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How does a light meter work?

Jim Mims , Oct 22, 2007; 12:20 p.m.

Or, how does MY light meter work? I understand the difference between spot, center weighted and matrix metering (I assume this is the same as what my camera calls "Multi-Segmented Metering") and through practice, I'm learning how they work. No, what I'm talking about is how does it work? Maybe an example will help me explain.

I'm using aperture priority. As I'm hand-holding the camera, the shutter speed displayed in my viewfinder is moving back and forth as I make minor adjustments in my composition. So, my light meter is constantly updating my shutter speed depending on where the camera is pointing, right? Is this what I want?

My camera (Samsung GX-1S, Pentax *ist-DS2 clone) has three settings for exposure lock (AE-L) when in Manual Mode:

1. Program - The aperture and shutter speed are adjusted automatically

2. Tv Shift - The aperture is locked and the shutter speed is adjusted automatically

3. Av Shift - The shutter speed is locked and the aperture is adjusted automatically

If I wanted aperture priority, it seems to me that I would set my camera on manual mode, dial in the aperture I want, and use Option 2 of AE-L to automatically adjust and then lock the shutter speed. Or should I stay on aperture priority and let the camera make the choice of shutter speed?

In reading "Understanding Exposure", Bryan Peterson talks about "metering the sky". So if I understand this correctly, in manual mode I would point the camera up at the sky and use exposure lock to set my shutter speed and then compose the shot?

Finally, my camera has the option of linking AF point and AE during Multi-Segmented Metering. If I allow the camera to automatically adjust shutter speed (using the example above), should I turn this on?

Thanks, Jim


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Justin Serpico , Oct 22, 2007; 02:22 p.m.

Your camera measures reflectance off the scene.

In spot it measures only the center area.

In center it weights the center and then gives decreasing weights to the essentially cone shaped area outside of the center spot. this means you get an "average" also know as center weighted average.

In matrix, the camera essentially guesses based on evaluating the whole scene. via each segment. It's a more complex average but a lot of test have shown center weighted is as accurrate in most cases as 6 or 45 metering. Actually, even more test have shown above a few segments exposure accuracy changes very little.

Your shutter speed or aperture change in Av or Tv modes because you are moving the camera to different areas of reflectance.

Your cameras meter only sees shades of grey. It trys to put whatver you point it at as a mid tone exposure (18%ish) grey. If you point it at a white wall, your camera makes the wall grey because it assumes the wall is mid tone since it is the only shade in the scene.

If you point it at a black wall it makes the wall? you guessed it, GREY.

So if you had a 1/15th second exposure on a white wall and you moved the camera to a black wall in Av mode you'd get a 1/60th exposure and still a grey wall. Really the correct exposure for the wall should be 1/4 second on the white wall and 1/250th on the black wall for the wall to be either black or white.

Finally, when you link the AF point to the AE it actually just center weights based on that segment. So it's essentially non center, center weighted.

Personally, I spot meter most of the time. Either select a mid tone and meter that, then lock in the settings in manual. Or select a lighter and darker area and adjust that. Such as the black wall, and close down 2 stops which would make you meter show -2EV in the meter on a black object.

I use center weighted for backlit subjects in rapid shooting with a +1 Exposure comp dialed in. And for just fly by wire rapid shooting I set it to multi segment/matrix and figure the camera can do the rest.

Also, for very even lighting with low tonal range multi segment does just fine.

Oh, as far as metering the sky, it's at a 45* angle to the horizon, and you should generally open up about 1/2-1 stop because the sky isn't usually true mid tone. Of course usually it's close enough. But this assumes mid day sun with even lighting. In a high contrast scene you need to make the call as to what your mid tone is, and if that mid tone fits into the 6-7EV range of the sensor/film. That EV range is usually 4 under mid tone and 3 over mid tone.

Jim Mims , Oct 22, 2007; 02:35 p.m.

Thanks Justin for the great explanation. Definitely filed for future reference.

That's what is confusing me. Unless you lock exposure down, the light meter is CONSTANTLY measuring reflectance, it never stops?

When you say you meter the sky and then "open up about 1/2-1 stop", what you're saying is at that point you've locked the aperture and shutter speed with AE-L (Auto Exposure Lock) and then adjusted EV compensation.

So for all this to work, You HAVE to use AE-L in manual mode. IOW, AE-L is a good thing and makes the camera much easier to use and learn?

Mel Unruh , Oct 22, 2007; 02:47 p.m.


This can get a little complex, so before we get too detailed, lets look at the basic principle of a meter.

Your camera meter works on a principal of light reflectance. It measures the amount of light being reflected from the surface of whatever you have the camera pointed at. Your in camera meter uses a reflectance value of 18% grey as a basis for exposure calculation. This is an important fact to remember. Someone figured out that if you took a reading off of everything and averaged it out it works out to 18% grey. So this is the standard that ALL meters use.

Now if your camera has spot metering available, you will notice that if you point it at a persons face, you will get one reading and if you point it at their clothing you may get a completely different reading. You may even notice that different parts of the same face give different readings. Same person, in the same light different readings. This has been an age old problem which camera manufactures have working to solve. Thus we now have multi-segment or matrix metering. This takes readings at different points in the scene and averages them out. Others take it a little further and allow additional weight being added for whatever is in the center of the viewfinder. All of these are strategies to try and give perfect exposure every time. They work well, but are not infallable. It all depends on the scene. A snow scene for example will throw everything off because of the brightness of the snow when using matrix of even center weighted. You would want to use spot metering on your subject (assuming a portrait here) and that only gets you close in that case. Ideally, you would want to meter something 18% grey and then lock that exposure (remember the 18% standard). You can buy what's called a grey card from any photo store for this purpose. Other things can work like the bark of a tree, dry asphalt, rocks or even WET beach sand (not the dry stuff - thats like snow). A grey card is best, but something close (and in the same light as your subject) will work. Another option is to buy an ambient light meter, these you just hold in front of your subject and point back at the camera. It measure the light falling ON your subject instead of light reflected by your subject.

