Controlling the ISO sensitivity setting on your camera is one of the fundamentals of photography, and affects both exposure and image quality of your pictures.
Once you’ve mastered the aperture and shutter speed controls of your camera, the next key element to achieving great pictures is to understand the benefits and pitfalls of controlling the light sensitivity of your camera’s sensor.
Let’s start by looking at the benefits.
By being able to increase the sensitivity of your camera’s sensor to light, less available light is needed to correctly expose the shot. This allows you much more freedom in the way you shot. It allows you to use faster shutter speeds or smaller apertures when they’d normally not be enough light to expose the shot correctly.
For instance, rather than being stuck with a slow shutter speed when shooting indoors, you can crank-up the sensitivity of the sensor, allowing you to use a faster shutter speed and still expose the shot correctly – without having to alter the aperture.
There’ll also be times though when you want a low sensitivity – long shutter speeds for blurring movement aren’t suited to high sensitivities. While it’s a similar story for shallow depth-of-field shots on a bright day. With your aperture wide open, you’ll want a low sensitivity selected to avoid overexposing the shot.
All this isn’t a problem because we can control the sensitivity of our camera’s sensor to light for each specific shooting situation, choosing whether we need it to be less or more sensitive.
Your camera’s sensitivity is referred to as ISO – a throwback to the days of film, but rather than referring to how sensitive the film is to light, it’s a measure of how amplified the electrical signals are that have been converted from light hitting the sensor.
ISO 100 is a typical baseline rating for a camera, with an increase to ISO 200 increasing the sensitivity by one stop. This will allow you to either use a shutter speed one stop faster (1/125th of a second at ISO 100 would be shortened to 1/250th of a second at ISO 200 for example). Or you can use an aperture one stop smaller – so rather than shooting at f/5.6 at ISO 100, you can shoot at f/8 at ISO 200 and maintain the same exposure as before.
Each time you double the ISO, the sensitivity of the sensor increases by one stop, so at ISO 3200, you can shoot with a shutter speed five times faster or an aperture five stops smaller than you could at ISO 100.
This means that 1/30th of a second at ISO 100 would be shortened to 1/1000th of a second at ISO 3200, or rather than shooting with an aperture of f/2.8 at ISO 100, you’ll be able to shoot at f/16 at ISO 3200.
What does vary is the ISO range from camera to camera – a typical ISO range of a modern DSLR is 100-6400, but some high-end models offer expanded ISO ranges up to 102,400!
Naturally, the broader the ISO range, the more flexibility you have when shooting in low light. There are downsides however…
Look at most specifications of modern DSLRs or Compact System cameras, and along with a standard ISO range being listed, you’ll often find a secondary expanded ISO range that goes beyond that.
This will vary from camera to camera, but lets take the ISO of the Nikon D5100 as an example. It has a native range from 100-6400, but also offers an ISO equivalent of 12,800 at Hi1 and 25,600 at Hi2. So if you can shoot at an equivalent of 25,600, why not call it ISO 25,600?
It’s all down to some clever stuff in-camera. The camera will capture an image at its maximum native sensitivity (in the case of the D5100, ISO 6400), but at a reduced exposure before processing the file to produce a correctly exposed shot.
As this differs from the standard way of amplifying a signal it is not considered to be part of the camera’s native ISO sensitivity range, although the camera is still capturing at the sensor’s full effective resolution.
Some models also have baseline ISO 200, with the option of an expanded range down to an equivalent of ISO 100 – handy for studio photography with flash, or long creative exposures where movement is desired.
Rather than shortening the exposure at its lowest native sensitivity, the exposure is lengthened, with processing again being undertaken to produce a correctly exposed shot.
Shooting at expanded ISO equivalents does have its disadvantages though, specifically around the dynamic range of the shot.
The in-camera processing leaves less headroom for highlight and shadow details to be recovered, so these settings should be only really used when the standard ISO range of your camera restricts you from capturing the shot you want.
