Originally posted by: Madwand1
Originally posted by: shocksyde
I'm sorry you're so broken up about it, but many people at POTN recommend a crop body for such uses. I guess they don't know what the hell they're talking about.
I'm not "broken up" -- I'm getting pretty excited about my 100x zoom sales. Now if you're arguing that a crop factor works as a teleconverter, then pony up and buy my 100x crop factor "teleconverter". I'll get you a deal.
For some applications, a crop-sensor camera is superior to a full-frame. For example: shooting relatively stationary birds from a long distance.
Let's use the example of a popular long lens for birding and sports: the Canon 400mm f/5.6L. If you put this lens an EOS 50D, then the effective focal length is 640mm. You also get less vignetting in the corners at wide-open aperture due to the smaller sensor. Now, from ISO 100-400, the most commonly used daylight ISOs (for birding, airshows, etc) there is not much (if any) difference between the image quality of an EOS 50D and an EOS 1Ds Mark III. It is only when you start getting into the higher ISOs of 1600 and above that full frame really starts to shine. The EOS 50D is actually also a little bit faster than an EOS 1 Ds Mark III: 6.3 FPS vs. 5 FPS.
So, let's say you wanted a 640mm lens on a 1Ds Mark III. Either you pony up the cash for a 600mm f/4L (which costs about 8 times more), or you slap a teleconverter onto the 1Ds Mark III.
Let's consider option 2, the TC.
Canon only makes a 1.4x and 2x TC, but other companies do make a 1.7x TCs that comes close to the 1.6x crop factor on Canon APS-C cameras. So, let's say you slap that 1.7x TC onto the 400mm f/5.6L and put the combination on a 1Ds Mark III.
The first issue is that you've no longer got an f/5.6 lens. Since a 1.7x TC takes away 1.5 stops of light, you've now got a 680mm f/9.5 Lens. This combination WILL NOT autofocus even on the 1Ds Mark III, which requires a minimum of f/8 to AF.
The second issue is image quality. Let's say you are just super awesome at manually focusing with a very dark f/9.5 viewfinder and somehow manage to get good focusing accuracy. You've still got to deal with the degradation of image quality imposed by the TC.
Or, you could have just stuck to using the 400 f/5.6L on an EOS 50D with no image quality loss and fully working AF. Take your pick...