Things I’ve Learned About the Craft of Photography: Lens Selection & Image Sharpness

Old Corner Post, Stornetta Public Lands, California Coastal National Monument, Mendocino County

Old Corner Post, Stornetta Public Lands, California Coastal National Monument, Mendocino County

(Craft of photography: part 3)

One of my objectives is to capture the rich texture and details in the landscape. Thus, I have an above average emphasis on resolution (short of capturing diffraction effects) and image sharpness.

When an image is sharp, you tend know it at first glance, even when it is reduced to a two-inch thumbnail size. Fine detail is well rendered, without artifacts. Edges are crisp without being pumped in a post-processing sharpening algorithm. Detail is subtly apparent without shouting at you. The image pleases the eye.

What do I think about when selecting a lens in the field?

Fixed focal length lenses, or prime lenses, are simply sharper than zoom lenses at the same focal length, nearly every time. Zoom lens design has progressed markedly over the last 30 years. We are at the point where image center sharpness is frankly just about the same for zooms and primes of equal focal length. But, primes tend to hold that critical sharpness across the frame to the edges and corners.

Longer focal length lenses, whether primes or zooms, are simply not as sharp as moderate focal length lenses. Telephoto lenses have a lot of glass in them. Moving closer to your subject and using a moderate focal length lens will produce a sharper image than standing back and using a telephoto lens.

Choose the lens with lower geometric distortion. This especially applies to barrel distortion seen in wide-angle primes and on the short end of wide-angle zooms. We have excellent algorithms today to correct for geometric distortion, both in-camera and in post processing. But, correction inevitably stretches parts of the image, effectively reducing available resolution especially out towards the edges and corners.

Wide-range zooms are noticeably less sharp on the long-end. The ratio of the longest to the shortest focal length of a zoom lens is called the zoom ratio. A 24 – 70 mm zoom has a zoom ratio of 70/24 or about 2.9. Lenses with zoom ratios of about 3 or less tend to hold up well across the full range of focal lengths. When that ratio stretches to 5 (or even 10 or more for “super zooms”), we see marked fall-off in lens sharpness out toward the long end. Do not expect to obtain the same sharpness with an 18-200 mm zoom at 200 mm as one does with an 80-200 mm zoom at 200 mm. A 200 mm prime will be sharpest of all.

If using zoom lenses, try to stay 10% away from either end of the focal length range. For my 80-200mm zoom, I try to use it between 88mm and 180mm. My own tests of multiple zooms show that the last 10% of focal length tends to bring a disproportionate loss of sharpness.

Use only top-quality filters and avoid stacking filters. You spend a lot of money on your lenses. Choose the best filters with the best anti-reflective coatings.

What do I actually use the most in the field? I’ve evolved my practice to keep the kit I take into the field as simple yet flexible as possible. This is especially true when I’m out hunting for new subjects (vs. returning to a spot and subject I’ve shot before). I’ll talk more about this in a later post but I find keeping the kit simple enables me to concentrate on the images rather than fiddling with too much gear. As such, I tend to carry a kit with a single body, a normal zoom, and a fixed focal length telephoto lens. That kit is one of the following:

  • Nikon D750 FX body with 24-120 mm f/4 zoom and 180 mm f/2.8 telephoto. I try to keep the 24-120 mm lens no longer than 85 mm or so.
  • Nikon D300 DX body with 16-85 mm f/3.5-5.6 zoom plus 180 mm f/2.8 telephoto. When using the zoom, I try keep the focal length shorter than 50 mm or so.
  • Nikon D300 DX body with 17-55 f/2.8 zoom, 85 mm f/1.8 prime, and 180 mm f/2.8 prime.
  • All filters are from B&W and I opt for their multi-coated filters when available.

Photographing along our coast, I do not always have full flexibility to “zoom with my feet.” I’m not fond of walking off the edge of the ocean bluff. Hence, the framing flexibility of a mid-range zoom is quite useful. Still, I value the extra edge in sharpness from prime lenses and if the subject is especially texture rich and accessible enough for me to zoom with my feet, I will take a set of prime lenses into the field in lieu of the zooms mentioned above.

Resources:

I tend to use three resources for evaluating and comparing lenses.

Photozone, a Germany-based website (published in English) with excellent test and evaluation data of lenses. Consistent test methodology and open, full disclosure of test results. I especially like the ability to look at how well lens resolution holds up at various focal lengths, at various apertures and across the image frame. The major limitation I see is that tests are usually with a single copy of a lens and thus lens to lens variation is not accounted for.

DxOMark is probably best known for camera sensor tests . But they also present interesting data on lens plus sensor performance. I find their Perceptual Megapixel measure to be useful though coarse measure of lens resolution. Its principal limitation is that it condenses a lot of information to a simple scalar measure.

My own lens tests. I use a consistent subject target and shoot with my own cameras to visually compare the performance of lenses I own.

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