Over the past few years, I’ve had several people ask me how to set bracketing exposures on camera bodies for multiple exposures. When I say multiple exposures here, I am talking in excess of 3 shots. The reason for this is because by default, most Canon cameras (at least all the ones I’ve put my hands on) only have a single over and underexposure allotment on each side of the initial exposure settings.
Let’s take a look at the numbers…say we meter a scene and it says that according to the center point, we should be at f8, with an ISO of 100 and a shutter speed of 250. So, if we want to expose this multiple times over and under that initial group of settings, since Canon only allows 1 over and 1 under, we will need to make additional adjustments if we want more than the 3 exposure set.
Here’s the entire f-stop range (well, not all of it, but enough for our purposes):
So, in order to get the full range of exposure, our aperture would need to manually be set to f2.8, f8.0, and then f22 and then let the camera capture the over/under.
For ease of manipulation I always start at the lowest end and work my way up, so in this scenario, I’d start with f2.8, press the shutter 3x, then move up to f8, repeat the shutter press 3x, then finally to f22 and press the shutter 3x. That would give me a nine stop range of the same image at different exposure settings. It goes without saying here that in such scenarios, you would want to:
- Be shooting on a tripod
- Be using a remote release (or timer)
- Be using mirror lockup
- All other settings remain constant
- and that lighting conditions aren’t changing appreciably
- You are set to manual focus
- Your lens is set to its hyperfocal distance
Things that are problematic with bracketing in aperture priority are:
- Your depth of field is changing considerably as you move from f2.0 to f32.o so much of what is out of focus at f2 will be considerably sharper at f32 even though you are set at your hyperfocal distance.
- Lighting conditions will change. Unless you are in a studio environment and flash is the only light source, ambient light is constantly changing,and this will have an impact on your exposure values. Granted if you move quickly and methodically, this can be minimized, but due to the ever-changing values of light, this is something to consider!
- Weather conditions will change. The scene that was perfectly still 30 seconds ago while you did the first bracket may not have perfect stillness for the second bracket set. Changes in wind speed, direction, and intensity can blur things like flowers, branches, grass, and even some objects that you otherwise would consider fixed. (Ever see a lamp post in a strong gusty wind? Or a street traffic signal?)
Any other mechanics, pros and cons or points of discussion I missed here? Feel free to chime in with your thoughts on bracketing exposures based on apertures. Don’t forget to tune in tomorrow too for the backeting exposures on shutter speed, along with its pros and cons. Happy shooting and don’t forget the Twitter v bloging poll from yesterday! If you’ve already voted, tell a friend!