Re: [Leica] Depth of field summary with regard to the argument at hand

[John Collier <jbcollier@home.com>]

Sorry about dragging you into this Henry,

For the purposes of discussion we are assuming that the lens chosen will
cover any format you place behind it. The angle of view is a function of
focal length and film format; that is, a 45mm lens on a 6x7 camera will
encompass a greater angle of view than a 45mm lens on a 35mm camera

Any subject taken with any lens, from any distance or film format will
display the same perceived depth of field if the same physical size of
aperture is used and subject magnification is the same in the observed
output. This is largely true (see the posted article for explanations about
when it is not) and possibly even interesting but not particularly useful in
the field. When comparing depth of field, you need to standardise what you
are comparing. You need to standardise your final output or your results are
not comparable.

You have correctly stated that if I take a photograph with two side by side
cameras of a subject, one a 35mm camera, the other a 6x7, both with the same
focal length of lens and the same aperture; and, view the resulting
negatives from the same distance, that what little of the subject area is
visible in the 35mm negative will have the same perceived depth of field as
the 6x7 negative. However you are not comparing standardised output. If I
want a 8x10 photo I do not run out and get an 8x10 camera automatically. I
have the option of choosing any format and enlarging, reducing or contact
printing to get my 8x10. Generally I want to use as much of my film area as
I can to keep the quality high. I need standardised depth of field tables to
help me judge what the perceived depth of field will be when I print my 8x10
with whatever format I choose. The different film formats will require
different magnifications of the resulting negatives to get my 8x10;
therefore there will be different perceived depth of field even if the same
lens, subject distance and aperture are used.

A standardised final output and viewing distance has to be a constant in
determining depth of field so that the differences in the variables we are
interested in finding out about are readily apparent.  If I am comparing
different sized outputs from each different size of film format, it would
not give me useable information in comparing perceived depth of field
differences across formats.

It has been unfortunately categorically stated that if I want to take a
picture of a subject with a 45mm lens that it does not matter whether I use
a 35mm camera or a 4x5, the depth of field is going to be the same. This is
only true if when making my final output, I crop out a very large section of
the 4x5 negative. You can operate this way but it does not sound quite right
to me.

I am always willing to learn, please correct me if I am wrong.

John Collier

Please note that I have not ever written or calculated a depth of field
table and when I mention 8x10s as a standard of comparison it is only as an
example. I am not familiar with what exactly is considered the "standard".
It is not as important a point as the one of actually having a standard as
this enables you to make comparisons.

> From: Henry Ambrose <digphoto@telalink.net>
> 
> I swore to myself that I would no longer participate in the DOF
> discussion.
> 
> BUT!!!!!!!!
> 
> John Collier wrote in part:
>> As the size of the film format
>> used with a particular focal length of lens affects the size that a subject
>> will be reproduced, the larger the film format gives a smaller subject size
>> in a standardised final output as compared to a smaller film format, it is
>> an important factor in calculating the depth of field.
> 
> If you change only the film size the image size does not change.
> Whether there is a 35mm film or a 4X5 film behind your 35 Summicron the
> image will still be the same 24X36mm at the plane of focus - so nothing
> changes.
> 
> Do you mean a 35mm focal length lens that would cover the larger piece of
> film ?
> Does angle of view have any thing to do with this film size issue?
> 
> Henry