Interesting focus question. Please Help!

If you focus on the eyes and then recompose, won't you get out of focus eyes because the distances differ. These lines make a triangle. The Hypotenuse(eyes) is always the longest line in a triangle. I am having problems with nailing the focus on the eyes, could this be why?

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Yes, you are right. I think you'll need to use a different method to focus.

Sincy I don't have my XTi yet, all I can say is that with my Sony I would set it to "adjustable spot AF" and then move the focus area on the screen to wherever I want to focus. I did a quick search on google and from website:

A 9-point autofocus system is found on the XTi. A thumb-controlled button on the back of the camera allows the user to quickly choose which focus points to use. Of course, the focus points can also be chosen automatically. The XTi has a AI Servo AF to better capture moving subjects, and a One-Shot mode for still photography.

I know i can use one of the other focus points, but I have found that the outer ones don't seem to focus as accurately as the center focus point. I keep hearing that you should use the center focs point and focus on the eyes and recompose. I am debating whether or not that is the best case now. I think the logic of it works.

I'm having the same issue shooting at f2.8 with the 70-200is, I'm going to try the spot focus. I love shooting at f2.8 but I shot some portraits of 2 people and w/2.8 only one person was in focus. You really have to nail the focus or stop down.

EOS MAN, check this article out. You are exactly correct in your thinking.

http://visual-vacations.com …focus-recompose_sucks.htm

Thats amazing. I guess great minds must think alike. Thanx.

This is taken from a 5D

Unless you're shooting with a wide aperture (2 or 1.8 for example), I don't think you have much to worry about. With a little math, we can figure out the distance change. Say we're shooting from 8ft away. And we'll use me as the sample subject (I'm 5'5"). The distance from my eyes to just below my sternum is 15 in. Using the Pythagorean theorem, we can calculate the distance of the hypotenuse as sqrt(15^2 + 96^2) and get a distance of about 97 inches.... a very minuscule change. And the farther away you shoot, the smaller the distance change... So unless you're shooting fairly close at a wide aperture, I wouldn't worry about it too much as your DOF should cover that small distance.

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Actually.... the more I think about, the distance from the eyes to the camera is never changing when you re-compose... only the angle is changing...

No, the distance of the point you focused on would change. However this leads me to thinking that the actual plane of focus is curved... is this right?

http://visual-vacations.com …focus-recompose_sucks.htm

ok... say I tie a piece of string to my camera, and have my subject hold the other end of it on say the tip of their nose while pointing my camera them. Are you saying that when I tilt my camera down (changing the angle), I'll need a different length of string??

René Damkot wrote in post #4250957
http://visual-vacations.com …focus-recompose_sucks.htm

Thanks for the article. I see now what Mr. B is saying...

But the article does support my first post (before I started over analyzing things... lol)... Some quotes from the article...

So again, I say...

I may way wrong, but if you use "focus lock" when you focus on the eyes (as per the example), even when you lower the aim and recompose, the focus is still locked at where you locked it and the eyes will still be in focus.
Assuming you don't accidently move the camera closer/further away by swaying a bit of course.
Or am i just fundamentally lost?
Greg

BiPolarBear wrote in post #4251003
I may way wrong, but if you use "focus lock" when you focus on the eyes (as per the example), even when you lower the aim and recompose, the focus is still locked at where you locked it and the eyes will still be in focus.
Assuming you don't accidently move the camera closer/further away by swaying a bit of course.
Or am i just fundamentally lost?
Greg

Give that article that Rene posted a good read... it explains how the focus plane is parallel to the camera sensor, and clearly explains how the distance changes.