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Tuesday, June 9, 2009

DPP Solutions: Lens Resolution

Have modern lenses exceeded the capabilities of D-SLR sensors?

This Article Features Photo Zoom

Do today’s lenses out-resolve today’s image sensors? Or do today’s sensors out-resolve today’s lenses? Does it matter? That’s the stuff of much debate. And the answer, really, is that it depends on the sensor and the lens involved. The best of today’s lenses certainly out-resolve the lower-resolution D-SLR sensors. But putting your prized film-SLR lens on a 21- or 24-megapixel D-SLR might well reveal some flaws you hadn’t noticed shooting film. Here are some considerations.

Currently, the highest-resolution 35mm-format D-SLRs are the 24.5-megapixel Nikon D3X and Sony DSLR-A900. Their images measure 6048x4032 pixels, and their sensors measure 35.9x24mm. Dividing the horizontal pixel count by the horizontal sensor measurement, we get 6048/35.9 = 168 pixels per millimeter.

Today’s highest-resolution APS-C D-SLR is Canon’s 15.1-megapixel EOS 50D. Its images measure 4752x3168 pixels, and the sensor mea-sures 22.3x14.9mm. Divid-ing 4752 by 22.3, we get 213 pixels per millimeter.

The highest-resolution Four Thirds System D-SLRs, Olympus’ 12.3-megapixel E-30 and E-620, produce images that measure 4032x3024 pixels, with a sensor measuring 17.3x13mm. Divide 4032 by 17.3, and we get 233 pixels per millimeter. Thus, the maximum possible resolution for these D-SLRs may seem to be 168 to 233 lp/mm. But the actual resolutions aren’t nearly that high.

Digital images are composed of millions of tiny picture elements called pixels. The more pixels an image contains, the more detail it can reproduce. But a single pixel can’t “resolve” anything. If a single line pair from a resolution test chart fell exactly over a single pixel, it wouldn’t be resolved at all. Pixels just collect photons. A certain number of photons would be collected by that pixel, and the result would be a gray pixel in the image, not a black line and a white line. The line pair would have to fall over more than one pixel to be “resolved,” thus cutting above the maximum potential resolution numbers considerably.

Then there’s noise. The production of digital images also produces “noise” (nonimage signal), and noise reduces resolution. Even the best sensors at their lowest ISO settings produce some noise. All other things being equal, the smaller the pixels and the higher the ISO, the worse the noise, and thus the resolution suffers.

Next, the anti-aliasing filter over the sensor blurs the image. That’s its job. (The blurring is necessary to reduce artifacts caused by the RGB filtration over the pixels.) This further reduces resolution.