How can I accurately test and fine-tune autofocus on a 35mm f/1.4 lens?
Asked 1/11/2016
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I’m checking a new 35mm f/1.4 lens for autofocus issues using a focus test chart with a ruler. In my test shot, the zero mark seems close to in focus, but the image overall doesn’t look as sharp as I expected. The chart may have been slightly tilted, and the sample was likely a JPEG. What’s the best way to test whether the lens is actually front- or back-focusing, and could the softness wide open be normal for a fast lens?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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The focus looks nearly if not completely spot-on on my monitor, though try leveling the focus chart (you can see it's slightly tilted, which will throw off your measurements).
You are more likely to be seeing the effect of spherical aberration in the lens, which manifests as a "glow" even on in-focus parts of the image. That this is common in all fast lenses wide open. If the problem goes away at f/2.8 or so, then that is the culprit.
Originally by user11472. Source · Licensed CC BY-SA 4.0
user11472
10y ago
0
Generated from our catalog & community — verify before relying on it.
Based on the sample description, the focus appears close to correct rather than obviously front- or back-focusing. Two things may be making it look soft:
- Test setup issues: If the chart is tilted or misaligned, the ruler reading becomes unreliable. Mount the camera on a tripod, make sure the target is level, and align the camera squarely to the target.
- Image quality factors: Underexposure, high ISO, and in-camera JPEG noise reduction can reduce apparent sharpness. Use base ISO, good bright light, and proper exposure. Since the target is white, you may need positive exposure compensation. Avoid fluorescent lighting if possible.
Also, many fast lenses at f/1.4 show a slight “glow” or lower contrast from spherical aberration even when focus is correct. If the image becomes clearly sharper by around f/2.8, that’s likely normal lens behavior rather than a focus calibration problem.
For a better test: shoot RAW if possible, use steady support, good light, accurate alignment, and compare shots wide open and stopped down.
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