Glad that I could help.
There are two basic formulas that can help with macro work. You probably
know them but if not - the first is that when you put a lens on
extension the magnification equals the extension divided by the lens's
focal length. So a 100mm lens on 200mm of extension = 2x lifesize
magnification.
There is a somewhat complicated formula to estimate the working
distance, but you can approximate working distance by dividing the lens
focal length by magnification. SO a 100mm lens on 200mm of extension
would have a working distance of 50mm. A 50mm lens on 200mm of extension
would have a magnification of 4x and a working distance of 12.5mm (50 /4).
The formulas are approximate - they would work for a single element lens
but that obviously not the case with a regular camera lens. But they
would get you to a good approximation.
I have never found a good explanation of what is going on when you
reverse mount a lens. After using reverse mounted lenses quite a bit, I
can say that reversing the lens allows you to focus closer. It also
seems like the subject to lens distance does not change as much as you'd
expect when you reverse mount and change extension. But I've never
learned the theory.
Apparently the D-FA is not using simple extension to help it focus more
closely which is why it does not focus once the internal focusing
mechanism is set beyond a certain point. I'm not sure what is going on
there.
Mark
On 7/8/2012 12:09 AM, John Celio wrote:
Mark, this is exactly the response I was hoping for. Thank you *very*
much. I never would have thought the lens would be the part causing
the problem, nor that wider angle lenses would produce greater
magnification (though that kinda makes sense now that I think about
it). I will try it with my FA 50 (my only other small prime with an
aperture ring) right now.
Thanks,
John
On Sat, Jul 7, 2012 at 6:57 PM, <[email protected]> wrote:
That is basically the same setup that I use for my snow crystal shots,
except I use a 50mm lens reverse mounted. In fact, it is so simlar I
just now pulled it out of the closet, set it up and put my D-FA 100mm
macro on it. I have 300mm of extension (tubes and bellows) with the lens
reverse mounted at the end.
Your focusing problem lies in how how the D-FA 100 close focuses. I had
no problem getting a shot in focus when the lens was set to infinity,
and actually had ample working room. As I adjusted the lens's focusing
mechanism I quickly was unable to get anything into focus. Obviously,
something about the internal focusing on the D-FA causes a problem with
it reverse mounted.
Focusing on a ruler, with the D-FA 100 at infinity, and using a full
frame 35mm film camera (also pulled from the closet), I saw 8 to 9 mm in
the view finder. SO I reckon it to be about 4x life sized. With the 50mm
I routinely use on this setup, the finder on the full frame camera shows
about 4 mm, so about 8x.
If you really want to get extreme get a short and fast lens and use it
on all that extension. A 24mm on 300mm of extension would result in 16x
or so magnification. Set the lens to infinity or use one that does not
use internal focusing.
Regarding protecting the delicate rear element of the lens - put a short
extension tube on the rear lens mount. It will serve as a hood and also
offer some protection to the rear element. To help with focusing - set
up your rig in a dark place. Put a bright flashlight on the finder
(where your eye would go). It will project a little rectangle of light
that you can use to align your subject and get a sense of focusing
distance with (when the rectangle is sharp, you are in focus.)
Have fun -
Mark
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