How does a light meter work?

Discussion in 'Pentax' started by jim_mims, Oct 22, 2007.

  1. 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

    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?

  2. mountainvisions

    mountainvisions Moderator

    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.
  3. 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?
  4. Jim,

    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
  5. mountainvisions

    mountainvisions Moderator


    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.
  6. Justin,

    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... :)

  7. 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.
  8. 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
  9. mountainvisions

    mountainvisions Moderator

    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.
  10. 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 (

    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
  11. I hate Matrix, or auto-everything metering. They give no sense of understanding light. Not all scenes are 18% gray, and not all scenes can fit in the Matrix bucket. Not all scenes can be even judged with reflective metering... but your brain can adjust and compensate.

    In general, a light meter functions as a reverse-light bulb: It sucks light and outputs current. This has nothing to do with 18% gray, or even with ISO settings. A light meter gives measures in EV, or similar empirical standards.

    The best way to learn how much EV is in a scene, is to get an ambient light meter. You will have to be close to your subject to use such a light meter, because it measures the actual light intensity that falls on your subject. Although this severely limits your light-metering capability of measuring exposure for far away subjects, this typical metering is not fooled by your subjects color, reflectiveness, or any other composition features that repeatedly fail "intelligent" exposure algorithms.

    Most people who master this type of light measurement, get extremely acquainted with light, and light features. This is not rocket science, no matter what "Matrix" metering freaks try to tell you. Most scenes fit within a limit range of EV's, and all you need to do is simply try to fit your camera's EV range within the that scene's EV range. That is all what exposure is about. Most people know for example, that most sunny days is 15-16 EV; that is all you need to know, and it doesn't matter if there's snow outside or if you're standing on black lava.

    This of course, renders most modern camera exposure modes unusual. I wish they had made a handheld ambient light meter that could transmit readings to a nearby camera (bluetooth?) but obviously, most camera designers have nothing to do with actual photography...
  12. "This of course, renders most modern camera exposure modes unusual" -- my typo, I meant unusable.
  13. mountainvisions

    mountainvisions Moderator


    I really recommend getting charles campbells backpackers guide to photography. from the title you can guess it's about outdoor photography but it covers every aspect of shooting and the really important part of it is understanding tonality. he explains it really well. and since the book is about 15 years old I bet you can find a used copy of it somewhere for a few bucks.

    Petrana, I disagree. Reflectance is everything, afterall your film records reflectance. seeing things in tones is the key to understanding exposure. From that point you can learn to make sense of reflectance values. Honestly, the B&W function on a LCD will help with this as well. Take the color out of a scene and tones become more clear.

    Also, I disagree about understanding exposure, I have a good grasp and all from reflectance metering. Really it's quite simple. You want your mid tone to be mid tone, you want your exposure to fall within the film/sensor latitude. spot meter off the highlight, spot meter off the shadow, spot off the mid tone. If you have over 2.5 stops over mid tone add filtration, or let the shadows go black, if you have less than 2 stops over and less then 3 under you have a unfiltered scene. obviously scenes with extreme tonal range throw curve balls but it boils down to adding filtration, adding light, or coming back later in the day. Adjust based on artistic needs (ie, black shadows are wanted, under exposure a bit and/or add some fill flash to the mid tones to keep the mid tone in range).

    Outside of a studio I've never understood the fascination with abient meters. Nor have i understood why people feel the in camera meter is not useable and must be supplemented by a handheld meter. Seems like the built in light meter is a solid part of the 35mm system keeping things simple, compact and portable. With digital even less so. Even with simple flash photograpy the ambient meter is going the way of the dodo. Take a test shot, look at preview with the flashing highlights, adjust aperture or flash output. All for the price of a DSLR. Amazing.
  14. Wow, that is an old, old controversy. When in-camera meters were first introduced way back in the 50's or 60's, for many years pro photographers turned up their noses at such an "amateurish" device, as they now do with having a built-in flash. But after so many fine photographs were produced using the new metering system, it was generally accepted, once more photographers became accustomed to the way the meters work. The main thing is they do work well when used correctly.