Hopefully, I explained this well enough that you can see how your camera thinks. From there, you need to experiment in order to understand your equipment.

Hope this helps,

Mel Unruh

Justin Serpico , Oct 22, 2007; 06:43 p.m.


Actually, in response to the final 2 paragraphs...No you don't need AE-L in manual.

AE-L is necessary in the other modes Av and Tv.

With manual, simply point the spot meter at a mid tone such as the sky at 45* (which we established isn't always quite mid tone, but close enough) set the aperture where you want it, and then set the shutter speed so the light meter falls to 0 EV (middle of the bar graph).

So if you are shooting an evenly lit landscape you'd probably decide to use f/8-16 (depending on how much depth of field you need), then get the sky to fall at 0 EV by adjusting the shutter speed.

Now if you found as I do that the sky is a bit lighter than mid tone, you would open it up 1/2 stop by increasing the shutter speed 1/2 stop.

With Av or Tv. you'd lock on the sky and then use the EV comp to do the same thing.

As far as what mel is saying about the ambient meter (also known as a incident meter) I've never found them to be useful in a outdoor setting. The reason being with a reflectance meter you can reach out an touch a distant landscape with a zoom. but sometimes walking 2 miles to get to the landscape and walking back with the incident meter just isn't useful. Thats a extreme example but even say going onto a field to measure the light falling onto a batters box just isn't always feasible. Of course if you are shooting studio work, like I believe Mel does, then a ambient meter tends to make a lot more sense.

Mel Unruh , Oct 22, 2007; 07:13 p.m.


You are correct. I speak in terms of people photos (or something similar that will hold still). Ambient/Incident meter is a waste of time if you are taking shots of landscapes or something "Out there". If it's "right here" use incident. When its "out there" use a spot or matrix :-)

Jim, are you geting this... :-)


Michael Kuhne , Oct 22, 2007; 07:30 p.m.

From my experience and understanding, the matrix metering systems represent a bit more than just averaging. They use a computer or biasing feedback technology which evaluates extreme lighting conditions to automatically compensate for them as opposed to the old centerweighted metering. Snow scenes will be less grey, and backlit subjects will be compensated for, shadows more open, or bright highlights toned down.

Of course, these systems are not perfect, and if there is time, it is always best to double check using the spot meter, as Justin indicates, for more refinement of exposure.

Sometimes matrix can work against your wishes. If wanting to create a silhouette effect, for instance, using the matrix meter would be inappropriate.

By Program, TV, AV- I presume you are referring to modes as set on the mode dial. TV means you select the shutter speed and the camera sets the appropriate aperture. AV- you select aperture, and the camera sets the shutter speed. Program- the camera sets both. The changes you see as you move the camera is the meter being constantly active and instantly responding to changes in reflective lighting entering the lens. These are AE (auto exposure) modes. Then there is metered manual mode, where you set both using the meter's reading as a guide.

Sometimes you don't want AE, and want the exposure you set to stay put, regardless. So you switch to manual. For example, when your main subject(s) move between a bright and a dark background.

AE modes can be very useful. But when the time is available, do a lot of shooting in the manual mode, and also use the metering techniques Mel and Justin have given here. Do manual focus as well, which will train you eye to monitor what the camera AF is doing when AF is engaged. Experience will familiarize you with the process, and your photography will be better controlled and enhanced.

Miserere Mei , Oct 22, 2007; 07:51 p.m.

Justin Serpico wrote: ...but sometimes walking 2 miles to get to the landscape and walking back with the incident meter just isn't useful.

But only *sometimes*, right Justin? :-D

Justin Serpico , Oct 22, 2007; 07:56 p.m.

Well, I mean if your out there to enjoy nature, why not walk a few extra miles to get a incident reading. Of course, in that time the light just might change a few stops causing you to walk back again.

This is where an eager assistant and some high powered GMRS or Ham Radios might come in handy.

Jim Mims , Oct 22, 2007; 09:40 p.m.

Excellent. Thanks guys, I really appreciate it. I'm doing too much reading and not enough practice. <g> From P&S, to Program Mode, to Av/Tv to full manual. Quite an evolution. I'm concentrating more on Av mode at the moment and I have been experimenting with AE-Lock because I can push the button to "home in on" the correct shutter speed, then adjust from there while I take practice shots and understand what Exposure Compensation really does.

Justin, you talked about EV tonality. I enjoyed reading the "Ultimate Exposure Computer" by Fred Parker (http://www.fredparker.com/ultexp1.htm)

That helped my understanding but left me wondering how much photographers use it. Does it help to develop a "feel" for which EV value to use in different situations from an Exposure Value Chart and compare that with real recorded values for aperture and shutter speed, keeping the chart with you all the time? Or is it better to use the intelligence of the camera and develop a personal feel for what and where that mid-range tone should be metered from?

Thanks Mel, that's exactly what I was talking about, when set to aperture priority, the shutter speed being displayed was all over the place just from slightly moving the camera. I think with some more practice I'll get much better at understanding what exposure works best for the shot at hand.

A friend suggested a fast prime lens to use as a learning tool, like you also mentioned, Michael. I have a Pentax 50mm f/1.4 on the camera now and, judging by Hin Man's photo's, the extreme depth of field control will cement in my mind how to control and react to aperture.

My Samsung can be set to use 1/2 or 1/3 stops in it's configuration. I'm thinking it would be easier to deal with 1/2 stops right now, to make it simpler.

- Jim

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