The way present HDR photography technology forces us to capture it, movement is a really big problem. When something moves between exposures the software gets really confused and will create what is known as ghosts. So if our monitor could show us the HDR photo i bet it would look funny. Lucky for us we are going to tone map our HDR photo and fix the moving sign.
It was really windy here in Copenhagen and i thought that it would be a good opportunity to do some HDR photography with movement, for an upcoming tutorial. If you look at the 9 photographs below you will see how the sign move from shot to shot. So when the software merges our 9 shots to one HDR photo, it will turn that moving sign into a ghost. Sometimes the software can fix it, but the sign here, moves to much and will turn into a ghost.
But why does it create ghosts? Well the answer is really technical. For a good HDR photo any given "pixel" must be present, in two shots. That way the software can merge them correctly. When you have movement the "pixel" is "moving". If you look at the chart below, you can visually see (i hope) how movement in the different exposures destroys the HDR photoI.
So to sum it up. Shoot your HDRs as fast as you can. If it means that you will have to use a f/4 then do that. Because you are shooting multiple exposures, DOF (Depth Of Field) works different than in normal photography. Even with an f/2 you can have reasonable sharpness throughout the image. Of course it depends on the size of the print and there are differences in the final outcome between a f/2 and f/32 shot.
So, you have to think about movement a lot when shooting for HDRI and the best way to learn it - is by trial and error. I have a good feeling for what will work and what will not and i also examine all my shots on the LCD of the camera after i have shot them. That way i can quickly determine if i need to shoot the scene again.
I just did a default tone mapping of the nine exposures, as you can see on the right. You can see the sign is looking really, really crappy and is generally just destroying the look of photograph. I like the light on the building and also the sun behind the clouds. It do think it could end up looking interesting, but we need to work a bit on it and we're probably also going to darken it up. All this takes place in Photomatix and Photoshop.
Today i was out shooting and i took seven photographs. That ended up being 108 exposures. Divide those by nine shots and you get 12 HDR images. That means that there was 3 locations where i shot the same motive more than once, probably because of movement or because i wasn't sure what aperture would work best. I am also still learning and question everything i have already learned.
Remember when shooting HDR photos: always carry extra memory cards, and always carry an spare battery.