Floris Vanhoof Posted March 15, 2021 Share Posted March 15, 2021 Next week I'm filming wave patterns on an oscilloscope from the 70's on 16mm with Kodak 250D. How should I measure the light coming from this oscilloscope screen to keep the waves from over exposing and the background black? Link to comment Share on other sites More sharing options...
Premium Member Frank Wylie Posted March 15, 2021 Premium Member Share Posted March 15, 2021 (edited) I would spot meter the trace from a pattern that would fill the spot meter target with a full intensity patch and open up 1/2 a stop. Since the meter is looking for 18% gray, the 1/2 stop extra light will allow the trace to be brighter than mid-gray, but should not blow-out the most intense areas. It depends upon how much detail you are trying to capture of the less intense portions of the trace, but if you are afraid of blowing out the brightest portion of the trace, I would be conservative and tend to use less than more exposure. I assume the phosphor will be green, but you didn't specify. The curves on 250D show that green is slightly less sensitive than Blue and much more sensitive than red, so a modern spot meter should give you a reasonable reading. You also don't mention your end delivery media; film or digital or what? If it is a film print, you can boost the gamma of the print coming off of the negative to assure proper blacks OR with a digital scan, just pull the toe of your curves down to crush the blacks a bit. Of course, nothing beats a bracketing test which ever way you are going... Edited March 15, 2021 by Frank Wylie correction Link to comment Share on other sites More sharing options...
Floris Vanhoof Posted March 15, 2021 Author Share Posted March 15, 2021 Thanks for the reply! I will film with the exact same scope as in the YouTube video. Indeed with more complex patterns, it would be great also to capture the trace while not burning in the shorter lines. I got good results exposing the Vision3 250D film at ISO 160 and filming double exposures one stop down. End medium will be 16mm film. I should also mention that I will probably do multiple exposures (rewind the camera) with different color filters on the oscilloscope, and my Bolex EL has a build in light meter. Also: what would give the best result: Color filters just in front of the lens or onto the screen of the oscilloscope? Link to comment Share on other sites More sharing options...
Premium Member Phil Rhodes Posted March 15, 2021 Premium Member Share Posted March 15, 2021 This is likely to be a bit tricky. Unless you're driving the scope tube really very carefully, the trace will vary in brightness as different patterns pass by and the total length of trace varies. Could be a bit tricky. Link to comment Share on other sites More sharing options...
Premium Member Frank Wylie Posted March 16, 2021 Premium Member Share Posted March 16, 2021 Multiple passes with filtration? Do a test. Link to comment Share on other sites More sharing options...
Floris Vanhoof Posted March 16, 2021 Author Share Posted March 16, 2021 Thanks for the reply. When doing Ektachrome, I got 1 meter of film as a test developed overnight at our the local photography lab. They could hang it next to the usual 35mm slide films. With negative, I don't really know other then 30 meters and having it scanned (above my budget this year). Link to comment Share on other sites More sharing options...
alexandre favre Posted March 16, 2021 Share Posted March 16, 2021 If you dont' have time to do a test, you can try to check exposure with a digital camera and overexpose one stop. I did this with a 5D some years ago and it was successful. Link to comment Share on other sites More sharing options...
Selinica Harbinger Posted April 7, 2021 Share Posted April 7, 2021 Check the manual for the oscilloscope too. You may need to do some added digging from there. The older test gear required film cameras to record the waveform, so there is information in the manuals about that. The trace focus and intensity will affect the exposure as well as the phosphor used-the old test gear used different phosphors for ones intended to be mostly just photographed and those intended to be primarily viewed. Then there were some slower and faster phosphors and some that were a mix of the two colors-a fast green and a slower blue for the camera that our eyes don't see as well. You will want to do a test, especially as compared to a regular CRT the oscilloscope will have the trace speed varying based on the signal settings rather than how TV/monitors work. You may need a viewing hood too and no grid illumination. I know for mine it's not easy to get the ideal hair lines that are the sweet spot of visible and fine enough. However mine is from the 60's so there is a decade difference. Do be a little cautious-if you don't turn down the intensity as you adjust the focus you can burn the phosphors and damage the screen. That's just my two cents though, Alexandre has a really good point with the checking using a digital camera too. Link to comment Share on other sites More sharing options...
Floris Vanhoof Posted April 7, 2021 Author Share Posted April 7, 2021 unfortunately nothing about this in the manual http://hparchive.com/Manuals/HP-120B-Manual.pdf still a great tip. I'll see if there's other scopes with info in the manual available. Link to comment Share on other sites More sharing options...
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