    But there are still some who prefer to carry separate meters. Like Justin, I have felt no need for that. I am sure Petrana's method works well too, for her purposes, although I think it impractical for may other situations.
  15. "B&W function on a LCD will help with this as well" -- Justin I respect your methodology, to each his/her own. I myself use the LCD on my DSLRs for one purpose only: hitting the "FORMAT CARD" menu option...
  16. "I wish they had made a handheld ambient light meter that could transmit readings to a nearby camera (bluetooth?) "

    Now, there's an interesting product idea! Seems do-able and not terribly expensive.

    Several people have made the statement that none of the metering modes is perfect. No one has mentioned one of the principle reasons for this: the meter has no idea of the photographer's intent. While the meter may make a good estimate and produce a perfectly recognizable image, the photographer may have intended to have highlights washed out or to have shadow detail fade to black. This is why it is imperative that a serious photographer truly understand exposure and the limitations of his/her meter. Only then can you translate the image in your mind to the one that is captured by your camera.

    Paul Noble
  17. mountainvisions

    mountainvisions Moderator


    I used to agree totally about the LCD. And for the most part still do. But I also realize if there is a tool in something why not use it. Digital isn't film and being rigid in shooting digital won't make it film, why not use all the features to your advantage.

    I typically don't review all my images. Actually, when I'm in the field I suspect I review as little as 10% of my shots, and often 0% when on the move. When shooting sports I tend to review more to erase the clearly missed shots right off the bat. No reason to lose 20 images per card.

    I was referencing the B&W feature as a teaching tool. Taking a black and white preview allows the scene to be broken down into tones only. Explaining tonality to a new photographer in color is ridiculously hard. After all, a normal eye sees colors, and lots of them. In B&W the scene is only seen in tones. Very useful teaching aid when explaining tonality. And the best part is that it's free, since the feature comes with most cameras these days.
  18. Another most useful feature in modern DSLRs is the RGB histogram. I find that invaluable, and would certainly not shun its use just be a "pure" photographer. Being rigid does not make you a better artist; using all the tools at your disposal to create the photo you want, quicker and better, does.
  19. I'll second Miserere on the histogram point. I check my LCD after almost every shot, but not to review the picture--I check the histogram to make sure the exposure came out the way I wanted it.
  20. A friend let me borrow his stack of Outdoor Photographer magazines and I found a good article that dovetails with what we're discussing here:

    - Jim
  21. "I check the histogram to make sure the exposure came out the way I wanted it" -- This is something I don't understand... What does the histogram have to do with how you want to expose an image? One may want to use the histogram and make sure nothing is being clipped, but if your intention is to clip-- what's the reason of using the histogram at all?

    I must admit, when I have started shooting digital (~2002), this was a nifty gadget to check on exposure. I usually shot a few test shots at the beginning of a location-shoot, then simply locked the exposure and forgot about the auto-everything modes... I'd rather keep my eye on the composition than look at the rear screen (which is always too small, and never usable in day light anyway).
  22. Jim, thank for the link.. However..

    "If the histogram doesn?t go all the way to the left, the image is overexposed and contains no true black or very dark tones. If the histogram doesn?t go all the way to the right, the image is underexposed and contains no white or very bright tones. A well-exposed image will have a histogram that goes from one edge to the other, even if most of the tones are to one side or the other (unless, of course, you?re shooting a very low-contrast scene, such as a foggy pier shot that contains no dark or light tones)."

    Again, I disagree with this observation. Why does one believe that you need to FIT the exposure into the histogram? there is no BAD exposure-- The right exposure is a product of the photographers composition. If I choose to clip my blacks, does that mean I mistakingly "underexposed" my image? It just so means that compared to the average 18% gray, this exposure is darker.
  23. I agree as well, Petrana. When I say that I'm checking to see if my exposure is how I want it, I just mean that I double check to make sure that I didn't WAY overexpose or WAY underexpose, unless that was my intent, of course. I'm still a noob, and still getting a feel for the whole subject of exposure (just now starting to use the spot meter instead of full-auto).
  24. mountainvisions

    mountainvisions Moderator

    I disagree again...for instance if I'm shooting hockey my histogram falls all the way right, guess ice is featureless and my white jerseys have no texture. Nothing short of extreme photoshopping every photo is going to fix that.

