[Leica] new Puts article

[henningw at archiphoto.com (Henning Wulff)]

At 3:38 PM -0400 6/29/05, Howard L Ritter, Jr wrote:
>I still believe that what I posted before is correct: If you take two photos
>at the same f/ ratio, from the same distance, with different focal length
>lenses and different size sensors (or not, it doesn't matter), and then
>print both pictures scaled and cropped to show the same area and the same
>subject size, the pictures will look identical including the DOF.
>
>The photo taken with the shorter FL lens will have a smaller CoC on the
>sensor, but will have to be enlarged proportionally more to make the subject
>the same size, offsetting the smaller CoC. The DoF with a smaller sensor and
>shorter FL lens will be deeper only if the print is enlarged to the same
>_degree_ (e.g., 15 x the sensor size) not the same image scale (e.g.,
>subject's head 2" high), because the viewer will be looking at detail that
>appears smaller and not see unsharp focus as well.
>
>Same distance + same f/ ratio + same subject size on print = same DoF.
>
>--howard

Unless I'm completely missing something in your response, the simple 
answer Howard, is NO.

A 300mm lens at f/8 on an 8x10 camera will produce a print whose DOF 
looks nothing like that from a 43mm lens at f/8 on 35mm film, 
neglecting completely the resolution and tonal differences, and 
neither will resemble a same sized print from a digital Canon SD400 
at 7.2mm FL at f/8 with a 1/2.5" sensor. They will all give the same 
angle of view, and the same subject size on print when the full 
film/sensor is used to make a print with the same diagonal. Objects 
that are significantly closer or further than the main subject will 
appear sharpest and with the most detail in the print from the SD400, 
in spite of, no; because of the small sensor size.

This is because the depth of field is proportional to the circle of 
confusion, but inversely proportional to the _square_ of the aperture 
ratio. Which therefore, mathematically leads directly to the first 
point below.


Here is a link to a fairly well written article on the subject:

http://www.photo.net/learn/optics/dofdigital/


The salient points in the article are:

"DOF is inversely proportional to format size"

and the summary points regarding 35mm film and lenses vs. APS-C sized 
sensors, specifically the Canon 10D which can form the basis for 
these comparisons:



        1.      For an equivalent field of view, the EOS 10D has at 
least 1.6x MORE depth of field that a 35mm film camera would have - 
when the focus distance is significantly less then the hyperfocal 
distance (but the 35mm format need a lens with 1.6x the focal length 
to give the same view).
        2.      Using the same lens on a EOS 10D and a 35mm film 
body, the 10D image has 1.6x LESS depth of field than the 35mm image 
would have (but they would be different images of course since the 
field of view would be different)
        3.      If you use the same lens on a EOS 10D and a 35mm film 
body and crop the 35mm image to give the same view as the digital 
image, the depth of field is IDENTICAL
        4.      If you use the same lens on an EOS 10D and a 35mm 
film body, then shoot from different distances so that the view is 
the same, the 10D image will have 1.6x MORE DOF then the film image.
        5.      Close to the hyperfocal distance, the EOS 10D has a 
much more than 1.6x the DOF of a 35mm film camera. The hyperfocal 
distance of the EOS 10D is 1.6x less than that of a 35mm film camera.


And note that the article also has the correct formala for verifying 
these things yourself.


>----- Original Message ----- From: "Adam Bridge" <abridge@gmail.com>
>To: "Leica Users Group" <lug@leica-users.org>
>Sent: Tuesday, June 28, 2005 4:16 PM
>Subject: Re: [Leica] new Puts article
>
>>I don't understand this, Henning.
>>
>>It seems to me that the depth of field is solely a fuction of the
>>geometry of the lens/film system. That the print and enlargement has
>>nothing to do with it.
>>
>>If I took an R8 with a 50 shot wide open at f1.4 I'd get some depth of
>>field. I replace the back with the DMR. Same shot. Same depth of field
>>but cropped.
>>
>>So then I open the images on photoshop and look at them. How does the
>>depth of field change?
>>
>>So I'm confused but I could certainly be enlightened! :)
>>
>>Adam
>>
>
>
>_______________________________________________
>Leica Users Group.
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    *            Henning J. Wulff
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