"BTW, a 1/400 shutter speed is not super fast, but it is the minimum that I would use with that lens photographing toddlers."
Have at it, there is no law or 'rule' against it. 
It just isn't necessary.
@ebiggs1 wrote:"I do not recommend “focus and recompose” techniques ..."
I totally disagree with this. I use focus and recompose all the time. Almost every shoot I go on will have a focus and recompose situation. Focus and recompose is not focusing on a distant mountain and then fixing on a close up tree.
It involves the subject, in perfect focus, and then simply moving the frame. The subject is just in a different place in the frame not further or closer.
Ernie, it depends on the depth of field (which is why I said I wouldn't do this unless I had a broad depth of field). But this is a well-understood issue. Focus & recompose with a shallow depth of field is a good way to get a lot of out-of-focus shots.
Here's a diagram that will hopefully clear things up.
This is a perfect circle with a camera located at the center. The camer is focused to the radius of the circle such that everything on the blue circle would be in perfect focus.
I've drawn a subject on the circle toward the right and indicated the position of the subject's eyes. I've also drawn a depth of field box ... green is in focus, red is out of focus. There are two of these boxes. The one over the subject is used to establish "focus". The one in the top center of the circle is used for "composition" (e.g. rule of thirds).
Notice when you look at this, that the subject's eyes are no longer on the focus plane.
This is why you really need to be careful when doing focus & recompose if using a shallow depth of field (which often is used for portraiture.)
Do not just rely on the center AF point for everything. There's a good reason the camera has more than one AF point.
Tim said,
"This news shouldn't be a surprise to anyone. Most experienced photographers know this. I didn't invent the idea."
WOW, perhaps I need to go live my life all over again. 
Most experienced photographers know what works and what doesn't. But than again most experienced photographers don't have the time to read pretty graphs and book learn about physics. They do what works. If they are out making money that is.
"There are no exceptions."
Oh, yes, there is. There are the hobbyists, the book learners, and those infatuated with the physics of photography. Then there are the rest of us that just do the work. Just like my little test images of the best lens vs a much less good lens. No reliable guesses but the charts tell us you should see the difference quite readily.
@ebiggs1 wrote:Tim said,
"This news shouldn't be a surprise to anyone. Most experienced photographers know this. I didn't invent the idea."
WOW, perhaps I need to go live my life all over again.
Most experienced photographers know what works and what doesn't. But than again most experienced photographers don't have the time to read pretty graphs and book learn about physics. They do what works. If they are out making money that is.
"There are no exceptions."
Oh, yes, there is. There are the hobbyists, the book learners, and those infatuated with the physics of photography. Then there are the rest of us that just do the work. Just like my little test images of the best lens vs a much less good lens. No reliable guesses but the charts tell us you should see the difference quite readily.
Ernie... I don't know why you're trying to defend this.
The diagram helps people see how when a subject distance has not changed, and only the camera composition changes, something that had critical focus will no longer be in critical focus after re-composing.
Technically lenses do not have perfectly flat focus fields, but they do endeavor to try to be flat.
So here's an example using actual photographs. This was taken using an 85mm f/1.4 lens. The camera is on a tripod. The focus card is on another tripod. This guarantees that the distance never actually changed, nothing moved, there was only a rotation of the tripod head to recompose for the 2nd shot. This is "straight out of the camera" (other than converting RAW to JPEG and sizing it for the message forum.)
In photo 1, I've centered my focus test target, used the center AF point, and I've put it on the center of the high-contrast test target. Check the diagonal scale to the right of the focus point and you can see that the "0" point is nicely focused.
Photo 2 was also focused using the same center AF point and using the same position on the target (the high-contrast target just left of the "0" position on the scale) BUT THIS TIME after the camera locked focus, I recomposed so that this position would be on the right side of the frame. Notice what happens to focus here:
Since these are uncropped images, but the size had to be reduced to fit on the forums, you might not be able to read the focus card very easily. So I've supplied the same images, but cropped in on the area of interest.
Here's the first image (cropped) where the target is at the center.
You can see in the image above that the camera did a pretty good job with the focus.
But here's the crop of image #2 where we re-composed.
The focus target is no longer in optimal focus. We have at least 1 full centimeter of back-focus and the detail we can view around the "0" position (which is blurry) is what we could expect if this were a subject's eye and we focused and recomposed with a shallow depth of field.
Now I did say that the laws of physics are the same everywhere, for everyone, and for everywhen ... and that there are no exceptions.
Ernie, you took issue with that statement. Are you suggesting that if we replace a focus test target with a human face and eye that, for some reason, the laws of physics will change?
I would have thought the diagram would have been enough for most people to visualize why this happens.
"I don't know why you're trying to defend this."
I am not trying to defend this, I am telling it like it is. You can show all the test graphs and charts you want. In reality it makes no difference in the finished work. That is the important part that all graph and chart readers don't realize.
Would you like for me to show you several thousand photos and have you tell me which were recomposed? This makes for a good coffee table discussion or perhaps a college lecture hall but that's about it. You see the big difference is in our background. You are a hobbyist and this stuff is important to you. To working photographers like me it doesn't. Why? Because it doesn't effect our work like it does your lab tests.
If you believe MTF charts you would think some lenses were so bad to be nearly useless. But in fact that couldn't be more untrue. It is the same thing.
Tim if you a worried this will ruin your photographs, make sure you follow the rules. I'll continue to shoot the same way as I always have until somebody complains about recomposing focus issues. Perhaps you would like to explain to this young lady her shot is ruined because it is recomposed?
It is all about how you use it. It is all about how you use your gear. It is not about charts and graphs that don't tell the true story. Tim I am done because we are too far apart. I am not a chart reader, you are. We are both happy so there is an end to it.
@ebiggs1 wrote:"I don't know why you're trying to defend this."
I am not trying to defend this, I am telling it like it is. You can show all the test graphs and charts you want. In reality it makes no difference in the finished work. That is the important part that all graph and chart readers don't realize.
Would you like for me to show you several thousand photos and have you tell me which were recomposed? This makes for a good coffee table discussion or perhaps a college lecture hall but that's about it. You see the big difference is in our background. You are a hobbyist and this stuff is important to you. To working photographers like me it doesn't. Why? Because it doesn't effect our work like it does your lab tests.
If you believe MTF charts you would think some lenses were so bad to be nearly useless. But in fact that couldn't be more untrue. It is the same thing.
Tim if you a worried this will ruin your photographs, make sure you follow the rules. I'll continue to shoot the same way as I always have until somebody complains about recomposing focus issues. Perhaps you would like to explain to this young lady her shot is ruined because it is recomposed?
It is all about how you use it. It is all about how you use your gear. It is not about charts and graphs that don't tell the true story. Tim I am done because we are too far apart. I am not a chart reader, you are. We are both happy so there is an end to it.
You're right. In that photo, with a depth of field so broad that we can read the name of the school on the building in the background, a short focal length (with wide-angle distortion and the vertials leaning apart due to an unlevel camera), shot in mid-day sun with no fill-lighting, and a subject placed so far apart and so small, we can't even see the subjects eyes... much less count the eye-lashes -- the focus & recompose just doesn't matter.
