"Robert P. VanNatta" wrote:
> That isn't an unreasonable amount of
> extension for a 200 mm lens, but
> you won't get the magnification you
> are expecting me thinks.
Since I'm new to macro and close-up photography, I have no
expectations, per se.
What magnification do you think I'd be able to achieve with a
150mm extension tube set and the Pentax A*200 macro lens?
> Longer focal length lenses require
> more extension to achieve any given
> magnification.
More extension than what? I'd need 200mmm with a 200mm lens
to get 1:1, is that correct? Since the lens in question
already allows for (I believe) 2X life size, wouldn't I get
about 4X life size with the addition of 200mm tubes?
> My good book says that image quality
> will degrade when you push magnification
> beyond 1:1 with extension unless you
> reverse the lens. Telephotos are not
> suitable for reversal.
To what book are you referring? Since the Pentax lens goes to
greater magnification than 1:1, and is considered to be
extremely sharp and offers quite high resolution, I don't
understand how extension tubes would degrade the image more
than just racking out the focus. Could you please explain how
that is?
> When you talk about image degradation
> that means different things to different
> people.
To me it means reduced sharpness, reduced contrast, reduced
resolution - anything that degrades the image from the optimum
performance of the lens. Does degradation mean something
different to you?
> With macro photography something will
> likely still be in focus and sharp, but
> the depth of field becomes progressively less.
Yes, I know that. Is that what you consider image
degradation?
> Shucks if you want to do super macro
> photography you can stick your camera
> on the end of a microscope
No, I don't want to do that. I just want to get the best
image possible at the highest magnification so I can fill the
frame with the subject and make large, sharp, highly resolved
prints.
> but you have to face reduced depth
> of field
DOF is already pretty shallow at max magnification with this
lens, and it's even a bit more shallow with a 68mm extension
tube set. However, it's adequate for my purposes at f/8.0 or
f/11.0
> reciprocity failure
> issues. when you compensate 19
> f stops you have strained the
> reciprocity characteristics
> of your film
Thus far reciprocity has been of no concern. Exposures are
all less than one second. With three times the extension
(68mm to 200mm) I can see where it might be a consideration,
although I think the LX meter will handle that. Plus I'll
bracket like crazy until I get everything down pat.
> unless you have unless you are
> using a critical mass of nuclear
> fissionable materials at close
> range as a light source.
I only use fissionable material when I barbecue.
--
Shel Belinkoff
mailto:[EMAIL PROTECTED]
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