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Old Aug 29, 2010, 7:09 PM   #61
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How about this:

For equivalent images, a larger image sensor will produce a more shallow depth of field than a smaller image sensor. ... and you can define "equivalent" any way you want.
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Old Aug 29, 2010, 7:12 PM   #62
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Originally Posted by fldspringer View Post
... so its a function of f-number and magnification. The thing to notice here is that its is largely independent of focal length.
Just to be clear, that's "f-number", not "aperture" and/or "focal length"?
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Old Aug 29, 2010, 7:18 PM   #63
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Originally Posted by chengbin View Post
@fldspringer

That's what I've noticed. The high ISO capabilities of the DSLR makes low light shooting possible.

Can anyone comment how good the 18-55mm lens that come with the kit?
If it is the new Canon 18-55mm IS lens, then it's pretty good. If it's the old Canon 18-55mm, then it's an awful lens!!!
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Old Aug 29, 2010, 7:24 PM   #64
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Originally Posted by JohnG View Post
OK - let's re-iterate the basics. Fstop is a ratio that involves the aperture size. It just so happens that the depth of field calculations use the same parameters - so you are really both correct - aperture size (and distance from lens to focal plane) both affect DOF. The relationship between those two variables is what we call the f-stop. If you alter the aperture opening without changing that distance you change the f-stop.

Math is funny that way.
as to COC being affected by sensor size - yes it is. Now if you perform operations on the image that approximate changing the physical attributes of sensor size, focal length, distance and f-stop then you're changing the values of the equation.

Again, if you've got algabraic formulas Greg that leave circle of confusion (and their dependence on sensor size) out of it I'd like to see them. But the formulas published widely do, in fact, use all those variables. Again assuming you do not alter the resulting image in software.

So, since you are fond of chastising people for not understanding the algebra - perhaps you can show us how using the different COC values produces the same results. Or are the formulas incorrect?

Again, let's deal in specifics - if you could address the example I posted earlier where dof calculations were different simply by changing sensor size I'd appreciate it. Or is it your assertion that the math used in the DOF calculater is incorrect?
I'm very confused by what your after.

COC as being specific to a format is something I don't really agree with. Its something you posted as a variable in a calculation, and I was showing where THEY came up with that figure.

COC varies. It varies with enlargement. It varies with resolution (its a common practice also to use 2x the pixel pitch), and more to the point, it has to vary with eyesight, distance to the print, lighting.... Its an important variable, in fact it supplies the unit of length into the equation of DOF. It just happens to not be dependent on format in the sense you want it to be. Its more of a display parameter than a format parameter.

Just for fun, type into the DOF formula, my Olympus 50mm f2 at f2, and then punch in 100mm at f4 (same 25mm aperture) on a 1D FF camera. The results should be the same. The advantage the larger format has is it can open the 100mm lens and restrict DOF more. Its because of the focal length, not the format.
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Last edited by fldspringer; Aug 29, 2010 at 9:33 PM.
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Old Aug 29, 2010, 7:24 PM   #65
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Just to stir the pot a little, and to air my pet peeve, I really wish we could all start using the correct terminology when referring to f/number. It is the focal length of the lens divided by number, not focal length number, or focal length minus number. Please- we are all supposed to be informed photographers and are giving advice to those less experienced. If we can't agree on how to refer to a common term, how in the world is someone new going to be able to follow this?

As to the assertion that the DOF is independent of focal length, the use of the f/number in the equation negates it.

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Old Aug 29, 2010, 7:34 PM   #66
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Originally Posted by TCav View Post
Just to be clear, that's "f-number", not "aperture" and/or "focal length"?
For macro, its the general DOF rule. f-number and independent of focal length to get the magnification. Its supposed to be quite accurate down to about a tenth life size.
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Old Aug 29, 2010, 8:06 PM   #67
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Originally Posted by VTphotog View Post
Just to stir the pot a little, and to air my pet peeve, I really wish we could all start using the correct terminology when referring to f/number. It is the focal length of the lens divided by number, not focal length number, or focal length minus number. Please- we are all supposed to be informed photographers and are giving advice to those less experienced. If we can't agree on how to refer to a common term, how in the world is someone new going to be able to follow this?

As to the assertion that the DOF is independent of focal length, the use of the f/number in the equation negates it.

brian
Its a hyphen, not a minus. I've seen it written without the hyphen, with the hyphen, but I'm not familiar with the "/" symbol being used that way.
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Old Aug 31, 2010, 5:03 PM   #68
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My friend just got the t1i with the 18-55mm kit. It is indeed MUCH BETTER. It is much much sharper.

http://img401.imageshack.us/img401/3328/img2796d.jpg
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Old Aug 31, 2010, 7:51 PM   #69
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Glad to hear it! Good luck to the two of you!
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Old Aug 31, 2010, 7:53 PM   #70
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That is good new. The 18-55 IS is a good lens. But way more to consider with a dslr then a megazoom.

I have read this thread, and it has been very interesting.
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