The Doubling Rule
There’s no such thing as a free lunch—or is there…?!
Dark Waters (taken at 1/60 of a second)
Wildlife photography is all about trade-offs, and one of the most frustrating is having to choose between a narrower aperture, a faster shutter speed and a lower ISO. There’s rarely enough light to allow you to dial in the ideal settings, so it’s a common problem—especially during the golden hour or at night!
One way to dig yourself out of this particular hole is to use the doubling rule (or what Tony Northrup calls the rule of doubles in this video), but what is it, and how does it work?
What’s the Problem?
Eland Horizon (taken at 1/20 of a second)
To begin at the beginning, let’s start by clarifying the impact of the aperture, shutter speed and ISO settings. For a full explanation, you can read my Basics of Photography article, but here’s a quick summary.
Aperture
The aperture is the opening in the lens that allows light to reach the film or sensor, depending on which kind of camera you have. The setting is represented by the f-stop, f-number or f-value, such as f/2.8 or f/16. The lower this number, the wider the aperture.
Aperture is most useful in photography for controlling the depth of field. This is the amount of the scene from front to back that remains ‘acceptably sharp’. Outside that area, the foreground and background will be blurred out. The higher the f-stop, the higher the depth of field (and vice versa).
Unfortunately, narrowing the aperture reduces the amount of light that can pass through the lens, which means that you have to compensate by reducing your shutter speed (which might result in motion blur) or increasing the ISO (which might result in unsightly noise).
Shutter Speed (or Time Value)
The shutter speed isn’t actually the ‘speed’ of the shutter at all. It’s the amount of time it stays open, such as 1/20 or 1/1000 of a second. This is reflected in Canon’s use of ‘Time Value’, but everyone else in the world says shutter speed, so let’s stick with that!
A fast shutter speed is helpful in removing motion blur—either caused by camera shake or the movement of your subject. However, some photographers deliberately use a slow shutter speed for slow pans. The idea is to create a greater sense of energy and movement by blurring the legs or wings and background while keeping the subject’s head sharp.
ISO
In the old days, the ISO was a linear scale that measured the sensitivity of the rolls of film you put in your camera. However, with the advent of digital photography, it’s come to mean the ‘gain’. Increasing the ISO means increasing the voltage applied to the sensor, making it more sensitive to light and therefore brightening the image.
The effect is similar to turning up the volume on your stereo—the music gets louder, but there’s a background hiss from all the static. This is the equivalent of the ‘noise’ you get from shooting in low light at a high ISO, such as 6400 or 12800. The low Signal-to-Noise Ratio (SNR) reduces image quality by making photos blotchier, introducing randomly coloured pixels and reducing the contrast and dynamic range, ie the brightness of the highlights and the darkness of the shadows.
In an ideal world with enough light to play with, you could shoot at the lowest possible ISO—usually 64 on Nikon cameras but 100 on other models. However, most wildlife photographers want to take advantage of the warm, soft light of the golden hour, either just after sunrise or just before sunset. That inevitably means you have to shoot at higher ISOs—but that’s fine as long as you don’t push it too far!
What is the Doubling Rule?
"Is this close enough...?" (taken at 1/250 of a second)
The doubling rule is a clever way of increasing the odds of getting a sharp shot in low light—as long as your subject is stationary! All you have to do is take a short burst at a ‘safe’ shutter speed, say 1/1000 of a second, and then double the exposure time while doubling the number of frames.
If you carry on doing this until the animal walks off or the bird flies away, you’ll end up with hundreds of shots at lower and lower ISOs. When you review them on your computer, you should do so in reverse order. That way, the first sharp image you see will by definition be the one with the lowest ISO—and therefore the least noise.
All you need is one, and it’s just a numbers game. Your hit rate at 1/1000 of a second will probably be close to 100%, but it might halve at 1/500 and halve again at 1/250. By doubling the length of your bursts, you make up for the lower hit rate and give yourself the best possible chance of a good, clean, noise-free image.
Just remember that you have to double the exposure time, NOT the shutter speed!
Alternative Methods
The doubling rule is not a perfect solution to the problem of low light and high ISOs, but it’s a big improvement on the reciprocal rule, especially when combined with the image stabilisation systems of modern mirrorless cameras.
The Reciprocal Rule
The reciprocal rule is a rule of thumb that helps you to avoid camera shake. It’s based on the idea that motion blur is proportional to your focal length. In other words, the longer your lens, the more any movement is magnified, leading to a loss of image sharpness.
All you have to do is make sure that your shutter speed is never slower than the focal length of your lens. For example, if you were shooting with a 500mm lens, your minimum shutter speed would be 1/500 of a second.
Again, this only helps with camera shake, not with a moving subject, but the main reason why it doesn’t really work any more is image stabilisation.
Image Stabilisation/Vibration Reduction
Modern cameras and lenses have exceptional image stabilisation—which Nikon calls Vibration Reduction. The best systems work in five axes: yaw, pitch, roll, horizontal and vertical. Mirrorless models like the Canon EOS R3, R5, R6 and R7 offer eight stops of shake correction—which is the difference between 1/1000 and 1/4 of a second!
The system works in different ways using a complicated assortment of gyroscopes and accelerometers. It’s either in the lens, the body or both. Sensor shift or In-Body Image Stabilisation (IBIS) usually works in conjunction with any stabilisation system in the lens, but it’s sometimes completely independent.
The point of all this is that it makes the reciprocal rule seem outdated and unnecessary. With so much image stabilisation on offer, there’s no need to limit yourself artificially to what’s likely to be a rather high shutter speed. I have a NIKKOR 600mm lens with a built-in 1.4x teleconverter, for instance. If I followed the reciprocal rule, I’d have to shoot at a minimum of 1/600 or 1/840 of a second!
The other thing to mention is that most lenses with image stabilisation systems have a slider on the barrel that lets you choose between up to three different options. These vary by manufacturer, but these are the Canon options:
1: Standard mode for stationary subjects, operating both vertically and horizontally
2: Panning mode for horizontal or vertical panning
3: The same as 1, except it’s activated only when the shutter release is pressed.
Exposing To The Right (ETTR)
Some photographers expose to the right to reduce noise levels at high ISOs. This means deliberately overexposing the image and then darkening it in post. The idea is that noise is usually worst in the shadows, so the extra brightness will help in those areas.
However, this only works if you reduce the shutter speed or widen the aperture. If you shoot in manual (or any other exposure mode) with auto ISO, the camera will simply increase the ISO. That doesn’t increase the amount of light hitting the sensor, so it won’t have any impact on the noise levels.
In any event, the value of exposing to the right is marginal at best. It’s hardly noticeable in the final image, and it also leaves you open to the danger of clipping your highlights if you’re not careful.
Noise Reduction in Post
These days, it’s possible to reduce noise very successfully in images with high ISOs by using a variety of photo editing programs, including Lightroom, Photoshop and Topaz Photo AI. However, the choice isn’t either/or. You can do both!
In any case, it’s always a good idea to get as much right in camera as possible. That means keeping your shutter speed as low as you can to give yourself the best possible chance of recovering a clean image.
Verdict
Pig-aboo
If you want to preserve image quality by keeping your ISO down, a useful trick is to use the doubling rule. It doesn’t work with moving subjects, but it can be very useful for portraits in low light. The image stabilisation systems in modern mirrorless cameras make it possible to use very slow shutter speeds successfully, so it’s often worth a try.
There are various other ways to reduce noise, but there’s nothing to stop you doing those things as well as following the doubling rule. If you get as much right in camera as possible, you’ll be more likely to end up with a good, clean image.
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