Last week we looked at custom white balance. This is a way to ensure that your image does not have blown out whites or crashed darks, by telling the camera what the “middle” tone is (r/g/b 127/127/127, aka grey). This week we chose an alternative route called exposure bracketing.
Exposure bracketing means to take an underexposed and overexposed image in addition to your Baby Bear image, the presumably perfect exposure. You would use it when you’re pretty sure you set the correct exposure, but the scene has difficult contrasts or changing light. Rather than take multiple shots and manually adjust exposure every time, you just tell the camera to try three variations on each shot and hope one of them works. You can make this variation minor (1/3EV) all the way to severe (3EV).
Exposure bracketing is especially useful for image compositing. You would take the image with the best tonal range of darks, and the best lights, and combine them. The (dynamic) range of your camera is limited. For an image with a large range of tones, with intricate details in both the darkest darks and lightest lights, you can take the bracketed photos and overlay them with a mask (or buy a program that does the work for you, or try to spontaneously mutate into Ansel Adams). Way back in my darkroom days, double-exposing photo paper like this required physically cutting a mask in order to dodge the light during the second exposure (or risk over-burning, meaning over exposing with light and making the paper too dark). The fact that programs can now take stacks of 300 images and average them all together to create perfectly detailed microscopy or surveillance photos still blows my mind.
Here are several images* taken in the ubiquitous frustration that is indoor fluorescent light. Photos were taken with a Panasonic Lumix, which does not allow for both custom white balance and exposure bracketing in the same shot. I varied ISO from 80 to 200, exposure time from 1/2s to 1/60s, and F-stop from 1.4 to 2.8.
Example 1: Blacks, whites, and greys
In this series, the correct exposure was probably +1/3EV. I set the camera to +/- 1EV to see the differences more clearly. The whites are blown at +1 EV but “normal” is a bit too grey (the pegboard is painted white, the ping pong ball is spray-painted black)
In this series -2/3EV would probably be best. Here, -1EV looks like an okay exposure and the other two look a bit blown out. Amusingly, because these are each about half a second apart, you can see a lot of hand movement on the keyboard. Something to keep in mind when exposure bracketing action shots!
*All images are the jpg co-copy of the original RAW files – RAW files available via dropbox, since they will not display on wordpress.
I can see why the camera defaults to +/- 1/3 EV, since in all my photos 1 EV was too far. I would save 3 EV for landscape photos with intent to combine, while 1/3EV is for still scenes where I am not sure the exposure is quite right. Now that I know how this works, I plan to use it often!