by David Tong
Figure 1: A Pretty Average Scene
After writing my previous articles discussing exposure (here and here), I received a couple of emails asking me “what is metering and how do I use it?”, and it made me realize that while the concept is simple, it’s not as easy to understand with all the jargon that goes with explaining what metering is.
Virtually all 35mm cameras that were sold after 1980’s have built-in meters. The built-in meter helps the user and/or the camera’s sensor to determine what exposure settings (shutter speed and aperture) the camera should use to obtain a “proper” exposure. As mentioned in my previous article, the camera’s meter can be pretty accurate for “average” scenes where the bright, middle, and dark shades of the scene are pretty well distributed, like the image above.
The image has bright areas (clouds, sky, and the light building), a lot of midtone (the front building, the road, the trees), and ample dark areas (the areas under the elevated highway). That is a pretty average scene, and most of our photos do fall under such lighting conditions, which makes sense to rely on automatic metering.
However, there are ways to “fool” the camera’s meter.
As I mentioned previously, the camera meter determines the midtone in the image, then averages everything out, and depending on the metering mode used, where you point your camera can yield dramatically different exposures on the same scene. Let’s use visuals to explain further. (The images below were shot indoors, just in case you see any glass reflections)
This is a gamma chart that shows the gaps between dark and light areas of an image. The whole area represents what your camera can see between black and white points, and the middle swatch is midtone.
Image from millercanada.com
Image 1: Original Image – Auto Exposure via “Evaluative” Metering
Figure 2
The red dot represents the midtone value of the scene, that’s the halfway point of our histogram represented as the dot in our graph. If we remove all the colors, this is what you’ll see.
Figure 3
The RGB values in the Info palette indicate R127,G127,B127 for that same spot (if 0 is black, and 255 is white, midtone would be 128, so we’re pretty close).
What would happen if we meter somewhere else? Let’s say we want to get as much details under the highway (the shadow areas) as possible. I pointed the center of my viewfinder towards the dark area under the bridge (red dot again), half-pressed the shutter button to lock the exposure, and then recompose to take the same shot.
Figure 4
You just told the camera that red dot is midtone, as a result, you exposed for that area “properly” while the rest of the scene is averaged. The result is more details in the shadows, but no details in the areas that are much brighter than the area you metered. Again, if you desaturated the image, you’ll see that the metered area is “midtone”, similar to Figure 3.
Figure 5
Metering for shadows is also known as “preserving shadow detail” or “sacrificing highlights”. Note that the histogram is leaning heavily to the right.
If you use the gamma chart as a basis, you shifted the middle grey swatch 3-4 slots to the right, and you lost 3-4 slots of bright image details as it falls outside your camera’s exposure range already.
Conversely, you can do the opposite, preserve the highlight details while sacrificing shadows. You’re now shifting the middle swatch to the left (blacks).
Figure 6
Note how the histogram is now leaning towards the left of the image. We metered the bright cloud (indicated in red) and by doing so, you told the camera’s meter that “that part should be midtone grey”. After desaturating, you’ll see that the cloud area now has a similar midtone RGB values as the previous two “metered” areas in Figures 3 and 5.
Figure 7
We can clearly see more details in the clouds, but the dark areas are now void of details due to the shift in exposure scale.
I decided to shift the original image’s exposure slightly higher to bring out more shadow detail and sacrifice just a little cloud detail for my final shot.
Final Shot
You, as the photographer and camera operator have the final say of how much detail you want to retain and sacrifice in the scene. You can instruct the camera where the “midtone” is in your image, not the other way around.
Found this article useful? Subscribe now for more updates!
October 14th, 2008 at 5:29 pm
hi david! thanks for sharing this. i have also read your other articles and learned a lot. more power to you and i hope you won’t get tired writing for others who want to learn the hobby.
October 14th, 2008 at 9:42 pm
You’re most welcome
Please subscribe so you won’t miss any new articles
October 22nd, 2008 at 11:23 pm
Hi david! The articles you’ve written here on this site are short but packs a lot of information. Very helpful for beginners like me who finds it difficult to browse thru tons of articles on the net. I just have a question though, in here you used evaluative metering, what about the other 2 types, in what lighting conditions would it be better to use them? Or should we just stick with evaluative?
October 23rd, 2008 at 12:07 am
Thanks for dropping by, Ritchie.
