Intermediate Camera Functions - Spot Metering & Exposure Lock

I wanted to share a little write up about Spot Metering and Exposure Lock. Sad to say I had shot with a dSLR for nearly two years before I ever figured this out and made use of it. (This instruction in for use in Av or Tv mode, does not apply to M mode.)

Many people will typically turn on their camera and shoot with Evaluative Metering, meaning that the camera adjusts for exposure based all available light. Evaluative metering looks like this.

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When shooting in this mode you can focus on your subject by pressing the shutter half way down the recompose your scene without changing the overall exposure.
However, once you start branching out you notice there are other metering modes, this diagram shoes in red the area the camera uses to determine exposure.
Soon you will discover how great spot metering can be. If your subject is back lit then the shadows on the subjects face will be dark. If you use spot metering on the face your camera will brighten the background because it will expose for the face. Here is a good example of this. The faces are exposed correctly while the background is bright.

Spot metering is great but soon you will find that your exposures start jumping all over the place. One the face will look great the other the face will look blown out and you start wondering why. If you lazy you may say to yourself, oh well I will just fix it in lightroom. But you are better to get it right in camera first, it will save you in the long run.

When you focus on a subject with spot metering by pressing your shutter button half way down your camera decides on an exposure. If your subject is in the center and you take the picture everything turns out just right.

In this picture I shot in Av mode with ISO 1600 f/1.8. I focused my spot meter on the white tip of the shoe, and left the subject in the middle. My camera decided on 1/400th as a shutter speed. I clicked the shutter button fully and this is the image I got.

In this picture I left my settings as Av mode with ISO 1600 f/1.8. This time I used the center Auto Focus point to spot meter with. I focused on the white part of the shoe, then decided I wanted to recompose the picture like I often do. This time the camera decided on a shutter speed of 1/60th, significantly more light was allowed in and it overexposed my subject.

What happened is when I moved my subject out of the center of the frame the camera then re-adjusted the exposure for the darker background. The camera thought I needed more light and because I am in Av mode it dropped the shutter speed significantly. But I wanted the white on the shoe exposed so what do I do?

This is where shutter lock comes in...


Same starting settings as before Av mode, 1600 ISO, f/1.8. But this time I composed my subject in the middle as the two times before. Now I focused and decided I wanted to recompose my subject. However just before I did this I pressed the Exposure Lock button at the back of the camera near my thumb, it looks like this
Once I press this button my exposure is now locked for this picture and I can recompose and keep the exposure that I originally wanted with my spot metering. No matter where the center point is the camera has locked the exposure to the white spot on the tip of the shoes.

If you want to cancel the exposure lock press the button to the right of the exposure lock button and try again!

Hope you found this helpful!


(This was my 700th post!)