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To start, here is some info about the following photo I took:

Photo: Shutter 1/80s, f/1.8, ISO 100

Lens: Canon EF 50mm f/1.8 STM, no lens filter

Camera: Canon T5i

I took some pictures at my school's rally with a brand new lens, but unfortunately some turned out slightly blurry and it's unclear to me why this happened. This photo is too large to attach directly, but here is a link to it. When zoomed in on the subject's face it is easy to see that the image is somewhat blurred and there is some lens aberration. I thought it could have been caused by either poor lens quality, bad auto-focus, or motion blur, or is it perhaps something else? I want to figure out whether if it is a problem that I can fix by changing my camera settings, or if it is a problem I cannot fix without buying new equipment.

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... I thought it could have been caused by either poor lens quality, bad auto-focus, or motion blur, or is it perhaps something else?

If the problem is motion blur, you typically see a slight double-image or slight translucent "ghosting" around moving parts. You'll also notice more blur where there's motion vs. where there isn't any. There may be some motion blur in this image, but it doesn't look to be the apparent cause of most of the softness. But your shutter speed is slow, especially for moving subjects.

If it's missed autofocus, you tend to see a sort of even level of blur across the entire image. If you used a setting, focal length, or subject distance that leads to thin depth of field and background blur, there may be parts of the image in front of or slightly in back of where you thought you focused that's in focus. This could be part of the issue, because of the uniform softness across the frame. In lower light, which this evidently is, it can be harder for the camera to lock on focus. Consider adding light with a flash or using a flash's AF assist light (if it's allowed), or aiming for an area of high contrast (where black meets white at a sharp edge--lettering on shirts is typically good for this). You may also want to try using the single center AF point, which is your only f/2.8 point and should be more accurate than the other points with a fast lens.

If the problem is noise, you'll see noise. :) This doesn't look to be a problem, but might be if you processed with aggressive noise reduction, but I'm not seeing the "painterly" effect that typically causes.

If the problem is lens performance, you can see if you get better results with another lens, or at different settings. Most lenses are at their softest, with the most distortion/CA/vignetting when they're used wide open. Which you are doing here. And I do see chromatic aberration, so, this may be one of your culprits as well. Stopping the lens down to at least f/2, if not f/4 could improve things. Play with the aperture setting on dpreview's interactive test results to the optically-identical EF 50mm f/1.8 II, to see how sharpness and chromatic aberration are affected by the aperture setting.

The one big setting error I think you made was using ISO 100. While you're taught that a lower ISO setting leads to less noise, the fact is that the ISO performance on current dSLRs is good enough that you really don't need to worry about that tradeoff until you're over ISO 1600, and underexposure will often cause you even more issues with noise than a higher ISO setting. Simply setting your ISO to 1600 would have let you use both a faster shutter speed and a smaller aperture setting. This is why you paid the big bucks for a bigger sensor; don't be afraid to crank the iso up.

  • Comments are not for extended discussion; this conversation has been moved to chat. – John Cavan Aug 28 '16 at 2:05
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All in all for the shutter time and subject matter you did very well to get a photo as clean as you did.

Here are the causes of the imperfections you notice in your photo:

  • The people in the photo are moving too fast for the shutter time of 1/80 second. Don't be afraid to push the ISO up to 400 or even 800 so that you can use a shutter time of 1/320 or 1/640 second to freeze moving/jumping people.
  • At f/1.8 your lens will be less sharp than stopped down a couple of clicks. This is true of the vast majority of lenses. Use some of that speed you gain by raising the ISO to 800 and set the aperture at f/2.2 or f/2.5 and adjust the shutter time to 1/400 or 1/320 second.
  • With such a wide aperture the depth of field will be fairly shallow, so the people in the background will not be as sharp as the person you are focused on. Closing the aperture a little will help some, but there's always only one distance that will be in sharpest focus. That's OK, though, as it looks natural to most viewers and helps guide the viewer's eye to the primary subject that would otherwise be lost in that sea of people.

I shoot similar photos in the gym at a local high school at pep rallies. I usually use ISO 2500 or so with f/2.8 on my 70-200mm telephoto lens (it is very good even wide open) and f/4 on my 24-105mm and/or 17-40mm lenses.

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Based on the edges of the shirt, it's obvious you have motion blur. Based on some other subtle clues (e.g not every high-contrast edge is blurry), it seems that it is the motion of the subject rather than the camera.

You may also have some focus blur, but the main blurring seems to be motion.

That slow of a shutter speed is pretty much guaranteed to have sketchy results. I would shoot no slower than about 1/500 sec. Boost the ISO if necessary.

You also have some fairly bad chromatic aberration going on. I love large apertures, but in this case you might consider a smaller aperture. In this context, at some point it will come down to a balancing act of evils between chromatic aberration, high-ISO noise, and motion blur.

In circumstances like this, I sometimes purposefully shoot several shots with a slow shutter speed, about 1/60s. The idea is that I might get a tiny % of those shots that are beautiful in terms of low noise, and might even have some artistic-looking motion blur. (Especially if the camera is tracking the subject and it is the background that has induced motion blur.) I also try shooting with fast shutter speed, reasonable aperture (e.g. f/8.0), and high ISO - so that I know I'll get some good "fallback" shots with the exception of ISO noise. (Otherwise, I prefer wide-open aperture for shallow depth-of-field.)

Personally, I usually lock the shutter speed and aperture, and let the auto-ISO do the adjustment. I also shoot with a full f-stop of underexposure bias, 1) for additional flexibility, and 2) you can pull detail out of shadows but you can't recover blown-out highlights. (With a full-frame sensor, I can afford to pull low-noise detail out of shadow, unless I am going for absolutely 100% color and detail fidelity under tightly controlled conditions.)

So, if you are almost always going to do post-processing, then yes do shoot in raw format, and try my trick of purposefully underexposing your shots. But a raw workflow is substantially slower - and gets exponentially slower with a linear regression backward in computer hardware time and cost. (I use raw but for cameras that can't shoot in raw, my workflow is so much faster.)

  • As someone who usually shoots similar events with both FF and APS-C bodies I can say the FF camera is much more tolerant of pulling exposure when shooting and pushing it in post. – Michael C Aug 27 '16 at 7:02

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