*Metering and Exposure Compensation. Which, when, why?

Metered off the wall…

Metering for a particular scene can be tricky.  The goal, in most cases, is to expose for your subject, telling your camera what it should consider the proper exposure for 18%, or “Middle Gray” and adjusting your exposure based on the light that is hitting your subject, or the light involved in the scene.  Most cameras will utilize an automatic, average metering as a default which most of the time will do a decent job at keeping the highlights and shadows in control.  First, before we delve into the different metering styles, we should examine one of the most helpful tools on your camera.  The histogram.

The Histogram for the picture above.

Most modern digital cameras will allow you to view your histogram.  The histogram is a measurement, in graph form, based on the amount of pixels involved in the tonal distribution value between absolute black to pure white (luminance) and in your Red, Green and Blue (RGB) channels.  The vertical axis is based on the number of pixels occupying the luminance value while the horizontal axis, consisting of 256 separate values, is the luminance value from black (value = 0) on the far left, middle gray (value = 128) directly in the middle and pure white (value = 255) to the far right.  The goal, most of the time and generally speaking, is to keep all of the information captured within this range avoiding “clipping”, or losing the information.  Clipping occurs with extreme over or underexposed elements and show by having the info on the histogram touching the right or left, ultimately clipping off of the chart.  If the info stacks up and “clips” on either side, the detail is gone in those pixels.  Whether that is what is intended is up to the photographer, but once that info is gone, it isn’t coming back, or what little does can be riddled with noise.  The way you control this info being recorded from your position behind the camera is by metering.

Depending on the tonal range of the scene you are capturing, it may be impossible to expose properly to include info in the darkest shadows and brightest highlights.  In these cases, one must choose which tonal values are most important to the image.  For instance, if taking a picture of a mountain with the sun setting behind it, it will likely create a dynamic range too great to capture in one exposure meaning the bright, vibrant sky is too much brighter than the areas in the scene not being lit by that light.  If exposure is made for the sky, the mountain will likely come out as a dark silhouetted mass.  If the exposure is adjusted to properly expose for the mountain, the sky may be blown out.  See below…

metered to expose for the sky

The dark mountains and shore are all stacked up on the left.

Metered to expose for the shore and mountains.

The blown highlights in the sky and water clipped on the right of the histogram.

There are ways, through software (which I will get into in a later post), to combine multiple exposures so that the final picture uses the info in the image with the properly exposed highlights and the image with the properly exposed shadows to show the entire dynamic range of the scene.  For now though, we are looking at metering so place a mental bookmark at high dynamic range imagery and follow me over here…

Keep in mind when studying metering, your camera is trying to achieve 18% gray based on what you are metering off of.  This is your ‘mid-tone’ and represented by “0” in the middle of your light meter.  Your camera should allow you to adjust your metering method.  Most cameras will have between two and four different metering modes, and they should be based on these following metering algorithms:

Matrix/Evaluative/Pattern: Taking the whole scene into account, it bases it’s exposure on the the midtone value in relation to the average of the whole scene.   This is usually the default/automatic metering mode.  Most cameras without the ability to change the metering mode will use something like this.

Partial/Average: Allows you to base your metering on a partial portion of the frame.

Center Weighted: It bases the metering on the subject in the center of the frame and averages the rest of the scene into account for the final reading.

Spot: Spot metering takes a small 3-5% spot, usually the center, or spot attached to a focus point, to meter the exposure for just what the spot is placed on.

