If you have read my tutorials you will have seen that I do recommend using Aperture priority mode and Exposure bracketing to get a 3 exposure HDR image quickly and simply.
But how you meter for this or even in manual mode doing your own bracketing can make a big difference in the quality of your final HDR image especially when it comes to noise.
Now first off, we should understand how the metering system works on our cameras and also how to lock the exposure.
Canon and Nikon cameras work similarly although they may use different terminology. ( The following assumes the factory Custom Functions setting, varying custom functions can change your camera’s Auto Focus and Exposure functions, please consult your camera manual for more information)
For metering modes, Canon’s Evaluative metering and Nikon’s Matrix metering work very similarly: They use the selected focus point as the main source of their readings and then take in the surrounding data to arrive at a correct exposure. a half press of the shutter will lock both the focus AND the exposure.
In other metering modes; Point, Center weighted, Partial, Use the center of the viewfinder for exposure ( in varying sizes) and a half push of the shutter button (or the back focus button) will lock ONLY focus. The exposure will vary with re-composition unless you press the Exposure Lock Button on the back of your camera (consult your manual)
So knowing the mode you are in and how it locks exposure is critical
So let’s see how in practice this can affect our HDR images
In the examples I am showing it is an extreme example but one that really shows how important proper metering and exposure lock is. These images contain the sun in the frame and as you may have seen in my tutorials I most recommend more than 3 exposures if the sun is in the frame because of the huge amount of dynamic range that needs to be covered. But we don’t always have time or a tripod to do the extra exposures needed. So let’s look at this extreme but pongient example of something that you may run across.
In this first set of images, the exposure was metered and locked on the sun itself. So the camera chose that as the first median exposure for the three exposures. (click on images to enlarge) (Excuse the Vine wires in the image, I wanted them there for a quick tip in my next post)
We can clearly see that while our exposure did a good job exposing the ball of the sun, our +2EV image that we would hope to have a good exposure for the shadow area, is extremely under-exposed. So what will that mean to our final tone mapped HDR? It will mean that our HDR program will try to make that a midtone area and will bring upo the exposure at least 2 stops. Bringing that area up two stops in luminance results in a huge amount of noise as it tries to bring out information that just isn’t there in our underexposed image.
Here is the tone mapped result for those images, you can clearly see the color noise in the flower area
In the next set of images, I meter and locked the exposure on the sky to the right of the ball of the sun, This resulted in our 0EV image being exposed 3 stops lighter, than in the first set ( as a rule of thumb, the sun itself is 3 – 4 stops brighter than the sky,depending on time of day)
We can see that the -2EV does a pretty good job of exposing the ball of the sun and our +2EV shot does a much better job of exposing the shadow area in the forground
The resulting tone mapped image shows a similar total exposure to the first image but with MUCH less noise in the shadow area of the flowers since the program did not have to raise the exposure of that area and bring up the noise floor of the color area
Now in a perfect scenario, I actually would have probably shot 5 frames to get the ball of the sun perfect and the shadow area even better exposed. But I think this shows you how much of a difference where you meter and how you lock it can make in your final image. As a general rule, we want to meter on a good mid-Toned area for our 0EV exposure, this can be blue sky, green grass or a gray cloud. Generally we do not want our 0EV meter to be a shadow area or a highlight area. Keep this and how your exposure locks in mind, especially if you focus and recompose your image,because what you focus on may NOT be what you want to expose for.
Hope that helps!