Auto Focus Points, Focus and Depth of Field

Mike D90

Senior Member
There is already one thread on Focus Distance and DOF and I would like to expand on that a bit with some things I have learned recently while photographing small bodied birds.

Focus points are the camera's way of achieving focus on the particular area you have chosen to be sharply in focus. With wider more open apertures, like f3.5 or f4, depth of field gets very thin (front to back).

In the photos below I wanted to point out, in detail images, just exactly how thin that DOF can be even at a few feet distance to subject while using a telephoto lens and big open apertures.

The first image below was taken at about 5 feet distance to subject, Nikkor 55-200mm ED VR lens at 200mm length, f5.6 aperture.

I captured screen shots from within Nikon ViewNX 2 to show the "focus point", a feature of this software, to show where lens/camera focus actually was.

Notice the focus point of the first image and the area within that focus point is sharper but the focus point was more so on the top of the head. Just millimeters outside that focus point marker are the eyes and beak, which is where I was trying to focus, and see the difference. I thin k I also have some motion blur in this image due to too low a shutter speed, but focus and DOF played a huge role in this image overall

Just a little bit of difference between where I wanted to focus and where focus really was prevented the eyes from being sharply in focus.


focus_point_01.jpg


































Now compare it with this shot below which is very very similar to the first one The focus point was dead on the eye and is where the DOF is best. Had I missed the proper focus point the distance between the eye and the beak tip is enough, even though small, to allow a narrow DOF to blur the eye.



focus_point_02.jpg





In this shot, even though I left off the Focus Point indicator, I totally missed focus, which was on the wing/chest area I believe, and a wide open aperture and high ISO left a grainy soft image.


focus_point_03.jpg
 
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How on earth did you get 5 foot away from a small bird without it freaking out and flying away? Haha!

Amazing pictures tho Mike even if they were just for tutorial use. :)
 

Mike D90

Senior Member
How on earth did you get 5 foot away from a small bird without it freaking out and flying away? Haha!

Amazing pictures tho Mike even if they were just for tutorial use. :)

I have no idea! I just walked up towards the feeder and stood still with camera up and ready. The little birds flew in, ate, and would fly away and come right back. Even the camera shutter did not scare them away. If I moved too much it did scare them away but they still came back. When they flew away I was able to feel the beat of air pressure from their wings I was so close.

But that is whats aggravating, getting that close and still cannot nail a sharp focused image.
 

Rick M

Senior Member
One issue I've found is that the focus box is too big and sometimes not exactly where you thought you placed it. With up close shots, the box has a lot to choose from and this is where auto-focus struggles.
 
I have no idea! I just walked up towards the feeder and stood still with camera up and ready. The little birds flew in, ate, and would fly away and come right back. Even the camera shutter did not scare them away. If I moved too much it did scare them away but they still came back. When they flew away I was able to feel the beat of air pressure from their wings I was so close.

But that is whats aggravating, getting that close and still cannot nail a sharp focused image.

I am a long way off getting that close to birds and stuff and getting anything close to what you have shown here. So respect to you sir :)
 
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