Canon 24-70mm not constant 2.8?

[Scott Crawley]

That's a really horrible lens IMHO, OK 70mm F4 but not much good anywhere else.

I appreciate your opinion, but I hope you didn't come to that conclusion based soley on my test shots, since they only represent 2 focal lengths and 2 individual stops. "Anywhere else" is simply not represented here.

What I see is vignette at the short end that is fairly significant wide open. It isn't uncommon in still zooms. I don't know which others are better in this regard. Does anyone? I'm curious now to k ow how it compares to others. Sure one might expect better from an L series lens, but I can deal with it myself. Heck I'm so used to it that it didn't even occur to me when I first read the OP.

It appears more pronounced when the iris is open. Would that indicate that it is related to the curvature of the elements?

Cheers

[HansSteinert]

I have the exact same problem with my 17-40 f4 L lens... except it's arguably even more extreme than your findings.

40mm image:

http://img15.imageshack.us/img15/2958/40mmq.jpg

17mm image:

http://img6.imageshack.us/img6/7382/17mmf.jpg

17mm 100 crop:

http://img263.imageshack.us/img263/2596/17mm100crop.jpg

All I did was stand in the same place with the same settings and zoom in/out.

Here's a video of it:

http://vimeo.com/27173912

[Jeff Kilgroe]

This "ramping" effect is very common on zooms, especially SLR zooms and other lenses rated in F-Stops, not designed to maintain constant light transmission across the entire zoom range. An F-Stop or F-Number represents the ratio between the diameter of the aperture and the focal length of the lens. Lots of things happen inside a lens as focal lengths change and elements move. Just because a lens may maintain a constant aperture ratio (F-Stop) across an entire zoom range, it does not mean that it maintains consistent exposure or light transmission, which are measured in T-Stops.

As Jim said earlier in this thread, over a year ago, this is one very good reason why people spend the extra money on a cine-lens that can maintain constant T-Stop or light transmission at all focal lengths. Keep in mind that the F-Number ratio can change over the zoom range as the lens adapts to maintain the desired T-Stop. Or what that means is a lens could be T2.8 across the entire zoom range, but could be F3.0 at one focal length and F3.3 at another. Likewise, if you take a lens that is constant F2.8 across the entire zoom range, it could be T2.9 at one end and T3.6 at the other end of the zoom range.