Image Alignment or Normal Optic Aberration?

[san.timoteo]

Hi, thanks for your site, very useful information here.

I recently acquired a new 50D, delivered with an EF 50mm 1.8, an EF-S 18-55mm 3.5-5.6 IS, and an EF 75-300mm 4-5.6 (III). Camera is for general consumer use, but I do have a copy stand and decided I wanted to photograph my record collection (easier to photograph them than to scan & stitch -- easier on the covers, too).

It's been awhile (1977?) since I used a copy stand, I recently found an old Beseler in good shape, got the photofloods, spent a lot of time mucking about with lighting, lining up the camera, setting manual exposure, white balance, etc.  Everything is good except one thing I can't seem to get right: alignment.  I've accused just about everything (starting and probably ending with my own incompetence!), but now I'm wondering about the 50D.

I've long known that with wide angle lenses, it is common to have visible aberrations, arcing and the like, around the edges. However, here the problem seems to be at the right and bottom of the image.  Normally, I would have expected a certain amount of symmetry in the arcing, but as you'll see in the test image, the problem is most noticeable at the right edge, whereas the left edge appears true.

Note that of the three lenses, the only one "suitable" for this job is the 18-55mm, as the standard 50mm has too small an angle to get the entire album cover, even when the camera is at the maximum distance at the top of the stand.  The camera reports the exposure was taken with zoom at 44mm, 1/60 at F5.0.

http://las-solanas.com/galeria/fauxtaux/IMG_0942_50pct.jpg

(A note about the above image: don't focus on lighting -- I just used the built-in flash for the test photo, not wanting to waste the life of the photofloods when unnecessary.)

Notice how the copy stand's grid lines on the right are straight, not arced, but on they right they angle in at the top.  As you move left, the angle diminishes until you get to the left where the lines are true 90 degrees.  I've mounted, remounted, tweaked all I can, and the results are consistent.  As best I can tell the camera body and lens are aligned to the copy stand, and the copy stand platform is flat and not warped.

I tried using the standard 50mm to determine if the problem came from the 18-55 zoom.  With the 50mm, there is a very slight amount of arcing at both left and right edges, but you'll notice the right edge continues to slightly tilt inward from bottom to top.  Here's a shot with the 50mm:

http://las-solanas.com/galeria/fauxtaux/IMG_0892_2.jpg

(again, ignore colors, it was probably on full-auto with AWB on, using the photofloods.)

The reason this annoys me?  I would have to use photo editing software to correct skew on all those photos... otherwise they turn out like this (leaving in the "gray space" so you'll "get the picture"):

http://las-solanas.com/galeria/fauxtaux/IMG_0953_1000.jpg

I tried my old cheap Konika 310 POS, and it exhibits the same problem.  However, my trusty old OM-1 from 1976 with 50mm Macro lense doesn't seem to have this issue.

So I'm wondering... is this phenomenon normal for these lenses?  The 1.6 magnifier is all relatively new to me.  I want to know if there may be a problem with the alignment of the sensor in the camera, or if this is common and I need to use a specific type of lens for copy work.

If a specific type of lens is required, what would you recommend? (That isn't "pro" expensive! :-)

Thanks for any advice.

Tim

[KeithB]

Ken Rockwell shows very little distortion in his 18-55 IS:
http://www.kenrockwell.com/canon/lenses/18-55mm-is.htm

Also, just a note, tethered shooting, by using the EOS utility on the computer and a USB cable connected to the camera can be very helpful here to let you see what you are shooting.

[Bob Atkins]

It's an odd problem. It's not a lens distortion since they are symmetric and cause lines to bow near the edges of the image. Your lines are straight and appear to be on one side.

If you had not said that you took lots of care over alignment, I'd simply say that the sensor plane isn't parallel to the copy suface. That's normally what produces effects like the one you are seeing. I'd double and tripple check alignment and I'd see if the 18-55 was any different from the 50 prime. Though the 50 may not cover the area you want to cover, at least you can see if you get the same effect when you're focusing on the grid pattern.

The 50D has Live View, so I'd use that, then tilt the camera around until I got the grid lines parallel to the sides of the frame. Another way to get alignment right is to put a mirror on the base and make sure that the image of the reflection of the lens is centered.

[san.timoteo]

Thank you for your input.  While I've heard some reviewers call "Live Shooting" a gadget, you have just identified one example where it is VERY USEFUL: copy work.  Using Live Shooting, I was able to play around with the positioning of the camera on the stand in ways I couldn't from just staring through the viewfinder -- I figured out where the problem was coming from, and thankfully it's not the camera.  I've now been able to get those gridlines perfectly straight.  Happy camper.