Despite we've been led to believe, the retina in your eye has a static contrast ratio of around 6 ½ f-stops, which is equivalent to that of today's top digital SLRs. The key word here however is "static". As the eye moves, it constantly readjusts its exposure, automatically giving our brains the perception of a much great tonal range – in essence, our brain is constantly doing HDR work. In theory, by constantly readjusting to the scene and the point we focus on, the human eye ends up being capable of as much as 20 f-stops.
We'll review f-stops in a bit, but to put what was just said into the most basic terms, our eyes constantly adjust by opening the iris to see into shadows and closing it down to see highlight details – our cameras can't do this. Frequently you have to pick between a correct exposure for either the sky or the land – you can't have both.
OK, let's talk about f-stops. This may be a real basic review for some of you, but bear with me a give me a moment to clarify some points that do confuse some folks. Assuming a constant ISO, two things determine how much light hits the camera sensor – the aperture and the shutter speed.
You can compare the aperture to the iris of your eye – a wider opening – comparable to a low aperture number – lets more light in. As the aperture numbers increase, the opening gets smaller, allowing less light in. Many of today's cameras' apertures are set up in increments of 1/3 of an f-stop, but each full f-stop lets in half as much light as its smaller neighbor – for example, f/4 lets in half as much light as f/2.8, and f/11 lets in half as much light as f/8.
For HDR photography we're going to leave the aperture alone and adjust exposure by varying the shutter speed. Now shutter speeds are easier to get a handle on – a 1/250th sec. exposure allows half as much light as a 1/125th sec. exposure, so this change in shutter speed is considered one full stop. Knowing the range in f-stops that our camera is capable of capturing will allow us to minimize the number of exposure we need to completely cover the tonal range of the scene.
Why do we care? Why not just blast away and capture the scene with nine or ten shots at half-stop increments? The two main reasons concern storage and processing. First off, ten captures of RAW files from my Canon 5D Mk II is somewhere between 250 and 300MB. That means a 4GB compact flash card will only hold around 13 individual scenes or capture groups. Secondly, these images need to be transferred to my hard drive and processed. Three correctly exposed captures will give me my HDR image much faster than if the software has to process nine or ten files – never mind my drives being filled up with hundreds of unnecessary image files.
The first thing we did was to test our camera's dynamic range. The details are a subject for another day, but by using a Sekonic L-758DR exposure meter and its companion DTS software, we calculated the exact exposure capabilities of our camera and lens combination. In one case we found that a camera system captured 2.8 stops brighter than a center or middle exposure and 2.6 stops down. Another camera and lens combination captured 3.1 stops up and 3.0 stops down. The conclusion you can safely make from these findings is that pretty much any professional level DSLR is going to safely capture more than 2 stops up and down from the middle or average reading. That means that spreading your captures out at 2-stop intervals is going to provide plenty of exposure overlap and produce a great HDR image. Let's take a look at a graphic that better shows this.
So if our middle reading is f/8 at 1/125th of a second, two stops up and down will be 1/30th and 1/500th of a second. Since each of these also has two stops of latitude up and down, we're actually safely covering from 1/8th second to 1/2000th of a second with these three exposures!
In most cases, this will be plenty of range for your HDR scene. Occasionally you may run into highlights that are much brighter than the scene average – 5 stops or more. Using our previous example, five stops brighter than 1/125th of a second would require a 1/4000th of a second additional exposure or a total of four shots.
Here's a scene in Bryce Canyon that had both very deep shadows and very bright highlights – to play it safe, I took four exposures to ensure sufficient highlight and shadow detail.
Taking these four exposures and processing them in Photomatix (www.HDRsoft.com) while adding a bit of saturation resulted in an interesting and super-realistic composite, which is what I was after for this image.
These guidelines are very conservative and provide enough overlap in exposure to ensure a great HDR composite photo. Stop wasting memory card space, hard disk space and processing time by spacing your exposures in 2-stop increments and enjoy the HDR experience!
@ Joe Brady for MAC Group
Joe Brady, Webinar Manager for MAC Group. Joe is an accomplished professional photographer, member of PPA who holds the Craftsman Degree and recipient of Fuji Masterpiece Award. Check out my archived webinars over at XritePhoto.com ().