The Risks and Rewards of Fast Lenses

I shot this with a Pentax 55mm f/1.4 DA lens at f/2.5. I was close to the minimum focusing distance and depth-of-field was obviously shallow. Click on this photo to enlarge it and you'll get a better idea of just how shallow it was. (Nice bokeh, eh?) Fortunately, thanks to the Pentax K-7's lens calibration feature, I could auto-focus on the eye nearest the camera and know that the focus would be there and nowhere else.

There was a time when practically every 35mm SLR sold around the world came with fast-aperture prime lens with a focal length somewhere between 50-58mm. The maximum aperture was at least f/2.0. If you were willing to spend a few extra bucks you could get an f/1.4 or even an f/1.2.

These lenses were seldom critically sharp at maximum aperture, but they allowed for bright focusing screens and shallow depth-of-field, which in turn made it easier to determine whether the image was in focus or not. This was quite important if you were using a manual focus camera, less so if the camera had autofocus, but even with autofocus cameras a fast-aperture lens improved the camera’s ability to focus in low light.

With time, as consumer preference shifted to zoom lenses and autofocus, it became more economical for manufacturers to produce slower zooms that were smaller, lighter, and had less critical tolerances than their speedier brethren. All other things being equal, the smaller the maximum aperture, the greater the depth-of-field and the more tolerance for slight focusing errors.

Digital was a game-changer. For the first time practically anyone could view an image at enormous magnification, only minutes after taking it. It became very easy to see whether a lens was front- or rear-focusing and by how much. As before, slow lenses had enough depth-of-field that the point of sharpest focus wasn’t particularly critical. It was a different story with fast lenses. If the camera and lens were out of tolerance with each other the lens would appear to be “bad” or “soft.”

You can find a full explanation of this phenomenon here, but the basic reason is manufacturing tolerances. If a camera and lens each have a tolerance of plus or minus 3 “focusing units” on either side of perfect, then a camera that is –3 short of perfect would cancel out a lens that was +3 over perfect to produce zero; i.e. spot-on. If, however, you mount a lens that is also –3, you’ll have a combination that consistently front-focuses. You may not notice it if you shoot at f/5.6 or smaller, but if you shoot at f/2.8 or larger it’s easy to see, especially if you’re looking for it.

Keep in mind that in the scenario where both the camera and lens are out of tolerance by the same amount and in the same direction, neither one is defective. Both are within manufacturing and focusing tolerances. It’s just that in this particular case the amounts combine to produce less than optimum results.

That’s why the better DSLRs these days (semi-pro and up) have a lens calibration feature that allows you to adjust the focusing tolerance for a specific lens and apply it every time that lens is mounted on the camera. Some companies sell focus calibration tools specifically designed for this purpose. If your camera has live view that allows 10X magnification (such as the Pentax K-7) I suggest that before you buy such a tool you try mounting your camera on a tripod and calibrate by comparing the AF through the viewfinder to what you see at 10X live view. If you’re using an f/1.4 lens you may want to stop down to f/2.0 or even f/2.8 to reduce chromatic aberrations and focus shift.

Once you get everything dialed in you may be amazed at what you can do with a lens that lets in at least eight times more light than the average kit zoom and is fiercely sharp as well. These are the rewards I spoke of in the title and which I will describe in greater detail in part two of this post.

Comments

[Bruce Robbins]

I've got a focusing problem, but in spades, with my K10D and the 21mm DA lens. The K10D lets you make an adjustment for a lens but doesn't store it. If you make some changes to get a lens to focus more accurately, it throws other lenses that you might subsequently mount completely off. In the case of the 21mm, I have to make an adjustment of 210 um or 0.22 mm (or 2.3 x 10 to the minus 20 light years if you prefer) to get it to focus. Thereafter, everything else is blurred unless I dial out the correction. My 70mm DA doesn't have a problem and the 40mm DA requires a fairly modest change.

I've checked with Pentax UK and it seems the only solution is to send the K10D and my three DA primes off to Pentax in Japan to have them all calibrated together. Apparently there is a way that up to four lenses can be calibrated with the K10D but it's obviously a closely-guarded secret. Since I no longer use the K10D much, I wonder if that would be worth the cost. Why I should have to pay for it at all is another argument.

[Jay]

Nice post and good to hear from you after a while, Gordon! Looking forward to part 2.

I believe contrast detection autofocusing systems like that in the micro four thirds cameras don't suffer from this problem. Read the autofocusing section here: http://www.steves-digicams.com/knowledge-center/micro-four-thirds-evolution-of-the-dslr.html

[Michael]

Excellent post. Keep 'em coming!

[John Roias]

I am looking forward to part two. I assume that you kept the Pentax K7. I am sorely tempted to trade my K10 for the K7 with features like that. To Bruce, I too have problems with both my zoom lenses and have not yet pinpointed the problem. I suspect sometimes the camera is focusing near or far and not where I want to. I seldom remember to think of focus as I concentrate on the scene.

JMR