• 02-20-2008, 10:50 AM
    How much does focal length range really detract from IQ?
    Hello out there-

    My place was recently robbed (or burgled, I guess, is the proper term for it), and my 30D and lenses are now gone. I'm in the process of replacing all of it now--perfect excuse to upgrade to a 40D!. I'm trying to get the fewest amount of quality lenses I can. The lens I previously used the most was my 28-135 IS lens.

    I was also thinking about getting the 70-200 IS lens along with a good wide angle. I'm just not sure if the 70-200 will give me as good of Image Quality as the 28-135, because I know that the quality deteriorates when the focal length range is so wide and you're at the end of the spectrum.

    Does anyone have any advice who uses both types of lenses consisently?

    Mucho appreciated!
  • 02-20-2008, 11:51 AM
    Re: How much does focal length range really detract from IQ?
    Focal length doesn't affect the quality.
    Unless you try to buy long focal length at the same price as shorter focal length.

    The rule of thumb used to be that 3x was OK (so 70-200) but more than that the quality would be lower (so 28-135 nearly 5x) and the more zoom range the worse it gets.

    You're right that quality may suffer at either end of the zoom range, and it will also be more noticeable the closer you get to the widest aperture of the lens, more often at the telephoto end.

    Are you comparing the 28-135 ordinary lens to the 70-200 L IS ?
    There is a world of difference between the L and non-L lenses !
    The 70-200 L IS will be far better than your 28-135 I think.
  • 02-20-2008, 12:39 PM
    Re: How much does focal length range really detract from IQ?
    Sorry to hear about the burglary. That is a rotten thing to happen to someone.

    I think you may be confusing image quality with depth-of-field or something else. Because image quality isn't affected at all by focal length. In fact, the Canon 70-200L lenses will blow away your 28-135 in terms of image quality. I know - I have both. However, depth-of-field does decrease at longer focal lengths. For instance, at f/5.6 at 200mm you will have shallower depth-of-field than you would at 135mm and f/5.6. But image quality has nothing to do with focal length. Image sharpness, contrast, color, saturation, etc. are mostly a result of the quality of glass used. That means how well the lens elements are ground, matched, the coatings used, and the actual quality of the materials used. And the Canon 70-200L lenses are among the best ever. You will notice the difference and probably wouldn't be happy with the 28-135mm after using the L lens.
  • 02-20-2008, 02:36 PM
    The 28-135 has a 4.8x focal length range (135/28=4.8). The 70-200 has a 2.8x focal length range. So by your own criteria the 70-200 should be superior.
  • 02-20-2008, 08:28 PM
    Re: How much does focal length range really detract from IQ?
    Ah, yes, I guess you are right. I wasn't thinking of it in terms of a ratio. I was just taking the actual difference in numbers (junior member here!). Thanks for the correction.

    One more question: Do you find it difficult to get consistently sharp images with the longer focal length? The rule of thumb I've heard is that in order to get sharp images, your shutter speed should be at least the focal length or faster (e.g.: if you're focusing at 200 mm, your shutter should be 1/250)

    OK, one more: What is the "L" for in 70-200L?

    Thanks, all.