Which you choose is entirely up to you.  Metering is merely measuring the light falling on or in a subject or scene.  I find that most of the time when I’m shooting outside without any added artificial light source, I use either spot or center weighted.  It is what I have gotten used to.  With a spot metering approach, I can find something on or around my subject that is close to the mid-tone or 18% gray in the scene, depress my shutter button half way which will give me an idea as to how I need to adjust my shutter speed, aperture, ISO (remember the Exposure Trifecta post?) or all three to get that spot to register to the middle of my light meter.  If I’m using an auto mode that will automatically adjust other settings (like Av, or Tv mode will) I then press my Exposure Lock button (designated by an * on my Canon cameras, read your camera’s manual regarding Exposure Lock) which will freeze my settings.  If I don’t press the Exposure Lock button, then when I recompose the image, my settings will change as the camera still thinks that I am spot metering depending on whatever that spot is on.  If you are using (M)anual mode, all you need to do with a spot, partial or center weighted metering mode is place the area that is reading the metering over something you want to meter off of, usually something falling into that medium gray/midtone range, adjust your aperture and shutter speed to get it to the middle of the light meter, then when you move to recompose the frame, your settings are already set so while your light meter might move around based on what the spot, partial or center weighted meter is on, you’re still set up to expose for what you’d metered off of because it isn’t automatically changing either your aperture or shutter speed.  If I’m feeling lazy, or just want to fire off quick shots, I’ll switch to the more automatic Matrix/Evaluative/Pattern style metering mode and not worry about it, keeping an eye on my histogram to make sure I’m not losing info in the shadows or highlights of course.

There are times where you may want to clip or lose info.  Low-key, or High-key portraiture are examples where clipping and losing info can help create a mood.  Here are a couple examples showing extreme under and over exposure.

Using under exposure in a low key portrait.

Using over exposure in a high key portrait.

Which metering mode should you use in different situations?  Well, that depends on your shooting style and desired outcome.  The best thing I can tell you is to explore the differences in how a particular metering mode will “see” the scene in front of the camera.  It all really comes down to understanding how to expose what you want to expose for in any particular scene.

Now that we have a basic foundation for different metering modes, there are situations where you may want to apply Exposure Compensation to the scene.  Exposure compensation basically says that you know your metering is going to take tonal values in the scene into account incorrectly.  A good example is shooting in the snow.  Bright, brilliant whites can turn into murky grays if you don’t compensate.  Because your metering is looking for 18%/ middle gray, and in many snow shots, the white snow provides the dominant tonal value, it can trick the metering into thinking it is averaging the scene when in fact it is just getting so much bright white, it brings everything down to “average” those tonal values out leaving you with an underexposed, murky image that doesn’t do the scene justice.  How do you compensate for something that is being processed as an underexposed picture?  You compensate by over exposing the image based on the idea that the metering is being fooled into thinking that the white is really gray.  Remember the -2..-1..0..+1..+2 diagram we talked about that represented your light meter, yes in that other post?  Each of those numeric values represent a stop.  So, by practice, normally a snow filled scene will need to be adjusted by about +1 to +2 stops.  That would mean that you would adjust your exposure compensation to the right.  The beauty, again, with digital is it is very easy to adjust on the fly.  Shoot, check, adjust, re-shoot.  Easy as pie.  If you want the image, or area being metered to become darker, set your exposure compensation to the left (underexpose), and to the right if you want to see the image or area being metered brighten up (overexpose).

 

Remember that the “0” on your light meter is wanting to achieve midtone or 18% gray (the very middle of the histogram).  When shooting certain scenes, you can use a couple quick mental notes to adjust your Exposure compensation that have helped me…

If metering off of snow (or a bright white wedding dress, etc), add +1 1/3 to +2 stops

If metering off of average caucasian skin tone, add +1/3 stop

If metering off of darker skin tones, subtract -1/3 to -1 stop

If metering off of green foliage in even light, subtract -2/3 to -1 stop

 

With all of these it is useful to check your histogram once you capture a shot and make sure everything you need is staying within the range.  Another trusted and true method is to shoot an 18% gray card.  To do so, simply spot meter off of the gray card itself when that gray card is being illuminated by the same light that the scene or subjects will be illuminated by, set your exposure so that your light meter shows the proper reading at “0” and viola!

 

Exercise Time!

