What is this talk about HDR all about?
High Dynamic Range (HDR) photography is something you hear digital photography enthusiasts speaking about a lot these days. Basically, it means combining two or more images — of the exact same scene — taken at different exposure levels into one finished image. One dark exposure can capture detail in bright areas of the scene, while a second, lighter exposure can give details in mid-tones and shadows. Combined, the two can give a finished image with far more overall tonal detail than is sometimes possible to capture with even the finest digital SLRs on the market today. This is especially true in very high-contrast scenes, such as scenics in bright sunlight, and indoor shots taken in the daytime with bright windows in the scene.
As old as photography is the search for the ideal light. Part of this search for light is related to the fact that some scenes just look better in certain (often warmer) light. But even more it is related to the fact that film and now modern digital DSLRs can only capture a certain dynamic range.
The Dynamic Range (DR) of a DSLR or film is measured by the ratio of the brightest detail in the highlights and the darkest detail in the shadows. The detail in the highlights is limited by pure highlight clipping and the detail in the shadows is limited in modern DSLRs by the noise level (or film grain).
It is common to measure the DR of cameras in the range of Exposure Values (E.V.) spanned, E.V. being often expressed as f-stops (even though photographers usually vary the exposure time rather than the aperture. Some data can illustrate this:
Black & White Negative Film
10-11 f-stops, or a DR ratio of about 1,000:1 – 2,000:1
6-7 f-stops, or a ratio of about 100:1
DSLRs (in 2008)
8-10 f-stops, or a ratio of about 250:1 to 1000:1
Daylight Scene (with full sun)
12-15 f-stops, or a ratio of 5,000:1 to 50,000:1 (depending on preferred amount of shadow detail)
Room Interior, with outside view from window into full daylight
At least 17 f-stops, or a ratio greater than 100,000:1
We have the following options:
- Control the light (only practical in the studio via flash or lights)
- Shoot only in low-contrast situations
- Overcome the limitation of a single exposure
Of course all three options are valid. This article concentrates on the last option. How can we overcome the limitations of our cameras. Because the problem is as old as photography we also had solutions for as long. We capture two or more separate photographs (from underexposed to overexposed) of the same scene and combine the images into a new photo. The classic solution is called Exposure Blending. Master photographers did this manually in the darkroom with enlargers, and today the same is done in the digital world in Photoshop™. Seamless manual blending is hard work because of the issue of:
- Aligning the source images
- Masking the images to get seamless blended results
However, today there are quite a few specialized software applications (one that I’ve had excellent results with is Photomatix Pro) that can are designed primarily for the single task of merging two or more exposures into a single image, which is LDR (Low Dynamic Range), but containing the details in highlights and shadows of the exposures from the sources images. There are different merging algorithms with different strengths and weaknesses.
Capturing multiple images with your camera and combining them into finished tonemapped HDR images is quite simple today. Getting the right artistic results will as always take some time practice and experimentation.
Notes of caution:
Movement is the enemy of HDR. This means that you have to watch for everything that moves. Here is a list of common ghosting* candidates:
- Cars, bicycles, and other vehicles
- Larger animals (dogs, cats,...)
- Small animals (birds, butterflies,..)
- Trees and plants in wind