COCs

[erker@xxxxxxxxxxxxxx (Greg Erker)]

>Can somebody point me to an online reference that deals with the circle of
>confusion concept??

  Here's something from dejanews that discusses
why MF has less DOF than 35mm and says why
you use different COCs for them.

---start---
>The same conclusion was reached in an earlier discussion with a more
>technical and rigorous explanation by Ben Weiner:
>
>From: bweiner@xxxxxxxxxxxxxxxx (Ben Weiner)
>Newsgroups: rec.photo.equipment.medium-format
>Subject: Re: MF depth of field
>Date: 18 Dec 1996 17:10:24 -0500
>
>Garry Lee <glee@xxxxxx> writes:
>
>>The point remains that the DOF is the same, view for view in any format.
>>For the same sharpness, the Circle of Confusion is bigger in a bigger
>>format BUT that is because the negative is bigger and needs less
>>enlargement.
>>It is a MYTH that larger formats have less DOF.
>>They don't.
>
>With all due respect to the participants in this discussion, it is
>a confusing subject, but the above is simply not true.  If you have
>ever struggled to get enough DOF with a reasonable f-stop while
>using a "normal" length lens on 4x5 or 8x10 you will know that
>larger formats DO have less DOF.
>
>Let's look at hyperfocal distance as an indicator of DOF.
>There is a simple formula for hyperfocal distance:
>
>     h = F^2 / (N*c)
>
>  h = hyperfocal distance
>  F = focal length
>  N = f-number
>  c = diameter of circle of confusion
>
>This formula comes from the Lens FAQ which David Jacobson posts
>to rec.photo.moderated and can also be found in any number of books.
>It is really simple to derive from similar triangles, but it
>is difficult to show this in a text medium, without pictures.
>
>OK.  Suppose we compare 35mm, and 6x7, which is about twice as big
>in linear size.  (No arguments over "4x as big in area", please!)
>Take a 50mm lens on 35mm, and a 100mm lens on 6x7.  If you stand in
>the same place with the two cameras, these give roughly the same
>"normal" perspective and angle of view.  I want to compare how
>the two formats do when taking the SAME picture.
>
>Let's suppose my subject matter extends from 5 meters to infinity.
>To get it all in sharp focus, I need to set the lens so it
>has a hyperfocal distance of 10 meters.  I want to know what
>f-stop I need.  I can solve the above formula for N, the f-number:
>
>      N = F^2 / (h*c)
>
>BUT I need to know the circle of confusion.  Let's take c = 0.025 mm
>(1/100 inch) for 35mm.  Since I only need to enlarge the 6x7 neg
>half as much, I can use a twice-as-big circle-of-confusion for
>6x7; c = 0.05 mm.
>
>OK, plug in the numbers (don't forget to convert h from meters to mm):
>
>For 35mm, N = (50*50) / (10,000 * 0.025).  N = 10, i.e. use f/10 (or f/11).
>
>For 6x7, N = (100*100) / (10,000 * 0.05).  N = 20, i.e. use f/20 (or f/22).
>
>See?  You need to stop down two extra stops, from f/11 to f/22, to get
>the same depth of field.
>
>Why is this?  Basically, it's because of the F^2 in the formula.
>The hyperfocal distance gets larger - and the depth of field gets
>smaller - by a factor of 4 when you double the film size and
>lens size.  The circle of confusion also gets larger, but only
>by a factor of 2.  There's a factor of 2 left over.  In order
>to get back the depth of field you've lost, you have to make
>the f-number go down by a factor of 2 (which is 2 f-stops).
>
>Another way to look at it is this: out-of-focus areas are blurred
>because of the physical diameter of the aperture.  That's why
>stopping down any lens (smaller diameter aperture) gives more depth
>of field.
>
>If you stand in the same place and photograph the same scene with
>two lenses that give the same angle of view, you will get the same
>DOF if the lenses have the same aperture diameter - physical
>diameter in mm, (NOT the same f/number).
>
>The 50mm lens at f/10 has an aperture of 5mm.
>
>The 100mm lens at f/10 has an aperture of 10mm, hence less DOF.
>To get the same DOF as the smaller lens, you need an aperture
>of 5mm, hence 100m lens at f/20.
>
>If you don't believe me, try it.  If you try taking a picture with a
>35mm camera and normal lens at f/8 or so, you will get respectable
>DOF.  With a 6x7, normal lens, f/8, you will get less DOF.  With a
>4x5 or 8x10 camera and its normal lens at f/8, you will get so
>little DOF it will make your head spin.  (Large format photographers
>rarely use apertures as fast as f/8 for this reason, but sometimes
>it can be put to creative use.)
>
>I have spoken.
>
><end quote by Ben Weiner>__________________________________________________
>
>
>In article <6d9im7$6nt@xxxxxxxxxxxxxxxxxxxxxx>,
>  "Charles Petzold" <charles@xxxxxxxxxxxx> wrote:
>>
>> Let's pretend that you own a lens that you can attach to either your 35mm
>> camera or your MF camera.  You place both cameras on tripods side by side
>> pointed at the same scene.  As you switch the lens between the two cameras,
>> what happens to the depth of field?
>>
>> Answer: The DOF obviously remains the same.
>>
>> HOWEVER, the problem is that the particular scene you're shooting is
>> rendered in the *same size* on the 35mm negative and the MF negative.
>> You're wasting part of the larger MF negative.  So what do you do?  You have
>> two choices:
>>
>> You could switch to a longer lens on the MF camera, and the depth of field
>> decreases.
>>
>> Or, you could move closer to your subject with the MF camera, and the depth
>> of field also decreases.
>>
>> So, what you're observing is normal and natural and an unavoidable
>> consequence of using a larger film format.
>>
>>         -- Charles Petzold
>>
>
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  Any questions?  I'll try to answer them.

Greg