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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

Finally, a digital image is just data until it’s de-mosaiced or “developed”—processed—either in-camera by the camera’s image processor (for JPEG images) or by you using RAW-conversion software (for RAW images). The conversion process also adds a number of variables that can affect resolution, including degrees of sharpening.

So, the maximum possible resolution at the focal plane is considerably less than the 168 to 233 pixels per millimeter on the sensors. The resolution of your digital images is the product of the pixel count, the noise level, the anti-aliasing filter, the processing of the image data, the lens, the aperture being used, focusing accuracy, camera steadiness and more.

(This is a greatly oversimplified explanation; for a much more thorough, technical and excellent one, do a web search for “Zeiss Camera Lens News No. 30,” and read the article “Measuring Lenses Objectively.”)

Just for reference, Zeiss produced a wide-angle lens for the 35mm format a few years back that resolved 400 lp/mm at the center at ƒ/4. The top lenses used with today’s D-SLRs resolve considerably less than that, but as we’ve seen, that’s fine. And the highest-resolution pictorial films resolve in the neighborhood of 200 lp/mm.

(The pixels-per-millimeter figures listed above might lead one to believe that the smaller-sensor cameras can produce greater resolution and better image quality than the larger-sensor cameras. This isn’t the case. The larger-sensor cameras generally have larger pixels that produce less noise and diffraction concerns, and higher pixel counts that require less magnification to produce a given-sized print. For example, even though the 21.1-megapixel, full-frame Canon EOS 5D Mark II has fewer pixels per millimeter on its sensor than the 15.1-megapixel APS-C EOS 50D, the Mark II’s image quality is superior, especially at larger print sizes.)

Another consideration is how we view images. When we shot slide film, we inspected the images with an 8x or 10x loupe, or projected them on a screen viewed from 10 to 20 feet. When we shot negative film, we printed the negatives on paper and viewed those prints from a suitable distance (generally, far enough away so that we could take in the whole image without having to swivel our head).

When we check a digital image on-screen, we easily can blow it up to 100% (or more) and actually examine the pixels themselves. With a 24-megapixel image, viewing it at 100% on-screen is akin to viewing an 84x56-inch print at arm’s length. We just didn’t do that with prints in the film days.

As cited in the “Time For An Upgrade” article in this issue, there are reasons why some lenses are better for digital imaging than others. But the resolution of today’s top lenses is quite sufficient for use with today’s top 35mm-format D-SLRs.