Imatest Documentation

Imatest Documentation
'' (overexposed 2
stops, with filter)
Canon 90mm f/2.8
TS-E (no filter)
Sigma 18-125mm
f/2.5-5.6 DC (hood,
multi-coated UV filter)
Canon 28-80mm
f/3.5-5/6 (no filter)
Canon 70-200mm f/4L
USM (UV filter, hood)
0.464%
6 5 90 8 0.291%
15 14 77 8 0.293%
10 9 80 8 0.634%
A very high quality tilt/shift lens with relatively few elements
and groups. Has beautiful tonality and color quality. Low
veiling glare expected.
A remarkably fine lens for the price. Surprisingly low veiling
glare. (If you're thinking of buying one: it has poor autofocus
performance; it's much better on manual.)
(1991 version) A cheap "kit" lens, designed for low cost.
Expectations were low. Strangely reddish white balance.
16 13 70 8 0.396% Excellent lightweight lens. The IS version has 20 elements in
15 groups, which will increase flare.
The table at the bottom shows the relationship between the number of groups (sets of attached elements) and the number of
reflections that contribute to veiling glare.
Most of the results are not surprizing. The Canon 90mm has the lowest flare, while the cheap Canon 28-80 is the worst. The only
surprise is the excellent performance of the Sigma 18-125: a lens know for its fine performance and value, though its autofocus
performance is mediocre. I use mine on manual.
Limitations
The veiling glare may be underestimated for telephoto lenses— especially if they are measured without lens hoods— because
telephoto lenses form an image of only a small fraction of the light reaching the front element (the portion is much larger with
normal and wide angle lenses). The target, as shown above, may not be large enough to simulate all the light that reaches the lens. If
a hood is used, this error is considerably reduced.
If there are nonlinearities in the camera response at low light levels, the results may be incorrect (although relative results, i.e.,
comparisons, will still be valid). It's always safest to work with RAW images and convert them with a "linear" (i.e., simple gamma
curve) setting. You can read many RAW formats into Imatest, using the dcraw converter.
ISO 9358
Lens elements, groups, reflections, and flare— some geeky math fun
Lens flare is closely related to the number of secondary reflections in a lens, i.e., light entering the lens that
is reflected off one lens surface, then another, then back to the image plane. For uncoated air-to-glass
surfaces, about 4% of the incident light is reflected. Simple coating reduces the reflection to around 2%;
muti-coating reduces it to 1% or less. For a simple coating, the amount of the secondary reflection is 0.02 *
0.02 = 0.0004, which doesn't seem like much until you calculate the number of reflections.
A lens consists of N elements in M groups, where a group may consist of several elements cemented
together. It's the number of groups M— actually the number of air-to-glass surfaces 2M— that really
counts. The first surface has no secondary reflection. The second surface has 1: light that bounces off the
second surface, then the first, then back to the image plane. Continuing with this reasoning, we see that the
m(th) surface has m-1 secondary reflections, i.e.,
Total reflections = R = (2M-1) + (2M-2) + (2M-3) + ... + 1 = M (2M-1) = 2M 2 -M
If you add a filter to a lens with M groups, you increase the number of reflections R by 4M+1.
Groups
M
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Reflections
R
1
6
15
28
45
66
91
120
153
190
231
276
325
378
Zooms typically have more elements than primes. Examples (easy to locate in the Canon Museum): The Canon 24-105mm f/4L
IS USM has 18 elements in 13 groups. The Canon 50mm f/1.4 USM has 7 elements in 6 groups. The Canon 90mm f/2.8 TS-E
has 6 elements in 5 groups. Now you know one reason primes are still used (others are large apertures, light weight, and excellent
sharpness (MTF)), as well as why some photographers avoid using filters (though a UV filter is useful protection for field work).
Veiling glare, as measured by Imatest, is very similar to the veiling glare index (VGI) specified in the integral (black patch)
test of the ISO 9358 standard, as described in Controlling Veiling Glare in an Optical Imaging System by Amber
Czajkowski (University of Arizona). The key differences are
The ISO standard calls for more rigorous testing conditions. In particular, an integrating sphere must be used. This means that
there will be more light from outside the field of view, which will sometimes result in higher readings.
The ISO standard does not include a step chart in the image. This means that only raw files (or files where the tonal response, i.e.,
OECF, is very well characterized) should be used.
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