Re: [Leica] Re: DOF -Optical vs Apparent

[Rob McElroy <idag@pce.net>]

Jim Brick et al.,

You are absolutely correct and I will repeat your statement again "Film format has absolutely NOTHING to do with DOF. Period!"  I don't know why this
thread had gone on for so long because it is a very simple concept.  No matter what size piece of film you stick behind a given lens, the depth of
field of that lens (focused at any single distance you choose) is constant - and does not change.  Also, making an enlargement (whether 2X or 20X)
from a negative has NOTHING to do with the depth of field on the original negative.  How far you plan on enlarging or magnifying a negative, is again,
not a factor in determining the optical depth of field.

Some of this confusion can be summed up by understanding the difference between OPTICAL depth of field (which is calculable and found on the DOF
charts that are on the internet and in countless reference books) and APPARENT depth of field (which is a subjective interpretation of what "appears"
to be in focus by a given observer).   Here is an example which illustrates the difference. Shoot a vertical picture using a 35mm camera and fill the
frame (top to bottom) with your subject (pick a flower, pick a person, pick anything).  Expose that photograph using a 28mm lens, 50mm lens, and 180mm
lens, with each lens set to the same aperture (pick any one you want-I'll pick a shallow f4.0 for this example). Make sure your subject fills the
frame (top to bottom) in each shot.  The OPTICAL depth of field in each photograph will be EXACTLY the same, but the APPARENT depth of field will seem
vastly different.  The telephoto shot will appear to have a very shallow depth of field while the wide-angle image will appear to have a lot more
DOF.  The out-of-focus areas in the telephoto shot are often unrecognizable and just fuzzy shapes in the background, so our brain interprets them as
WAY out of focus.  The out-of-focus areas in the wide-angle photograph are more easily recognizable and our brain interprets them as only a LITTLE out
of focus.  That is why apparent depth of field is so subjective.

If we assume, for this example, that the optical depth of field was say 6", if you look critically (using a loupe) at all three images, you will see
that even with the wide-angle lens - anything falling outside that 6" DOF, IS technically out of focus (not critically sharp).  Here's the big BUT
though.  With the wide-angle lens, our brain recognizes many OOF objects and doesn't think that those out of focus areas look too bad (not too fuzzy),
but with the telephoto lens the OOF objects are almost unrecognizable (way too fuzzy) and our brain tells us that they are definitely OOF.  And no,
this isn't fuzzy logic.

In response to Jim Brick, austin@darkroom.com wrote "I believe you are 'confusing' DOF on the film with DOF on the printed image at viewed distance.
They are different.  Even on the print, then again at viewed distance have different 'DOF' too."

This theory confuses the issue even more.  "DOF on the printed image at viewed distance" - there is no such thing!  Depth of field was fixed the
moment the negative or transparency was initially exposed - and enlarging the image can NEVER alter or change the DOF.  Make a 4"x6" print and a 4'x6'
print. View them at any distance you choose.  The DOF is always the same.  Our eyes and brain are the real culprits here.  If you make a very large
print of something and hang it on the wall for people to view from a comfortable distance away, the image might APPEAR to have more DOF, but it truly
does not.  The enlargement process itself introduces an overall flattening of fine detail when large prints are made.  This flattening helps to make
the sharpest (critically in-focus) areas look more like the slightly out of focus areas, thus giving the whole image the FALSE APPEARANCE of greater
DOF.

Another example is:  if you put a slight diffusing filter over your lens when you expose a scene, the DOF isn't changed by the filter - but visually,
the scene takes on a more uniform level of sharpness (or lack of it) and our brain THINKS there's more depth of field.

Oh, I've fueled the fire now.......and just when we thought it was almost out.

Regards,
Rob McElroy
Buffalo, NY


Jim Brick wrote:

> At 03:06 PM 2/7/01 -0700, shan@montana.net wrote:
> >For those of us who think that film format doesn't matter - here's another
> >DOF calculator.
> >
> >http://www.photostuff.co.uk/dof2.htm
> >
> >shan
>
> So where does one enter the film format?
>
> One doesn't because it does NOT matter.
>
> Film format does not matter. Focal length, aperture, and image size are the
> DOF determining factors.
>
> Many folks use a roll film back on their LF cameras. Let's say you have an
> 8x10 camera. You take a photograph on 8x10 Tri-X film. Now you slip in a
> 6x7 roll film holder and take another photograph on 120 Tri-X film. Nothing
> has moved. Same camera. Same lens. Nothing tweaked.
>
> Guess what!
>
> The DOF of both the 8x10 and the 6x7 images are exactly the same. Same
> image size. Same lens. Same aperture. But the 8x10 obviously has more
> coverage. But the same image size. Hence the same DOF.
>
> Same is true if you use a 6x6 back, then the rare 35mm back on a
> Hasselblad, or Rolleikin (sp) on a Rollei. Or the Mamiya 7-II with 120/35mm.
>
> Film format has absolutely NOTHING to do with DOF.
>
> Period!
>
> Jim
>
> ps... for fun, on that DOF calculator, enter 75mm, f/1.4, 2 feet, and see
> what the DOF is...  :-)
>       then enter 100 l/mm, and recalculate.  :-)