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Theory n Technique

The Basics of Light Metering

The Basics of Light Metering Ever since my post on Exposure Compensation (and Exposure Value), I’ve been meaning to write this post on light metering, and how it works, to give you all a more complete view of how to get the exposure right using every tool available to you in your camera. Read the exposure compensation post together with this one and I think it’ll combine to make a lot of sense In the exposure compensation post, I wrote about how the camera’s light meter does a pretty good job in most instances, but how in some more extreme scenarios it can be tricked by difficult lighting conditions; I then showed you how to work around these inaccurate exposures by using exposure compensation – but here I am trying to show you how to fix the metering itself! Making sure your meter gets the right reading for the scenario is the best way to get consistently good exposures. But to get the most out of your light meter – be it your camera’s built-in light meter or an external one – you need to understand how light metering works. Understand this, and you will know the right metering technique to use for the situation, and you will get the perfect exposure for every single situation, every time! Or at least, you’ll get it pretty close! TIP: In some instances, it might be faster to simply meter and compensate, rather than changing your metering technique…so as always, use the right technique for the job…with a bit of common sense! Quite simply, there are two main techniques used to meter light. I’ll go through those to begin with, after which I’ll cover the common metering modes available in most digital cameras. Alright? Alright!

Incident Light Metering vs. Reflected light Metering

Maybe you’ve heard of these terms before. Maybe not. Either way, these are the two main methods you can use to meter light for your photograph. If you have never used an external light meter before i.e. you’ve only used your camera’s built-in light meter, then you’ve never metered incident light before. Ok, here’s what they are, how they differ, and when you should use what! Incident light is light that falls directly from your light source, onto your subject. Reflected light is light that reflects off your subject and towards the camera. In any situation, both reflected and incident light will be present – you just need to know which to meter for your shot, and how exactly to do that Look at the basic ‘diagram’ below to get a better idea of what I’m talking about…it’s very easy to understand

I know - not the most professional-looking diagram ever drawn - but you get the idea! That's reflected vs. incident metering (in MS Paint!)

I know – not the most professional-looking diagram ever drawn – but you get the idea! That’s reflected vs. incident metering (in MS Paint!)

Metering Reflected Light

This is pretty easy. And this is how you’ve been metering light from the beginning, even if you didn’t realize it. Most often, the camera has a built-in light meter, and this is used. Just point your camera at the subject, and the built-in meter detects the light being reflected off the subject, and tells the camera what exposure to set. Easy. If by some chance you’re using a really old camera with no built-in meter (like my 1950s rangefinder), or you don’t trust your camera’s light meter, and you decide to use an external handheld meter, place it beside or above your camera’s lens, and point it directly at the subject. This also measures the light being reflected off the subject and gives you the proper exposure

Metering Incident Light

This one’s pretty easy too – however, it requires the use of an external light meter. You can’t do this with your camera’s built-in one. To meter incident light, place the light meter beside the subject, or right in front of it, point the meter directly at the camera, and measure. If you’ve seen a model at work in a studio, and you see the assistant guy holding a device under the model’s chin – that’s the guy measuring incident light falling onto the model’s face

And that’s how metering incident light differs from metering reflected light. Pretty straightforward, right? Yeah. But when to measure incident light and when to measure reflected light? Yeah, that’s what you need to know. I’ll go through both techniques and try to help you decide how and when to use either technique

Most often, you will be metering reflected light. Why? Because your camera’s built-in meter can only measure reflected light, and that’s what you use most often, isn’t it? Indeed. This works fine for most scenarios, and with a bit of exposure compensation, you can get very good exposures pretty consistently. However, this doesn’t work perfectly – not all the time. Why? Because all subjects do not reflect light the same way. White objects reflect more light, dark objects reflect less, and everything in between reflect varying amounts of light. Because of this, reflected light measurements can be thrown off by subjects with different colors, contrast, tones, and so on…and in extreme cases, the meter can get it completely off. Remember my exposure compensation post, where I showed you how the built-in meter (measuring reflected light) finds it so difficult to get the right exposure when shooting predominantly pure white/pure black images? That’s what I’m talking about

