Archived posting to the Leica Users Group, 1999/11/30

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Subject: Re: [Leica] RE: 75mm f1.4 DOF
From: "Jacques Bilinski" <jbilin@axionet.com>
Date: Tue, 30 Nov 1999 23:03:57 -0800

> At 09:07 PM 11/30/1999 -0800, Jacques Bilinski wrote:
> >So if it is right: "generally yes, but not at long focal lengths and
small
> >f stops".
>
> You believe everything you read on the web? What we have been saying is
> correct, depth of field is independent of focal length along.
Magnification
> and aperture determine it.
>
No I don't believe everything I read on the Web or in user groups for that
matter. For one thing there are plenty of conflicting statements on any
topic, as there is in this thread.

The web site I refered to states: " Note: When using small apertures (say
f/16 or smaller), long lenses do result in slightly less depth of field than
shorter focal lengths yielding the same image size." Yes, I happen to
believe this statement (until such time as someone can convince me
otherwise).

I recall reading many years ago a rigorous explanation, as to why this
'rule' which many photographers believe to be 100% accurate is only an
aproximation (but nevertheless a useful one). It made sense at the time but
unfortunately I've long since forgoten the details.

Intutively it makes sense to me that the depth of field for the 2 cases
might not necessarilly be the same. Take your statement "A 50mm Macro and a
100 macro at the same magnification and aperture will give the exact same
depth of field"  By the "same magnification" I assume you mean the same
magnification in the plane of sharpest focus. Say the 50mm is acceptably
sharp X feet closer than the plane of sharpest focus, and Y feet beyond the
plane of acceptable focus. The 100 mm would not have the same magnification
as the 50mm at these 2 distances, so it does seem reasonable to me that
maybe what matters is some king of 'range in magnification' rather than
simply a set distance in front and behind the plane of sharpest focus.