This is part of a four-part series on exposure.
Last week we talked about basics of exposure – essentially what does what. I’d like to begin going into these in some more detail, starting today with Shutter Speed.
Shutter speed, aperture and ISO are inextricably linked together. To achieve your perfect image, all of them are equally important and they have to balance each other out. Sometimes this link is more or less obvious than at other times, but it’s always there.
There are countless combinations of these three elements that will grant you a technically accurate exposure… that is, provide enough light to show all the detail in the picture, without it being under or over-exposed. This is generally pretty easy to achieve; set your camera to auto and press the shutter button.
But the real fun begins when you start to look at possible photo opportunities with some understanding of the way each of these elements will affect your image. Then, instead of simply trying to get the right amount of light to create an “okay” looking photo, you can choose your settings creatively to gain the best result from the subject in front of you.
Before taking each photo you can think, “Do I want this whole scene in focus? Or do I want to isolate just one area and blur the rest?”, “Do I want to freeze the motion in my photo? Or would I prefer a stretch of sweeping motion-blur to create a different, more dynamic effect?”, and even, “Do I want this image to have a completely smooth finish, or would this subject be complimented by a more gritty, noisy texture?”
As you make a practice of thinking like this, and of experimenting whenever possible, more and more possibilities become obvious with every photo opportunity.
Selecting the shutter speed, for me, is like making a choice of how much time I want to capture and bottle up into my photograph.
With photography, you can capture a tiny millisecond of time, and keep it to look at for all eternity (or something less dramatic than “all eternity” if you prefer…), or you can choose to capture more time – a second, several seconds, a minute – and compress it into a single image.
This works in the same way for light as well as motion. When you choose a long shutter speed for a stationary low-light photograph, you’re catching all the light spread across the time your shutter is open and compressing it into one image, as if that much light had been available all along.
Okay… so how does your choice of shutter speed affect your choice of aperture and ISO settings?
Say, for instance, you have a photo of some water pouring through a canal lock. It’s accurately exposed at a shutter speed of 1/500, but now instead of being able to see all the water droplets, you want to capture the rush and movement as water the forces its way through the barriers.
The obvious thing to do is to slow your shutter speed down (increase the time the shutter is open) to blur the movement, but wait… your aperture is open wide at f/1.4. If you simply increase the shutter speed, too much light will hit your sensor and you’ll get an over-exposed image. All the beautiful detail in the water will be bleached out.
So what do you do? You close the aperture (in this case you close it all the way down to f/16). This shuts out a lot of the light entering the lens, allowing the slow shutter speed to capture the water’s movement without letting in too much light, therefore saving the highlights from being over-exposed.
With this photo you started with your ISO set at 100 (the setting with low light-sensitivity). If you had begun with your ISO on something higher, like 400, you would have brought it down to 100 in addition to closing your aperture, because a low ISO means less of the light that’s allowed through will be caught on the sensor, meaning a longer shutter speed is possible than with a higher ISO. In this case it was already low, so there was no need to change it. (If this bit sounds like confusing gibberish, fear not, it’ll all make perfect sense soon, I promise!)
And in reverse. If you need to take a photo at a high shutter speed, but there is not enough light, you can open the aperture up (to a smaller f/stop number), and/or increase your ISO setting, to capture more light and be able to take photos with a higher shutter speed.
I took this next photo while the sun was setting and the light was fading fast. By increasing the ISO to 400, I was able to take the photos at a shutter speed of 1/200, just fast enough to catch the seagulls in mid-air as they swooped in to catch some bread being thrown. If the ISO were any lower, I would not have been able to use a fast enough shutter speed. I could also have opened my aperture a little more than f/6.3 to let in more light, but that would have decreased my Depth of Field, making it even harder to render all of the gulls in focus.
Next week we’ll be looking at using your aperture to affect the Depth of Field of an image, as well as how your aperture affects your choice of ISO and shutter speed settings.