Due to this problem of different objects reflecting things differently, reflected light meters are made to treat all subjects as one color: neutral grey. This neutral grey is supposed to reflect 18% of reflected light – it’s often known as 18% grey – and is the standard upon which all reflected light meters are based on. These meters are designed such so that correct readings can be achieved for average subjects in average lighting conditions. Note the word “average” – it works in average conditions, for average subjects, but the camera isn’t smart enough to detect different colors and tones and treat them differently as it meters. So while it works in average conditions, it tends to falter as the conditions move away from average towards extreme. Look back to the exposure compensation post of mine, where my example shots of pure black/white subjects both ended up looking a mid-toned grey. Makes sense now, doesn’t it? The meter thinks the pure white subject is a grey that’s too bright, and underexposes till it looks grey. With the black subject, the meter thinks it’s a grey that’s too bright, and overexposes it till it looks grey. That’s how it’s designed! Before you start to think that this is stupid…think again. It works very well for most conditions you could think to throw at it – it just might not work in some of these extreme cases

In these extreme cases, it makes sense to measure incident light, using an handheld external light meter. This measure the light falling onto the subject, and not the light reflected off it. What does this mean? It means that it doesn’t matter what the subject is and how reflective it is. The light meter reads the light falling onto it, regardless of the subject, and gives you an accurate exposure all the time! No compensation necessary

Measuring reflected light would give you fairly accurate exposures, but could give you varying exposure readings for different subjects, in the same light. Measuring incident light will give you an accurate exposure reading for any subject, all the time. Therefore, if you want the most accurate exposure, along with accurate tone and color reproduction, meter incident light!

Alright, so it seems like incident light reading is the best, and reflected light metering is lame, right? So why bother with reflected light at all? You want the best exposure all the time, right? Why bother with reflected light then? Well, unfortunately, incident light metering is not always practical. It’s not that you might not own an external meter – although that might be the case too…I didn’t have one for a while  – but the real fact is that you might not have the time to go up to the subject and take an incident light reading. You might not be able to REACH the subject to take an incident light reading.  Stop and think about it for a bit now: how do you plan on taking an incident light reading when shooting a landscape, when the subject could be anywhere between 1 to 100 miles away? Or when shooting a car flying around a racetrack at 100mph? You see? This technique of light metering is fantastic, but it’s just impossible sometimes. In such a situation, your only option is to use reflected light metering – and don’t fret…reflected light metering has the ability to get very accurate exposures too, like I already mentioned…and with a bit of exposure compensation, it can be spot-on!

To summarize these two techniques: incident light metering is a more accurate form of metering light. It is not affected by the subject’s color and how much light it reflects, it reproduces colors and tones very accurately, and I consider it a better way of metering light. However, this requires an external light meter, and it also needs you to be able to go right up close to the subject to take your reading. Not always possible

If it isn’t possible, reflected light metering is your other option. It may seem a bit unpredictable, compared to incident light measurement, but it works well most of the time. Looking at my work, I realize that more than 80% of my photographs have been taken after measuring reflected light. If you’ve never used an external light meter before, all your shots have been metered by this method! It just struggles a bit in extreme conditions, where incident light metering doesn’t struggle, but that’s about it. Use it. If you find it not working for you, use exposure compensation – or maybe try a different metering MODE

Camera Metering Modes

Cameras, and camera-makers, aren’t stupid. They know that reflected light meters can fail. They know it’s not the best form of metering there is. So they include a couple of metering modes in your camera, to get the most accurate exposure even in difficult circumstances. Nice of them! Some people have probably never changed their metering mode from the default one. The default one is usually the one that’s really designed for average subjects in average light. If it goes a bit more extreme, this mode will immediately struggle. However, there are a few other options to try, even when using your built-in light meter. Know what they do, how they work, and you’ll know when to use which mode, and get a good exposure without any trouble. Here’s  a few modes that I’ve found exist on all my digital cameras:

Evaluative/Matrix/Average/Zone Metering Mode

This the default metering mode I was talking about. Based on your camera brand/model, it’ll be called one of the above names. This mode works by measuring the light reflecting off objects in the entire frame. It splits the frame into a matrix of different ‘zones’, evaluates the light in each of these zones, and averages them out to give an overall exposure that should cover the entire frame and get everything looking as close to perfect as possible! Note how I used each of the titles (evaluative, zone, etc.) in my description! Wasn’t easy!

