[Leica] Re: 12mm Heliar UW

[Henning Wulff <henningw@archiphoto.com>]

At 9:54 PM -0800 3/20/01, leica@rakitzis.com wrote:
>  >With gels the amount of focus shift is negligible. In practice other
>>errors will be greater and mask any actual focus shift.
>
>That's good to know -- but what about glass filters? I have a 4x5 lens
>with a rear filter thread, and I do take care to focus with the filter
>in place.

If you put a glass filter behind a lens, you will have to re-focus. 
Obviously not a big deal with 4x5. I have a spring loaded clamp on 
gel filter holder that I use at times, because my 3x3 gel selection 
is larger than my 4x4 filter selection that many of my lenses need in 
front. I don't use any glass filters behind the lens, because optical 
faults show up more in behind the lens filters than in those put in 
front. Gels are generally very good.

>  >No, it's the same for any f-stop. It's purely a geometric function.
>
>I think I see what you are saying (but it took me a long time to think
>about it): the error in focus due to any film buckling (for example)
>will be the same for any f-stop. i.e., if you are in focus from 3ft to
>infinity with a wide angle lens at f8, or from 100ft to infinity with
>a tele at f8, a buckle which harms the focus with the wide angle lens
>will harm it for the tele just as well.

That's right. Sorry I wasn't very clear the first time. If the lens 
is at f/8, the depth of focus is the same for any focal length lens.

>However, in practice I often stop down more with longer lenses or on
>larger formats, so the "same for any f-stop" business doesn't tell
>the whole story.
>
>For example, the Alpa website has an interesting quote:
>
>	12. Why the need for extreme precision and robustness?
>
>        The laws of physics make wide angle lenses with their large depth
>        of field and their tiny depth of focus much more demanding in
>        their tolerances than lenses with a narrower field of view.

This is not true in actual fact, but due to the way we use wideangles 
and telephotos, it becomes true in practice. If we shift the back 
focus slightly, we are effectively focussing on something different 
than we intended.If we are using a wideangle lens, and we are now out 
of the object depth of field, usually nothing is left in focus and 
the picture is total junk. We just demand a very high degree of depth 
of field in most wideangle shots. However, if we shift the object 
focus slightly on a telephoto picture, we are usually still focussed 
on something; not any more on the thing that we are supposed to be 
focussed on, but still something, and usually something that seems 
very close to the object that is supposed to be in focus. This 
sometimes results in a useful picture as well. We just aren't always 
as critical about the plane of focus with telephotos as we are with 
wideangles.

- -- 
    *            Henning J. Wulff
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