Lens setting question

I have read a lot of threads about taking sports pictures and have a question....

Many posts say to use AV and set the lens wide open to get a good DOF. I use a 70-200 2.8 and when I set it wide open (at 70mm) it is 2.8; however, when I zoom the 2.8 changes.....

My question is that if I am shooting a baseball game and need to zoom to catch an outfield play, my DOF is wider. Am I setting something wrong or is this just how the lens works? Is there anything I can do to get the nice DOF (blurred background) when I need to zoom to the 200mm level?

drmac wrote in post #5270307
I have read a lot of threads about taking sports pictures and have a question....

Many posts say to use AV and set the lens wide open to get a good DOF. I use a 70-200 2.8 and when I set it wide open (at 70mm) it is 2.8; however, when I zoom the 2.8 changes.....

My question is that if I am shooting a baseball game and need to zoom to catch an outfield play, my DOF is wider. Am I setting something wrong or is this just how the lens works? Is there anything I can do to get the nice DOF (blurred background) when I need to zoom to the 200mm level?

Precisely what lens are you using?

If you are using any of the Canon 70-200 "L" lenses, there should be no forced change of the aperture when changing the focal length. Other manufacturer's 70-200 lenses may not be constant-aperture lenses, though.

Your terminology regarding depth of field is sort of backwards. "Good" depth of field is usually thought of as "deeper" depth of field - in other words, using smaller apertures (larger f-stop numbers) to get more of the subject in focus. A better term to describe what you are trying to do is "shallow" depth of field rather than "good" DOF.

What he means is that the camera exposure changes I think. That's normal in every mode except M.
How the subject affects the exposure in Post # 47
First set the f-stop & shutter speed you need. Then adjust the ISO. Need an exposure crutch?

I meant good as in what I want in the picture..... blurred background........

I understand the good DOF usually means different.......

drmac wrote in post #5270307
I use a 70-200 2.8 and when I set it wide open (at 70mm) it is 2.8; however, when I zoom the 2.8 changes.....

That shouldn't happen - the 70-200 has a constant aperture throughout its focal range, in other words if you set it at f/2.8 while at 70mm, it should stay at f/2.8 all the way up to 200mm.

Regarding the blurred backgroud, as you zoom in to your subject the background should become more blurred not less.

PhotosGuy wrote in post #5271100
What he means is that the camera exposure changes I think. That's normal in every mode except M.
How the subject affects the exposure in Post # 47

Frank has a good point here.

drmac - If you are, in fact, using one of the Canon 70-200 "L" lenses (I noted that you didn't answer my previous question about that) then if you change the focal length the aperture should not change - especially if you are using Av mode (setting the aperture and allowing the shutter speed to change).

However, if you are using the camera's built-in light meter (especially if you're using it in the "evaluative" mode) with the camera set to Tv mode (you set the shutter speed and the camera chooses the aperture) and the average brightness of the scene is different between a wide and a narrow angle of view (changing the focal length radically) then the difference in aperture may just be because of the camera's decisions when metering the scene.

Got it... I am using an "L" lens and I did check it out... and it works as mentioned. When I originally posted I was looking at a different lens that did adjust....


thanks for all the help