

The reason I originally got into Pentax 645 and not Mamiya was Mamiya lenses only went down to f/22, while Pentax offered f/32. And that's also why I traded in my Pentax 67 soon after I bought it, because the near focus distance (equal to half the hyperfocal distance) is horrendous if you're trying to emphasize subjects in a scene that are very near the camera.

05-24-2011, 10:53 PM   #9
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Correction. I forgot that the new lens is a 25mm (not a 28mm). That means that at f22 on the 645D, the near focus is 17.1". Very good. On the film camera at:
f/16 = 19.1"
f/22 = 13.5"
f/32 = 9.5"
10-13-2011, 07:15 PM   #10
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Hi guys reviving an older thread, found the DOFMASTER website and from what I am seeing the Pentax 645D has a CoC of 0.050mm. This stuff is kind of new to me but I want to learn, can anyone dispute this?

Online Depth of Field Calculator
10-13-2011, 08:59 PM - 1 Like   #11
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The more I read about Depth of Focus the greater my circle of confusion becomes.
10-13-2011, 09:08 PM   #12
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The Circle of Confusion is expressed as d / C
where d is the diagonal measure of the sensor, and C is a constant.

There are 3 common standards for C: 1000, 1500 and 1730. Depending on which 'standard' you choose you may get wildly different DoF calculations. Which one is "correct" is a highly subjective.

The 645D has a diagonal sensor measurement of 55mm.

55 / 1000 = 0.055
55 / 1500 = 0.037
55 / 1730 = 0.032

To illustrate how the CoC formula effects calculations, here are the DoF numbers for a 75mm lens @ f/8 from 25ft:
0.055 == 45.7ft
0.037 == 23.5ft
0.032 == 19.5ft
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