Blog readers and others occasionally send me questions, and I like to reply publicly so that others who are interested in these topics can also see the answers. Here are a few from the past few weeks. (Some posts are edited slightly here.)

Dave writes:

“I recently was reading a thread here where you posted about the 24-105 vs the 24-70. First I wanted to say thanks for your perspective. I had every intention of selling the 24-105 after much of the hoopla I’ve read about it not being so great and ordering the 24-70 MKII. My first day out with the 5D3 was in a museum (No flash) and the IS on the 24-105 in combination with the ISO on the 5D3 gave me shots I never would have thought possible. In short I’m very happy with the 24-105 when I had zero expectation of keeping it.

The question I have is that my 24-105 does have a fair amount of distortion @ 24mm and I noticed you also use the 17-40. Do you find the 17-40 a better performer @ 24mm and do you like the 17-40 in general?”

There are several interesting questions here, so let me try to take them one at at time.

First, though, an article I wrote a while back that looked at the pluses of the 24-105mm lens: In Praise of the Canon EF 24-105mm f/4 IS L lens

Image stabilization (IS) versus larger apertures – The maximum aperture of the Canon 24-105mm f/4L IS is f/4. An f/2.8 maximum aperture is available on a Canon 24-70mm zoom and larger apertures come on a number of primes. This would seem to leave the f/4 zoom in the dust for low light photography – but maybe not. In situations where the limiting factor is camera stability (and the photographer’s ability to hold the camera steady at low shutter speeds), IS can provide several stops of additional low light performance if the subject is not moving too fast and/or if some subject motion blur is acceptable or even desired. In a situation like this one – photographing in a museum – the IS can work in light that would require a f/1.4 lens without IS. In addition, working at f/4 gives you slightly more depth of field. This can make focus just a bit less critical – important when working handheld with AF – let you keep more of the subject in focus.

Distortion at 24mm on the 24-105mm f/4L IS – One of the perceived negatives of this lens is that it shows more barrel distortion (a bowing outward of the image along the sides of the frame) at 24mm. It also exhibits more corner light fall off (or vignetting) at the largest apertures and especially at wide angles. In most cases you won’t notice either of these, but if you image contains lines parallel to the frame edges you may well see the barrel distortion when shooting at 24mm and you may well see the vignetting at f/4. (All lenses vignette to some extent – so it isn’t a question of whether a lens vignettes, but of how much. Most wide zooms will also show at least some barrel distortion.)

I rarely notice the barrel distortion. First, most of the time I shoot at focal lengths that don’t exhibit the issue. Second, when I do shoot at 24mm the issue is not visible with most subjects. But sometimes it will be visible at 24mm. However, most current raw conversion software auto-corrects for this. Some worry that this will diminish image resolution, but I’ve look for degradation and I cannot see it. Likewise, the software can automatically correct for vignetting and do so in ways that are essentially imperceptible. However, keep in mind that vignetting can be a beneficial effect in your photograph – in fact, it is not uncommon for photographers to add some in post! It serves to diminish the importance of peripheral parts of your image and can bring more attention to central elements.

Continue reading Reader Questions (April 2013) →