EF 50mm 1.8 ii focused on top edge not on bottom

[jeremiah_romano]

I have three EF 50mm 1.8 ii lenses purchased at different times over the course of a few years. I am working in an archive setting taking reprographic style captures of documents. The issue I am running into is there is a gradient of focus sharpness from the top of my images (sharp) to the bottom (blurry). I have used all three lenses on a 7D, 5D Mk3, and a 6D which all produce the same problem to slightly different degrees (the 7D's being the worst). I have calibrated the camera columns and mountings for paralellism with the document table so I dont think that is an issue. I also have an EF 100mm 2.8 L IS USM , and a EF 14mm 2.8L ii USM. And neither of these lenses give me the focus gradient on any of my four capture stations. Does anyone else have this type of focus issue with the 50mm 1.8 ii? I am at the point where I feel out of my depth in terms of troubleshooting because the problem seems to be isolated to the 50mm lenses but I cant understand why.

thanks

-Jeremiah

[Skirball]

I’ve never seen anything like that, though I don’t spend much time shooting documents where such an issue might be more observable.  Are you using wide apertures (i.e. thin depth of fields, which would highlight the lens and subject not being parallel)?  Conversely, are you using really small apertures and suffering from diffraction? You mention it’s sharp at the top, are you setting focus on the top of the image as opposed to the center?  What kind of lighting are you using?

It’s kind of amazing that all three lenses would show the same issue, it makes me question the cause.  Furthermore, the crop sensor cameras, such as the 7D, would show less of the distortion if it was caused by the lens element since it uses the central part of the image.   It doesn’t make sense that it’s the lens element, and all three have this issue. 

As an aside, if I was trying to do what you’re doing then I would use the 100mm macro lens, it’s far sharper.  If you were looking to use 100mm macros you don’t need the L version for this use.  I realize this doesn’t solve your problem, just pointing it out.  Though I don’t know what kind of resolution you need for archiving.

[jeremiah_romano]

Thanks for responding to my post.

I strictly use f5.6 for the 50mm 1.8 ii based on some performance analysis I found here. I assume that f5.6 is pretty middle of the road for subject matter 20"-60" from the camera. I use two opposing banks of 5000K flourescent studio lights for illumination on all of my stations.

I am working mainly from EOS Utility and use the movable focus frame from within the liveview window to set the position for focus (which is generally centered).

I agree that if the problem area was originating from distortion occuring at the edge of the lens than the crop sensor on the 7D should suffer less than the larger sensor on the 5D or 6D. however that is not the case.

I do appreciate the suggestion about the 100mm lens and whenever possible I do use that lens becasue it gives me the best results, however the materials that I am capturing can range in size up to 30"x40" which the 100mm cannot fit in its frame at 60" away from the camera.

might there be a better lens option that could provide better focus properties and accomodate larger sized items. 80mm...?

these cameras and lenses spend their whole lives pointed downward and I some times wonder if this causes wear that might produce this issue. however the lenses vary in age by a few years so that seems unlikely

[Skirball]

Very interesting.  Sure, there are better lenses than the 50mm, but they should be fully capable of this task.  Corner sharpness should be totally acceptable at f/5.6.  Something is off, I just can’t imagine what.  The chances of all 3 lenses having the same flaw seems unlikely.

I would try a different method to focus just to make sure you have the center: use the AF on centerpoint without Live View, then switch the lens to manual.  If what you’re shooting is flat you shouldn’t really have to mess with focus during a session unless you change the camera height.  At f/5.6 the depth of field should be plenty big that you really shouldn’t even have to worry about being totally parallel.

I don’t know, maybe the nifty fifty does suffer from pointing down after a time?  Can you post an example so we can have  a look?

[jeremiah_romano]

Here is a test shot done on a 7D with a 50mm. the bottom edge of the test page is where the sharpness problem is.

[jeremiah_romano]

Let me make a quick 100% corner comparison image for inspection.

[jeremiah_romano]

Here is a comparison of the corners from the previous image I posted, it is the size of a standard piece of copy paper . the test image was made from a capture done with a 50mm on a 7D. the test image was about 40" from the camera.