I don't understand why the Canon 70-200 2.8 has a filter size of 77mm while the 200 2.8 has a filter size of 72mm. At identical apertures/focal lengths wouldn't they need the same size front element?
I don't understand why the Canon 70-200 2.8 has a filter size of 77mm while the 200 2.8 has a filter size of 72mm. At identical apertures/focal lengths wouldn't they need the same size front element?
Guess not...
By the way these aren't the same kind of lenses. You compare a zoom vs a prime. They will probably have different setups with types of glass that might differ as well. Also there has to be done some internal zooming, so all those things can change a lens.
And of course, probably the smaller=lighter=less product=cheaper?
I'm just thinking and so guessing, but I do think you can't compare them as being the same while having one focal length/aperture in common while having other options as well.
Jan
The length of the lens body may play into the engineering, I'm no scientist but I did have hyper focal cheerios for breakfast.
-Shea
Originally Posted by deviousalex
Clearly not... [:P]
Seriously, though, while the size of the aperture diaphragm will be the same the 200mm f/2.8 and the 70-200mm zoom set to 200mm, the sizeof the front element is not determined solely by the aperture. Optically, the aperture is placed as near as possible to the focal point of the lens. Depending on where that point is, the front element may need to be larger to accommodate that (and, for example, to avoid vignetting at the wider end of the range for a zoom lens - consider that the EF-S 17-55mm f/2.8 also uses a 77mm filter size, and that's a much shorter focal length). Finally, the diameter of the front element is not the sole determining factor for the size of the filter threading. Consider the EF 100mm f/2.8 USM Macro with a 58mm filter vs. theEF 100mm f/2.8L IS USM Macro with a 67mm filter (Bryan's comparison tool) - the front elements are pretty similar in diameter, but the L-series lens has a lot more 'dead space' around the front element.
If I understand correctly the aperture reported may actually not be precisely focal length/aperture number. Example: theoretically f/2.8 at 200mm is just over 71mm in diameter 200/2.8=71.428 As I understand it that would be the diameter of the diaphragm if it were placed as the front element. (or precisely 200mm from the focal plane, something like that) As you can see both the 77 and 72 mm pieces of glass would cover this width nicely. However because the diaphragm is internal, it can be smaller than the math would show it to be because the light is being bent already. Now, I don't know that this is true, but I think it is close. There was a daniel browning (where is he anyway, this is a perfect question for him) post that explained that from the perspective of someone who opened up a lens and found their actual aperture smaller than what they expected.
(In other words don't quote this...its speculation)
But they do not have the same focal length. One zooms from 70 to 200mm and the other is fixed at 200mm.
The largest aperture necessary will be at 200mm, however the larger glass may be to prevent vignetting at 70mm seeing as it is an internal-zoom lens.
Posting Permissions
Reply With Quote