Lens Testing

[LOTOC Mike]

I was a little bored this afternoon, and did some reading on lens testing.

I don't want to say too much first- in fear of sounding like an idiot and I want to get honest feedback.

Here is the link to these photos on Flickr: (http://www.flickr.com/photos/mikehutchison/sets/72157623555239388/)

*All photos were taken with a single AF on the top 9" mark on the ruler.*

Rebel XT
ISO 100

Canon 50mm 1.8 II

50mm
1.8
Taken from 18"
http://farm5.static.flickr.com/4069/4407232986_e324ea0f47_b.jpg

50mm
1.8
Taken from 30"
http://farm3.static.flickr.com/2693/4406467087_db2c395ff5_b.jpg


Canon 24-70mm 2.8L

70mm
2.8
Taken from 18"
http://farm5.static.flickr.com/4015/4407233446_bcbf52d64e_b.jpg

24mm
2.8
Taken from 18"
http://farm5.static.flickr.com/4065/4407233822_6d6f2ef842_b.jpg

70mm
2.8
Taken from 30"
http://farm5.static.flickr.com/4030/4407233626_2e3ac8604b_b.jpg

24mm
2.8
Taken from 30"
*It appears the AF missed on this one* I could retake, but the conditions may not be exactly the same as before.
http://farm5.static.flickr.com/4008/4407233674_12b1de6c21_o.jpg

Sorry, if posting the photos large like this threw the forum out of whack, as DJ requested no to do, but I have a big monitor, so I can't really tell.

[Sea Esta]

I would think you'd want to be a bit further back and then shoot as at these distances you are close to minimum distance of the lens and DOF is going to be quite thin. You can also tape a newspaper page to the wall and focus on that. I say anywhere from 6 to 10 back. Then take multiple shots at different apertures and find where your sweet spot is. On your 1.8 you might have to stop down to f2.8 or f4 and on your 2.8 it might be f4 or 5.6. Just mount your camera to a tripod and take shots and then look at them. Make sure you are shooting in M mode to keep everything consistent.

[Dr. Jeff]

Mike, what is it exactly you're testing? Is it an attempt to find the sweet spot as Glenn eluded to, or are you testing something else?

[LOTOC Mike]

Mike, what is it exactly you're testing? Is it an attempt to find the sweet spot as Glenn eluded to, or are you testing something else?

Sorry I didn't explain that better. I was really testing to see if the lens is focusing correctly. Sometimes you get a bad version of the lens, it happens.

So I was really trying to see if it is front or back focusing.

This is what I was mainly trying to do: http://photo.net/learn/focustest/

[Sea Esta]

Ok, I see what your doing now. I take it you were using autofocus for these tests. From what I am seeing, if you are focusing on the 9" mark it appears both of your lenes are sharper on the 10" mark as opposed to the 8" mark except for your 50@1.8 @ 18". There it seems to be dead on. I would rerun the test on manual focus and see if the results are close to the same.

IMO your lens aren't that bad off and are probably within specs. If your still concerned then I'd still put the newspaper on the wall and take a few shots, find your sweet spot and then check the sharpness between the center of the newspaper and the edges of the newspaper. ( You want the edges of the newspaper to be at the edges of your frame). The sharpness should be the same from top to bottom and left to right. If everything looks good then you have a good copy.