What's the "frame rate" of...

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Beatsy
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What's the "frame rate" of...

Post by Beatsy »

...my eyes? I guess it's a continuum of some kind rather than discrete frames per-se. Just curious if it's been measured - or is even measurable.

rjlittlefield
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Post by rjlittlefield »


Beatsy
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Post by Beatsy »

Thanks Rik. Because Google was more interested in showing me "great deals" on spectacle frames, I missed that one. Interesting read. Cheers.

rjlittlefield
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Post by rjlittlefield »

Part of the trick is knowing good search terms. That one is near the top for flicker fusion frequency.

This was an issue of personal interest for me back in the days of CRT monitors. It turns out that my personal flicker frequency is a bit higher than 60 Hz. Easily solved by just setting my graphics adapter to 70 Hz. Easily solved, that is, until one day I found myself in the back of a classroom that was full of 60 Hz monitors. The experience was agony until I spoke up and got everybody else to bump theirs too. That problem disappeared with LCD monitors, which have still have refresh rates but never flicker no matter how slow the refresh is.

--Rik

Beatsy
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Post by Beatsy »

I know what you mean. Mine's somewhere between 50 Hz and 60 Hz I think. When I lived in the USA the TV drove me batty with incessant flickering (50 Hz?). In the UK, it's fine (60 Hz).

ChrisR
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Post by ChrisR »

Um... :? isn't the US a hair under 30Hz , and the UK 25Hz, both effectively x 2 thanks to interlacing?
Chris R

Beatsy
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Post by Beatsy »

Crikey, you're right! Now I'm totally confused. TV did flicker horribly for me in the states. Must have been the set. :oops:

mawyatt
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Post by mawyatt »

I think the old TV's were synched to the line frequency (60Hz in US, 50Hz in UK) and interlaced. This was to help prevent slow rolling flicker from beat frequencies from florescence and sodium/mercury vapor lights which I understand go on and off at 2X the line frequency.

If you've ever tried to shoot sports with fast shutter speeds inside a gym with sodium or mercury vapor lighting this can cause a huge variation in the image illumination depending on where the shutter is open relative to the illumination cycle of the light sources. Some of the newer cameras have a mode to synch up with the lights and open the shutter during the light peaks.

Best,

Mike

rjlittlefield
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Post by rjlittlefield »

ChrisR wrote:Um... :? isn't the US a hair under 30Hz , and the UK 25Hz, both effectively x 2 thanks to interlacing?
I don't know about UK. But yes, in the US the analog TV standard for color transmission is NTSC, which specifies 29.97 Hz refresh. With 2:1 interlacing, that means that most images effectively flicker at 59.94 Hz. However, sharp horizontal features that are not anti-aliased will flicker at 29.97 Hz. That used to be particularly common with computer generated images.

Ordinary TV's never bothered me much. The difficulty started when CRT monitor manufacturers switched to faster phosphors that could support higher refresh rates. Those faster phosphors, when driven at only 60 Hz, had a high amplitude flicker that drove me nuts. Pushing the refresh up to 70 Hz solved the problem quite nicely.
I think the old TV's were synched to the line frequency (60Hz in US, 50Hz in UK) and interlaced. This was to help prevent slow rolling flicker from beat frequencies from florescence and sodium/mercury vapor lights which I understand go on and off at 2X the line frequency.
That was true for the original B/W version of NTSC. However, when color was added, there were other issues with beat frequencies that required slightly tweaking the refresh rate to avoid even worse difficulties. This is explained in detail at https://en.wikipedia.org/wiki/NTSC#Color_encoding .

The combination of side effects could get pretty interesting. Back in the mid 1990's I used a consumer-grade video camera to record some macro stuff against a very neutral background. When I played back the video I was surprised to see that the background color had a color balance that slowly cycled in a small range around neutral. For a while I thought it was some problem in auto color balance, but then I realized that I was using fluorescent lighting, and the frequency of cycling exactly matched beating between NTSC and the 60 Hz line frequency. Apparently the fluorescent tube I was using had some phosphors that were a hair faster than others, so indeed the color balance did vary depending on where in the cycle the camera happened to sample. Once I figured out what the cause was, it stopped bothering me.

--Rik

ChrisR
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Post by ChrisR »

NTSC, = "Never The Same Color". PAL is much better ;)
Chris R

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