any relevence between Lens diameter and focal length?

I guess the real question is this:

if I took a picture at 200mm with a 58mm lens, would it be the same exact picture as if I took one at 200mm with a 77mm lens? with the same body of course...

- Ben

To answer your question, Yes both photos would appear the same as the 200mm provides the same field of view.

The objective lens on some lenses is larger because the lens is faster and therefore takes in more light. A 200mm f/2.8 lens will be wider (and therefore heavier) than a 200mm f/4 because the f/2.8 gathers more light with that big glass lens element.

200mm is 200mm period....

The only difference is apt to be in the speed of the lens...the 77mm is probably an f/2.8 and the 58mm is probably f/4 or f/5.6.

f/number = focal length/aperture diameter.

perryge wrote in post #5258947
f/number = focal length/aperture diameter.

Exactly. And this explains why a smaller size aperture is a larger number and a larger size aperture is a smaller number. If you have a 200mm lens and it had the ability to use an f/2 aperture the physical size of that aperture opening would be 100mm. (200mm/2) An f/4 would be 50mm (200mm/4), and so on.

So from this we can then realize that the physical size of an f/2 opening on a 50mm (25mm) lens is much smaller than on a 200mm (100mm). So what's with this system? Well, its ingenious really. Given the same amount of light, the same ISO setting, and the same aperture it does not make a difference which focal length lens you use the shutter will always be the same.

By making the aperture number be a ratio of the focal length in this manner it makes it so the same amount of light transmits through all focal lengths at the same aperture.

This further explains why it is so hard to get a blurred background with a point and shoot at f/2 but with an SLR and a 200mm at f/2 it's hard to NOT blur the background. Because a physically large aperture gives you a shallow DOF. I figured out one time that on a typical point and shoot camera the physical size of an f/2 aperture opening was smaller than an f/22 opening on my 70-200mm at 200mm! So it figures that the DOF is different!

I'm not talking about Apeture's

I'm talking only about the diameter of the Lens vs. Focal length.

we'll say EXIF reads this:

f5.6
ISO200
Focal Length 200mm
Shutter speed 1/600 or whatever

if I had 2 lens's:

a Canon 70-200L 2.8 and a Canon EF 75-300 4.5

since the 2.8L's lens diameter is 77mm and the EF's is 58mm

would the 77mm, because of being wider cover more area then the 58mm @ 200mm?

beezwax wrote in post #5259370
would the 77mm, because of being wider cover more area then the 58mm @ 200mm?

First off, you are saying "lens diameter" when the 58mm and 77mm are really "filter thread diameter" values.

Larger glass diameter on a lens will NOT give you any different angle of view as another lens of the same focal length but a smaller glass diameter.

The primary difference between the two lenses, as stated several times above, will ONLY be the maximum aperture. The lens with larger diameter glass will probably have a smaller minimum f-stop number (which is the same as saying it has a larger maximum aperture).

Obviously, there would likely be other quality-related differences because the lenses with larger diameter glass are usually from the manufacturer's more expensive line of lenses. Significantly more expensive lenses usually have better quality in several areas - optical, physical build, ruggedness, etc.

ok...cool my question has been answered

Thanks