Camera as an Analytical Light-Meter - page 2 - Creative Uses of CHDK - CHDK Forum
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Camera as an Analytical Light-Meter

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Offline kwf

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Re: Camera as an Analytical Light-Meter
« Reply #10 on: 26 / March / 2008, 16:28:15 »
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@Barney Fife: wow what a long post, lets see if i can beat that :)

Currently the Spyder is not with me, so i can't test it. But as soon as it comes back and i have time , I'll play a bit with it.

Only 100 adjustment is not too bad, if it does really do something every step :) What do you mean with, how do they look to me on my monitor? They look quite okay for me, but honestly for good eye calibration i need the right patterns, e.g. comparing different whites and choose the most neutral.

About the DVD as spectroscope, i bought a cheap hand held toy spectroscope, thats not bad to see spectral lines as well ... you can see the extra spectral line of white color gamut displays with it :)

About the accuracy of various color calibration tools, i am a bit confused. I was so naive and thought that these colorimeters have color filters closely matching the spectral sensitivity of the CIE1931 observer. Thus i thought they should be able to measure all by human perceivable colors. Opposed to that the color filters of camera sensors are a compromise between color accuracy and quantum efficiency. So they cannot distinguish all colors. But it seems that these color sensors of the Spyder (and probably other similar cheap devices) are also just a compromise. So they probably need to assume some spectral distribution of the light source which should be calibrated, the LCD attachment of the Spyder is already a hint to this. This explains why the Spyder does not work with wide gamut displays .... Maybe the Spyder3 is better (with 7 color filters), but i don't want to waste more money. Currently i have the feeling that a Canon camera may be similar effective or even more effective to calibrate LCDs...

Photographing the screen through different known color filters (with known spectral response) would be a cool idea to improve the color accuracy to calibrate monitors :) Or i should just photograph the LCD through my cheap spectroscope, but this would need a lot of work to be calibrated to be able to get useful results.

Btw. i have spyder2, and as i said, i have the feeling it is not accurate, because does not work with wide gamut lcds...
« Last Edit: 26 / March / 2008, 16:53:50 by kwf »
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Offline dvornik

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Re: Camera as an Analytical Light-Meter
« Reply #11 on: 26 / March / 2008, 18:44:45 »
Quote from: kwf on 26 / March / 2008, 16:28:15
Btw. i have spyder2, and as i said, i have the feeling it is not accurate, because does not work with wide gamut lcds...


Have you tried using it with professional-grade software like Coloreyes or Basiccolor? You can download a demo. I don't have access to wide gamut monitors so I have no idea if it works with them.
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Offline Barney Fife

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Re: Camera as an Analytical Light-Meter
« Reply #12 on: 26 / March / 2008, 19:23:38 »
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« Last Edit: 22 / April / 2008, 14:35:33 by Barney Fife »
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[acseven/admin commented out: please refrain from more direct offensive language to any user. FW complaints to me] I felt it imperative to withdraw my TOTAL participation. Nobody has my permission, nor the right, to reinstate MY posts. Make-do with my quoted text in others' replies only. Bye
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databoy

Re: Camera as an Analytical Light-Meter
« Reply #13 on: 27 / March / 2008, 04:29:24 »
Regarding calibrating cameras, printers and monitors.
The following web site contains calibration details and accurate calibration charts.

Accurate Image Manipulation

Quote from the site:
The purpose of the AIM site is to provide help in setting up an accurate desktop image manipulation system for digital photographic images. AIM contains currently (about) 212 pages and is 100MB in file-size.

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Offline Racenviper

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Re: Camera as an Analytical Light-Meter
« Reply #14 on: 06 / May / 2008, 07:24:15 »
Can some repost this script?
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Offline PS

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Re: Camera as an Analytical Light-Meter
« Reply #15 on: 06 / May / 2008, 13:06:49 »
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Offline Racenviper

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Re: Camera as an Analytical Light-Meter
« Reply #16 on: 06 / May / 2008, 13:11:23 »
The point of this response is?
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Offline PS

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Re: Camera as an Analytical Light-Meter
« Reply #17 on: 06 / May / 2008, 13:40:14 »
The thread is wasted anyway.
« Last Edit: 31 / January / 2011, 20:34:58 by PS »
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Offline Racenviper

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Re: Camera as an Analytical Light-Meter
« Reply #18 on: 06 / May / 2008, 14:23:33 »
Now that Barney is gone everyone is afraid to post or repost. I'm sorry but all the information deleted, just is not right and it almost this site useless to read half of the posts. Someone was kind enough to send me the info relative to the title of this post.

