Thursday, November 26, 2009

Q: When/Why Do You Use Spot Metering Mode?

Q:  I was reading your processing info you included on your blog. You said you used the Spot Metering Mode when you took THIS PICTURE. I have never used that feature. Do you use it often? Why did you use it on that particular picture?

A:  Great question!  I use the Spot Metering Mode almost exclusively which, admittedly, is a bit unusual. I find it gives me the most accurate exposure value based on the exact area of the scene I am metering.

If I know the scene has a wider range of light than my camera can capture in one image (typically more than five or six stops of light) then I know I'll probably end up blending two exposures in Photoshop. So, I take one exposure metered off the sky and one metered off the shadow areas - that way I have an adequate tonal range to work with when I get to the digital darkroom. If the tonal range of the scene is wide but a little bit less than five or six stops of light (or if I'm feeling lazy) I'll take one shot and meter off the part I think is most important to properly expose (make middle gray) and run it through Topaz Adjust in the digital darkroom. Topaz Adjust does a very nice job of tone mapping a single exposure. (The image you asked about was a single exposure.)

[Note:  One other thing I should probably mention is that I almost always slightly underexpose my landscape images and tone map them in the digital darkroom. (I try to use a subtle approach as opposed to giving them the classic HDR look.) On the other hand, I almost always slightly overexpose and meter on the skin when I'm doing portraits. Some people called it ETTR for "expose to the right".]

For anyone who wants more information on metering, read on…

Have you ever wondered how your camera knows what aperture and shutter speed to select to give a 'proper' exposure? The answer is: the camera's light meter. The light meter inside your camera is what calculates the 'proper' exposure (balance of ISO, shutter speed, and aperture).

TWO TYPES OF LIGHT METERS:  INCIDENT VS. REFLECTIVE

First, it is necessary to back up a little bit and talk about the two different types of light meters.

A hand-held INCIDENT LIGHT METER measures the amount of light falling on the subject. I have one LIKE THIS which I use mainly when I'm working with studio lights. It works by placing the meter exactly where the subject will be standing (in the case of a portrait) and pointing the dome toward the camera so that the meter is measuring the actual amount of light falling onto the subject.

Your camera contains a completely different type of light meter, a REFLECTIVE LIGHT METER.

A reflective light meter does exactly what its name implies: It measures the light reflected off a scene/subject. A reflective light meter (like the one in your camera) does a pretty good job of measuring light in most circumstances.  However, there are situations where your camera's reflective light meter is fooled which results in an under- or over-exposed image.

HOW YOUR CAMERA'S LIGHT METER GETS FOOLED

Here's the tricky part: Your camera's light meter wants to measure the reflected light of an object that appears to be middle gray (12-18% gray). If you point your camera at something darker or lighter than middle gray the camera's light meter will incorrectly calculate the exposure! If there is an even spread between dark and light objects in a scene your camera's meter will do fine. Zoom in on a zebra and your camera's meter will have no problem metering the scene and giving you a proper exposure.

On the other hand, your camera's light meter will be fooled when there is (overall) more dark or light in a scene. Try taking a picture of a black dog on a dark background and your camera's meter will (most likely) overexpose the scene.  Take a picture of a white bird in the snow and your camera will (most likely) underexpose the scene.

If you really want to understand how your camera's meter works - how it tries to "see" everything as middle gray - try the following exercise.  (This works best if you actually DO the experiment instead of just reading about it. So, get your camera out and do it!)

METERING EXPERIMENT

  • Set your camera on Manual Mode
  • Turn the Auto Focus (AF) OFF.  (There's a switch on the lens which you'll need to move from AF to MF… just don't forget to put it back on AF when you're finished with this experiment!)
  • Tape a WHITE piece of paper on the wall and take a 'properly exposed' picture of it.  (Make sure the paper is well lit. Fill the frame so that you are only seeing the white paper & not the edges of the paper or the wall or the tape… all you want to see in the frame is the white paper. Adjust the shutter speed, aperture & ISO until the Exposure Scale in the viewfinder reads zero, meaning not overexposed & not underexposed.)
  • Observe the results.  Notice that the WHITE piece of paper doesn't look white at all… it looks GRAY!  That's because your camera's meter wants to make it middle gray. (Now you can understand why the photos of snow scenes look dingy gray instead of bright white.  Because your camera's reflective meter is being fooled by all that light.)
  • Now repeat the process with a BLACK piece of paper.
  • Observe the results.  The BLACK piece of paper you just photographed looks GRAY on your LCD screen or computer. (It should look almost identical to the results of the white piece of paper.)  Again, it's because your camera's light meter is trying to make the black paper middle gray. (Now you know why your camera overexposes the black dog on the black background. Your camera's reflective meter is being fooled by the overall dark scene.)
It's an enlightening experiment!

METERING OPTIONS

In order to accurately expose a wide variety of lighting scenarios our digital cameras offer several metering options. Here are some of the metering modes available on today's digital cameras:

Matrix/Evaluative Mode-

Matrix Mode (also called Evaluative on some camera brands) is usually the default setting on most cameras. It uses a grid overlay to meter and all areas of the grid are averaged.  This mode uses complex algorithms based on literally thousands of different scenes programed into the camera. The camera's meter "knows" what you are taking a picture of and can make a good guess about how to accurately measure the scene. Matrix/Evaluative Mode works best when the light is fairly even. It does NOT work well for strong backlighting or sunsets.

Spot Mode-

Spot Mode is the opposite of Matrix/Evaluative Mode. Instead of averaging the entire scene it only meters a very small area (2-5%) of the entire scene and is based on the center focus point. (This makes it necessary to treat focus & metering as two separate issues which can be done in a variety of ways depending on your camera. For instance, you can set your camera up to use back button focus.) 

Spot Metering Mode works well in very contrasty light where you need to properly expose one important area. For instance, when doing a portrait I ALWAYS want the face to be properly exposed (no matter what is going on in the rest of the scene) so, I spot meter on the subject's cheek. Spot Meter is the most difficult metering mode to master but it is also the most accurate. (Your mileage may vary.) It does NOT work well for sports or moving objects.

Partial Mode-

Partial Metering Mode is similar to Spot Mode but a larger area (about 10%) is being metered. It is still based on the center focus point but a lesser degree of accuracy is required because a larger area is being metered. This mode is useful for a backlit portrait. As with Spot Mode it does not work well for sports or moving objects.

Center-weighted Mode-

Center-weighted Metering Mode measures the entire scene but weighs the center of the scene more heavily by about 80%. This mode is good to use when there is a strong contrast between the foreground and background light. It also works well for portraits. Again, it may not work well with moving objects such as sports or wildlife.

CONCLUSIONS

Which metering mode should you use? That depends. It depends on the lighting. It depends on your camera. It depends on how comfortable/familiar you are with the different metering modes. Experiment and figure out which one(s) work best for you.  Generally speaking, Matrix/Evaluative will probably be your 'go-to' metering mode (unless you are weird like me).  I always choose Matrix/Evaluative when shooting moving objects. If you are shooting a sunset or something backlit try one of the other metering modes (spot, partial, or center-weighted) to see which one gives you the best result.

So, when do I use Spot Metering Mode?

1)  When I want to shoot in Manual Mode & need to do a 'test' shot with a gray card to set my exposure.
2)  For portraits, so the skin on the face is properly expose no matter how the rest of the scene is lit.
3) For tone mapping (particularly landscapes) in the digital darkroom

I hope that answered your question!