    Yeah, the histogram doesn't actually tell you if the exposure is correct but generally, in color photography, blown out highlights are a no no. So keeping it in off the extreme right is a good thing. And while you can generally lighten a photo a little bit since the shadows tend to have more latitude, you cannot "recover" something you never had with digital or slide film.

    Actually though. I prefer the flashing highlights/shadows...this is much more accurate in telling me if I got the general exposure I was aiming for.

    Of course I never agreed with the expose to one side of the histogram either. I believe every exposure has it's own needs (which I believe Petrana is essentially saying) but generally avoiding blowing the hightlights in digital or slide film is a goal.

    Petrana, one thing to remember, and it took me a while to understand this myself, at this juncture of the digital age, there are many people who never shot a film camera. There are kids who are now in their teens who no more about photoshop than the "professionals" teaching it. So you have to be really conscious of the fact that digital changed the way photography is done. I was looking at a guys work recently, and he admitted he never shot a roll of film, he went on to say he's have no clue as to how even use a film camera. His images were beautiful and virtually straight from the camera.
  25. Jim: I think some of your confusion is about simply how the hardware in your GX-1S works, AE-L in particular. In general this can all be observed by operating the camera without even capturing any images.
    When in an auto-exposure mode (P, Tv, Av), AE-L "freezes" the metering at whatever EV level is being detected at the time the button is pressed--otherwise, the exposure settings are constantly being adjusted to be the camera's guess at "correct". So if you're pointing it into the sky, as far as the camera's exposure logic is concerned, you're still pointing it into the sky after you re-compose.
    In manual exposure mode (M) on the GX-1S, AE-L implements "HyperManual" (like with the green button on K10D/GX-10). In manual mode, the exposure settings (shutter/aperture/ISO) are always locked, only changing when you adjust them yourself. AE-L (HyperManual) is a shortcut to instantly change these settings to the camera's guessed "correct" settings (like an instant 'burst' of auto-exposure) based on what the camera is pointing at when you press the button. The custom setting for HyperManual (AE-L in M) that you describe defines what kind of adjustment it will make--either only shutter, only aperture, or both aperture and shutter. Another way of looking at this is that when using Manual exposure mode, it is always "locked", except when pressing the AE-L button briefly unlocks it!
    I don't have my DS2 with me at the moment, but I'm trying to remember if EV Comp is even functional in M mode. Typically in M mode, if you want to overexpose by one stop, simply adjust the shutter (or aperture, or ISO) so that the meter reads +1 and shoot. As already noted, a shortcut for this would be to 1. press AE-L, 2. turn e-dial two or three clicks (depending on custom setting that specifies 1/2EV or 1/3EV adjustments) until meter reads +1, 3. shoot.
    So--AE-L is not necessary in 'M' mode, it is simply a shortcut for convenience. AE-L is important and useful when using P/Tv/Av AE modes, though not completely necessary there either as you can also use EV Comp to alter the automatic exposure. However you will need AE-L if you're going to try to apply techniques such as those described in Peterson's book while using AE (P/Tv/Av) modes. With these modes, that's the only straightforward way to meter and shoot two different subjects.
  26. You nailed it Andrew.

    And you're right, EV Comp is not available in Manual mode.

    Like you said, in manual mode it sounds like AE-L(ock) is useful in getting quickly to a correct exposure, then manually adjusting it from there.

    And right again, as I learn with the different modes, AE-L will once again prove useful by "freezing" what I'm seeing and giving me an opportunity to think about it.


    - Jim
  27. Hope that it helped. I saw that the thread seemed to have wandered off into arguments about using the LCD, types of light meters & all sorts of other stuff you didn't ask about. While I am personally comfortable with the camera features, I also dug out the Peterson book again for a second read recently, hoping to learn to use them more intelligently to get things right on the first try more often.

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