Time to be with your loved ones

Time to be with loved ones

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I recently bought a Canon T5i. I covered an event the day I got it, and the photos turned out nicely. Since then, I haven't taken a large batch of photos, until today. My friends and I had a photoshoot, and once I looked at the photos, I noticed any areas that were out of focus were duplicated. I don't know how to explain it. It's like the 3D effect with the overlayed red/cyan. I'm not sure what the issue is, but it's freaking me out. The first set of pictures turned out so nicely and this second set is really messed up? What can I do? Is there a problem with my camera, my lens, or the way I'm shooting? Here's an example photo:

enter image description here

and another

enter image description here

Thanks for any help!

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Maybe it's just me, but I can't see it too much in that photo. Can you give a couple others? –  Tortilla Jul 19 '13 at 6:37
    
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The other thing that comes to mind when people say they see red and cyan tones, is to check around to see if the lens suffers from chromatic aberration issues at certain settings. Worth a try. –  TroyR Jul 19 '13 at 7:35
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Troy, I guess you could describe the effect as a sort of "ghosting". The upper left of this picture shows it pretty clearly, more so than the grass: fbcdn-sphotos-g-a.akamaihd.net/hphotos-ak-ash3/… –  Tortilla Jul 19 '13 at 7:38
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@TroyR I am not familiar with that particular lens, but to me they have a "weird" feel in the out of focus areas... it's not like the typical Gaussian-like blur you usually get, but closer to a motion blur. –  fortran Jul 19 '13 at 7:58
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5 Answers

Based strictly on visual observation of the photos without any EXIF data, my eyes see a combination of the following issues:

  • Most of the shots are slightly front-focused. That means the area of sharpest focus is slightly in front of your subjects' faces and eyes. Try manually selecting a single focus point instead of allowing the AF system to select one from all of the points, as that usually results in the nearest object being in focus. If you are already using a single focus point and centering it on your subjects eyes, then the lens and camera need to be calibrated to each other. Unfortunately, your T5i does not have the capability to correct the problem using Auto Focus Micro Adjustment. The lens and body would need a trip to a Canon Service Center to be adjusted. If both are still under warranty, this adjustment should be covered.
  • The edges and corners show a moderate amount of Chromatic Aberration. This is to be expected with the EF-S 18-135mm f/3.5-5.6. It is part of the price you pay for a wide focal length range in a single, inexpensive, lightweight lens. If you save your images as RAW files, you can apply correction profiles for that lens using Canon's Digital Photo Professional software that was included with your camera. Other RAW convertors, such as Adobe Camera Raw or DxO Optics may also include profiles for your camera/lens combination.
  • The harsh Bokeh associated with a seven-bladed diaphragm. The fewer blades an aperture diaphragm has, the less circular the aperture will be and the rougher the bokeh will appear. Most premium lenses use 8, 9, or even more blades to smooth out the bokeh.
  • Some of the shots may demonstrate minor diffraction issues. The 4.3µm pixel pitch of the T5i means the diffraction limited aperture (DLA), or the point at which diffraction begins to affect image sharpness, is at f/6.8. To avoid diffraction, use an aperture wider than f/6.8.

All of these things appear to be combining to one degree or another to create the bluriness you are seeing. The hard bokeh makes the slighlty out of focus faces look worse. If you stop down to increase the DoF, you also make the diaphragm less circular, which makes the bokeh even harsher and introduces diffraction if you go above f/6.8. Other than the front-focus issue, the pictures are about what I would expect from that lens.

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18-135mm stm has 7 circular blades –  Michael Nielsen Jul 20 '13 at 14:08
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I must have looked at the earlier non-STM specs. With only 7 blades though, "circular" is still fairly relative and at narrower apertures will still be noticeably non-round. –  Michael Clark Jul 20 '13 at 16:24
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As the images do not have the EXIF data (Facebook strips it after upload), I cannot be sure if this is the cause... But the look of the pictures reminds me when I shot with really small apertures beyond the diffraction limit (around F11).

As they seem well lit outdoors, it's not something to be ruled out in the first place. Was that the case?

