How do you test lens focus?

JClishe · Jan 4, 2012

Title pretty much says it all. Is there a standard set of procedures, or more specifically, a standard lens test chart or something along those lines that can be used to test a lens' sharpness and identify any potential flaws?

Robin Usagani · Jan 4, 2012

You need something like this.. BUT can your 50D be micro adjusted?
http://farm6.static.flickr.com/5103/5850317938_f47caa5648.jpg

Rephargotohp · Jan 4, 2012

If you want to test sharpness you can get a ISO12233 Chart and shoot that properly (Tripod, Trigger, Square , Plumb to the chart etc)
If you want to test focus, Camera on Tripod, Tape Measure laid out going away from Camera, Camera at 45 degrees down to Tape Measure Focus On a set Inch mark and see if actual focus falls at or in front or behind that point. Test with AF and Manual Focus.

Rephargotohp · Jan 4, 2012

And a lens can be in focus but not sharp or can be sharp but not in focus

mjhoward · Jan 4, 2012

I use this one for testing the AF for front a back focusing, and then micro adjust from there: Jeffrey Friedl's Blog » Jeffreys Autofocus Test Chart

Derrel · Jan 4, 2012

I go out an shoot photos for about one hour. Set the in-camera sharpening to HIGH. As high as it will go. That makes it easy to spot slightly OOF frames. Shoot real-world photos at multiple distances. Focus charts shot at five feet in doors do not tell you much. Many lenses, and I mean MANY lenses, will suffer from a slight focus shift when the lens is stopped down from wide-open to smaller apertures like f/4 to f/11. It's not a good idea to base lens microadjustment, or your opinion of a lens's focusing qualities, on a test chart shot 5 to 10 frames from 6 feet away...

The autofocus brackets inside a camera are often not aimed EXACTLY where the AF SENSORS are reading, and at close distances, this can lead to false results.

Bottom line: take the lens in hand, mount it on a camera, and shoot 200-300 frames and see how it FOCUSES. Not see how it shoots a test chart, but see how the lens actually focuses across a wide range of situations, and distances. Trust me, if a lens has "issues" or "tendencies", the JPEG slide show will show them after 100-150 frames.