While doing some Wikia editing, I noticed a feature I hadn't played with before. In the OSD Misc. Values there's the option to display "Show Scene Luminance". I had also recently read a thread somewhere else on the net where people were using their cameras to test the NIT value of their monitors (1 NIT = 1 candela per m^2, a new name for an old-bird for monitor brightness).

By pointing their cameras at the monitor, setting the lens to f/5.0 in Av mode, set ISO at 100, and filling the FOV with nothing but a blank white region, you can plug in the resulting shutter-speed into this formula:  7200 / Tv = cd/m^2 = NITs. They found this value by using a monitor calibration tool called a Spyder and comparing the results.

Well, using that formula I got pretty close to 144 on my monitor (I have it tweaked for use in a dim room, very accurate for my needs). Keeping in mind that Av mode's selected shutter speed may be off by as much as 1/6th EV. But I got to wondering just how accurate is that? And saw that "Show Scene Luminance" feature of CHDK. I decided to turn it on. It rates the same monitor screen as 180.7. Which is almost exactly 1/3EV more (or less, depending on how you are using EV values).

That's not too far off from that brightness level from the monitor tests. It would be interesting to find out just how accurate that "Show Scene Luminance" value truly is. I don't happen to have any of my hand-held light meters anymore. Could anyone test to make sure? If so, that's an amazing feature to have at your disposal. Not only for photography but for all kinds of research needs.

Now comes amazing Light Analysis tool #2.

I noticed when pointing my camera at a fluorescent light (not the new compact ones, the tube-style lights that run on a ballast), as it would cycle in and out of its 60Hz cycle, the light would brighten and dim to completely off. You have to set your shutter speed anywhere from 1/200 to 1/400 to see the effect the best. And half-press your shutter so the scene in your EVF is truly matching your shutter speed. The faster shutter will slowly go in and out of sync with the 50Hz or 60Hz AC line voltage cycle.

Do you have your color histograms enabled? Now watch as the peaks of the blues and reds swap positions during each phase of your fluorescent light's cycling on and off. Pretty cool eh? This is why taking images by fluorescent light are so difficult. (And why reviews online of fluorescent light tests using Auto White-Balance are pretty foolish. NONE of those so-called "experts" take this into account.) This white-balance problem is especially troublesome for video taken by tube-style fluorescent lights. If your camera is trying to lock onto a color balance and the fluorescent light is at the red or blue end of its output cycle, your image's white-balance will be wrong. 1Any shutter speeds faster than 1/2 the Hz cycle (i.e. > 1/30th of a second) will adversely affect not only white-balance but exposure metering because they will average in only part of one cycle. Shutter speeds of 1/60" or more will only read part of ONE cycle. (The light flashes on and off twice every AC line-voltage cycle.) Vastly affecting both white-balance and exposure reading.  A curse to all videos taken in fluorescent lights, the white-balance shifting wildly during your video. For a quick analogy of how this works, just point your camera at your TV set and try to view a whole frame in your EVF during a half-shutter press preview of that shutter speed. Change your shutter speeds to see how much of each 1/30" cycle is captured. Only with shutter speeds 1/15" and slower will you get a true avarage across the whole screen. Shutter speeds of 1/30" and above will only start to average in or capture partial frames.

Light analysis tools like these used to only be in labs. Now you have one in your own hands. BTW: The newer compact fluorescents aren't subject to this slow 50/60Hz hue & luminosity shift as they operate at much higher frequencies. Your white-balance won't be affected as easily, or at all. If you don't believe me you can use your camera to test that. Big Grin


Sample Usage for Scene Luminance value: Take your camera with you the next time you are shopping for a computer monitor. The box says 500 NITs as max output. Is it really? Smiley


Edit: Whoa! Hold the presses. I just found that missing 1/3EV. I leave my camera set on -1/3EV as default. When I set my camera back to 0EV, then I get a monitor NIT value of 180. And Scene Luminance value is at 180.7

1Edit2: Added in some clarification on when this problem is most likely to cause white-balance problems and missed exposure readings.
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