Lens Contrast (Part 7 : MTF Chart)
_DSC4099, originally uploaded by shutterhack.
Taken with a Nikon D50 and AF Zoom-Nikkor 70-300mm f/4-5.6G lens
To get a really good idea of a lens's performance using MTF, you'd need a "family" of charts. For starters, every lens will perform differently at different apertures and at different distances. Just charting an F/16 lens for three different object distances — say, infinity, close focus, and perhaps 20 x F, where F = focal length, would mean you'd need 21 different charts. Really, you should have charts for at least six (and ideally, thirty!) randomly-chosen production samples, too, to account for sample variation. There are a dozen or so other conditions you should measure at every aperture and taking distance. You can see how the volume of data would quickly get out of hand for enthusiasts. But do bear in mind that when manufacturers give you one chart, it only measures performance at one aperture and one distance. That doesn't really tell you much, except comparatively, and it may not tell you want you need to know.
Often, usefully (sort of), they'll provide two charts; one for the lens stopped down, and one for full aperture. The more closely these two charts resemble each other, the better and more consistent the performance of the lens is likely to be across the range of apertures. (I say "sort of" usefully because open-aperture charts for an F/1.4 lens and an F/2 lens wouldn't tell you how the same lenses compare when they're both at F/2, which might be practical information to know.)
Incidentally, as an aside for those of you who may have seen the articles on "bokeh" (bo-ke, the Japanese word meaning "blur") in the March/April 1997 issue of PHOTO Techniques, off-axis aberrations are typically the cause of "bad" or confused-looking blur. The relative superimposition of the sagittal and tangential lines of an MTF chart are one predictor of "good" or smooth bokeh.
Labels: AF Zoom-Nikkor 70-300mm f/4-5.6G, Nikon D50, Photography
