Am I crazy? XL2 shallower DOF than XL1s? at DVinfo.net

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Old March 26th, 2007, 04:19 PM   #1
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Am I crazy? XL2 shallower DOF than XL1s?

Due to some recent experiences on set, and because I recalled, from this page:

http://www.dvinfo.net/canonxl2/articles/article06.php

this paragraph:

"Second, because these target areas are smaller in the XL2 than they were in the XL1 and XL1S, there is a slight magnification in the field of view for all XL-mount lenses. The multiplication factor for using an XL lens on the XL2 is 1.105 in the widescreen 16:9 aspect ratio and 1.35 in the standard 4:3 aspect ratio. Basically this means that all XL lenses are just slightly more telephoto on the XL2 than they are on the XL1 or XL1S."

I ask does the XL2, then, have shallower depth of field at any given focal length/aperture than the XL1s/XL1? It would seem so. If the field of view is more telephoto, that equates to a shallower depth of field. Note that it says that in 4:3, the field of view is magnified by 1.35x. That's quite a bit, more than a third.

I had a shoot a few days ago, shooting in 4:3. I was shooting some kids in a classroom, about 6-8 feet away, with the XL2 and 16x manual lens Sometimes I'd zoom in to get CUs/XCUs of them, as they sat in a row. As I would pan from one face to the next, they would tend to go slightly out of focus, though they were almost the same distance from the lens, and I was stopped down to around between f2.8 and f4. I wasn't THAT close to the telephoto end of the zoom range on the lens, so it doesn't seem the DOF should have been that shallow.

I've messed with the back focus, and it's fine, so that's not it. That which I focus on stays in critical focus throughout the zoom range, but things that don't seem much closer or farther away are slightly soft.

Seems weird is all.

Last edited by Josh Bass; March 26th, 2007 at 04:20 PM. Reason: grammar
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Old March 26th, 2007, 07:32 PM   #2
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Hi Josh. I think it doesn't work as you imagine. The depth of field and the field of view are not the same. Here's a simplistic way to think about it.

The depth of field is related to the focal length and the aperture of the lens. Let's say these are fixed, this means the depth of field is fixed too. If we use a large sensor to capture the image from this lens, we will have a wide field of view, with whatever the depth of field happens to be.

Now if we replace the large sensor with a smaller sensor at the same position, the field of view will be much narrower, because we are only capturing a small part of the image. However, it is the same lens with the same settings, therefore there is no change in the depth of field.

Since the image from the small sensor still produces a full size video frame, we see a magnification of the image which makes many people think that we had somehow changed to a more telephoto lens. If we had changed the lens, the depth of field would have changed accordingly. But we didn't, so the depth of field stays the same as the original lens.

Richard
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Old March 26th, 2007, 07:43 PM   #3
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Then is there some kind of test I could do myself to see if there's something "off" with my lens. . .aside from back focus?
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Old March 26th, 2007, 09:24 PM   #4
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This actually means that at the same field of view, the XL1 would have a shallower depth of focus. Since the XL2 is sharper, though, it might have a smaller effective circle of confusion, meaning that the depth of focus might seem a bit shallower. Since the XL2 is natively 16:9, it would be better for 16:9 stuff regardless, though.

In the end, it probably doesn't matter much.
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