Quote Originally Posted by Richard Lane


Just remember, it is more important where focus lock was initiated, and not where the red square ends up, as occurs in re-composition.
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Rich:


This is kind of the discussion we got into about a week ago in the "post your best bird shoots" post. You were nice enough to try and help me out, and I said I would post some examples. I have not had a lot of time to do so, but this post seems to be addressing the same issue... so here is what I have found.


All shots below were with taken with a 7D, 100-400L @ 400mm and f/7.1


My problem has been when trying to focus when the background is cluttered (or has better contrasting feature as compared to the subject). In my case I was trying to shot Sand Cranes in a cornfield.


What seems to be happening is the spot size is bigger than the square, and it tends to focus on the higher contrast background. This is true even at wide open aperture... I closed it down some just in hopes that I could get a better DOF so I could focus on the body and get a sharp eye. (did not work to good though).


The shots below were not intended to be great pictures. I was just trying to figure out the AF problems in a difficult situation.


This first shot was in "One-Shot" mode and "Spot Focus". The focus point was confirmed to be centered on the head of the bird in the middle. It is totally out of focus. In this case the camera was well supported, and I pushed the shutterbutton half way a dozen times and got focus confirmation but was always out of focus.


[img]/resized-image.ashx/__size/800x1200/__key/CommunityServer-Components-UserFiles/00-00-00-66-81/8124_2800_centered-on-head_29002800_one_2D00_shot_2900_.JPG[/img]


Next I switched to AI-Servo with Spot Focus. This next shot was AI Servo and the spot was centered on the head. However, I had to move the spot back and forth over the head until it would final register the head as the focal point only for an instant, and then I would quick take a quick picture to get something half way decent. Still difficult to get a good focus, and it is fleeting.Also, the bokeh is horrible.


[img]/resized-image.ashx/__size/800x1200/__key/CommunityServer-Components-UserFiles/00-00-00-66-81/7904-_2800_on-head_2900_.JPG[/img]


At this point I pointed at the base of the neck and could get it to focus pretty easily... so this next one is focused on the base of the neck (spot smaller than subject). Still horrible bokeh.


[img]/resized-image.ashx/__size/800x1200/__key/CommunityServer-Components-UserFiles/00-00-00-66-81/7907_2800_Base-of-neck_2900_.JPG[/img]


Since the bokeh was so distracting, I then spent some time blurring the back ground in post processing. This helped, but still not great (see next one)


[img]/resized-image.ashx/__size/800x533/__key/CommunityServer-Components-UserFiles/00-00-00-66-81/7883_2800_Blured-background_2900_2.jpg[/img]


To give you an idea of the lens ability without the distracting background confusing the AF - below is the same lens, but the background is nice blue water and the spot was just under the eye (but the head was covering most of it).


[img]/resized-image.ashx/__size/800x1200/__key/CommunityServer-Components-UserFiles/00-00-00-66-81/7698_2800_ai-under-eye_2900_.JPG[/img]


I must say, I do not usually have a problem with AF, but the spot needs to be smaller than the subject (which means getting close). If the spot is larger than the subject, then the background and type of background can effect the focus a lot. Also, AI-Servo does help in cluttered background situation, but it is hit and miss and does not hold focus long.


Sorry for the long post. If anyone has a good idea on how to set up a shot better to overcome these problems in corn fields... I am open.


Pat