[photo-3d] Re: Digest 348: depth of focus

[Olivier Cahen <o_cahen@xxxxxxxxxxxxxxxx>]

    The question raised by Michael looks very strange, and  Allan and George are
right.
The diameter of the spot on the film with a given change of focus is a function of
only of the focal length and the diameter of the aperture.
    If you want to get the finest depth of focus, set the focusing distance to
twice the distance of the nearest object. With most cameras, it will be sharp
enough from the window to the infinity, except in macro-photo.


>
> Message: 7
>    Date: Mon, 13 Nov 2000 17:16:44 -0600
>    From: "Michael Galazin" <RexLion@xxxxxxxxxxxxxxxx>
> Subject: depth of field
>
> Can anyone tell me what factors govern a 35mm lens' depth of field?  Does a
> greater DOF indicate a better quality lens, or one of a greater diameter, or
> what?  I was trying to predict what my DOF would be with my Wirgin stereo,
> so I looked at my other cameras to see if they all run about the same.  They
> are similar, but far from identical in this regard.  I see that my Realist
> 3.5 at (for example) F8 will give good DOF from infinity to 9 feet; by
> comparison my FED B-O-Y at F8 is sharp from infinity to 10 feet; and my
> Realist 45 DOF at F8 extends from infinity to about 6.5 feet.  All of these
> lenses are supposed to be 35mm.  So I'm wondering, does the greater DOF of
> the 45's Steinheil Munchen Cassars indicate better lens sharpness overall
> than the David Whites on my Stereo Realist?  If not, what is the reason for
> this DOF variation?
>
> Best regards,
>
> Mike Galazin
>
> Message: 10
>    Date: Tue, 14 Nov 2000 10:44:20 +1100
>    From: "Allan Griffin" <agriffin@xxxxxxxxxx>
> Subject: Re: depth of field
>
> Mike, all my experience teaches me that ANY lens of a given focal length
> produces the same depth-of-field characteristics for a given f. stop.
> Variations in the actual scale on specific lenses would appear to be
> different interpretations of exactly where the "acceptable degree of sharp
> focus" drops off.  I expect we have Group members on this List who can quote
> the theoretical basis for such calculations reference the "circle of
> confusion" or other.  It would be profitable for you to actually conduct
> your own comparison tests and set up your own "acceptable" DOF scale for any
> of your cameras.
>
> Allan
> Message: 13
>    Date: Mon, 13 Nov 2000 21:12:14 -0500
>    From: "Dr. George A. Themelis" <drt-3d@xxxxxxx>
> Subject: Re: depth of field
>
> DOF tables are calculated, not measured.  Three factors
> enter in the calculations:
>
> 1) Focal length
> 2) Aperture size (f-stop)
> 3) Circle of confusion
>
> Since being out of focus is something gradual and only one
> plane at a time is in focus, factor no. 3 is an arbitrary
> constant which determines how much "out of focusiness" is
> acceptable.
> Lens manufacturers have more or less agreed in given
> circle of confusion values to be used as a function of
> the film format.
> Based on these facts, I would expect that all 35mm lenses
> will list the same DOF at the same aperture if the same
> circle of confusion is used in the calculations.
> It is known (and has been discussed in photo-3d) that
> David White used a smaller (more strict) circle of confusion
> in earlier calculations so earlier Realists show LESS
> DOF than later Realist.  Realist changed this value and
> their DOF tables later on and explained this change in
> their newsletter.  This is only a calculation issue.
> The lenses will not show different DOF.
> You mention that for F8 the Realist and the FED BOY give
> different DOF, from infinity to 9 feet and to 10 feet.
> Is this something measured or taken out of tables?
> Where is the lens focused?  At infinity?  (You cannot
> quote a DOF range without indicating where the lens is
> focused unless if you are talking about the Hyperfocal
> Distance. If infinity is the outer range of the DOF
> then most probably you are talking about the Hyperfocal
> Distance, which is the twice the shorter DOF limit and
> that's where the lens if focused.)
>
> George Themelis