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For that matter, how is magnification measured? I have the 50mm Macro and get
what seems to be lifesize on a 4X6 print. Obviously, prints are not the
measuring stick for magnification, but that is how I think when I imagine the
finished product.
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magnification is measured on the slide or negative. 1:1 means the image
on a slide is the same size as the subject. You are right that you don't
need anything close to 1:1 to get life-side on an enlargement, and people
who want 1:1 are using it to get much bigger than lifesize on a print.
There is nothing special that happens at 1:1 really other than it is the
point where distance from lens to subject and distance from lens to film
are equal. But from the point of view of practical macro photography,
you can think of the magnification range as a continuum and 1:1 is just
one point on that continuum like any other.
This issue of enlargement is one reason it is *much* easier to do macro
work in 35mm than in medium or large format. If you work with 6x7,
you are enlarging a 35mm slide 2.3x as much as a 6x7 slide. If you
want an 8x10 with 8x lifesize reproduction of subject on print, you
will be working at 1:1 in 35mm, but at 3.7X in 6x7. you lose 2 stops
of light to bellows factor (if focusing by extension) for the 35mm lens,
(and have a 2-stop darker image to focus) but you lose more than 4 stops
to bellows factor with the 6x7 lens (that also has a slower max aperture
to start with) and will thus have a very dark image to focus. A Pentax 67
macro lens is either 100/4 or 135/4, and at 3X you'll be trying to focus an
f/16 image on the ground glass. 645 is a little more forgiving, but not
much. This is the main reason I still have a 35mm outfit-- macro work
in medium format takes much patience and is difficult at best.
Cheers,
Joseph
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