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All You Need To Know About ISO Speeds In Photography
ISO is the final point on the exposure triangle. Controlling ISO speed is incredibly useful when it comes to shooting in low light. But like aperture and shutter speed, ISO does not just impact exposure value; it has side-effects too.
What is ISO?
ISO stands for International Organization for Standardization (I don't know why the letters are the wrong way around!). This organization standardizes the scale against which a digital camera sensor's sensitivity to light is measured. This sensitivity is measured in terms of speed: ISO speeds. The faster the speed, the greater the sensitivity to light and the faster an image is created.
ISO speeds are the direct equivalent of film speed in film photography. But the great advantage of the digital system is that we don't have to use different roles of film for different degrees of sensitivity. We can just switch the speed up or down at the press of a button.
How is ISO speed measured?
ISO speed is measured on a scale of numbers that begin at 100 or 200 on most cameras. At each step up on the scale the camera sensor's sensitivity to light doubles (just like shutter speeds and aperture sizes double with each setting). Digital cameras are capable of faster and faster ISO speeds every year, but the scale often looks like this:
Interaction with aperture and shutter speed
We've seen how aperture and shutter speed interact in the previous 2 lessons. Well, ISO adds an additional element of control by allowing us to speed up the time it takes for an image to be exposed, without having to open up the aperture or slow down the shutter speed. This is a great asset when taking hand held shots in low light, where sometimes a wide aperture and slow shutter speed just aren't enough to capture the scene clearly.
So for example, say you're at a party and your friends gather for a photo. You set the shutter speed to 1/50 (about the slowest possible for hand holding without risking camera shake) and the aperture to a wide f 2.8. The light meter tells you that the shot will be under-exposed, but you take it and see. Sure enough it's way too dark, and the histogram shows most of the information is in the darks/shadows (left hand side of the graph).
Instead of just reducing the shutter speed and taking a blurry photo, all you have to do is increase the ISO speed and leave the other 2 exposure variables exactly as they are. So the ISO speed goes from 100 to 800 and you've captured your moment! But...there's a catch.
Side-effect of increasing ISO
The higher the ISO speed the greater the level of noise, which appears as a grainy texture in your photo. What is noise? Well, firstly let's understand how your camera's sensor works:
Digital image sensors gather light through millions of photosites (which ultimately turn into pixels; several photosites amounting to 1 pixel). As this light is gathered it is converted into an electronic current. This current is carefully processed by your camera and ends up being stored in the memory card. The electronic current that contains all of the light information for a photograph is described as the 'signal'.
The problem is that all electronic products create a litte bit of current from within themselves. It's this current that we call noise. In the case of digital cameras the noise is primarily generated by the heat from the sensor. So the unwanted noise competes with the current that we do want, the signal. ISO concerns the relative strength, or ratio, of the signal and the noise. The higher the ISO ratio, the stronger the signal and the better the image quality. The lower the ratio, the greater the amount of noise in the image.
In low light, where the intensity of the light hitting the sensor (and so the strength of the signal) is weak, the ratio of signal to noise drops. In good light, where the signal is strong, the ratio increases. So shooting in dim light decreases the signal to noise ratio. But the ratio can also be decreased by increasing the ISO speed.
As we've seen, increasing ISO helps us to create an image faster in low light. This works by ramping up the speed at which the photosites can gather light and convert it into electric current (signal gain). But it also increases the sensitivity to unwanted current: noise. So as we increase ISO, we drop the signal to noise ratio, and that is the major side effect.
IS0 800 (quite fast, some noise visible):
ISO 3200 (very fast, far too much noise visible):
What ISO speeds should you shoot at?
As a rule of thumb start at ISO 100 and stay there whenever possible. Although there is still noise at this low speed, it is completely undetectable in the photos that you will take. From then on, it's really a matter of 2 things:
1) The ISO performance of your specific camera. Cameras with larger sensors, where the individual photosites are larger, perform better at high ISO speeds than cropped sensor cameras. Test out how high you can push the ISO before the noise takes over too much.
2) Personal taste. Some pictures demand high quality and low ISO, such as portraits, landscapes and still lifes. But, in my opinion, there is too much concern over increasing ISO some of the time. When it comes to freezing action and capturing moments in time, fast ISO speeds are a great tool, and we shouldn't become too obsessed with chasing technical 'perfection' in photos.
Warning: A mistake that every photographer makes at some point is to shoot at a high ISO speed in low light one day, then the next day, when shooting in great light, forget to reset the ISO back to 100. The result? A shoot in strong lighting that's riddled with noise! I can almost guarantee that you will do this at least once - every photographer I know has. But just try to be aware of it and check the ISO before you start any shoot as a habit.
Typical situations where ISO needs increasing
Here are some of the most common situations that require us to increase the ISO speed in order to get a sharp photo:
1. Indoors, when you do not have flash, do not wish to use flash, or flash is not effective because of the distance of the subject.
2. At sports matches, where a slight increase may be needed even with a wide aperture to enable a really fast shutter speed.
3. At parties, where there is limited available light and shooting with a fast lens isn't enough.
4. At sunset, when you don't have a tripod but want to capture the beautiful light with a hand held shot.
5. Photographing bands at a concert, where the light is dim and the subject is too distant for flash to work.
So, in this lesson we've learnt: what ISO means, how ISO speed is measured, how it interacts with aperture and shutter speed, how ISO effects the signal to noise ratio, what ISO speeds to shoot at and when to increase ISO. I hope you found this useful.