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May 12, 2017

Exposure – There are 3 main variables that effect how the image gets exposed by your camera. The two most important are the shutter speed and the aperture or f-stop. The third is the ISO, or how sensitive your camera’s sensor is to the light that’s falling on it.
Since understanding the ISO is the least complicated of these three I’ll talk about it first. Essentially the ISO represents a trade-off between how well the camera can capture an image under low-light conditions, and image quality. The higher the ISO the greater the camera’s sensitivity to light, but also the higher the ISO the more ‘noise’ will degrade the quality of the image.
Think of noise as the digital equivalent to grain in film. The higher ISO films could capture images in low light but they showed more grain. Digital photography has the same issue, with grain being replaced by noise.
Noise really only matters if you’re going to be enlarging your file. If all you plan on doing with the photo is to look at it on a small monitor or post to your Facebook page then it really doesn’t make any difference. But for those shots that you’re going to want to enlarge or use on a larger monitor then noise can become distracting.
So which ISO setting should you use on your camera? Well for images that you’re going to want in high quality I’d keep the ISO in the low range, around 100-200. For general shooting you should be safe with an ISO setting of 400, but for those low-light situations where getting the shot is more important than optimum image quality, then you can use an ISO in the range of 800 and higher.
Your selection of ISO also comes into play when dealing with the other aspects of exposure, the shutter speed and aperture. I’ll discuss shutter speed in this post and take up the subject of the aperture in the next one.
The shutter speed represents the amount of time that the shutter is open and light is reaching the camera’s sensor. Whenever you double the shutter speed twice as much light reaches the sensor, and whenever you cut the shutter speed in half then half as much light is used for the exposure. So changing your shutter speed from 1/125th of a second to 1/60th of a second lets in twice as much light, but shooting at 1/250th of a second would only let in half as much light as the 1/125th of a second shutter speed.
Why does this matter? It all has to do with motion, both that of your subject but also how well you can hold your camera steady. Is your subject in motion? Do you want to freeze that motion or do you want a blur? Think of a car speeding across the scene, if you use a fast shutter speed its motion will be frozen, but if you use a slower shutter speed the car will be blurred. There isn’t really a right or wrong answer here, it’s just what you’re looking for that matters.
1/400th of a second freezes the car's motion

1/50th of a second creates a slight blur in the car's motion

1/15th of a second creates much more blur
The direction of that motion also comes into play. What if the car in the above example was coming towards you instead of across the scene? Motion that is coming towards you appears to be moving much less than motion that is moving across the frame, so to freeze motion for an object that is moving towards the camera you won’t need as short a shutter speed as you would for an object that is moving from one side to the other.
But what about your motion? We’ve all seen blurry photos that have nothing to do with the moving of the subject but have everything to do with the moving of the camera. Hand-holding a camera is only good down to a certain shutter speed, any slower than that and the image will show signs of camera shake. With full-frame 35mm cameras the rule for hand-holding the camera is that the shutter speed should be no slower than the reciprocal of the lens length, so if you’re using a 50mm lens you can hand-hold the camera down to 1/60th of a second (the closest shutter speed to 1/50th.) Then using this rule, other examples would be if you’re using a 200mm lens you shouldn’t hand-hold below 1/200th of a second, and if you’re using a 28mm lens you can hand-hold all the way down to 1/30th of a second.
However most consumer-grade digital cameras don’t have a full-frame sensor, and the lens lengths might be listed in 35mm equivalents or they might be listed as their actual lengths, so it makes this rule a little murky. Given this I’d err on the side of caution, and hand-hold using a shutter speed setting that is one step faster than what the rule for full-frame cameras allows. Experiment! The worst that can happen is you delete the mistakes.
If you’re shooting in a situation where you need or want to use a slower shutter speed than hand-holding will allow, and for the moment ignoring the effect that changing your aperture will have on the exposure, it might be possible to change the ISO to a higher setting until you reach a shutter speed that is high enough for you to hand-hold the camera. Just keep in mind the trade-off between the ISO setting and noise in the image that I described above.
Another option would be to find a more stable way to support the camera. While a tripod would be the best solution it isn’t always the most practical, but bracing your camera against a solid object like a door or tree, or propping it up on a table might give you just the added support you need.
Next up…there’s a hole in my lens! Well there had better be, and the size of that hole has a whole lot to do with exposure!