Re: [Leica] how can the 75 Lux focus go bad?

[John Collier <jbcollier@home.com>]

Not at the same print size. The image size on the negative is the same, as I
already pointed out, but the final image size is different when the print
size is the same. Which is why the DOF tables vary according to film format
as well.

For example: 

A 4x5 camera and a 35mm camera, both with 90mm lenses, take a picture of a
subject at the same subject distance. Now make a 4x5 print of both negatives
using as much of the negative as possible. This is a contact print for the
4x5 negative and a, approximately, 4x enlargement of the 35mm negative. The
resulting image size is "smaller" on the 4x5 contact print regardless of the
fact that image size is exactly the same on the negative. A smaller image
size will result, if the same relative aperture is used, in greater
perceived ‹thank you, Martin‹ DOF.

John Collier

> From: "Austin Franklin" <austin@darkroom.com>
> 
>>> I disagree.  If you take the same lense, and use it on ANY size
>> film, at the same distance, you will get the same DOF.  >Optics
>> doesn't know what the film size is, and adjust it self accordingly.
>> 3' IS 3', no matter what the film size is.
>> 
>> That is like saying that a 24mm lens @f8 on 35mm has the same depth of
>> field as a 500mm lens @f8 on 35mm if you enlarge the 24mm image
>> sufficiently.
> 
> No it is not what I said.  I said, quite clearly, that the same 50mm lense
> at 3' gives the same DOF on both 35mm and 4x5 film.
>