Wrote Stuart Goggins:
>I picked up a set of hoya close up filters. Any advice on using these. For
>example how much does the lens you attach it to affect the close focus
>distance. I have only one lens in the 55mm filter ring, a 135 f2.8. I am
>considering purchasing a step up ring for the 49mm lenses, would this be
>worth it?
Hoya is OK. This is what close-up lenses (not filters!) do: They shorten
the focal length of the combo. But as extension (focal plane to optical main
plane) remains the same, the effect is the same as increasing actual
extension with an extension ring or a bellows. The difference is that you
do not lose any light. In fact, you gain some speed, as the shorter optical
focal length does not decrease the apertures! However, resolution and
contrast will fall off a bit, especially in the corners, and there may be some
distorsion. But you have to stop down quite a lot in any case to gain depth
of field, so this may not matter.
Your c-u lenses will be marked in dioptries. These are 1000 mm (one meter)
divided by the focal length of the lens, so that + 1 dioptries means a focal
length of 1000/1 = 1000 mm, 2 dioptries 1000/2 = 500 mm and so forth. The
shorter the focal length of the lenses, the stronger they are of course. Now
a 135 mm prime lens is 1000/135 = 7.4 dioptries. Adding 3 dioptries to this
by way of a c-u lens makes 10.4 dioptries = a combined focal length of
1000/10.4 = about 96 mm. So this is more or less the same as adding an
extension tube of 135 - 96 = 39 mm! You can compute things for yourself and
see that the effect will be proportionally greater with a shorter prime lens.
A step-up ring can come handy with filters and other front-mounted
accessories too of course. So go ahead and have some fun!
Vänliga hälsningar/Best regards
Lars Bergquist
Välkommen till/Welcome to ...
<http://www.bahnhof.se/~timberwolf/>
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