How many shots do you need to make an HDR merge?
The most common answer is three.
The real answer is … it depends. First, it depends on the contrast ratio of the scene. Second, it depends on the exposure value (EV) increments you use between exposures. Third, it depends on the camera you use.
Many scenes only need 2 exposures. Most scenes need 3. Some scenes need 5. Only a few scenes need 7 or more.
How far apart in EV (exposure value) should separate exposures be?
1.5 stops. (Really any value between 1 and 2 stops.) While you won’t get better image quality if you use more shots separated by less exposure value (less than 1 stop), you also won’t compromise it. While you can also use higher increments (more than 2 stops) be careful – you may produce banding in smooth areas, particularly those with gradations.
Do you need to make HDR merges more frequently with some cameras and less with others?
Yes. Cameras that have a greater dynamic range can capture a higher contrast ratio and so don’t require HDR bracketing as frequently. While this can make a difference for images that would require two and occassionally three shots, for scenes with more extreme contrast ratios HDR merges will be necessary for all cameras.
Today’s cameras (including smart phones) can create great looking HDR images on the fly, but to get optimum results it’s best to do this manually. In camera solutions render artifacted JPEGs and give you little or no control over how the results look. For optimum results, make separate Raw exposures and render them manually. While the technology at work is wizardry, this four step process is easy to practice. It’s an essential skill for all photographers.
Today’s fast burst auto-bracketing cameras combined with software alignment make hand held HDR possible. However, it’s recommended that whenever practical you use a tripod to eliminate any alignment issues between frames that might arise; it’s necessary if exposures are long.
HDR merges require multiple bracketed exposures. The goal is to produce at least one exposure with great highlight detail and another with great shadow detail. You may need additional exposures in between your lightest and darkest exposures to help smooth tonal transitions between shadows and highlights. The most common number of images used is three, because this is the default number for auto-bracketing on DSLRs. However, there is no ideal number of exposures for all scenes. Some scenes need as few as two, while others need as many as eight. In general, it’s best to have more than you need, not less. The wider the dynamic range of the scene the more exposures you’ll need. Make sure that separate exposures are between 1 and 2 EV (exposure value)(equivalent to one f-stop) apart. It’s typically recommended that you fix f-stop and change shutter speed to avoid depth of field issues, but other changes in EV will work.
Merge in Lightroom
After producing multiple bracketed exposures, the next step is to combine them with software into a single 32 bit file.
Simply select the exposures you wish to include (You don’t have to use them all.) and use the software of your choice. The software you use to merge exposures will compensate for alignment and ghosting, from motion of either camera or subject. (Lightroom and Photoshop do excellent jobs.)
Rather than rushing to render this file at the same time, save it – you may want to render it multiple times.
Like the human eye, film has a nonlinear response to light. For film, we adjust the EV to fit the amount and contrast ratio of the available light into the most useful area of its curve response. Using film, you expose generally, and when compromises need to be made, you favor shadows or highlights. Details lost at the point of capture are irrecoverable.