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Hardware

The Complete Guide to Advanced In-Camera Metering

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In my previous tutorial "The Ultimate Beginner’s Introduction to Exposure", someone brought up that I should have included a section on Metering. And while they were probably right, I thought it was too advanced for a beginner's guide and over-complicated for someone just starting out.


In-Camera Meters - What Are They?

The meters in most cameras are "reflective" meters. Meaning they measure the light that is reflected off our subject and back into our lens. This is different then some modes on standalone handheld meters which, while they can measure reflective light, are best known for metering the ambient or incident light from the light source itself, whether that is the sun or a flash/studio strobe.


Metering Modes

All Digital SLRs have a few different metering modes for different situations, and are very helpful to get a correct exposure. They're much improved over earlier meters, and superior to reading settings off of a film box (in the olden days) or knowing the "Sunny 16 Rule" (highly debatable, but go with me here!)


Matrix or Evaluative Metering


Example showing right focus point selected

This is pretty much the default metering mode on DSLRs. Nikon calls it Matrix, Canon calls it Evaluative. Other brands have other names. This mode uses the area under the focus point on your subject, and then also samples the rest of the image area to make a determination of what would be the best exposure to satisfy both needs.

This is very often used in portrait photography to get a good balance between the person and the background.

Bear in mind something to use this mode correctly. Since it uses the chosen focus point for a large part of the calculation, you must, if you focus and recompose (using the center focus point), lock the exposure also at the time you lock focus. Or, if you use focus points other than center, use one that covers your subject and keep it there.

Moving the focus point to another area than intended can change the metering significantly. If that focus point ends up on a bright sky instead of a person, you're bound to encounter problems.

This is the most oft-used Metering Mode, but is actually the one I use the least. But make up this decision for yourself. Do not do as I do, because you may not shoot like me.


Spot and Partial Modes

Spot


Partial


These modes both work the same way, they just cover different amounts of area. Spot and partial cover a small area just in the center of the viewfinder, and essentially disregard the rest of the screen. Spot metering covers an area of about 3% of the screen, Partial covers an area of about 9-10%. Which one to choose will depend upon the scale of your subject in the framing of the image.

The important consideration when using these two modes is that you meter on something that is mid-toned - but more about that in detail later in this piece.

This mode is best used if there is a wide difference between the subject and the background in brightness, such as shooting a person backlit by a bright sky. Or, if the mid-toned area is small in comparison to the rest of the image area.

Because these modes are so small and precise, you will need to make an artistic decision on what is actually metered for and what you want to accomplish in your shot. If you are looking for a silhouette, you need to meter the background not your subject. If you want your subject well exposed you need to meter on them.

You have to make a conscious decision and compromise on what needs to be exposed the most correctly (silhouette, expose the background correctly, vastly underexpose your subject, or backlight, expose your subject correctly and blow out the background).


Center Weighted Average


This mode takes an overall meter of the entire scene, but puts more emphasis on the center of the viewfinder. This is probably the least used mode by most people, but the mode I end up using around half the time for my landscape work.

It just works for me and the way I shoot. It works for me because most times I have equal light between my foreground subject and the background, but since I want my foreground subject to be the most prominent/perfect exposure, it takes that into consideration.


Let This Button Be Your Friend

I mentioned before about "locking your exposure" before recomposing. A little used button by most DSLR users is the Exposure Lock button. Now it may be a stand-alone button, or it may be nested under another button. In truth it can also be set to be locked when you lock focus by choosing that in the Custom Function menu of your camera. Either way, find this function and use it.


As shown on a Canon Camera. Consult your camera manual for the location and options on your camera.

Now if you shoot manual, you won't need to use it, since you essentially lock your exposure all the time anyway. Just remember to set your exposure before you recompose.

However, if you shoot one of the Program Auto Modes or a Semi Auto Modes, like Aperture or Shutter priority, this button should become your best buddy. Find the area you want to expose for and press this button to lock exposure and then lock focus so that it does not change as you shift your composition giving you a less than desirable exposure.

Like I said, modern exposure meters are a modern wonder, but they need to be used correctly for you to make the most of them.


When Meters Get It All Wrong!

In-camera meters are designed to get a correct exposure on a mid-tone (think of green grass or blue skies), but more precisely an 18% reflective grey area. In most cases, this works out great since there are so many mid-tones in most scenes we shoot. Even people's skin tones can be a mid-tone.

