XSI or used 5D?

[Daniel Browning]

by thinner dof , do you mean the area in focus will seem smaller?

Yes. If we set aside bellows factor, resolution, and post-processing differences, the only factor that affects the DOF is the lens (or, you could think of it as "lens, sensor size, and print size", but I prefer to think of it as just the lens).


It's important to remember that aperture is not the same as f-number. Aperture is the focal length divided by f-number, so that a 100mm f/4 lens has an aperture of 25mm (100/4).


There are several ways this principle can be stated:

1. The lens with the widest aperture has the thinnest DOF, no matter what camera or focal length.
2. When aperture is the same, the lenses have the same DOF, no matter what camera or focal length.
3. When f-number *and* composition are the same, DOF gets thinner with larger sensors.
   

What I mean by "composition" is: field of view, perspective, and focus distance.


It's possible to compare two different cameras (e.g. 5D and XSi) using two different compositions. For example, if you put a tripod in a certain spot and don't move it, then put the 50mm f/1.4 on the XSi, set it to f/1.4, and have a focus distance of 5 meters, then you have a certain 25-degree horizontal angle of view. If you do the exact same thing with the 5D, then everything is the same except for the angle of view (field of view), which will be 45 degrees instead of 25.


The DOF will be the same, but the angle of view will be different. I don't think it makes sense to compare different angles of view, because that's a fundamental component of the photographer's composition, and he isn't going to start shooting wider just because he upgraded to full frame: instead, he's going to find a new lens that will give him the same capability that 50mm did on his XSi: such as the 85mm f/1.8.


100mm f/4 on the XSi has the same DOF as 160mm f/6.4 on the 5D. Both have the same composition and both have an aperture of 25mm.


160mm f/4 on the 5D has the same composition as 100mm f/4 on the XSi, but it has thinner DOF. Another way to look at it is this: the 5D will give all your full frame lenses a wider field of view and thinner DOF, unless you stop down to the same aperture for a given composition.


By the way, DOF isn't the only factor that scales with aperture. Light gathering power and diffraction do too.

[Daniel Browning]

I'm confused (easily achieved!). So please can some one answer this: I have a FF and a 1.6. If I photograph the same subject at the same distance with the same lens and aperature, which sensor gives the shallower DOF?
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Neither, it's the same. But since you're comparing two totally different compositions, it's not a useful comparison.


What has thinner DOF: 50mm f/2 on a 5D, or 50mm f/2 on a digicam?


Neither, they are the same. But on the 5D it's a nice "normal" lens that's usable indoors. On the Digicam it's a very long super telephoto focal length. It doesn't really make sense to compare them that way.


Instead, I prefer this:


What has thinner DOF: 50mm f/14 on a 5D or 7mm f/2 on a digicam?


They are the same. Same composition (including field of view) *and* same DOF.

[Steve Eisenberg]

I'm confused (easily achieved!). So please can some one answer this: I have a FF and a 1.6. If I photograph the same subject at the same distance with the same lens and aperature, which sensor gives the shallower DOF?
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All things being equal, the depth of field is identical. But the composition (as Daniel points out) will be totally different. Looking at those two photos in the link earlier, you can see that the depth of field measures the same, but the full frame has more area captured in the image.


That being said, If you wanted those two photos to have the identical composition, you would have to change the distance from the subject, or change the lens you are using. In doing so, you will change the depth of field, and end up with a different looking picture.

[Daniel Browning]

many people said that you get more "reach" with 70-200 on cropped lens, when I think about it, I'd imagine that's "perceivedreach" since your imagesensordoes the cropping for you... I mean I'd like to make the decision or rather have theabilitycrop on my own if I do want that "extra perceived reach" on the final print out. am I missing something?

The "reach" is real. The way it works is like this: if you crop a 5D to the same size as 1.6X, you're left with much fewer pixels on the subject than if you had just shot a 1.6X camera in the first place. Here's another way to look at it:


If you bought the $2,700 5D2, shot it at 200mm, then cropped it to 1.6X, you would have an 8 MP image.


if you bought a used $300 20D, shot it at 200mm from the same distance as above, you will get the exact same image (and 8 MP to boot).


In other words: it does you no good to pay all that extra money for "full frame" unless you're going to use the "full frame" in your images. Wildlife photography and macrophotography often benefit from the crop factor for that reason.

[HiFiGuy1]

To answer the OP, I would think it would be germane to know the price range of the used 5D he's seen. I would think that it would be significantly more than a new XSi. Aren't they only $700-800?


A more apples-to-apples might be a used 5D vs a new 50D. In that case, I would suggest the 50D, as I've seen them for $1200-1300, which is roughlyequivalent toa good used 5D.


Also, as has been pointed out, all the OP's current lenses will work, whereas the FF body could only work with two of them, although those two are very nice.

[ultima16888]

yes, if it's xsi vs 50 D i'd pick 50D for sure, because i'm more concerned about image quality then price difference, and not suggesting that I have all the capital resource to buy a medium format camera.. I guess used 5D is the max I can afford on a new body... hope that's clear why i'm wondering which one has better image quality..

[George Slusher]

I would ignore the technical discussion of "thin" DOF, as only your 50mm f/1.4 lens will have a particularly "thin" DOF on any camera. (The 60mm f/2.8 will be a bit better than the 70-200 f/4L IS and 17-85mm f/4-5.6 IS.


However, there's a more important consideration: neither the EFS-60 nor the EFS 17-85 will even MOUNT on the 5D. They can be used ONLY on the 1.6 crop factor bodies, like the Rebels and 20D/30D/40D/50D. That would leave you with only the 50mm and 70-200mm lenses for the 5D. You'd have to get another wide-angle zoom, like the 16-35mm or 24-70mm, for the 5D. If you want to do macro shots, you'd be best off with the 100mm f/2.8 Macro lens.


Instead, as someone else said, you might consider a used 40D or 50D.