Can anyone else see the optical illusion it creates?

To me it looks like the top-right hand corner is stretched diagonally up and right, making the top look like it's slanting up a little from left to right, and the right side slanting out to the right a little.

As John says, very interesting, Steve.

I noticed some time ago, when taking pictures of the moon, that accuracy of focus was incredibly critical - I kept getting unexplained duds, even though I had focus confirmation every time.

As you have, I also noticed that there's a range of a few millimeters of focus ring position over which the green hexagon is displayed.

I found that if I took the picture when the focus ring was right in the middle of that range, I got more shots in focus. Still only about 1 in 3, but a lot better than the 1 in 10 or so I was getting before.

Well, I can see them as two separate flashes.

I think most people can, but they are very close together - if you didn't know there were two, you might not notice if you weren't particularly observant.

P-TTL has one important drawback with portraits - a lot of people react so fast to the pre-flash that they blink - and by the time the shutter opens, their eyes are half closed. It's very annoying, and with some subjects makes P-TTL unusable.

Page 166 in the manual.

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All the 'Red eye reduction' setting does is discharge the pop-up flash a short while before (and in addition to) its normal pre-flash/main flash cycle.

The first flash reduces (but does not eliminate) red eye by making the subject's pupil contract before taking the picture.

Then it does exactly what it does without the 'Red eye reduction' setting - it sends a preflash first, which allows the camera to do its metering, and then sends the main flash while the shutter is actually open.

Try this:

- switch your camera to 'Red eye reduction' mode
- watch the popup flash from the front (and off to the side a bit, so you don't get blinded)
- take a picture

You'll see three flashes. The first (anti red eye) one about half a second before the second two, which take place very close together.

Now switch the camera out of 'Red eye reduction' mode, and do the same thing.

This time you'll just see the final two flashes, very close together.

The first of these second two is still early enough to trigger the Xtra flash, which will be all over and done with by the time the shutter opens for the actual main flash.

Edit: ...and if it's dark enough for the camera to decide it wants to use a burst of flashes from the popup to allow it to see to focus, that happens before all the above as well.

The 'pre-flash' we talk about in the context of P-TTL refers to the metering pre-flash, that is, the one that takes place a small fraction of a second before the main (exposure) flash.

The AF flash burst and the red-eye flash are both 'pre' the exposure, but the 'P' in P-TTL refers to the metering one.

I don't think the way the AF system works out whether the shot is in focus is any different whether you focus it manually, or allow the screw drive (or lens motor) to focus it.

So if you have the green hexagon focus confirmation, that shows that the AF system thinks the picture is focused.

Given that, I would imagine that any adjustment (and I don't know what it is that's actually adjusted) applies equally to AF and MF lenses.

I'm only guessing though - I don't know much about how it works.

Another thought:

How central are the gliders in your original frames? Is the one you posted to the gallery just resized, or is it cropped as well?

The reason I ask is that the glider is nowhere near the centre of the frame in the one you posted, and it may simply be that the camera was focusing on the clouds behind. The sky in the background is not featureless enough, as I mentioned above, to be confident that the camera won't pick it as the subject.

If that's the case, and you don't want to focus manually, you'll definitely need to force it to use the Centre AF point, and sort out your composition in the crop.

I'm sure you're aware, but just in case, the "half-press to lock focus, then reframe to compose" technique will not work at all with AF-C - because the focus will be changing all the time while the subject is moving and you have the button half-pressed.

That can be quite an ask though, sometimes, especially with fast jets.

With something as slow as a glider, it should be reasonably easy to keep the centre AF point on the subject.

However, I'm not convinced that if the background is featureless, Auto-AF point selection is any worse than Centre-AF point. I've used both quite a lot, and not seen anything much between them.

Of course, if there are background features to confuse the situation, Auto-AF can indeed go horribly wrong, but I don't think this case is like that (edit: actually, it might be - see below).

Very interesting information, thanks, Steve. I did wonder if it was possible for the adjustment to itself require adjustment, and from what you say, perhaps it does.