But what if the predominate subject in our viewfinder is not a mid-tone? What if we have almost all black or almost all white in an image? (the proverbial black labrador in a coal mine, polar bear in snowstorm). The meter will still make them grey, because that is all it knows to do.

In the example below I shot pieces of white, black and grey paper. In each case I shot the paper at a center meter for "correct" exposure and then also exposed each at -2EV (exposure value) and +2EV - or in other words, under and over exposed each by two stops.




In every case, 0 Meter made each piece of paper grey, (the grey paper correctly), -2 made everything black, + 2 made everything white. Now we probably would have had to go to +-3 to get pure black and pure white but I didn't want to go past my meter and use a calculation. I wanted to see what the meter does.

So like I said before, the meter is set to expose a mid-toned subject and this proves it.

Just as a little aside here, remember back a few paragraphs when I talk about the olden days and the "Sunny 16" rule? Well this image was shot of all three papers at once. No meter. Just using the Sunny 16 rule (on a sunny day set your aperture to f16 and your shutter speed a reciprocal of your ISO i.e. 1/ISO)


Interesting, it got all three exactly right - so much for old technology, huh? But what it also shows is that if I were to meter and I metered on the grey mid-tone area, it would get the exposure correct everywhere from black to white (shadow to highlight). So remember that, if you can meter on something mid-tone, do it. If not - well see the next paragraph.

Getting back to our 3 papers shot at 0 meter. So what does that tell us in real life use? Well, if we are shooting a scene with a predominately black or white composition, we need to compensate for that or our images will be either underexposed in the case of predominate white or over-exposed in the case of predominate black. You can either do that easily in manual mode or by using exposure compensation if you're shooting in P, TV(S) or AV(A) modes.

Exposure compensation (see your camera's manual on how to set this) allows you to preset a given amount of negative or positive exposure into the camera. So say you know you will need -1 exposure, You can dial in -1 into exposure compensation and it will keep it there until your scene changes and you no longer need to compensate.


In Real Life

So when are you likely to encounter this situation in real life? Shooting snow is an obvious one - try shooting +1 to +2 on it just making sure you don't overexpose so that you lose detail.

Shooting a sky with large puffy white clouds is another. If there are some blue breaks in the clouds switch to spot metering and try to meter off that area. Or once again try +1 or more to make your white white, and not grey.


Another common occurrence is shooting a white wedding dress. If it is just the dress, well you can try just shooting +2 again. But suppose there is a person in the dress, as there usually is. In this situation the most important thing to have the correct exposure for is the bride's face. You can image if you metered on the dress how dark her face would be!

So in this case it may once again be a good time to switch to spot metering and meter the bride's face. Trying to strike a balance between good exposure for her skin, but keeping some detail in the dress. If you have to make a choice, go for the face.

But what about overexposing black? Of course a black dog on a dark background is the easy one. But how about this; concert shots. Most concerts are poorly lit with the exception of a large arena show which may have as much light as daylight. But think of most concert shots or small clubs, and even the big arena shows where you have a solo spotlight on a single performer. You have a performer in the middle of a sea of black.

While most people think because of the low light they will get an underexposed shot, in most cases people over-expose the performers. You can see this by the glowing performer and the background (you guessed it) turning grey.

Once again you can correct this by knowing and understanding your meter, and choosing to underexpose by 1 to 2 stops. Or, once again, you can choose spot metering and try to correctly expose the performer (if mid-toned) and not let the abundance of black influence your exposure.


How Can I Tell if I Am Getting It Right?

Well without relying on your LCD (which can lie to you), try using your histogram. If you are shooting white or black and you get a big spike in the middle of the histogram, that tells you there is a good chance you are under or over-exposed.


If the spike is to the left on a black image, it is probably correct.


If the image is predominately white the spike should be to the right.


And then to help you on a white image if you start to see "Blinkies" in your histogram display, you know you have most likely overexposed the whites.

So in a real world situation, like shooting my white fence - excuse the subject, it was too hot out for hot models today - you would see something like this. The first was shot at 0 Meter, the second at +2 meter.



In Summary

So be sure to learn about, and experiment with, the different metering modes on your camera. Understand when they get it all right and when they get it all wrong, and what to do when this happens. You'll soon find yourself with outstanding exposures, far less blow out images, less noise, and less work to do in post production.

Remember to:

  • Meter on a mid-toned object.
  • Use the correct metering mode for the situation at hand.
  • Lock your exposure before recomposing.
  • Know when the meter will be fooled and how to compensate for that!
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