After testing over hundreds of lenses…I have come out with a focus test method that I felt is most convincing to myself. I call it:
INDEPTH FOCUS TEST
There are many focus test chart flying around in the internet, I have printed out some and test them using my Nikon D200 DSLR. However, I found those test chart to be NOT ACCURATE, as it mainly influenced by the position and accuracy of the focus sensor of your DSLR. Some DSLR tends to focus on the subject that is of higher contrast or nearer to the sensor. Some differs.
In order to eliminate the camera focusing sensor position “error”, I developed the INDEPTH Focus Test method, there the camera focus sensor will have no problem in locking focus onto the target, and can give accurate focus photo for evaluation.
Below is how I tested lenses that I ordered for my customers at ShaShinKi.com.
Put two cards (with wordings, eg. namecard, so that you can check sharpness of the wordings later) on your table, one with a lens cap at the bottom so that it is around 1cm higher than the other. Focus on the lower card and take a shot (make sure your shutter speed is above 1/200s, or use flash to freeze motion). Take another shot with focus on the upper card.
Distance between card is around 5mm, separated by a lens cap.
When you focus on Card A, Card B should be blurred (out of focus).
When you focused on Card B, Card A should be blurred (out of focus).
Check the sharpness of both shots. It should show sharp on your focused card and blur on the opposite one. Same as the second shot.
Repeat the above test at different focal length if your lens is a zoom.
After the close distance test, try take a few shots at infinity or far distance. Again, choose a subject which have some clear wordings on it so that you can check the sharpness and clarity on your camera LCD screen later.
If your lens is good, all should be sharp.
(Updated on 20th March 2008 with picture. A picture is worth a thousand words…)