How To Get Excellent Sharpness From Your Digital SLR

Last week, suddenly, one year after I bought it, my Canon 1DsII with the 85/1.2 started producing the sharpest images I've ever seen from any camera. I figured out why, and what I learned from studying this unexpected boon falls into two categories: Things you need to do to make your camera sharp — that's hardware. And then there's what you need to do later in software.

I'll give you the executive summary right away: If you want sharpness, there's a careful setup to adopt, outlined below. However in my case there were two key steps: Ditching the lens protection filter which was perturbing the auto focus of my 1DsII, and adopting Canon's new DPP 2.0 Raw converter which has a superb capture sharpening algorithm.

Let's start with the hardware. Most SLR cameras nowadays have auto focus based on phase contrast. You need a fast lens to make this work properly. It's the physical diameter of the lens which counts, not the amount of light. Slow lenses won't focus as accurately as fast ones.

Of course, a slow lens has an intrinsic depth of field that may hide the fact that it's not perfectly focused. On the other hand, a fast lens such as the 1.2 has a razor-thin depth of field when open, and thus relies on perfect operation of the auto focus if it is to yield sharp results. But, believe me, the smooth transitions to out-of-focus that are characteristic of images made with a fast lens shot wide-open are worth the pain of focusing accurately.

Now you need to make sure your camera is focusing correctly, with the lens you're using. Unfortunately, tolerances for digital are stretching the old SLR designs a bit, so if you have problems achieving accurate focus, your camera may need to go back to service for a so-called focus check. If you do this, send the lens in for the check too.

And that lens should be fast, but it should also be a very good one: The resolution of digital requires the very best lenses. Full-frame digital like the Canon 1DsII is particularly demanding in the corners, while crop-frame cameras like the Nikon D2x and D70s are more forgiving: The small sensor of a crop-frame camera sits in the sweet spot near the center of the image circle, where every lens is sharpest.

Oh, and did I remind you to adjust the viewfinder to your sight? After all you want to see that your subject is right in focus. There's a trick to diopter adjustment : Tune it so that the various readouts at the edge are clear. Trust me, if your diopter is misadjusted, you'll get blurry pictures.

One last thing — I knew that filters degrade the lens performance slightly.— but it took me a year to figure out that the filter on my lens caused the AF to misfocus slightly, and that was seriously degrading image sharpness. Ditch that "protection" filter!

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