|
Mark: I think you are confusing close-up lenses
with add-on convertors. For example: the TL-H27 27mm 1.7x Telephoto
Converter Lens for Canon Optura Digital Video Cameras is
an add-on lens that provides a 1.7x effect when connected to a main
lens. This particular on has a 27mm filter size
but others are available in sizes up to 58mm I think, Back to the original question about image
quality differences between tele-convertors and add-on convertors;
teleconvertors cost lens speed as you mention while add-on convertors seem not
to. I’m not at all sure regarding image
quality between the two approaches. Cheers, James From: I suspect that there
are two things you are a getting a bit confused, and that its easy to do. One is to multiply the
focal length (and I really don't necessarily like this analogy but its the best
I can come up with before coffee) where you make a 200 mm lens have the
magnification of a 400 mm lens (or a 2x) that goes between the lens and the body.
These usually add some softness, but have some advantages and
disadvantages. They take up a lot of light. The common ones are
1.4, 2.0 and 3.0 and they need, 1 stop, 2 stops or 3 stops of additional
light. Not always a problem but occasionally is a major problem.
Because of the nature of the glass, a 1.4 converter is usually sharper than a
2x and the 2x sharper than the 3x. Close up or macro
photography has a bit of a different challenge. They are designed to get
the ability to let the lens focus closer than it would be able to do so
otherwise. In many cases the challenge is to get something small the same
size on film (or digital sensor) as it is in real life. There are 2
options there. The cheaper is close up filters that go on to the front of
the lens. Because of the glass they tend to be softer than the other
option, but they are much cheaper. The other method is called extension
tubes which go between the lens and body. These are more expensive,
especially on auto focus because the electronics have to pass through the
tube. This is really just a hollow tube and has no effect on sharpness at
close up distances (which is why its usually preferred) but it changes where a
lens can focus by moving it away from the camera body. The longer tubes
move the lens further away from the body and changes where the focus is. |
