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Archive 2012 · ? minimum practical subject size vs image size for pro ca...
  
 
Alan321
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p.1 #1 · p.1 #1 · ? minimum practical subject size vs image size for pro cameras


Nowadays I only use 1-series Canon cameras. They have 45 AF sensors packed in fairly close together. Sometimes when I am out wandering I take a photo of something such as a bird up in a tree or a kangaroos (on the ground ) at what is no doubt too far away for a high quality shot but I wanted to capture something rather than nothing. The results are usually doubly disappointing because so many of the photos do not achieve focus on the desired subject and often don't even grab the background properly. Shutter speed is not the issue. Nor is lack of light, although I'm often shooting at around late afternoon or dusk without direct sunlight on the subject.

This raises a couple of questions for me:

1. As a percentage of picture height, what is the smallest target/subject size that you would reasonably expect the pro cameras to get in correct focus when there is other stuff in the distant background (not plain blue sky, for example).

2. In some cases my subject is only about 10% of the picture height. Now that might seem ridiculously small at first, but if you were to shoot a portrait with the focus on the subject's eye and the eye is only about that size you would expect it to work, so why not a little bird in the distance ?

Any help will be much appreciated.

- Alan



Jan 17, 2012 at 07:09 AM
trumpet_guy
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p.1 #2 · p.1 #2 · ? minimum practical subject size vs image size for pro cameras


Alan,

What kind of depth of field are we talking about? Are you shooting with 200mm at f/2.8
or something like that?

In general, the distance the lens elements move is greater for a given change in
subject distance when the subject is close versus when the subject is far. It's
harder for the camera/lens combo to get it right when the subject is far away.
However, this is contradicted by the fact that depth of field increases with subject
distance.

Maybe it's a question of contrast. The light reflected from a subject's eye into
your focus sensor is brighter than the light from a bird in a tree, for instance.
I'm guessing here, though.



Jan 17, 2012 at 07:21 AM
trumpet_guy
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p.1 #3 · p.1 #3 · ? minimum practical subject size vs image size for pro cameras


Maybe the issue is simply lens resolution. A far subject at 10% of picture height
doesn't look as sharp as a near subject at that same percentable of picture height.



Jan 17, 2012 at 07:23 AM
snapsy
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p.1 #4 · p.1 #4 · ? minimum practical subject size vs image size for pro cameras


If I'm understanding your situation correctly I think the issue you're facing is the size of the subject under the AF point vs the AF point size itself. In the viewfinder the AF point is represented as a square but in reality the AF point can be an area that is smaller, larger, and/or of a different shape/vector than just a "box". At longer focus/focal distances the small area of detail you're attempting to focus on represents a smaller percentage of the coverage under the AF point, and if there are other objects spatially near your intended subject but at different distances than the likelihood of focus missing your intended target is higher.


Jan 17, 2012 at 07:45 AM
Alan321
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p.1 #5 · p.1 #5 · ? minimum practical subject size vs image size for pro cameras


When I have seen this problem it doesn't seem to be a lens issue. It has happened with a 70-200 f/4 L IS, a 100-400 L IS, a 300 f/2.8 L IS, and an 85 f/1.2L II. I'm usually shooting at or near maximum aperture because the distance is such that DOF should be OK.

I haven't been able to identify faulty technique on my part but I won't rule it out just yet. Nor have I identified a lack of resolution or DOF - in fact the dead giveaway is that some photos will look a whole lot sharper than others in a set and that could not happen if it were resolution or DOF. I'll post some examples later on if I can find them in my backups but those I had on tap were discarded a while ago as rubbish. Another good reason never to tidy up

- Alan



Jan 17, 2012 at 02:14 PM
Alan321
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p.1 #6 · p.1 #6 · ? minimum practical subject size vs image size for pro cameras


snapsy wrote:
If I'm understanding your situation correctly I think the issue you're facing is the size of the subject under the AF point vs the AF point size itself. In the viewfinder the AF point is represented as a square but in reality the AF point can be an area that is smaller, larger, and/or of a different shape/vector than just a "box". At longer focus/focal distances the small area of detail you're attempting to focus on represents a smaller percentage of the coverage under the AF point, and if there are other objects spatially near your intended subject but at
...Show more

Spot on, snapsy, but often when the focus misses the intended target it does not focus on an alternative either I end up with nothing in focus. I'm sure this should not happen but that's what I end up with. I'd expect at least something that was covered by the AF sensor at whatever distance would be presented in focus even if other parts were ignored.

I'm aware that the focus sensors are not really box-shaped but are cross-shaped and I have a rough idea of how tall and wide the linear segments are relative to the boxes in the viewfinder, but I don't know how thick they are. Canon won't tell us how many image pixels are covered by a single AF pixel. No matter what size they are, I'd still expect that the camera should give me something in focus instead of nothing.

Apart from these technical aspects the philosophical issue I'm facing is whether or not I just expect too much from the AF system.