Anyway, you should make some more thorough testing:

  • Try a different lens (or the same lens in a different body)
  • Try with IS on and off
  • Try different focal lengths
  • Try different apertures

That would help to isolate the cause of the problem at least.

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They did try with IS on and off. I agree with your suspicion of diffraction. –  Tortilla Jul 19 '13 at 7:34
    
yeah, but without knowing which pictures are taken with each settings it's difficult to tell anything... :( –  fortran Jul 19 '13 at 7:37
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I understand, that's why I've been writing so many comments, lol. I agree with the diffraction though. dominiquealexi, can you tell us what apertures these were taken at, and/or run some tests with various other apertures? –  Tortilla Jul 19 '13 at 7:42
    
I took a few pictures with each, and found this: Using my T5i body and a lens from an old XT body (18-55mm), I don't get the effect. Using the XT body and the 18-135mm STM lens, I still get the effect. So I guess the issue is with my lens? edit: the EXIF data from the photos before all have about F5 or F5.6 as far as I can tell. –  dominiquealexi Jul 19 '13 at 7:45
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It seems so, which is what I think we all suspected. f/5 and f/5.6 are awfully large to be causing much diffraction. –  Tortilla Jul 19 '13 at 7:49
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You may just be experiencing the different bokeh for the first time. Bokeh is generally described as the quality of the background blur. It comes from the shape of the iris as the light passes through and is thus altered by the aperture, the lens design and the camera it is being used on (due to the difference in projection distance). It may just be that you are seeing that.

Another possibility is that you are seeing chromatic aberrations. Lenses, particularly cheaper lenses, will tend to have a slight prisming effect, particularly near the outside edges of the lens. This results in red lines on one side of sharp lines and blue lines on the other. There is chromatic aberration correction support in Canon Digital Photo Professional if you shoot raw and import them in there to apply the correction. Canon takes in to account the lens design when performing the correction.

If you can post a crop of what you see as the problem, that might make it easier to narrow down exactly what is happening.

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The effect you're seeing is probably a combination of chromatic aberration and spherical aberration. Both exist to some extent in all lenses, but different lenses vary greatly in how susceptible they are to these effects. Not surprisingly, more expensive lenses are usually more resistant to such optical imperfections, containing lens elements specifically designed to counteract them.

Chromatic aberration is caused by dispersion, the fact that different wavelengths of light have subtly different refractive indices, as seen in a rainbow. It usually manifests as coloured "fringing" around objects in a picture, particularly near the corners. This might be the "cyan-red glasses" effect you mentioned. In high-quality lens designs, it is combated with lens elements partially canceling out another element's dispersion, as well as using special low-dispersion materials such as fluorite glass in critical elements.

Spherical aberration affects the bokeh - the "evenness" or "smoothness" of the blur in out-of-focus areas of the image. In an ideal world, the out-of-focus image of a point would be an evenly-shaded disc. In reality, however, the discs look more like the ones in this image. In particular, if the lens has positive spherical aberration, the perimeter of the bokeh discs of objects behind the focal plane appears brighter than the center. Now, if the image contains long and thin details, such as grass or thin branches, you might see how this may cause the sort of "duplication" you mentioned. In lens design, spherical aberration is typically compensated for by using one or more nonspherical lens elements. Note that negative spherical aberration in a lens design may be desirable, as it may result in smoother, more pleasing background bokeh.

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The problem is not "out of focus". This is caused by slow shutter speed. When you use a long exposure things that moves tend to be "duplicated" in the photo.

The solution: Faster shutter speed. You can achieved it by increasing the ISO, using a bigger aperture (smaller number is bigger) or shooting when there is more light in the scene and then the exposure will be shorter.

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I'm not entirely convinced it's shutter speed. The blur seems too isolated, not universal. –  Tortilla Jul 19 '13 at 7:07
    
If he's using a flash that can explain it. –  Itay Gal Jul 19 '13 at 7:08
    
I rarely use flash--I don't like the oily look it gives people. I have yet to move to a filler flash. –  dominiquealexi Jul 19 '13 at 7:14
    
What shutter speed did you use? –  Itay Gal Jul 19 '13 at 8:18
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