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Tuesday, April 27, 2010

Resolution Vs. Sharpness


Crowded Pixels • Extender Percentages • Shedding Light On Adobe Lightroom • Protection • Laminates • Storage Is In The Cards

Labels: How-ToColumnTech Tips

An example of the possibilities of the Canon EOS 7D with an APS-C image sensor having 18 megapixels. The image of a flying brown pelican was captured using a Canon EF 500mm lens with an EF 1.4x tele-extender attached (1120mm with 1.6x crop factor), ISO 400, 1/350 sec. at f/11. Note the sharpness in the enlarged inset.

Crowded Pixels
Q I’m interested in stepping up to the highest-resolution APS-C-sensor camera (18 megapixels) available, but I’m concerned about reports of image softness and muddiness from some reviewers and blog sites on the Internet. They blame the problem on excessive numbers of pixels on such a small surface. Can you offer your insights and test results on this topic?
G. Corbett
Via the Internet


A The 18-megapixel APS-C sensor has the advantage of higher resolution if you photograph with quality optics. I’ve frequently photographed over the last few months with the Canon EOS 7D, which has this sensor and consistently yields images with excellent sharpness and color.

The smaller sensor, combined with higher resolution and fast capture, has some real advantages for wildlife photography. When you want to go long, the smaller sensor adds a crop factor of 1.6x, a significant boost that I used on my recent trip to Africa. More pixels means you can crop the image even further in postprocessing without loss of image quality. But there’s a trade-off with another technological advancement that’s important for wildlife photographers: expanded ISO. The cameras with the best response at high ISO are those with full-frame sensors and lower pixel counts because the pixels are larger and spaced further apart, minimizing cross-pixel contamination and noise. With the 7D, I’m able to use ISOs of 400 to 800 with excellent results, but I’d be reluctant to go higher in most situations.

If you’re looking to solve both problems, that is, you want the crop factor of the smaller sensor and the high speed of expanded ISOs needed to photograph wildlife, you should consider the Canon EOS-1D Mark IV, the Nikon D3X or the Nikon D3S (using the DX format). If you shoot mostly landscape and highly detailed design, you want to stay with the full-frame cameras.

Extender Percentages
Q When I shoot with my 500mm lens, I get the tack-sharp images I need 90% or more of the time. When I add the 1.4x tele-extender, the number of sharp images drops to about 70% to 80%. But when I use the 2x, the success rate drops to no more than 10%. The images, even when in focus, are very soft, almost like a soft-focus filter was used. I use the 500mm on Gitzo tripods with Wimberley heads, mirror lockup and a cable release. Is there a trick to using the 2x that I’m missing?
T. Wilkes
Via the Internet

A The percentages you’re achieving with the 500mm lens alone and with the 1.4x tele-extender are realistic when using a tripod under field conditions, so I’m pretty sure your technique isn’t the culprit. The results you’re getting with the 2x tele-extender are bothersome. You should be able to do better. I would expect a sharp result 60% or more of the time in the field, even considering that you’re shooting at 1000mm. I’m assuming that you’re using the camera manufacturer’s matched tele-extender for optimum results. Still, it’s my opinion that even under the best conditions, you’ll lose approximately 20% of the overall quality (sharpness and contrast) with the tele-extender. That said, if you start with a high-quality prime lens coupled with a matching 2x, the quality loss should be barely visible.

So here’s the question: Is the problem with the tele-extender or your technique? One way to answer that question is to ask another: Have you ever achieved even one sharp image using your 2x tele-extender? If the answer is yes, the problem is probably the photographer. If the answer is no, your 2x is possibly faulty. The way to find out is to borrow an identical 2x and set up a controlled comparative test on a stationary, finely detailed subject. Use your tripod, lock up the mirror, and shoot both wide open (ƒ/4) and stopped down (ƒ/8 to ƒ/11) with both tele-extenders. Choose a lower ISO and a faster shutter speed; that means there must be reasonable light on the subject. With all of these variables controlled, you can achieve the best possible result from each combination. Review the results on your monitor at 100%, and you should be able to tell if there’s something wrong with your tele-extender and proceed accordingly.

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