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Old Oct 13, 2005, 7:45 AM   #1
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I am looking at moving from my Dimage 7 to a 5 or 7D and believe that the multipliers/crop factor is 1.5x. Along with increasing the perceived lens length (compared to 35mm) does it do the same to f stops? i.e. 100mm = 150mm and f3 = f4.5?

Please let me know so that I can work out what lenses I would like in my package.
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Old Oct 13, 2005, 8:05 AM   #2
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It doesn't impact the actual aperture of the lens (which isa ratio between the size of the aperture iris opening and the focal length of the lens).

But, yes, you'll have a difference in Depth of Field with a smaller sensor for any given 35mm equivalent focal length, just as you have greater depth of field with your DiMAGE models (only you had a far greater difference with those, since their sensors were very small compared to a DSLR).

The subject will occupy a greater percentage of the frame at any given actual focal length with a model using a smaller sensor, and there are some circle of confusion differences due to the size of theof the image circle.

Here is a handy online DOF calculator you can use to get an idea of how it compares. Plug in a camera model, select an actual (not equivalent) focal length, then play around with the focus distance and aperture and you can see how they compare (keeping in mind that your subjects will occupy a greater percentage of the frame with a camera using a smaller sensor, so you'd need to shoot from further away for the same composition compared to a 35mm camera).

http://www.dofmaster.com/dofjs.html



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Old Oct 13, 2005, 8:42 AM   #3
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Cool, so it is the opposite way around. The 5/7D will actually have a lower dof for the same stop which is cool as was concerned that I might need to get more expensive lenses to make up for this.

Thanks for that!!!!
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Old Oct 13, 2005, 8:47 AM   #4
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Mark1616 wrote:
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Cool, so it is the opposite way around. The 5/7D will actually have a lower dof for the same stop which is cool as was concerned that I might need to get more expensive lenses to make up for this.
You're not considering framing. Remember, any given focal length lens on a DSLR like the KM 5D or 7D will appear to be longer from an angle of view perspective (narrower angle of view for the same focal length on a DSLR)..

Again, you'll need to shoot from further away for your subject to occupy the same percentage of the frame.

In other words, if you shot from 20 feet away with a 100mm lens on a 35mm camera, you'd need to shoot from 30 feet away with the same lens on a DSLR (because of the difference in angle of view), for your subject to occupy the same percentage of the frame.

Further distances at the same aperture and focal length will give you more depth of field.

Another way to look at the differences:

When selecting a DSLR, pick a focal length, focus distance and aperture to compute DOF.

Then, select a 35mm model, pick an actualfocal length that's 1.5x longer (for example 75mm versus 50mm). Then, you'd be taking compositon differences into consideration for DOF computation purposes (shooting the same subject at the same distance would give you the same composition using a lens 50% longer on a 35mm camera).


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Old Oct 17, 2005, 2:29 AM   #5
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Another way of looking at this is that to take the same picture with a 7D as a 35mm camera with a 50mm lens you'll need a 33mm focal length on the 7D. (I'm assuming you're standing at the same spot. If not, you're not going to get the same picture whatever lens you use). The 33m lens will have an inherently greater DOF (as defined by the equation which I've forgotten). However, the picture you take with the 7D will be physically smaller (24mm as against 36mm) so you'll need to blow it up moreto get the same end result. When you blow the picture upyou diminish the DOF. However the former effect outweighs the latter and thesmaller sensor camera will always have a greater DOF than the larger sensor camera, for any given aperture.l
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