The part highlighted in grey is metered. Yeah, pretty much the entire frame is covered, isn't it? The boxes indicate the zones. It's all put together, evaluated and averaged out to give a correct reading - Image from canoncompact.com

The part highlighted in grey is metered. Yeah, pretty much the entire frame is covered, isn’t it? The boxes indicate the zones. It’s all put together, evaluated and averaged out to give a correct reading – Image from canoncompact.com

This mode basically puts no priority on anything – it aims to get everything properly exposed. Use this mode in average lighting conditions, for average subjects. Yeah, I keep saying that a lot about reflected light, but it’s especially true for this mode. Put something a bit outside the ‘average’ box, and it’s going to struggle. Use this mode also when you think everything in the frame is important and should be properly exposed. I use this mode quite a lot

Center Weighted Metering Mode

This mode works very similarly to the above one. It measures light reflected off the entire frame, but unlike the evaluative/zone metering mode, this mode measures the scene, but gives priority to subjects in the center area of the frame. Hence the title ‘center weighted’ metering

Again, the grey-shaded area is metered. The entire frame is shaded lightly grey, if you look closely, with more focus on the center - Image from canoncompact.com

Again, the grey-shaded area is metered. The entire frame is shaded lightly grey, if you look closely, with more focus on the center – Image from canoncompact.com

When you have something of importance in the center of the frame, use this mode. The camera will put priority on that area of the frame, while still trying to get the rest of the frame looking right too. It could be a useful mode, but I can’t say that I use it much

Spot Metering Mode

This mode, the last I’ll be discussing today, measures light from a very small ‘spot’, right in the center of the frame. This little spot usually covers just around 3-5% of the entire frame, and is the only area the meter uses to measure light. It disregards the rest of the frame. Spot metering really comes in handy when you’re shooting a scene with objects of vastly different brightness, and where you want 100% priority on one particular subject, and nothing else. Just point the spot at that particular area, and meter. That’s it. Do I use this mode often? Yes

Only that little spot in the center is shaded grey. That's the only bit used to meter the scene - Image from canoncompact.com

Only that little spot in the center is shaded grey. That’s the only bit used to meter the scene – Image from canoncompact.com

Once again, I go back to the exposure compensation post. Remember the backlit subject in that post, the camera in front of a bright window? I used exposure compensation to fix that, right? Well, maybe I mentioned it back in that post (maybe I didn’t) but the alternative solution to that problem would have been to spot meter. What does that do? It meters light only from the area I want – so I point the little spot directly at the subject, in this case the camera in the window, and meter. That’d give me an exposure that is right just to get the camera well exposed, completely disregarding the bright backlit window area. Using that exposure, I’d get a result where the subject looks perfect, even if the background doesn’t. See the sample images below:

First shot - evaluative metering - shows how the camera tried to get the entire scene right, but failed to exposure the subject properly. Second shot - spot metering - ignored the background, but got the subject properly exposed

First shot – evaluative metering – shows how the camera tried to get the entire scene right, but failed to exposure the subject properly. Second shot – spot metering – ignored the background, but got the subject properly exposed

To give you a complete idea of metering, and how to use various techniques…think of the options you have for this backlit scene. You could use reflected light metering and the evaluative mode, and compensate by a great deal, to get the right exposure. You could use spot metering to get the right exposure. What else? Incident light metering! Measuring the light falling onto the subject, and using that reading to expose the shot, would’ve got the perfect exposure too! It measures only the light falling onto the subject, and ignores the loads of backlight. Makes sense, right? I hope so! Your camera might have a few more metering modes, but these are a few I can be sure all cameras would have, and in any case, these are the most useful. I usually have my camera set to the evaluative (default) metering mode, but I often switch to spot metering in certain circumstances. I also use my Sekonic L-308s handheld meter if it’s practical, and use the readings it gives me on my camera’s Manual mode

