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.
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.
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 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.
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.
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.
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