Sunday, February 12, 2012

Resolution and Wishful Thinking...

I've been on conversations with fellow engineering colleagues where, over the years, we tend to note just how disconnected from reality some people are who make claims about resolution and it's effect on photographic image making. Typically, arguments arise in on-line forums where someone will claim that Zeiss lenses are "the best" and someone else says Leica "can't be beaten", and all parties claiming they can tell the difference in images made with different lenses. DP Review is pretty typical of this effect.

The claim that really takes the cake is the one Zeiss and other manufacturers use when they say a lens "writes" well. What the hell is that? Optics are optics and physics is still physics. It's all bunk. There's no truth to those kinds of claims, no matter what some folk would like to believe.

In optics, lens design, when well-executed in manufacturing, tends to not be the limiting factor in resolution. It's either the film or the sensor (depending on which technology you use).

The following is a chart of theoretic limits of resolution of a perfect lens at various apertures.

Resolving Power

This presents the theoretic resolving power of an ideal lens where the light's
wavelength is 589.3mu (green).

Tangential lines/mm
f-number Angular distance from axis (in degrees)
0 10 25
1 1391 1329 1035
2 695 665 518
4 348 332 259
5.6 246 235 183
8 174 166 130
11 123 117 92 <--- approx. limits of film/sensor resolution
16 87 83 65
22 61 59 46
32 43 41 32
45 31 29 23
64 22 21 16

Radial lines/mm
f-number Angular distance from axis (in degrees)
0 10 25
1 1391 1370 1260
2 695 685 630
4 348 343 315
5.6 246 243 223
8 174 171 158
11 123 121 111 <--- approx. limits of film/sensor resolution

16 87 86 79
22 61 61 56
32 43 43 39
45 31 30 28
64 22 21 20

Which means that diffraction limits really can't come into play in any meaningful manner until a lens is stopped way down to at least f/11. Even then, most modern digital sensors are resolution limited to around 70 lp/mm. So with those systems (such as with either Canon's, Nikon's, or Sony's full frame sensor'd wonder boxes), diffraction in the pure optical physical sense will not come into play until you're well beyond the limits of what a lens offers. ie: f/16

As sensor resolution rises, diffraction will still not come into play as many photographer shoot at apertures brighter than f/11.

Said another way, I've never met a lens I didn't like. Some, yes, I liked more than others. But seldom, if ever, were the issues of "liking" a lens related to it's ability to be sharp.

I've shot them all. Leica. Zeiss (small, medium, and large formats). Canon. Nikon. Pentax. Mamiya (medium format), Schneider (medium format, large format). Sigma (I know people claim to hate these lenses, but I can't find fault with them, they're wonderful optics, regardless of money). Dagor (large format). Kodak (small, medium, large formats - brilliant optics from WWII until the mid-1960's).

It's not a matter of the lens. It's the nut behind the eyepiece.

So there. Now go out and make some fine images. Forget about the blather on those forums. OK?

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