- Alan



Jan 17, 2012 at 02:25 PM
 

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Gunzorro
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p.1 #7 · p.1 #7 · ? minimum practical subject size vs image size for pro cameras


Alan -- I find Canon is quite accurate on the center AF point -- in most of my cameras (including 60D and T2)i, the area within the box is nearly perfect, 100% of the time.

A few things that I do that may help in your situation.

1) Center point AF

2) Focus lock on a larger subject/area at the same distance, then recompose

3) Manual focus

4) If these fail to resolve the focus issue, send to Canon for adjustment

Hope this helps you nail those fleeting shots.



Jan 17, 2012 at 02:40 PM
Jeff
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p.1 #8 · p.1 #8 · ? minimum practical subject size vs image size for pro cameras


Just out of curiosity Alan, have you checked EXIF data to see what focusing distance the lens is reporting? If I'm reading you correctly, I've seen some similar things from all sorts of camera and lens combinations, and it usually seems to manifest itself when most of the detail in the image is approximately the same distance away (and, I'm presuming from the EXIF data I've seen, the AF chooses to front focus for no apparent reason).

Not that it's definitive, but it might help you at least identify some patterns in AF behavior. I've often wondered if AF arrays can be influenced by interference patterns of complex subjects (branches in trees, for example, which we often assume to be a good, high contrast subject that AF 'should' be able to lock onto).

Regardless, I'm with you...



Jan 17, 2012 at 02:46 PM
mark fadely
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p.1 #9 · p.1 #9 · ? minimum practical subject size vs image size for pro cameras


Various lenses and camera bodies will respond differently of course, but with a 1-series body you should expect to have a focus lock with a small subject like this and busy background:


This was testing a fixed MKIII after the whole AF debacle - lens is a 400 5.6 @ 5.6 centerpoint only in ai-servo

Full Frame






100% crop








Jan 17, 2012 at 03:44 PM
Alan321
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p.1 #10 · p.1 #10 · ? minimum practical subject size vs image size for pro cameras


Jeff wrote:
Just out of curiosity Alan, have you checked EXIF data to see what focusing distance the lens is reporting? If I'm reading you correctly, I've seen some similar things from all sorts of camera and lens combinations, and it usually seems to manifest itself when most of the detail in the image is approximately the same distance away (and, I'm presuming from the EXIF data I've seen, the AF chooses to front focus for no apparent reason).

Not that it's definitive, but it might help you at least identify some patterns in AF behavior. I've often wondered if AF arrays can
...Show more


Hi Jeff. Long time no contact.

What you suggest makes sense but I'll have to put in some effort to do the tests properly. That cannot happen with my Canon for at least several weeks because once again I'm on the opposite side of the country from my gear. I've got some Nikon gear with me that has much the same performance limitations but I have no tripod with me.

I have not yet observed distance as a significant factor but maybe I was not looking carefully enough. It seems to be more related to subject detail size vs AF sensor size.

I'm certain that some contrast patterns will upset any AF system. It would be useful if Canon and Nikon would tell us how big their AF pixels are so that we could know what size of details are invisible to AF sensors and hence which patterns are more likely to be problematic. It's also relevant that some patterns that are visible to linear AF sensors are not so obvious to us when we see an image area rather than image strips.

- Alan



Jan 21, 2012 at 06:00 PM
Alan321
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p.1 #11 · p.1 #11 · ? minimum practical subject size vs image size for pro cameras


Gunzorro wrote:
Alan -- I find Canon is quite accurate on the center AF point -- in most of my cameras (including 60D and T2)i, the area within the box is nearly perfect, 100% of the time.

A few things that I do that may help in your situation.

1) Center point AF

2) Focus lock on a larger subject/area at the same distance, then recompose

3) Manual focus

4) If these fail to resolve the focus issue, send to Canon for adjustment

Hope this helps you nail those fleeting shots.


Thanks for this.
I've tried the first two tips without appreciable success. I can't explain it simply.
Manual focus is a problem for me because I use eye glasses and because with small subjects it is too easy to think something is well focused when in fact it is not.

Something you did not mention is Live View. When the subjects are static and I have adequate support for the lens and camera I have used Live View AF with a level of success that rules out non-AF limitations such as lens quality, sensor resolution, etc.

- Alan



Jan 21, 2012 at 06:24 PM
jwp721
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p.1 #12 · p.1 #12 · ? minimum practical subject size vs image size for pro cameras


mark fadely wrote:
Various lenses and camera bodies will respond differently of course, but with a 1-series body you should expect to have a focus lock with a small subject like this and busy background:

100% crop
http://fadely.smugmug.com/photos/498254899_nDYdm-XL.jpg


Your example was such an ideal situation that I would expect my 50D to be to focus lock on a bright white object in front of a dark background. However, I am certain that your shooting technique is much better than mine!

Thanks for sharing.



Jan 22, 2012 at 04:08 PM





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