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Tuesday, June 9, 2009

Hi-Tech Studio: Exact Focus

Even high-end lenses can exhibit some slight front and back focus. A simple device can have you dialed in for perfection.

This Article Features Photo Zoom

Depth-of-field test positions
While photographers have enjoyed the benefit of autofocus for 25 years now, many still struggle with its accuracy and repeatability. You may have heard the terms “front focus” and “back focus.” In fact, you may be struggling with this phenomenon in your own kit.

Put simply, front focus and back focus mean that if you intend to focus on a particular subject or spot, your camera ends up focusing either in front of or behind your intended target. For example, let’s say you shoot a close-up headshot, wide-open. If your lens is back-focusing, you’ll end up with your subject’s ear in sharp focus instead of her eyes. This can be especially frustrating if you’re using professional glass on a top-of-the-line D-SLR system.

Like any production item, cameras and lenses must meet a certain manufacturing tolerance specification. If we use an imaginary scale of +/-10 and say that any lens from manufacturer A must be accurate to +/-5, a lens that measures +4 is considered acceptable, just as one that’s measured at -4. The complication comes when you combine products together in use. Suppose that your camera’s AF sensor is +4 and your lens is +5, both within specs. This camera/lens combination is now +9 on our scale, which is pretty far out of the accepted range of tolerance.

Depending on the price and expected quality level, the degree of tolerance may be tighter on one product than another or between various manufacturers. So, lens X that costs $100 might have a tolerance range of +/-7, while lens Y of the same focal length costing $1,000 more would be produced to within +/-2. As a consumer, you don’t know the particular tolerances for any given manufacturer or their products. And while we’d like to assume that the higher-end camera bodies and pro-level optics are immune to this issue, the reality is they’re not, despite a higher level of precision.

Generally, with wider apertures and longer focal lengths comes shallower depth of field. For many of us, this is a large part of why we spent the extra cash for the fast, pro glass. But if the lens is front- or back-focusing even a small amount, there isn’t enough depth of field to render the intended point of focus sharply. The resulting image will be soft and often leads us to believe that we have a bad lens sample. For cameras, as the number of pixels increases by making the pixel pitch smaller, a greater demand is placed on the entire imaging system, lenses included. For medium-format digital backs, a larger sensor area also results in less forgiveness of front or back focus.


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