As in previous tests, the numbers that follow the aperture are resolution in line pairs per millimeter (aka lines per millimeter). For this lens I tested both center (at some focal lengths) as well as the edge of the frame (in selected focal lengths). As in the 70-200mm f/4 L testing, as I got to the longer lengths I started to use the Canon 40D's Live View mode with 10x magnification to carefully focus on the USAF Resolution Test Chart.
One thing to note before we get to the numbers; I strapped this lens on the heaviest tripod I own, a Bogen 3033 set of legs on a Bogen #3057 head. This rig has firmly held a Folmer and Schwing 12x20 inch view camera in the field. It easily holds an 8x10 Deardorff quite solidly. Yet when I was attempting to focus in Live View mode it was very easy to tell that the 100-400L gets "shaky" when I barely touched the lens.
This illustrated very very clearly to me the need for careful technique when using the 100-400L in the field. IS may play an important role. High shutter speeds will also play an important role. And, finally, I may need to strap this lens down to the 3033/3057 Bogen and trigger the shutter with a remote release. All this in an attempt to get the highest possible resolution out of the Canon 40D/100-400L setup.
(100mm) AF focused - center measurement only
(200mm) AF - center measurement only
(200mm) Live View, careful 10x focusing, center + edge measurement
f/5.0 65 82
f/5.6 65 82
f/8.0 65 82
(400mm) AF - center measurement only
(400mm) Live View, careful 10X focusing, center measurement only
At 100mm, the lens appears to test "OK". It's certainly not what the 70-200 f/4L measured at the same 114 inches from the target. I wasn't entirely thrilled. But I kept going, just to see what else this lens might have in store for me.
At 200mm I tested in both AF and Live View modes. I was also able to capture edge resolution performance readings. Here again, the lens tested "OK" against the 2D test chart. But then something caught my attention. When I read the edge performance of the Live View tests I realized that 82l/mm is outstanding! performance. I had to wonder what I'd done wrong. Perhaps the Live View focusing wasn't as accurate as I'd hoped. And, of course, there's the fact that at these higher magnifications, AF and manual focusing with the wee-Canon 40D is nothing like focusing a 120 format film camera. The brightness of the display as well as the distance to the subject must come into play.
It seems that the resolution drops off on my 100-400L at 400mm. But it's not far behind the 200mm center measurements on that lens when using either AF or Live View to focus this 100-400L. I wasn't sure what this meant, even after looking at DPReview's performance tests of this lens at 400mm and comparing it against the 400mm EF f/5.6 L. There was one last test I wanted to undertake before I headed into the field.
I took the nice and sharp 70-200L, set it at 200mm, and moved the tripod to 54 inches from the target. This made the subject size the same as the 100-400L at 400mm. I then Live View focused images. Testing f/5.6 on both lenses I put the results into an image viewer (Gimp or Canon's own software worked nicely in my tests) and observed the differences in apparent resolution at 200x.
The results are really quite interesting. The 100-400L was tested at 400mm and 114 inches from the target. The 70-200L was tested at 200mm and 54 inches from the target. In the following image, take a close look at the results. Resolution is really nice on the 100-400. Looks at the test bars for both the 70-200L and 100-400L at f/5.6. They look very nearly the same, don't they? Then look at how the performance degrades from f/5.6 thru f/11 on the 100-400L. This appears consistent with a few user observations I read on DPReview.
- Click on the following image to view it full size -
What's interesting to me is that when operating near the closest focus of either lens (6 feet for the 100-400, and slight farther but still near the close focusing limits of the 70-200L at roughly 4 feet) that performance between the supposedly less sharp 100-400L and the know to be brilliant 70-200 f/4L is identical! What this leads me to believe is that close focus performance of Canon's zooms might not be as good as their ability to resolve scenes at something greater than their close focus limits.
With this in mind, could it be that the 100-400L's performance is actually quite good at greater distances from the subject? To find out, I'm headed out into the field...