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View Full Version : focus and recompose issues.



westy79
10-24-2009, 07:15 AM
As my D40 has 3 focus points i find myself using centre focus and recompose for my shots off centre. I thought this was a good technique to use from various articles etc. But after stumbling across some more articles on the net the system seems flawed. Because the sensor moves off the parellel plane of the focus point when you move the camera left or right it brings the focus point "Out of Focus"

I always assumed that once focused locked it stayed locked even when you recomposed. Isnt that why you use focus lock, to lock focus?

Just when you think you have found a good system an article on the net can confuse things again!!!!!

Elisha
10-24-2009, 07:20 AM
The focus lock only works within a certain range and angle I believe.
It may be able to maintain the lock if you recompose 15 degrees off the center but may be OOF if you are 20 degrees off center. The numbers are just an example.

I realized this when I was using my 50mm f/1.7 and since then I just used the off center AF points and recomposed to the minimum.

But unfortunately you do not have that option with the limited AF points.

westy79
10-24-2009, 09:43 AM
i can see why the multi focus points are an advantage on higher spec cameras. Surely if the dof was alot deeper you could more the camera either way and keep within the focus area.

So what is focus lock then? I thought it just locked focus for the photographer to recompose a more pleasing off centre shot!

Elisha
10-24-2009, 09:44 AM
i can see why the multi focus points are an advantage on higher spec cameras. Surely if the dof was alot deeper you could more the camera either way and keep within the focus area.

So what is focus lock then? I thought it just locked focus for the photographer to recompose a more pleasing off centre shot!

It is but like you said, the shallower the depth of field, the more OOF it will be when you recompose.

erichlund
10-24-2009, 10:10 AM
First of all, depending on how conservative you are using it, the impact may be minimal. The farther away the subject is, the less the impact. It's simple geometry if you really want to do the math.

There's a whole lot of factors that go into it. Lenses naturally focus spherically, but designers try to make then focus on a plane, so really, you don't know where the focus point is off your actual point of focus.

The point is, if you want to focus and recompose, you will not have perfect focus. It will probably be darn close, and if you use smaller apertures, you will likely not notice unless you try to view or print very large.

All of this leads to the real advantage of full frame cameras. Big, bright viewfinders. You can compose and then focus manually, assuming your subject is not covered by one of the focus points. Only then do you actually know that your subject is in focus.

westy79
10-24-2009, 10:57 AM
sorry to remake the point but shouldnt focus lock actually lock focus depending on here you recompose, providing its within the dof range, no matter how deep or shallow.

the art of photography sure keeps getting harder to grasp!!

XaiLo
10-24-2009, 11:09 AM
All of this leads to the real advantage of full frame cameras. Big, bright viewfinders. You can compose and then focus manually, assuming your subject is not covered by one of the focus points. Only then do you actually know that your subject is in focus.

erichlund, oh to dream of that blissful day :)

Westy79, I'm so glad you brought this up... I shoot at 2.8 a lot and don't take notes. I've wondered many a time why a certain picture was out of focus when it shouldn't have been, and as soon as I read your problem I knew the answer to the problem. But I never attributed it to my mystery issue. Two lessons learned today take notes and think objectively.

westy79
10-24-2009, 11:15 AM
xailo, glad this problem came to your attention. To be honest i have never really noticed a problem, it was only from reading yet more internet articles that i noticed. Frustrating because i thought once focus was locked that was it! How wrong was I.

So will the image stay sharp providing focus is within the dof range?

Elisha
10-24-2009, 11:15 AM
Look at this link and you will get a better understanding: http://www.outbackphoto.com/workshop/phototechnique/essay06/essay.html

I was trying to find a diagram someone posted on one of the Sony forums a few months back about the same issue but I'm not able to track it down. It illustrated the point much better!

Paradox
10-24-2009, 12:02 PM
The focus lock locks the focus where it is. It stops the camera changing the focus. If you move the camera, then the point the focus is locked on is no longer where it was originally, and you've told the camera not to compensate for it. Therefore, depending on how far you move the camera, your subject will be out of focus.