Alright, that’s it for this post. Two basic techniques…reflected and incident light reading…and a set of different modes that you can use if you’re using your camera’s built-in reflected light meter. That’s all. Pretty basic. There’s a lot more to metering, and a lot more techniques – especially if you’re shooting in a studio with many lights, with flashes and strobes, and all that – but this should be enough to get you started, right? If you can get yourself an external meter (check out the L308S or L208 from Sekonic, the industry standard for light meters), use one, and compare the results it gives with the ones you get using your built-in meter. If you can’t get an external one, try out the different metering modes of your camera, in different conditions, and experiment. After knowing how each one works, and after having experience with these different modes, you will be able to better judge when to use which…and along with a bit of exposure compensation, you will soon be getting the perfect exposure…consistently! Affiliate links – Buy your light meter here! Sekonic Light Meters on Amazon Sekonic Light Meters at B&H Photo I recommend the Sekonic L-308s that I own (and reviewed) If you have any tips, tricks…anything at all…to share with us here, please leave a comment. Same goes for questions. Subscribe if you like the stuff you’ve been reading here! RSS or email works. And if you’re buying a light meter, buying from my affiliate links will be much appreciated! Until next time Did you know that I’m currently working on this site full-time? Please consider making a small donation if you can – thank you!

By Heshan Jayakody
All content here is my own, except where noted
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Discussion

20 Responses to “The Basics of Light Metering”

  1. Great great article. Keep up the good work :-)

    Posted by Torben Christiansen | January 25, 2013, 14:14
  2. Nice work! I enjoyed this post – especially the incident and reflected light measuring bit. I knew a bit about the different modes, but I find it very interesting to read about incident light measuring

    Posted by Chris | February 3, 2013, 09:39
  3. Thank you for sharing this!! The incident/reflected light metering area is something I was finding hard to understand before I found your post. Your diagram really helps! Cheers

    Posted by Anna | February 3, 2013, 15:15
  4. I learned somethings new today! Thank you for this!

    Posted by Conrad | February 3, 2013, 17:57
  5. terrific stuff. really good information here. thanks for sharing! i always wondered why people use external meters and all that when their camera has a perfectly good one built-in! now i know

    Posted by Robert | February 19, 2013, 07:52
    • Thanks Robert! Yeah, external light meters measuring incident light is a very accurate way of measuring exposure! but you already know that – haha. Cheers!

      Posted by pixelogist | February 19, 2013, 08:11
  6. Thank you for the information. I was wondering if the specs of the Sekonic L-758DR comparable to the Cl-200A Chroma Meter ? I’ve recently caught the light meter< bug, but I could definitely use a hand.

    Posted by Natasha Lambert | February 21, 2013, 00:39
    • Ni Natasha – i’m afraid i’m not too familiar with the Chroma Meter you refer to, but from what I have read about it, it seems to be quite a fancy one, capable of not only measuring the luminance of light but also color temperature and other aspects of light. I guess it depends on what you want: if you need to measure all this, you could check out the Chroma…but knowing much more about the Sekonic one you mention, I can definitely recommend this as being one of the best light meters you could get your hands on. Sorry if i couldn’t answer your question directly!

      Posted by pixelogist | February 21, 2013, 06:19
  7. I learned a lot by reading this post. I have a compact camera, but I know that it has these different metering modes, and only now I know how they work and when to use them. I have a feeling this is gonna help me improve my pics! Thanks

    Posted by Nathan | March 7, 2013, 07:47
    • Metering modes are there on every camera, yes…even on your mobile phone, if it has a decent camera and camera app :) I’m happy this post helped!

      Posted by pixelogist | March 7, 2013, 12:42
  8. Another really helpful post that helped me – reading this, plus my camera manual, and I was shootign like a pro! Hahah

    Posted by Yod | May 8, 2013, 15:35
    • Good point – always read the manual! :D Learn the theory, learn the techniques, but learn how to apply them to your specific camera brand/model too! Thanks for stopping by

      Posted by pixelogist | May 8, 2013, 18:13
  9. I’ve been using a mirrorless cam for a few months now, and I thought the camera just does all this automatically. I never knew there was so much I could add to the process! Wow…

    Posted by Wendy | June 24, 2013, 07:08
    • There is quite a lot to learn about metering, indeed! These are just the very basics…using an external meter, flashes, strobes, even natural light…things can get much more complex! But these basics should do for now :)

      Posted by pixelogist | June 24, 2013, 09:36
  10. Wow, I learned a lot here. Just wanted to stop and say thanks for takin the time to write all this…it really helped somebody out, hope you realize that! :) Cheers from Italy

    Posted by NV | July 17, 2013, 19:27

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