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01-22-2019, 08:11 PM   #1
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What interval does a light meter read?

I am wondering the speed of the light meter reading because of electric lighting. I would think you could change it to read in 1/50, 1/60 increments. I looked around and haven't found anyone addressing this. Does anyone know?

01-22-2019, 09:08 PM   #2
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Are you wondering if the frequency (50Hz or 60Hz) matters to your light meter? I don't think it does.
01-22-2019, 09:16 PM   #3
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It's what started me wondering when I had changing readings. Now I just want the answer.
01-22-2019, 09:49 PM - 1 Like   #4
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Canon has an "anti-flicker" setting that is sort of what you're looking for. It looks like it detects a light source that changes at 100 or 120Hz, then picks a time to shoot where the light output is highest, which is also the best color temperature. A neat idea, but it may mean some shutter lag as it analyzes and waits for the right point of the cycle. The camera tells you when your light source is flickering, which is useful enough by itself.

01-23-2019, 12:06 AM   #5
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QuoteOriginally posted by swanlefitte Quote
I am wondering the speed of the light meter reading because of electric lighting. I would think you could change it to read in 1/50, 1/60 increments. I looked around and haven't found anyone addressing this. Does anyone know?
Can you clarify your question? The title of your post, "What interval does a light meter read?" would imply you're asking whole EV, 1/2 or 1/3, with some incident and handheld meters at tenths of an EV.

But if you're asking because of non-continuous light sources such as the flicker from fluorescent, neon, sodium vapor, LED, etc, I would say it is rather slow and is just averaging the light intensity over a fairly 'long' period of time....such as 1/4 second. From an engineering perspective, I'm sure someone at Sekonic or Gossen or even Spectra Cine could answer this technical query.

In cinematography and video, whenever shooting with non-continuous light sources, to avoid flicker due to the shutter being out of sync with the light source, we had to use high frequency ballasts that pulse at extremely high rates.

Are you wondering this just from a purely theoretical curiosity or is there an exposure concern?

---------- Post added 01-23-19 at 01:20 AM ----------

QuoteOriginally posted by MarkJerling Quote
Are you wondering if the frequency (50Hz or 60Hz) matters to your light meter? I don't think it does.
QuoteOriginally posted by swanlefitte Quote
It's what started me wondering when I had changing readings. Now I just want the answer.
From a very simplified analysis, at 50Hz, the light is on 50x/second and off 50x/second. There are different ballasts that may increase this, but regardless, of the Hz, it's on half the time and off half the time. So although it may be on more frequently, it's also off more frequently and the amount of lumens or foot candles is going to be the same....unless you're shooting at an extremely high shutter speed with the possibility of missing a burst, at which point, the light meter is not going to adjust to the darkness.

Practical answer: Avoid high shutter speeds (or bracket) when shooting with predominantly non-continuous light sources.

If you want a specific number, you'll need to find an electrical engineer familiar with this particular technology, and even then, I'm sure this is not a constant value on all light meters.
01-23-2019, 12:54 AM   #6
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Do you mean what is the sampling rate of the meter in continuous mode? You'd have to ask Ricoh because it is not stated in the specifications. Just doing a google search on "light meter sampling rate" turns up very few with this listed. 2 to 4 times a second are what I find at a glance for a couple scientific meters.
01-23-2019, 01:35 AM   #7
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QuoteOriginally posted by Not a Number Quote
Do you mean what is the sampling rate of the meter in continuous mode? You'd have to ask Ricoh because it is not stated in the specifications. Just doing a google search on "light meter sampling rate" turns up very few with this listed. 2 to 4 times a second are what I find at a glance for a couple scientific meters.
That's what I meant. The time it gathers light for a measurement and the time between measurements. If it measures 4 times a second it's 1/4sec if no delay between measurements. Light flicker is 50/60 per second so it would read 12-15 cycles.

I really only care for information not for any reason. To me it's as useful as an mtf chart for a lens I have. I can just take a picture and see how it looks but the mtf chart is interesting.

Now from thinking about it I want to know what range of the spectrum it measures and how the rest is filtered out. Is it more sensitive to red or blue, etc. I can't see it working like the sensor; 2 parts green, 1 blue, 1 red but maybe it does.
01-23-2019, 08:39 AM   #8
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The long answer is, it depends on what light meter you're talking about... A handheld light meter might offer a few different modes, including instantaneous (flash mode) and average.

If you're talking about the inbuilt meter in your camera, the short answer is that it averages over a period that will even out artificial sources (around the 1/4 second mark), but the issue is not the measurement period, but the shutter speed you are using, which is much more likely to be in the same ballpark as the light source.

The even shorter answer is that the meter movement you are seeing is probably the product of something other than light source. Small movements in where you are pointing it can create large fluctuations, especially if you're using spot metering, or with weighted metering if there is a light source near the edge of frame.

01-23-2019, 09:13 AM   #9
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The "interval" depends on the camera but all modern cameras almost certainly use a variable integration period (what you called the "interval") that depends on the light level. As for RGB, the newer Pentax cameras have a RGB array sensor not unlike a tiny camera so they meter each color channel independently. Older cameras had a simple photodiode which has the default spectral sensitivity of silicon and no ability to distinguish color although they may put a slightly colored filter on the sensor to help match it to the main sensor.

The point is that the exposure meter is essentially a tiny camera with an electronically-controlled shutter speed. The shutter speed the meter uses is almost certainly faster than 1/60 second in very bright conditions and definitely slower than that in very dim conditions. That's the only way the system could get accurate readings over the huge range of EV levels (from -3 to +20 for the K-1).

As for light source flicker, there are many clever tricks for detecting it, filtering or averaging it out, and timing the main exposure to occur in synchrony with the flicker.

P.S. The AF system does the same thing.
01-23-2019, 02:49 PM   #10
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These answers are helping me to fill in my understanding so thank you.

Googling is like asking how cameras work and getting. Power it on. Look through view finder. Aim. Press on botton. Which sucks when I want to know about bayer sensor, foven sensor, image circle, mirror box and the like.
01-23-2019, 08:14 PM   #11
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QuoteOriginally posted by swanlefitte Quote
These answers are helping me to fill in my understanding so thank you.

Googling is like asking how cameras work and getting. Power it on. Look through view finder. Aim. Press on botton. Which sucks when I want to know about bayer sensor, foven sensor, image circle, mirror box and the like.
Camerapedia <http://camerapedia.wikia.com/wiki/Camerapedia> may have useful answers than Wikipedia on camera/photo questions.

Regards, Jim
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