Light meter?

Myu · Apr 9, 2012

Okay so I am looking to get a light meter just for playing around with, a cheap one at $25 or so which I have found. I won't be using it for photography as my camera has a built in one that reads in stops rather than lux. My cousin is getting into film photography, and he would have use for a light meter for his 35mm camera (that has none). If I got it, it would be to play with it and see how much brighter my art lights get as they warm up. Then just give it away for him to use. So the question is, a meter with a +/-3% and resolution of 1 lux, how well will it be reading f-stop? If it would have an f-stop error grater than 1/2 a stop it wouldn't be worth getting cause my reason is stupid and he has an iphone app that I bet could get with in 1 f-stop.

When I searched google I kept getting over complicated answers that said everything but what tolerance lux measurements need to be to have use. I just want to know how accurate a light meter has to be to work well for photography.

KmH · Apr 9, 2012

The light meter in the camera can only meter reflected light.

A hand-held meter is often useful to meter the incident light falling on your subject, and the very short duration of strobed light.

Good light meters measure in 1/10 stop increments.

Haemp · Apr 9, 2012

KmH said:
The light meter in the camera can only meter reflected light.

It is often useful to meter the incident light falling on your subject, and the very short duration of strobed light.

Good light meters measure in 1/10 stop increments.

Thanks for the explanation.

TCampbell · Apr 9, 2012

A hand held light meter measures the amount of light falling on the meter (the "incident" light). You use this meter by placing in the light where you intend to shoot your subject (or just hold the meter in front of the subject.)

An in-camera light meter measures the amount of light REFLECTED off the subject -- which then bounces into the camera. The "problem" with this is that not everything reflects light equally. If I have a 100 watt light bulb illuminating a person wearing all back with a black background, I won't get very much light "reflected" into the camera. But that same 100 watt light bulb illuminating a person wearing all white standing front of a white background will reflect more light. And yet... you realize that both subjects were actually in the identical lighting (the same 100 watt light bulb at the same distance.) An incident light meter would tell you the correct exposure. The reflected meter built into the camera will only guess at the exposure. It makes assumptions about how much light the "average" scene should reflect. The problem is... when the scene isn't behaving according to the meter's assumptions then the camera will be wrong.

I have an incident light meter in my bag. I only occasionally use it. I typically don't use it when shooting candids or out and about for fun. But if I'm shooting a shot that matters -- maybe something that'll get published, I meter the shot with the incident meter.

Here's an EXCELLENT (albiet a bit lengthy) video that demonstrates why an incident light meter will get you a correct exposure when the built-in light meter will give you an incorrect exposure (sponsored by a company that makes light meters.) Control the Light and Improve Your Photography: Part I

Myu · Apr 9, 2012

Good information, but still not quite answering my question if a meter that measures in lux units will work and how accurate its measurement has to be. About to watch the video and see what is said.

As for all the internal camera metering, that is not a option in this situation due to the camera he has doesn't have a functioning meter.

Edit:
Only part way through that video. Heck I might get a meter for myself. Many of the issues he is talking about I deal with often myself. Didn't know how useful one can be over the internal meter of my own canon..

TCampbell · Apr 10, 2012

Incident light meters typically measure in EVs or display the exposure as an f-stop & shutter speed (typically you set the ISO and either the shutter speed for f-stop and the meter will display whatever you didn't set.) Light meters which measure in lux are usually not used for setting photographic exposures. But you can convert them... and you can even get a calculator or slide-rule that shows the conversion.

1 Ev = 5 Lux
2 Ev = 10 Lux
3 Ev = 20 Lux
4 Ev = 40 Lux
5 Ev = 80 Lux

You can see the pattern that each time the Ev increases by 1 stop, the Lux doubles.

Lux = 2^ev * 2.5 (e.g. 2^5 = 32. 32 * 2.5 = 80) So 5 Ev = 80 Lux

If you convert Lux to Ev you can find the exposure based on an Ev lookup table. The rule is Ev = Av + Tv. Av is the "Aperture value" and Tv is the "Time value". The Av's and Tv's are integers (starting at 0) based on the full-stops of shutter speeds and f-stops.

1 second = 0 Tv
1/2 second = 1 Tv
1/4 second = 2 Tv
etc.

f/1 = 0 Av
f/1.4 = 1 Av
f/2 = 2 Av
f/2.8 = 3 Av
etc.

So if you're meter tells you (on a bright sunny day) that you have 82,000 Lux then translates to 15 Ev.

Any combination of Av and Tv that adds up to 15 will be a correct exposure (all this assumes ISO 100).

So 1/125th (Tv=7) and f/16 (Av=8) add up to Ev 15. That means 1/125h at f/16 (and ISO 100) would be a correct exposure on a bright sunny day (and coincidentally, that's also the Sunny 16 rule).

Is the conversion a pain? I think so. I'd suggest you get a light meter that measures in exposures that you can immediately enter into the camera without doing the math.

Myu · Apr 10, 2012

Yeah, while the math is fine for me (currently college student with engineering major. You learn to like math to survive.) However my cousin would not like having to deal with converting to ev then looking at av and tv tables.

The only meters I have found that convert and do the grunt work for you cost in the hundreds, guess I won't bother for now.

edit:
Also I crunched the numbers at 500lux with +/-3% error the ev would be +/- about .5 if rounded to the tenth. Seems to be plenty of room for guess work. Which is fine for someone who knows how to meter and has plenty of experience to know which way to guess.

Thanks for showing me the formula and how ev related to the settings, that was what hung me up when I first searched. I couldn't get from ev to what the camera should be set at.

Oh and 500lux is about 7.6 ev if someone wanted to know.

2WheelPhoto · Apr 10, 2012

Cut all the BS and get one of these. It'll even figure out your ambient to strobe ratio. Sekonic L-358 Flash Master Meter - Silver 401-358 B&H Photo

With a STUDENT discount you can pick it up brand new for $249, thats a steal.

chuasam · Apr 11, 2012

Unless you're shooting slide, you generally have a 1 or 2 stop latitude in your exposure anyway.

Myu · Apr 11, 2012

$249! I'd rather get a lens. Heck that is more than I paid for my cannon 20d. Aren't there meters that have the calculations for photography use in the $20-$50 range?