FStop / Lens Question

[fntzlnd]

Here is a question for you...

Say for example, you have two lenses (these are fictional, but better illustrate the question):
Lens #1: 100mm f2.8
Lens #2: 100mm f8

...lets say they are of equal caliber (both are L).

Is f8 on Lens #1 going to have the same depth of field range as f8 on Lens #2, or will f8 on lens #1 be greater since the lens 'starts' at f2.8?

...oh by the way, I got my first L lens. 17-40. Woo Hoo!!! I love it. I know that the images would be better, but what i noticed on the "L" lens vs. my Tamron 28-300, is that the Program mode on the 10D is more reliable. Using the Tameron, I always had to be careful that I was at f8. Anyone else have this same experience?

Thanks
Greg

[Jesper]

f/8 at 100mm is going to have the same depth of field, no matter what lens you use. A Tedious explanation of the f-stop (http://www.uscoles.com/fstop.htm).

I also have the 17-40L and also the Canon 28-135 IS USM. The 17-40L is really nice, but the difference in image quality is not spectacular. Well, the 28-135 is not a bad lens either (even though it's not an L).... :wink:

Long ago I had a Minolta camera with two kit lenses: 28-80 and 75-300. I did a course and the teacher advised us to get a 50mm prime lens. So I got a 50 f/1.7 Minolta lens and it was MUCH sharper than the kit lenses. Even on a small 4 x 6 print you could easily see the difference. :shock:

[Scottes]

f/8 at 100mm gives DoF the same on any lens. DoF is a measurement of lens length, aperture, and frame or sensor size - not manufacturer or glass quality.

DoF may appear to be *slightly* less on a cheap lens if that lens is optically inferior - DoF shows the range of image that's "acceptably sharp" - the meaning of "acceptable" can change from person to person.

I have a zoom lens with absolutely no DoF because EVERY picture it takes looks like crap, and it's never acceptable. :) Just kidding.

[maderito]

For a given camera, the depth of field (DOF) is a function of the lens focal length and the aperture. Thus at f/8, the DOF would be the same for both lenses at any given focal distance (camera to subject distance) since the two hypothetical lenses have identical focal lengths.

On the other hand ... the two lenses would be quite different in size. Fast lenses - f/2.8 - are always bigger than slow ones -- again, all other things being equal.

I don't understand your queston about "program mode" and "more reliable" function of the Canon 17-40L vs. the Tamaron 28-300. More reliable at what? - focusing? producing quality shots?