focus lock?

[ricstew]

hi all
I posted in the ntp .......have you got your exposure locked to the focus......and recieved some excellent advice.
I am driving a pentax km/k2000......
As advised I changed the focus to the a/f button on the back.....and some members were saying this is how they lock the focus and then lock exposure with the shutter button. They also focus and recompose like this.
So I am in AF.C mode......focus switched to the back button......but after playing with this afternoon, it wont lock focus so I can recompose.....I must be in AF.S for this....according to the book.....
Have I got this right? AF.C to track a moving object ( where it constantly changes focus ) and AF.S to lock focus and recompose........or am I missing something?
cheers Jan

[I @ M]

If all else fails ---- blame Arthur = AK83

Using the AF button as opposed to the shutter is ( as has been pointed out previously ) a very sound idea but to utilise it the way that has been described you must have the ability to TELL your camera to take the shot whether focus has been achieved or not especially operating in AF.C mode.

If you are not trying to keep focus on rapidly moving objects / people then AF.S would be the appropriate way to work it.

[ricstew]

Thankyou Andrew...........I was starting to think I had missed something totally......
cheers
Jan

[arthurking83]

one problem with AF-S and focus locking that I found I had trouble with on the odd occasion was that you have to be quick to expose one focus was confirmed, or you lose the exact focus point. Of course this is all too easy to do with fast lenses that give super shallow DOF, but if you're shooting under conditions where you have a bit of DOF to play with then it may not be an issue.
First up, we'll define focus lock as the audible beep/confirmation tone that the camera gives the operator.(and remember this is more obvious with fast aperture settings, or long focal lengths at short working distances)
if you shoot, where you half press to achieve focus and then full press to expose then in the fraction of a second that you do this, you could, or the subject could move a few millimeters and you've lost that focus point, hence focus confirmation is rendered useless.
I just stopped using AF-S mode altogether because where I thought I should have focus.. I find out later that it just didn't happen. this is one of my main pet hates about screw driven lenses (on Nikon cameras).
So I started using AF-C mode almost exclusively(this is with the D70s).
NOTE: What I subsequently figured out all my my lonesome self was to full press all the time.. even if the camera didn't expose for a few seconds. This may sound obvious to those that use that method, but I wasn't used to AF for a long time.

but what you'd do in AF-C mode is just have faith that the camera has achieved focus, even though you can't hear the fact. they're usually pretty good. You can also visualise this fact, and with screw driven lenses you can easily hear it, as the 'zzzt' 'zzzt' 'zzzt' sounds from the lens dissipate and you just know that the subject is focused.
With using the AF-On method and AF-C mode, once you're confident that the subject is focused, you have to immediately release the AF-On button and quickly recompose and release. Remember that you can expose at any time, as the camera is now not bound to expose only when focus is confirmed.

the real problem with focus lock, and focus and recompose is that you're relying on the fact that you're not swaying to and fro too far, and that the lens has no, or minimal field curvature, both of which can lead to missed focus points.

I hardly use the focus and recompose method tho. The D300(D3/D700) has a better way to do this in that, it has focus tracking mode. With a D300 set this way, the operator focuses using any AF point on the screen and while maintaining focus recomposes, and the AF point follows the subject originally focused on as the scene is recomposed. That is focus is always maintained, and operator or subject movement, and/or field curvature is then not an issue when missing focus.
It's not a 100% cure for focus and recompose issues, as it's not 100% reliable. but when it works, it's really cool