I thought this topic would be helpful to all of you who are interested in DSLR photography but fairly new into this field. I spent few hours and days to understand and implement Auto Exposure Lock feature and I feel like it worth to share with you all. Although the idea is same for every DSLR camera, I am taking reference of Nikon D90 for menu settings and have included couple of photographs I have taken with my D90. If you are using other DSLR camera than D90, you may have to go through different menu options but you can always refer your camera manual for quick search.
What is an Auto Exposure?
Camera automatically controls your camera exposure setting under different environmental situation depending upon lighting condition. For example, if you are shooting any dark subject as a main subject with white background (enormous amount of light), your camera sets the exposure as per background and your main subject will come dark. There is a separate post demonstrating that effect with snow background. If you are interested looking that example, please follow my post from reader’s question on how to use AE-L/AF-L correctly? That’s why you have to lock the exposure to proper place to expose our main subject properly.
When should I use AE-L function?
This can be useful when taking photos so that consistent exposure is achieved across all images, or when you wish to expose for the background and then recompose and shoot a photo so that you can achieve better lighting effect on your subject over different environmental condition.
How to set Nikon D90 for AE-L?
1. Press the Menu button on the left hand of your camera.
2. Navigate to your Custom Settings Menu (Pencil icon).
3. Go to f Controls menu, option f4 is for AE-L/AF-L button.
4. Set option according to your need. Every option is self described.
I recommend using following setting under f4 menu.
AE lock (hold) : It will only lock AE and hold that setting till you pushed AE-L/AF-L button again.
Now after the setting is done, This function locks the camera exposure for as long as the AE-L button is pressed. You can see Exposure Lock button to the right of your viewfinder, which is labeled with AE-L/AF-L. Once you pressed this button, you can see AE-L icon on the left corner when you look through your viewfinder.
Now let me describe when to use it with couple of photographs.
Without Auto Exposure Lock
I took this picture without locking exposure. Camera controlled exposure according to white sky background and my main subject (Building) came out dark.
With Auto Exposure Lock
Now what I did is I focused on walkway (you can see walkway at the left bottom of picture) only (because it matches to that building color) and locked the exposure to that settings so that any bright background won’t affect exposure anymore. Now no matter what the background will be, exposure is locked with desired settings.