Q) In the quest for magnification and image resolution, is it better to crop an image in the computer rather than resorting to the use of teleconverters (1.4x – 2x) in the field? I am aware that there will be a pixel reduction with the former as against more glass with the latter and the possibility of a poor optical match.
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A) It depends on the quality of your optics—both lens and converter—and the pixel resolution of the camera used in the capture. In the past, I would always have suggested using the best-quality lens and teleconverter available to try to get the largest image possible in the frame. What’s changed is that the captured image with newer higher-resolution D-SLRs actually allow for some cropping in the computer. I’ve found that I can make significant crops of 21-megapixel captures and still generate enormous prints. If you’re using a zoom lens that doesn’t lend itself to teleconverters, the solution will definitely be a reasonable crop within a computer image-processing program. If you have an excellent prime lens and a matched tele-extender of either 1.4 or 2X, then your first course of action would be to use the converters to get as large an image as possible, and you might still be able to crop a small amount in the computer if you have an 8+-megapixel camera. The more megapixels, the more information and the more ability to crop. With this said, the need for achieving the maximum sharpness by using a tripod and techniques for quality capture cannot be overstated. One of the best things about the digital age is that with high-quality optics and larger, better sensors, I have many more options in my quest for magnification and sharpness.
The first image of the young marmots is full frame from a 21 MP Canon EOS-1Ds MK III. The second image is severely cropped from the same image, yet it will still make a decent print and be very usable in a slideshow or on the Internet. The final cropped result is still the equivalent of approximately an 8 MP camera.