they say: focus, recompose, then shoot....

[sanmich]

Ok, I hope it's not the dummiest question on earth:

The focal distance defines a plane of focus in front of the camera.

That means that the actual distance of all the points in the plane is different, depending on how far the point is from the optical axis.

That also means that when recomposing after having focused, I rotate the camera body and while I don't change the radial distance to the subject, I put the subject in a different plabe of focus.

My understanding is that when recomposing, you actually loose focus on your subject, and the more excentric the sunject in your frame, the more this problem has chances to occur.

Or....

 

[benno]

Ok, I hope it's not the dummiest question on earth:

The focal distance defines a plane of focus in front of the camera.

That means that the actual distance of all the points in the plane is different, depending on how far the point is from the optical axis.

That also means that when recomposing after having focused, I rotate the camera body and while I don't change the radial distance to the subject, I put the subject in a different plabe of focus.

My understanding is that when recomposing, you actually loose focus on your subject, and the more excentric the sunject in your frame, the more this problem has chances to occur.

Or....

If you shoot at f1.2 at your minimum focus distance it might be a problem. In practice, I've never had an issue with it.

 

[pvdhaar]

I've never noticed a problem either.

Somehow the mix of aperture, field of view, and focal length work together to mitigate the problem.

You would only have one with an extremely fast lens with a wide field of view. That is, where depth of field is shallow and you'd have to turn the camera a lot to recompose. But typically, lenses in the 1.2 to 1.4 aperture range (where you'd find extremely thin DOF) aren't really wider than 35mm.. And for lenses in the 35mm and larger focal length range, you'll find that the amount that distance changes when recomposing is less than other factors affecting sharpness, like field curvature for example and lower mtf off center..

 

[papasnap]

ah, I'd been a bit curious about this too. some pretty feasible sounding explanations there - thanks!

 

[waileong]

Sanmich-- you worry too much.

PS If you're obssesed about this, you should pose groups in a curve (following the curvature of your front lens element) instead of a straight line.

 

[Abbazz]

If you're obssesed about this, you should pose groups in a curve (following the curvature of your front lens element) instead of a straight line.

On the contrary, you need to put everybody in line so they are in focus, because camera lenses define a plane of sharp focus, not a curve -- at least for a well corrected lens.

When the subject lies in a plane orthogonal to the lens axis, focusing and recomposing is possible without losing the focus, but this is not the case for a subject forming a curve whose points are all equidistant to the lens.

The question of the original poster is very pertinent. The inability to recompose without losing the focus in some shooting situations is the main drawback of using rangefinder cameras, or manual focus SLRs with a centered focusing aid. This is the reason why autofocus cameras have all these focusing points disseminated across the whole focusing screen, to allow the user to select a focusing point anywhere in the frame.

When will we have rangefinder cameras with a joystick to move the focusing spot across the whole viewfinder? 😀

Cheers!

Abbazz

 

[waileong]

I was engaging in a little hyberbole...

Look, in most cases, it doesn't make a difference. There are so many factors-- mechanical tolerances, DOF, depth of focus, film flatness, etc.

 

[ferider]

My understanding is that when recomposing, you actually loose focus on your subject, and the more excentric the sunject in your frame, the more this problem has chances to occur.

Or....

You are correct. The error goes with the cos(recomposition angle).
Say, for instance you use a 50mm lens (40deg FOV), and you recompose
using the 1/3rd rule. Then the error at 1m distance is

1m * cos(40deg/6) = .993...m.

Which means you make less than 1cm error, well within the DOF of the
lens. This is typical: the wider the lens the worse, but on the other
hand the wider the lens the more DOF. So unless you recompose with
a big angle (into the corner of the picture) and a very fast wide angle
(like 35/1.2), you won't notice at all.

Best,

Roland.