BetterPhoto Q&A
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MTF & Megapixels

First, how do you read an MTF chart? Like how can you look at two lenses & determine which has better optics from the chart. I read the canon explanation but it was all greek.

Now what I'm really wondering: I have a 1Ds to resell. I could keep it, but that's more horsepower than I need. So I could sell it & either buy the mkII or save some money & go for the 10d, & maybe get a real nice lens, f2.8 something with it. I was gonna opt for the latter, but is 6.3 megapixels enough, if you're planning on selling prints, for example? Or should I go with the 8Mpixel markII? This is really an opinion thing, but there's some people out there with sound judgement so I'd like to hear what you have to say.

If I hit a real nice shot, I want to be able to blow it up to 18x24 or 20x30ish or something maybe.

I shoot wildlife & travel mostly.

-Steven
-www.vinrock.i8.com/photos

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July 07, 2004

Re: MTF charts
(a) You can only compare charts compiled by the same source due to different methodologies used. The charts derived by Canon for their lenses cannot be directly compared to those published by Nikon. Photodo.com is the only source that allegedly uses the same methodology for a variety of lens makers and can be compared, though their's seems subject to sampling error, and it hasn't been maintained for several years now.

(b) very generally, the higher and flatter the curve, the better. Up and down scale is % resolution (100% best), left to right scale is distance from the center of the film frame. Graphs that are high on the left side and drop on the right mean that the lens produces good sharpness in the center of the frame, but that resolution falls off toward the outer edges.

(c) There are several lines graphed, which makes things confusing. There is a line graphed for each combination of aperture (wide open and f/8), detail level (line pairs/mm: 10 and 30; or 10, 20, and 40), and line orientation (sagital - line pairs arranged radially, like spokes on a wheel; and tangential - in rings like a bulls eye target) - a total of 8 to 12 lines per chart. Zooms lenses also have a chart for the short and long (and maybe some intermediate) focal lengths. Generally the highest line is at f/8 and for 10 lp/mm. The lowest will be at the lens's largest aperture for 30 or 40 line pairs/mm. This bears out the common knowledge that most lenses are sharpest when stopped down a few stops from wide open, and it's easier to resolve 10 lp/mm than 30 lp/mm.

(d) MTFs are virtually always measured with the lens focused at infinity, so they aren't even relevant to macro lenses (though macros tend to score well).

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July 07, 2004