Just a quick question on behalf of a work colleague. I should say I'm very sceptical, but anyway.....
He uses a 5D3 and 70-200mm f2.8 mk2 to shoot football matches. Normally he would keep things at f2.8 (thanks to the near-constant Scottish gloom) and let the camera do the rest. Whilst pretty happy with those results, for whatever reason decided to go fully manual and constantly juggle aperture and shutter speed to suit. Auto ISO is not used. He claims (RAW) exposures need next to no tweaking regarding over/under exposure. Now here's the thing, he says images appear sharper as a result of using manual exposure compared to Av priority.
I said I doubted things being sharper unless he's just using an "on average" faster shutter speed thus reducing any shake. He's been taking football pics for years and should know the minimum shutter speed he can get away with and seems quite convinced of the improvement.
So, sharper images when using manual exposure - is he just fooling himself? The obvious thing is for him to set up a test chart and do an Av priority shot compared to a manual exposure one. Personally, I ain't buying it.
Well, I have not read the entire thread, but there could be some truth to his story, problem is, he isn't giving you the whole story. Let's try to fill in the holes. There are three things that control exposure: Shutter, Aperture, ISO. None of these things are "neutral" in their impact to IQ, however. Shutter not only controls the duration of exposure, but also has an impact on the amount of motion blur. Aperture not only controls the quantity of light over time, but also depth of field. ISO not only controls the rate at which the image saturates, but can also exacerbate noise.
In an auto exposure mode, you control ONE of these things (or, in the case of M+Auto ISO, two). Lets say your friend used to use Tv. He could then control shutter, but aperture and ISO are arbitrary factors. Depending on the needs of the scene, even a lens that is sharp at f/2.8 may not be sharp in a deep enough field, so if your shooting in lower light, you may end up with images that are sharp...in an extremely thin depth around part of your subject. Additionally, you might find that your images are noisier than you might have been able to achieve if you had total control over your exposure settings.
Av mode is similar. Again, you control one factor out of the three, and shutter speed and ISO become arbitrary factors. If you stop down your aperture to achieve the proper depth of field, your shutter might automatically end up too low, increasing motion blur. If you need f/8 for the proper DOF, depending on the capabilities of your camera and how the custom functions are configured, you might end up with a shutter that is MUCH too low, rather than an increase in ISO. You might have a low-noise image, but now you also have blur from camera shake.
Now, before I move on, better cameras, like the 5D III and 1D line, often offer much more configurability with custom functions to guide the camera into making the kinds of choices you would more often, thereby reducing the chance that you might fall into one of the situations where the camera chooses wrong, and your IQ suffers as a result.
There IS an argument for using manual mode to achieve sharper results. When you control every aspect of your exposure, you are also controlling every aspect of IQ. YOU control the shutter speed, and therefor have total control over whether you get motion blur or not. YOU control the aperture, and therefor have total control over whether the entire thickness of your subject is within your DOF. YOU control ISO, and therefor have total control over noise levels.
If your friend was really referring to his ability to exactly choose, via instinct or that natural sense that accumulates along with experience, the right shutter speed, aperture, and ISO to maximize his IQ....then he is absolutely correct. You CAN get sharper images when using manual exposure. The kicker is that you have to have the skill to gauge all the various aspects of your scene and your exposure settings to actually set them right and achieve that maximum level of IQ.
Personally, I use manual exposure mode for my bird and wildlife photography. I used to use Av, but I then I learned a bunch of new things, and I became aware of how to use manual mode properly to maximize IQ, on the fly, without having to actively think about it all that much. It's a technique that kind of comes along with a certain level of experience I think. With birds, I eventually got a sense of what apertures with what lenses I needed to get the right amount of the bird in the DOF, and at what minimum shutter I needed to freeze the bird's motion. I learned the range of ISO settings within which noise was acceptable and controllable in post. You also learn how to use the exposure meter at the bottom of the VF to help you, along with the brightness of various background factors, to gauge what your exposure needs to be (where the marker on the exposure meter SHOULD be pointing in order for exposure to be "correct" for that scene).
Once you reach that point, you don't have to think as much about what your doing when it comes to exposure. You make quick, momentary judgments about where you think the exposure meter indicator should be, what DOF you need, roll some dials, and start taking shots. You also begin to understand that once you set your exposure for a given scene in known lighting, you don't need to change your exposure once it's set. Only when the lighting changes, or you change your scene (i.e. point your lens somewhere else) do you then have to recalculate and choose new settings...but again, that should all happen in a couple of seconds and be done before you really realize you did it.
So, your friend isn't wrong, he just wasn't telling you everything.