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Tuesday, February 9, 2010

What Lens Would Adams Use?

To get the very best sharpness, colors, contrast and overall image quality, you need to use the best lens possible

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Shoot at an intermediate aperture unless you really need extreme depth of field or very fast shutter speeds. At wide apertures, various aberrations reduce image sharpness. At small apertures, diffraction (the bending of light around the edges of the aperture) reduces sharpness. Adams and other large-format photographers often shot at ƒ/45 or ƒ/64, but large-format film images are much larger than 35mm or APS-C images. While the width of the Airy Disk (the diffraction pattern produced by a given aperture) is the same for ƒ/64 regardless of focal length, it becomes much larger relative to the size of the image (and pixels) as format decreases. Thus, at a given aperture, the degree of apparent blurring due to diffraction increases as image format decreases. That’s why compact digital cameras rarely stop down beyond ƒ/8—diffraction with their tiny sensors and pixels would render images unacceptably blurred.

Each lens has its sweet spot, where sharpness is greatest. This is usually one to three stops down from maximum aperture. Test your lens and camera to see what’s sharpest with your system.

If your camera has an “AF fine-tune” feature, use it to make sure all your lenses are being focused as precisely as possible. In film and early digital SLR days, if you got a body/lens mismatch, you were stuck with it. Today, many DSLRs let you adjust the focus plane in fine increments to compensate for a lens that’s a bit off. You can store in the camera’s memory AF corrections for a number of lenses you use most often. (Unfortunately, you can’t store different corrections for different focal lengths of a zoom lens, but the feature can still help maximize sharpness.) LensAlign from RawWorkflow.com is a great help in using AF fine-tune.

Shoot RAW, and use the RAW-processing software’s lens-correction features. You can correct chromatic aberration, distortion and peripheral light falloff (vignetting). And, of course, you can optimize sharpening for each image.

If you want really wide shots, consider stitching together a series of shots made with a longer lens, rather than using a really wide lens. There are a number of stitching software programs that make it relatively easy to do, and the results turn even a modest megapixel-count DSLR into a monster-megapixel device with excellent detail.

Designed For Digital
There’s more to it than just how sharp a lens is, however, because there are some basic differences between film and digital sensors. Film consists of three-dimensional silver-halide crystals (gains) embedded in an emulsion, while a digital sensor consists of millions of tiny photodiodes (pixels) covered by an RGB Bayer Array filter grid, a layer of micro-lenses and an anti-aliasing low-pass filter. The three-dimensional film grain can accept light rays striking them at an angle, but the pixels are like tiny “buckets” and light must strike them more nearly square-on. The low-pass filter is very flat and far more reflective than film, so lenses designed for film can suffer from reflections from the filter surface.

To take the second point first, lens manufacturers use special anti-reflective coatings, baffle designs and meniscus protective glass over the front of large-diameter lenses to minimize the effects of sensor reflections. Older lenses without these technologies are more prone to reflections when used with DSLRs, and those reflections reduce image quality.

As far as the angle at which light rays strike the film or image sensor, again, newer lenses designed for DSLRs produce better results, but high-end pro lenses also produce very good results, even if designed for film; they’re well corrected for aberrations and distortions.


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