Wednesday, January 09, 2008

DSLR lens testing - Canon EF 70-200mm f/4 L

Shooting a Canon 40D, mounted on a heavy tripod, looking at the smallest resolvable block of information from a USAF Resolution Test Chart, at 114 inches from the target, I took a look at a 70-200mm f/4 L (non-IS) resolution. In general, I like this lens for it's size and weight. So I was looking forward to seeing just how sharp it could be.

The numbers from left to right after the test aperture are the resolution figures noted in Lines Per Millimeter (aka line pairs per mm - they're the same thing). My comments follow.

70-200mm f/4 L (non-IS)
(70mm) AF enabled
f/4.0 65 65 58
f/5.6 74 74 65
f/8.0 93 83 74

(100mm) AF enabled

f/4.0 73 73 65
f/5.6 73 73 73
f/8.0 73 73 73

(200mm) AF enabled, center measurement only
f/4.0 40
f/5.6 58
f/8.0 58

(200mm) Live View - careful focusing at 10x, center measurement only
f/4.0 82
f/5.6 72
f/8.0 72

For the first pass I let the camera autofocus on the USAF Resolution Test Chart. The resolution numbers you see here reflect this.

Looking at resolution at 70mm the lens performs very nicely. As some people feel, the resolution improves from f/4 through to f/8 as the lens is stopped down. At f/4 the numbers look good compared with the two previous tests of fixed focus lenses. By f/8 the 70mm resolution is the best I've seen so far. Simply brilliant.

Looking at resolution at 100mm the lens performs very consistently at all apertures all across the field. Very nice and even, but no improvement in resolution by stopping down. I measured this length specifically to compare with a soon to be acquired 100-400mm f/4.5-5.6 Canon EF-L.

Looking at the top end of the zoom range I performed two test passes. The first pass let the autofocus select the focus point. The resolution of the 2D subject (the USAF chart is flat, after all) is down from 100mm resolution. For a second pass I used the 40D's Live View and 10x magnification to hand focus the lens. I did this to see if there was any improvement to be had in the lens measured resolution performance, and there is. As you can see, the lens is just as sharp at 200mm as it is at 70mm, depending upon the aperture.

The second test pass taught me that the 40D's autofocus, while good, might not be the absolute best in all cases when shooting a 2D subject. I'm sure there are many factors which come into play with regards to the 40D's AF performance. I just happened to stumble upon on sample/observation. This is a good learning and will be applied to the 100-400L tests when I get there.

Please Note: Resolution tests should not be confused with Modulation Transfer Function testing. MTF looks at a lens' ability to preserve contrast from the original scene. Resolution tests look for the finest possible sharpness, independent of contrast. The two tests are somewhat related in that the human eye perceives resolution as contrast. But the nuance is important as many folks feel that MTF is the only valid optical test. It's not. There are many ways of looking at lens performance. These happen to be just two, and I have chosen the simpler to implement resolution test process.

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