I found out something interesting this morning. I went to the river to shoot some leaves--but not enough has changed here. But there were all these geese, so I put on my 70-300 IS DO.
Then I got really worried. I was at 300mm on my lens, but I couldn't focus correctly. The geese just weren't sharp. If I backed out to 200, it was fine. I was pretty convinced my lens was broken.
Now, I thought about other factors, even whether or not having a filter in front of my lens should matter. But why only at 300mm if that was the case. Argh, I just shot a bunch of test shots at different focal lengths and figured I'd look into it later.
But I never tried removing the filter. Until I was about to leave. Then, I discovered that it WAS INDEED the filter. Somehow--I'm not sure how--it was distorting the image at 300mm, causing it to look out of focus. It may be related to the fact that it's a cokin filter--further out in front of the lens, plastic, etc. (It was a GND.)
But I suspect it's more related to a combination of factors. Normally, you would think the longer the lens, the less anything right in front of the lens is going to matter. But it's a Canon DO lens (diffractive optics--mirror lens of sorts). Perhaps it's related to that? Otherwise, why at 300mm and not 200mm?
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