In article , kctan
writes
>Apologize for your misinterpretation of the term "extension". What I mean is
>the extension alone is equal to the focal length of the lens for 1:1
>magnification. Therefore total distance from lens to film plane is 2X focal
>length of the lens. Your term of total extension includes the lens dimension
>which is not an extension.
Sorry, I misunderstood you on this one.
>DOF depends on 3 factors.
>1) Lens to subject distance(Magnification)
>2) Aperture(smaller aperture reduces diameter size of circle of confusion)
>3)Focal length(due to hyperfocal issue).
>
>This thread is about macro in norm photography and not macro using the
>ultraphot system lens which is a reversed lens technique meant for
>photomicrography. So whatever logic I'm trying to say is based on norm
>photography. If you think there is no diff then it is an egg and chicken
>issue 
>
Now, I have a lot of sympathy with someone who comes here for
enlightenment, and takes the trouble to read and, if necessary, check
the credibility of what is said.
However, I do find it trying when someone has pointed out misconceptions
in your statements, and instead of either agreeing and saying thank you,
or checking (if you don't understand or believe the explanation) or
asking for further particulars, you just state the same errors as if
nothing happened.
Listen up: depth of field in macro work is determined by (i)
magnification, (ii) lens aperture and (iii) the size you choose to
define as an acceptable size for the circle of confusion.
No other factors come into the equations*. None.
In particular, lens focal length does not come into the equation. Lens
focal length is a factor which you select to give you the right
magnification for the focus extension capabilities of your equipment and
possibly for the working distance you need. But if you want 1x
magnification, or 20x magnification, it does not matter whether you get
it with a 100mm lens or a 16mm lens, the DoF is the same.
This applies at 1:1 as well as at 20:1; there is a crossover to the
equations you are probably more familiar as you approach 1:10 (0.1
magnification) (see footnote*). Incidentally, one reason why the use of
the correct terminology is important is that it gives clear clues as to
which region of approximation you are working in.
Now, if you don't understand that, ask (or better, go do your own
research - we can give you some good references if you need them). If
you don't believe it, go do the research anyway and find out where you
are misunderstanding the equations.
Please, just don't just carry on making the same wrong assertions as if
you heard nothing.
Your sentence about "macro in norm photography and not macro using
ultraphot system lens which is reversed lens technique meant for
photomicrography" seems to miss the point. The explanations I have given
you are quite applicable for 1x work with a 35mm camera and macro lens.
Also, BTW, the original question in this thread was about macro lenses,
not "normal" photography.
David
[*Footnote for the technical:
The equation for DoF is very complex. In order to make it usable, it is
usual to simplify it. There are some terms which are of negligible size
at "normal" distances (i.e. u >> f). These will show DoF is proportional
to f^2. However, at close-up and macro ranges, where u approaches f in
size, those factors become large, and they cannot be eliminated.
Instead, others become negligible, and can be discarded. The two
simplified equations are:
Normal range T = 2u^2*N*C/f^2
Macro range T = 2*C*N*(1+m)/m^2]
Where T = DoF, u = object distance, C = circle of confusion size, N =
marked aperture (f-number), f = focal length of lens, and m =
magnification.]
--
David Littlewood
>> Stay informed about: Why do only primes have macro 
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