Camera Labs

During my latest venture into lens-test-land (see e.g. here and there) I encountered the obvious problem of shooting a standard-target to judge lens performance. I experimented with two targets: The Siemens-star and patterns on a monitor.
Some first tests with monitor-patterns easily and immediately revealed geometric distortions, light fall-off and sharpness.
Look at this example (clickable for larger views!):


Let's see whether these results hold under closer scrutinity:

Well, let's kick the first out: Light fall-off! With a shot from an LCD-monitor you're more likely to record differences in lighting of the minotor than the lens. Especially when you're shooting 30cm away from the monitor with a wide-angle lens, the angle of view from the camera to the edges of the LCD-monitor will produce visible light fall-off. So forget about this measure!

Ok, on to the easy one: Geometric distortions. That is clear as the sky that an absolutely flat LCD-Monitor with a grid on it is a perfect target to test the lens. Easy!

Now to the last point: Well, if you see differences in centre- vs corner-sharpness, that is valid. Only thing you have to make absoultely sure is that the camera is aligned parallel to the monitor, as any misalignments will result in one corner (or the other) falling out of the very limited dof that you have when shooting a 21" monitor "full-screen". That is a magnification of about 1:20 and dof is around 1.2mm (!) (for f4.0 and 25cm viewing-distance of an A3-print) in this case.
But what about pixel-pitch on the monitor and pixel-pitch of the sensor and projected pixel-pitch of the monitor on the sensor? Well, here's a little math: My monitor has a pixel-pitch (pp) of around 100dpi, my camera's sensor a pp of 4100dpi. So long as you're at a magnification of 1:10 (projected monitor-res = 1000dpi) you end up with 16-18 sensor pixels peeping at one full color monitor-pixel (consisting of red/green/blue subpixels). As you need 4 sensor pixels to analyse all colors (Bayer-matrix) that's a comfortable ratio to say the resolution of the sensor outperforms the projected monitor resolution and thus you should be able to "see" all monitor pixels perfectly sharp - if you have a perfect lens! So any deterioration in perfect sharpness of a screenshot at 1:10 can be attributed to the IQ of the lens!
At 1:22 you're still at a ratio of 4 sensor-pixels per projected full-color monitor-pixel. That is still pretty safe and gives you a full view of the 24"monitor for a camera with a 1.1" APS-C sensor.

If you shoot at a magnification of 1:100 (camera at 100xfocal length from the monitor) the circumstances turn around. In this case you've got 5-6 full-color monitor-pixels projected on each sensor-pixel. So it is safe to say that the camera cannot see any single monitor-pixels any longer. So tests with single-line-width structures on a monitor are irrelevant from this distance.

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Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews, My Pictures, My Photography Blog
D800+assorted lenses

Wow, that is a lot of info there Tom. Thanks for going into this detail to explain.

from what im seeing your using a CRT monitor there?

I really can reccomend a TFT preferably 20" or above.

For bright vivid realistic interpretations the Samsung 226BW is a very good choise although some people dont like the widescreen aspect.

Also you could splash out in the profeesioanl monitors, but they seem to charge you an extra $2000 for a little hood that will stop the glare

If i have missed your point here somehow please correct me

Jay

I have the BenQ 24" TFT and its the best thing I ever got for my PC.

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Interesting that you would say this, Jay! Because the curvature on the test-pic does not come from the monitor but from the lens. So this is easy proof that geometric distortions can be easily seen with monitor shots from a perfectly flat and rectangular LCD-monitor. In my case it's a 1200x1600 21" NEC. But as I'm not testing colors but sharpness, distortions and color-fringing, the IQ of the LCD-monitor really plays no role in this type of test!
For color-fringing, see this test which is also good for testing corner-sharpness and geometric distortions.

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Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews, My Pictures, My Photography Blog
D800+assorted lenses

Ohhhh , i see now. From the picture it just looked identical to a spread sheets i use ony my old CRt monitor.
Yes the curvature of the lens would explain that , and yes thats 1 point thats been proven!

I have to add one word of warning though:
From a test the forum-member eL did it becomes clear that you should have a digital connection between your graphics-card and the monitor (DVI)! Otherwise it's hard to get real sharp letters and signs on your monitor.
You can really see the difference between an analogue- and a digital interface

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Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews, My Pictures, My Photography Blog
D800+assorted lenses