Ckw From UK - England, joined Aug 2010, 545 posts, RR: 17 Reply 4, posted (10 years 6 months 4 days 23 hours ago) and read 1219 times:
DSLRs have a CROP factor, not a focal length factor ... there is a big difference. The crop factor of a DSLR is exactly the same as enlarging the central portion of a 35mm - it does not decrease the depth of field or add the optical compression effect of a true focal length multiplier (such as a teleconvertor) - a picture taken with a 400mm on a DSLR (with a crop factor bringing it up to 600mm) is not the same as a shot taken with a 600mm on a 35mm.
Photopilot From Cuba, joined Jul 2002, 2441 posts, RR: 20 Reply 6, posted (10 years 6 months 4 days 19 hours ago) and read 1196 times:
One slight correction of what Wietse says.
A tele-converter does not "double" the aperature of a lens. It adds effective f:stops. A 2x converter adds 2 f:stops to the exposure. Therefore a 75-300mm F4-5.6 lens becomes a 150-600mm F8-f:11 lens... not 11.2
A 3x converter generally adds 3 f:stops. eg. a 2.8 lens will become an f:8 lens.
Each converter comes with an exposure compensation value written down somewhere in the instructions. At least it should, though with through-the-lens metering this generallly isn't a problem. It is only us guys that use a hand-held incident meter that have to take this into account.
One other thing. Converters tend to break down in quality most noticeable at the frame corners first. With the "crop" factor of DSLR's, this doesn't affect you as soon. Another benefit to DSLR's.
Ckw From UK - England, joined Aug 2010, 545 posts, RR: 17 Reply 8, posted (10 years 6 months 4 days 7 hours ago) and read 1147 times:
Another thing which stays the same is the minimum focusing distance - this can be a real boon if you're into insect or flower photography. My 300mm has a so called "macro" capability, which is OK - but stick a 2x convertor on it, and I've got both a much more powerful macro capability PLUS the ability to work at a comfortable distance from the subject ... and, because I could be a meter or more from my subject, I can easily use fill-in flash if necessary.