I can't see that there's much magical about stopping a lens down. The
optical configuration doesn't change, the refractive indices of the
glasses don't change...
You really need to be at a place where you're stopped down enough to
avoid the outer area of the glass (The edges) and NOT enough so that the
aperture becomes an optical device of its own (Pinhole) Also, if you're
shooting negs, They need to be FLAT and you need to be really
anal-precise about focus because at that magnification there's zero
depth of field.
The old adage of closing down 3 stops from wide open should get you
close In any case.
This is my opinion (Mythology?) and I can't refer to any research except
in MTF curves which support my theory to some extent. Karl, I'm sure,
has actually done the groundwork and could tell us how valid the -3
stops thang is.
Herschel
karl shah-jenner wrote:
Steve writes:
I use ND material (neutral density, gray filter) in sheet form, ..
<clipped>.. It probably works better > than trying to mount an ND filter on an enlarging lens.
a much better way for sure. I have sheets from .15 to 1.2 I bought from Lee for various purposes - handy stuff to have. And anything between the lens and the subject or sensor like a lens mounted ND filter is best left out if it can be managed.
The Bowens illumitran slide
duplicator has a knob that adjusts the flash intensity, and a system to
meter the exposure, which also helps speed up the process. Since the
Illumitran was built for ASA 50 or slower duplicating film the light
intensity needs to be reduced to obtain the optimum aperture.
I have one of those too, I was going to sell it but they seem to go for near nothing these days judging by the ebay prices (good news for buyers) but I mostly now use a continuous light source made from an old LCD display backlight, lit with white light LEDs which I've made up so I can vary the intensity as needed instead of the cathode light. Not an 'off the shelf' product though, so the illumitran (which I already suggested early in '07 for such things) is probably a better option
The manufacture or others may have already worked out the best aperture
for the lens.
good point and definately true for taking lenses.. but often with enlarging lenses we read the 'best when stopped down one or two stops' on the net, and at the college they were mad keen on trying to get students to print at time = X, varying the aperture to avoid reciprocity failures in the paper (!) The Schneider site: http://www.schneideroptics.com/info/photography.htm gives plenty of information about their EL lenses, though none I could find http://www.schneiderkreuznach.com/pdf/foto/vergroesserung_e.pdf on the optimum aperture. MTF graphs are available, but not many can interpret those. As it is, I have found EL lenses are often (not always) actually sharpest opened up two stops rather than closed down two.
karl
