Focusing fast lenses wide open.

I sometimes find myself disappointed with the sharpness of my fast lenses, which is presumably down to technique.
I've had a think about it and I suspect that the only way to get the focal point sharp is to use centre focusing on the eye or whatever, and shoot right away without recomposing in-camera.
To get the composition back, the image must be cropped in post, thus moving the sharp bit (from the centre of the lens) to where you want it in the frame. Time for a K3
Normally I frame up in camera using select focus and don't crop (I have never used centre focus and recompose, that's clearly wrong).
Am I on the right lines?
I should point out that this isn't a problem for commercial shots, as I would take several shots and I'm rarely wide open, or that close. It just messes with artistic shots, or shots of the children etc.

I may not be understanding your comments clearly, but why is it wrong to use centre focus and then recompose?

Probably because the DOF is so shallow when using wide apertures that re-composing can alter the lens to subject distance and make the intended focal point go out of focus.

Regards
David

That's it David.
But equally problematic, I suspect that wide open, select focus asks the lens to achieve focus using areas of the lens away from the centre that are soft, so the subject of the image will always be soft, and the best part of the lens has been squandered in an out-of-focus area ??

So everyone who used an ME, MX, K2, etc did it wrong? Just look at any old camera manual and it shows you how to focus - you had to use a central split-image circle or central microprism, there was no alternative.




Sorry you believe all the images they took are out of focus but most seem fine to me!

I suppose it depends on how radically you recompose. I've not had problems with a 1.8 lens wide open, using centre focus then adjusting composition. I suppose it could be difficult using a faster lens still (?). I have never really got on with the faff of moving the focus points (on a K5).

It is absolute fact that recomposing will move the plane of focus off your subject, but only at the close distances I am talking about. i.e. sitting next to your child at the table or macro work.
The example from your manual is so far away it would not be affected by recomposing.

Focusing and recomposing would also bring us back to my original point that I think it's best to keep the subject in the centre and crop it later.

David, I am finding the above a lot easier with the K3. I think the key to re-composing is to keep the micro focus plane the same.

A little tip for anyone who is not getting the sharpness how they would like, try a slight side step in the direction in which you want to re-compose rather than moving the camera by hand.

Yes if its a close / macro shot of a bird or animal keeping the focus point central (usually on the subject's eye) and sorting out the composition afterwards makes sense - it would be easy to loose the focus point while recomposing. It depends on the lens whether it would loose focus; some have a curved field, some flat. Can't quite decide which would work better!

Ok - I get it for close stuff, but for general use it's the technique I use most of the time. Recomposing usually consists of moving the camera (with my eye against the viewfinder) sideways by a couple of inches - I wouldn't have thought this would have any significant impact on the focus plane?

It doesn't have any relevant effect LennyBloke, unless we are at macro lens distances.

The loss of focus will be much more likely to be camera and subject movement as we recompose. This only matters when using very wide apertures, but with lots of practice the success rate will increase.

Dammmmm - I thought I'd found an excuse for my poor photography

I think you've had a bit of experience LennyBloke, just a bit....

You can see it happening in the viewfinder. You have to move back and forth to re-establish focus. Whether that is because of movement of the camera, or the focal plane (or whatever) shifting because the camera was moved in an arc, is largely irrelevant. The fact is the subject has gone out of focus.
But this is off-topic really, I'm still trying to establish if we can't expect the subject to pop out of the frame unless we use the centre of the lens (wide open) and crop later.