Recently Wieland Willker wrote:
>
> But, the 50mm has no FREE system and it works good. So, with the 100macro
> when I'm near 1:1, I work at around 60-70mm. For better image quality you
> sacrifice focal length, is this correct? Or better, you sacrifice it to
> get an all-in-one lens (good infinity focus + good macro focus).
Here is my advice. A 100/2.8 macro is an expensive lens, and you should
definitely not buy it "blind," without seeing it first. You should compare
it carefully to your F/FA 50/2.8 Macro and see if it indeed has advantages
that justify the price difference.
The assumption of 60-70 mm lens-to-subject distance with the A 100/2.8
Macro at 1:1 is not correct. It is more like 110 mm, but I need to
double-check. What you are missing in your assumption is, is that the 100
lens is of telephoto design and that because of the FREE, the fornt nodal
point moves. It is not correct to think that if you take two lenses with
very different optical designs but with the same focal length and set them
to the same magnification that you will get the same lens-to-subject
distance.
So maybe this is the answer to your query. The 100 macro may be 60 mm at
1:1, but this is from field-of-view and not from position of the front
nodal point.
If you are interested in more detailed discussions, see my article on
finding out the focal length of the A 100/2.8 Macro. THere is a formula
for finding the location of the nodal points too, but you need to perform a
few experiments and measure a few distances first.
Cheers,
Boz
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