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Thread: Brightness Box?

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    xo-whiplock's Avatar
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    Brightness Box?

    I'm trying to sort though some ideas I have to make a brightness box for testing and/or adjusting the exposure meters on my used cameras. I'm hoping someone has built a brightness box and if so, could tell me what is needed to do it right? I'm looking for help in adapting what may already be laying around... dimmer switch, house light bulbs, old lamp w/socket, wood to make the box... etc. I only have an old GE light meter, and a DSLR to find light/exposure output from the box and then mark switch positions... Need EV 4 thru 15 if possible, but not sure of what bulb to use... this I can buy if needed. Pointers, links, schematics, anything to help would be appreciated. I tried both apug and google searches, but nothing I found helped.

    Thanks in advance.

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    xo-whiplock's Avatar
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    So I take it, it's best to just use a sunny day? I was hoping to come up with a DIY bench tester... I've seen commercial units, but can't afford one. I made a simple shutter tester, and just need to make a light source I can set EV's on to complete the bench testing. What about using a bulb from a movie projector and bouncing that off a white surface and using a dimmer switch?

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    xo-whiplock's Avatar
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    Okay... good point. A standard is good. I can use your method to verify a single EV between my handheld light meter and my DSLR's metering at any given time outside in daylight... I'm pretty sure they are within a stop of each other, and I can zero out my hand held to match my DSLR's metering, as the DSLR is most likely spot on. So, assuming that the accuracy of my DSLR is adequate, I would simply need to use a bulb that is bright enough at full power to meet or exceed an EV of 15, and then by reducing voltage, I would reduce luminescence... as my light meter reads a full drop in EV, I would simply mark that position of the dimer switch with that EV... and then re-test to verify... in theory anyway. Should I be worried about color temperatures? I would think just how bright and what my meter says the exposure is for shutter, aperture, at a given ASA/ISO and look up EV on a chart (work backwards really from EV desired, until I get shutter and aperture the same on my DSLR)... I'm just not sure a projector bulb will respond well to having a dimmer switch in series???

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    xo-whiplock's Avatar
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    Okay... scrap the dimmer switch and projector bulb... I get what you are saying now with regard to "fixing" a camera's metering... can't do it if the photocell is worn out and lost its ability to produce liner output. So, if I am able to make a brightness box and my meter output falls outside of desired linearity, replace the bad parts with good ones. I guess we are back to the beginning of this thread, in that going outside and using daylight at different EV's is better then trying to mimic it in a box... without some specialized skills and parts. The old KISS principle. Thanks for helping me out.

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    Quote Originally Posted by xo-whiplock View Post
    Okay... scrap the dimmer switch and projector bulb... I get what you are saying now with regard to "fixing" a camera's metering... can't do it if the photocell is worn out and lost its ability to produce liner output. So, if I am able to make a brightness box and my meter output falls outside of desired linearity, replace the bad parts with good ones. I guess we are back to the beginning of this thread, in that going outside and using daylight at different EV's is better then trying to mimic it in a box... without some specialized skills and parts. The old KISS principle. Thanks for helping me out.
    I am very much interested in this subject. If it's not possible to calibrate an old worn out meter, it's still possible to check the performance of the meter. I am not sure as to which color temperature to use. Kyoritsu Electric Co. Ltd made a good number for standard light source as well as meter and shutter testers. Many of them have color temperature of 2800K although a number of newer LED based ones specified as only white LED and I don't know the color temperature. Since most film are daylight balance film it makes sense for the light source to be daylight equivalent but then I am not so sure about this. Anyone has any idea about this?

    Using a dimmer changes the color temperature as you dim it down so it's not a good thing here.

    What I did in the past is to use a dichroic color head. I used a Beseler 45 computerized color head which has accurate filter display. I turn all the filter off and then check the brightness at the diffuser which is a 6'' circle with a spot meter. It's about EV15 in this condition (I have to dial in equal amount of filters a bit to get it exactly EV15). Checking eveness of the brightness with the spot meter and found within 1/10 stop so it's ok there. Dial in equal amount of filtration of C, M, Y and checking the brightness with the spot meter I get to about EV9 with all the filter dialed in.

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    Bill Burk's Avatar
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    Check out Post #36 of this thread for a good choice color temperature for meter calibration...

    http://www.apug.org/forums/forum37/1...g-match-4.html

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    Quote Originally Posted by Bill Burk View Post
    Check out Post #36 of this thread for a good choice color temperature for meter calibration...

    http://www.apug.org/forums/forum37/1...g-match-4.html
    Thanks Bill. According to Stephen in that thread the color temperature for the calibration light source changes from 2800K to 4700K in 1960. That's fine. So an LED with white LED would be a good choice. The color temperature has to be significantly off for the meter to read differently. Now I think the LED doesn't change color temperature as you dim it down. How would I dim the LED. There is current control and then pulse modulated technique. Something for me to work on...

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    Bill Burk's Avatar
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    Quote Originally Posted by Chan Tran View Post
    Thanks Bill. According to Stephen in that thread the color temperature for the calibration light source changes from 2800K to 4700K in 1960. That's fine. So an LED with white LED would be a good choice. The color temperature has to be significantly off for the meter to read differently. Now I think the LED doesn't change color temperature as you dim it down. How would I dim the LED. There is current control and then pulse modulated technique. Something for me to work on...
    Then there are physical modifiers (screens) and unit count (fewer / more LED's in a bank)

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    The following was in response to Chan Tran explaining his use of his optical color printer/enlarger... The issue of light meters and exposure meters is a different direction...

    Color temperature changes throughout the day... from sun rise to mid day, to sun set again. So color temperature is not a constant in daylight, but a variable. You'll see in most editing software you have the option to select the temperature according to the effect you want or to match better the time of day you took the shot so it looks like what you had seen when taking the picture... sometimes just changing a photo to cooler or warmer helps with the effect you are trying achieve, that may not have been present when you shot the photo for affect. I'm not experienced in optical printing but I'm guessing you want to reach the exact or as close as possible to the film negative's color temperature in order to render accurate color reproduction initially and thereby reduce the need for post editing to fix it, and one may not be able to fix it, if initial setup is too far off.... just a guess. So, even though your EV is reduced after applying the color filters, it just means you have to expose the photo paper with the new EV. Am I correct in this?

    So from reading post up to this point I take it that in order to check the exposure metering on a camera, one needs to use a stable light source, and simply using a dimer will not provide that stability. Hence, the LED's with physical modifiers to achieve desired output for a given or desired EV output. Interesting.

    I got side tracked reading all this info... I came back to this thread to comment that I installed a little program onto my computer that allows brightness adjustment of my computer's LED monitor... Although with note pad in full screen mode and brightness at 100%, I get an EV from my DSLR of EV10, and as I drop brightness in 10% increments, I see a full stop of change in shutter speeds, indicating EV droping 1EV/10% reduction. I may be able to simply use my computer's monitor to check my exposure meters? And above EV10, just use daylight?
    Last edited by xo-whiplock; 03-30-2013 at 02:34 PM. Click to view previous post history.

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    Bill Burk's Avatar
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    The issue with daylight is that it fluctuates, as you said, it's not easily controlled. A 4700K Incandescent bulb, on the other hand, might be something you can control with a feedback circuit or voltage regulator. If I were setting up a standard box, I'd make it one brightness, a standard, and then modify its brightness outside the box on the bench. You could use the old GE meter to monitor the brightness inside the box.

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