Feel free to play along.  If you’re not familiar with how to view your histogram on your camera, or change your metering, check in with your camera manual.  It’s funny that as intuitive as things are now-a-days, they are so much more complex.  I’ve read each camera manual from cover to cover for each camera I’ve ever had.  It may seem as if I have no life when I admit this, but really it takes an afternoon, or Sunday morning with some coffee and I get a much better grasp on how to manipulate my camera.  I highly suggest at least keeping it handy to refer to when needs arise.  Okay, lets start with the metering.  If you have a tripod, grab that too.  Find a scene.  It can be a room shot, or in my case I chose the back yard.  Set your camera to it’s Aperture Priority mode and stick with one aperture setting.  Find a scene with some good tonal range (a pretty large difference between the bright areas and dark areas which we want to use to try and trick our camera’s metering mode for this exercise) and set your camera to its Matrix/Evaluative/Pattern metering setting.  Take a shot and look at the histogram.  Assuming you have a large tonal range, you should be getting little peaks and valleys throughout your histogram, or at least bumps in different areas.  Here’s mine:

f/5.6, 1/60sec, ISO100

Evaluative Metering taking the whole scene into account.

Here’s the Evaluative Metering Histogram.

Because the scene has too much dynamic range (basically bright brights and dark darks) I’ve clipped the highlights and lost the shadows which you can see on both sides of my histogram, but otherwise it seems to have averaged out the scene pretty well with that considered.

Now, change your metering mode, I chose center weighted, and take the same shot.

f/5.6, 1/40sec, ISO100

Center Weighted Metering

The Histogram for the Center Weighted Metering frame.

If you take a look at the histogram, you can see that I’ve clipped the highlights quite a bit more using center weighted metering.  The reason becomes a little more obvious when you look at what is in the center of the frame and take into consideration how the center weighted metering averages out the rest of the scene.  In the center of my frame (where the center weighted metering meters from) there is a bunch of dark greenery.  My camera tried to expose that dark greenery as if it were the mid-tone in my scene and then average out the rest from the center out.  This is why the highlights were pushed that much further to the right effectively overexposing the sky.  It did however pull more of the shadow areas up with it which makes it look better on that side of the graph.

Finally, I shot the scene with spot metering on my center AF spot.

f/5.6, 1/20sec, ISO100

The same scene using Spot Metering on center.

The Spot Metering Histogram

Now, because the spot was centered, it was taking the deep, dark greenery as it’s point of mid-tone exposing it for 18% gray and not averaging out anything else, or even taking anything else into account when setting the exposure.  The highlights in the sky are still blown, but you see the shadow values shift more to the middle.

Now, shooting on Aperture Priority, and keeping the ISO at 100 throughout the series, you can see that when the metering was changed, it adjusted the shutter speed accordingly to accommodate what the camera felt it needed based on the metering mode I’d set it on.  Because during the Center Weighted and Spot metering, the areas being metered were placed primarily on the dark green in the center of the scene.

Here is another one to exercise Exposure Compensation.

I took a picture, using spot metering and with my camera still set to Aperture Priority with the center AF spot (where my Spot Metering was being taken from) on the white wall of my garage.  As you can see in the first picture without any Exposure Compensation, the white is a little muddled and gray.

EC ‘0’ – The white turns gray.

The Spot Meter was taken directly below the yellow thing which told the camera that I wanted that area to represent my mid-tone/18% gray.  That yellow thing has been on our garage for years and I still don’t know what purpose it serves.

Now, I adjusted my Exposure Compensation by +1 full stop.  The white became white again.

EC ‘+1’ – Good bye 18% gray.

Now, I know I’ve given you some compelling and award worthy pictures to ponder while playing with your metering and exposure compensation, but give it a try.  The best way to get used to it is by doing it.  Enjoy!

10 thoughts on “*Metering and Exposure Compensation. Which, when, why?

  1. Pingback: The Exposure Trifecta. Or, how to shoot on Manual. « Tyson Robichaud Photo-blography

  2. Pingback: High Dynamic Range with Photomatix « Tyson Robichaud Photo-blography

  3. this post is So helpful. you made this concept so easy to understand. i am going to go grab my rebel and try some things out…thanks T!

    Like

  4. Pingback: *HDR 101, pt 1 – Capturing Dynamic Range «

  5. Very informative for those trying to improve our end result. Keep up the good work and I hope you can cover other areas, as well.

    Like

  6. Pingback: *Everything you need to know about digital photography. Two years of tips, tricks and various freebies, revisited. «

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