Fixing Front and Back Focus
Conclusion
Now that you've seen how to fix front- and back-focus with lens alignment products, we'd like to finish with a few more remarks about autofocus performance.
Naturally, if you observe that your photos are consistently not sharp where you want them to be, you will suspect that there's something wrong with your lens. However, you should be aware that front- and back- focus isn't necessarily the only thing that could be getting in the way.
Other Causes of Auto Focus Errors
You may think that your lens has front or back focus but focus errors can have several more likely causes:
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The focus points are not as small as indicated by the red overlay squares in the viewfinder. A somewhat larger area is taken into account when the system determines "correct" focus
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The placement of the actual focus points in the AF module and the overlay points may not match completely, or at all, so the camera doesn't focus where you think it does
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Letting the camera decide where to place focus (i.e. using more than one focusing point): the camera may pick another point as the active focus point than you expected/intended
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Too low contrast or too little light to achieve focus with any degree of accuracy
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Low light combined with a heavy color cast
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A slow lens (small max. aperture). The performance of the AF system deteriorates with slower lenses. Those with a max. aperture of f/6.3 or f/8 are especially problematic. Some camera models may already have difficulty at f/5.6
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Auto focus systems are not as accurate as you may think they are in the first place. This becomes painfully evident when shooting at a large aperture like 1.4 where there is too little depth of field to mask the error
Pros and Cons
| LensAlign | Spyder LensCal |
| +Meticulously-labeled ruler | +One-piece unit |
| +Parallel alignment System | +Sturdy |
| +Sturdy once assembled | +Bubble leveler |
| +Tripod mount | +Tripod mount |
| +Available extended ruler | +Much cheaper than the LensAlign |
| -Expensive | -Difficult to align camera |
| -Difficult to disassemble | -Simple ruler and markings |
| -Subject to wear-and-tear | -Could easily make one yourself |
Unfortunately, neither system ships with detailed instructions on how to test your lenses and interpret the tests, so but that's why we're here!
The Bottom Line
So, which is better- the LensAlign or the LensCal? Well, if you're planning on spending money on a lens calibration tool, we would without a doubt recommend the best system, and that would be the LensAlign. Assuming your tripod has a built-in bubble leveler, it's a breeze to align your camera to the LensAlign unit, and you'll be able to test your lenses without having second thoughts about the setup. The tests will be more accurate and you'll be able to test a wider variety of distance/aperture combinations with the LensAlign than with a LensCal.
We hope this article saves you from the pain of exchanging your lenses or getting them serviced! If you have any further questions, feel free to post them in our Photographic Technique forum.