While shooting at higher ISOs does provide you with flexibility under a range of tricky lighting conditions to get shots that wouldn’t be possible at lower sensitivities, it does have its downsides, specifically image noise.
Image noise is the digital equivalent of film grain. With higher sensitivity film, the silver crystals used were larger to improve their light gathering capabilities, which resulted in a grainy appearance compared to the smaller, less sensitive silver crystals used in slower ISO film.
And it’s a similar story with digital. As you increase the sensitivity, amplifying the signal, image noise is generated.
This appears as random speckles across the entire image, with fluctuations in colour, not dissimilar to film grain, though probably not quite as aesthetically pleasing. The more you increase the ISO sensitivity, the more pronounced image noise becomes and the more image quality degrades.
Image Noise Variations
At an identical ISO setting, different cameras will display varying levels of noise, with some handling image noise better than others.
The main reason for this is down to the physical size of the sensor used by the camera. A compact has a much smaller sensor than that used by a DSLR, if both models share a similar resolution, it’ll be the DSLR sensor that’ll deliver the better results of the two. This is because the photosites (pixels) have more space on the sensor on the DSLR, are larger and have better light gathering capabilities.
This provides a stronger signal compared to the smaller, more compact photosites used by the compact camera, resulting in better image noise handling.
It’s not the physical size of the sensor that plays its part with image noise, its also the design of the sensor and the complex in-camera algorithms that can make the difference too. Cameras with virtually identical sensors sizes and resolutions can see one model control image noise better than the other.
Combating Image Noise
There are ways of controlling image noise, allowing you to get the best possible results. Some models offer in-camera high ISO noise reduction settings, which are applied to your JPEG files.
You can choose to have it deactivated, or set to Low, Normal or High. As you increase the strength of the setting, the random speckling is reduced, but the trade-off is image sharpness. Fine detail can suffer slightly, though you may have to zoom into the image quite a bit to notice any differences compared to shooting without High ISO NR.
If you’re shooting in Raw, then you can control image noise once you’ve opened the file up in a Camera Raw converter, such as Adobe Camera Raw that’s part of Photoshop and Elements.
All you have to do is click on the Detail tab, zoom into 100% and then use the Luminance and Color slides to correct image noise. The Luminance slider is to control the grain-like look of the image noise. Increase it, and the effect is reduced, though push it too far and images will tend to look ‘plasticky’, with and unnaturally smooth finish. The Color slider controls the colour artifacts that can appear in high ISO noise images, reducing it as the slider is increased, though sharpness can be affected.
While you won’t eradicate image noise from your shots, you’ll be able to control its impact on your image, allowing you to control the final look of the shot.
Other Noise Reduction
Image noise can also be a factor at lower ISOs as well, though you’ll probably only encounter it if you’re shooting a long exposure. Slightly different in appearance to random image noise at higher ISOs, fixed pattern noise is the result of heated generated by the sensor being active for a longer than usual period. This produces image noise, with what commonly termed ‘hot’ or ‘stuck’ pixels, which are brighter than normal and degrade the image quality. This can be resolved with an in-camera feature known as Long Exposure Noise Reduction, which most cameras have. By taking a duplicate image with no light – known as a Dark Frame – the camera can plot where these hot pixels are, and then correct for this in the final image. You’ll find that if you take an image with a 30 second exposure, you’ll have to wait another 30 seconds while the camera takes a dark frame and corrects the image before you can see the final result.
Knowing What ISO to Shoot With
There will be times though when you need to use a tripod to avoid camera shake, but you’ll be rewarded with noise free images.
Using a tripod is not always practical or possible however, and a high ISO will be needed. Many modern cameras control noise very well up to ISO 800, so using these settings for general photography shouldn’t be too much of an issue.
Above that, and you’ll start to see your image degrade as you push the sensitivity further, but it may mean the difference between getting a shot and walking away with nothing.
The best advice would be to start at your camera’s lowest ISO sensitivity, and if you’re unable to achieve the shutter speed or aperture you desire, increase the sensitivity until you do.
Author: Matt Tuffin