AFs tip

[dgktkr]

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One of the things I like about the SL is that it yields a good hit rate with quick pictures on hikes with the missus. Some of the things that continue to fascinate me are the patterns, shapes and colors of wildflowers. With the AFs mode and the 24-90 I can get good shots in without disturbing the hiking pace too much. One of the issues that frequently occurs, however, is that the SL doesn't focus on the intended target. Using single point AF, frequently the SL would focus on the background and refuse to focus on the closer subject (i.e. a flower) that is at the center of the cross in the viewfinder, even after multiple tries.

 

I have discovered a method that seems to deal with this problem. If the camera doesn't focus on the object where the cross is, I half press and hold the shutter button. Then I use my left hand to manually bring the object into approximate focus. Now I can release the shutter and press again to have the camera get a better (and faster!) focus using AFs. 

 

It seems that the issue occurs because the previous shot was of a distant scene. When the following shot is to be a closeup, the AF system seems to be blind to the closeup object and is happy focussing on the background. But when the closeup object is manually brought into approximate focus, the AF system can then execute the expected action.

 

Here are some images to illustrate the point. First, the distant shot. (Tuolumne Meadows, 24-90, 43.0mm, f/8.0, 1/250 s, ISO 50)

 

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Then, a few minutes later, the closeup. At first, the camera refused to AFs on the blossom in the center, but after applying the technique above, AFs worked great. (Tuolumne Meadows, 24-90, 90.0mm, f/8.0, 1/320 s, ISO 50)

 

 

dgktkr

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[LocalHero1953]

I think this is an inevitable consequence of contrast AF systems: if the SL can't "see" a contrasty edge because the camera is focused at infinity, it will miss focus on a close object, unless you manually pull the focus towards you. Once it can see there's a bit of contrast there, it finds the focus. I've had similar effects on other AF cameras.

[scott kirkpatrick]

Good point.  It happens to me as well, not just distant followed by close but flowers close with foliage a foot or two behind can be problematic.  (With several other contrast-detect cameras.)

 

scott

[sillbeers15]

I disagree. I have had several of these experiences and I was not on 'infinity focus' or far away subjects on previous shots. Even when I had been on almost consistant focusing distance, the same issue can happen. The AF 'cannot see' your subject of intent due to its 'size' or/and 'contrast'. The flower and branches are too small in the frame against the AF sensor, therefore it gets confused on the subject of focus. Same problem I hv encountered trying to focus on flying birds against a clear blue sky. I experienced the AF sensor going the opposite direction until the unclear image of the bird gets completely out of sight due to 'out of focus', all due to the AF sensor not able to see my intended subject.

[LocalHero1953]

Sure, yes, small objects create their own focussing challenges as well. In the end, if you use a camera enough you understand its limitations, and compensate for them.

[ramarren]

What I don't understand is that, as long as you have "almost" focused the lens, why don't you finish the job and just make the exposure? Why 'rough in' the focus and then let the AF system re-focus? 

 

This is why, when I use the SL24-90, I most often have the camera set to MF with the joystick button set to AF. Press the button and the AF system roughs in the focus, or hits it precisely; otherwise, just focus the lens and make the exposure. 

 

I'm sure this is my problem ... I've never trusted AF in any camera to be anything more than occasional convenience. 

[dgktkr]

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What I don't understand is that, as long as you have "almost" focused the lens, why don't you finish the job and just make the exposure? Why 'rough in' the focus and then let the AF system re-focus? 

 

This is why, when I use the SL24-90, I most often have the camera set to MF with the joystick button set to AF. Press the button and the AF system roughs in the focus, or hits it precisely; otherwise, just focus the lens and make the exposure. 

 

I'm sure this is my problem ... I've never trusted AF in any camera to be anything more than occasional convenience. 

 

 

Mainly for speed. 

 

If I use MF to achieve final focus, I prefer to use magnification, but that takes time. Until I gain more experience, I'm willing to trust AF. So far, I'm happy with what results from AF.

 

If there is plenty of time to take the shot, I like your method.

 

dgktkr

[sillbeers15]

Mainly for speed. 

 

If I use MF to achieve final focus, I prefer to use magnification, but that takes time. Until I gain more experience, I'm willing to trust AF. So far, I'm happy with what results from AF.

 

If there is plenty of time to take the shot, I like your method.

 

dgktkr

I have nothing against you. However in the interest of obtaining speed, we move the function of 'AF' to the back button separating it from the shutter release button. With both finger and thumb, one can lock focus and release shutter at ease without loosing the focus instantaneously (at times the AF locks and moves onto wrong subject the moment you fully release the shutter) and avoid the camera not allowing the shutter to be released as it thinks the subject has lost focus on moving subjects and tricky back lighting situations. On SL system, there is no other way to separate the AF function away from the shutter release button than to select MF. However the beauty of the SL system is such that the camera still functions as AF on native lenses the moment the back button if depressed. You may try and judge for yourself if the 'back button focusing' method is any slower than full AF function when subject moves rapidly. This is a common method employed by sports and wildlife photographers.