I chose evaluative for most of my images in this blog because I think it has the least amount of user-induced variables compared to average, center-partial, or spot. Evaluative analyzes the whole scene and a lot of camera-computing is used so it takes my human metering errors or preferences out of the equation.
A smaller area of metering (basically anything other than matrix/evaluative, really) is preferred if the contrast is high. High noon, back-lit, shooting someone wearing predominantly white or black, shooting at night, shooting in front of a dark or white background, etc.
Scenes where the “bulk” of the tones are either too bright, or too dark.
Over time, you’ll pretty much know how your camera’s meter behaves. Whether it usually over or under-exposes a scene when the indicator is pointed at “0″. Stick to a single mode first, and every time you take a shot, check the histogram. If there’s always an empty gap in the left of the histogram when your indicator points at “0″, then your cam has a tendency to over-expose, the reverse applies if your cam prefers to under-expose.
October 23rd, 2008 at 6:34 pm
Thanks for the detailed response! I’m bookmarking your site. *click*
December 7th, 2008 at 4:35 am
Again, a very informative subject which you tackled very well. Thanks again.
December 9th, 2008 at 3:28 pm
hi david! i stopped shooting around 1995 and now i’m back to this hobby again… your articles are very informative and they help me refresh my knowledge because i’m trying to learn again the things i have forgotten about photography…
December 9th, 2008 at 4:21 pm
Jigs, nice to see you here bud…
December 15th, 2008 at 12:21 am
So, How Do I Meter? I still do not understand after reading this. This article is speaking still far away in the sky than on practical thing how do metering in a real camera. How do I metering in my Canon PS A62? What procedure and menu, setting I shall do or press?
December 15th, 2008 at 12:44 am
Tascas: You half-press the shutter button to lock meter, that’s how it is with virtually any auto-exposure cameras since the 90s. Refer to your user’s manual.
December 19th, 2008 at 7:39 am
Sir, I am regular reader for your site for past couple of weeks….and it is simply great.
So please pardon me, as the first link you provide in this page, may contain some error….some extra “http://”
I think it must be “http//reviews.davidleetong.com/?p=255″ rather than “http://http//reviews.davidleetong.com/?p=255″
Sorry for pointing out that………..Please don’t get me wrong.
December 19th, 2008 at 7:42 am
*Sorry the correct one is :-
http://reviews.davidleetong.com/?p=255″
December 19th, 2008 at 9:26 am
Thank you so much Arijit for pointing that out!
Much appreciated 
January 29th, 2009 at 4:14 pm
dave, have read almost all your articles. they’re all very informative especially for newbies like me. you must be a pro to come up with these well-written articles. you can hit the nail right on the head with these short articles. congrats dude…..continue writing and more power…
April 27th, 2009 at 10:53 pm
That was some article David, very informative. I now have a better grasp of what metering is. I shall look forward for more of this. Thanks a lot.
April 28th, 2009 at 1:55 am
Thanks
October 9th, 2009 at 1:02 pm
David, we will appreciate very much if you can have an article on spot and center-weighted metering.
December 14th, 2009 at 7:24 pm
Hi Sir, I would just like to thank you for helping me understand the metering process….being new to photgraphy, i often get lost even with the my manual as well as the tons of hits google provides…. just so I got it right, if I want my subject’s features defined (without it looking like silhouette) with the sun behind it, i would have to find the midtone first, lock it, then recompose and then take the shot?… did I get it right? thanks again
December 14th, 2009 at 11:43 am
Hi Martin, it's my pleasure to provide these articles to everyone. Regarding your question, yes, you got it right, if you meter on your subject's face by either using your camera's spot metering or center-weighted metering, or physically walking up or zooming into your subject's face and take a reading, you'll get a more proper exposure for your subject.
I got some other articles here if you're interested. http://www.howitookit.com/category/tutorials/basi...
December 15th, 2009 at 1:43 am
awesome Sir David! I will read through your articles and reply to some if I have some clarifications which may be far in between since your approach is really simple, direct and practical. Thanks again and more power.
PS, if you have some email group or updates, it would be much appreciated if i can subscribe to it…. thanks again
December 15th, 2009 at 8:06 am
Hi Martin, there's a subscribe via RSS or via Email on the top-right of this page