The Downside of Fast Shutterspeed
I have recently purchased a DSLR and I am still learning how to use the camera. I have begun by using the Shutter Priority mode, and set the shutterspeed to the highest value, that is 1/4000sec. I want to know what shutterspeeds should I use for nature and urban shots, where all I really need is a tack sharp image. Should I just keep the shutterspeed at 1/4000sec and lower it when I want to convey movement?
Pravin Khullar, New Delhi
It’s a good thing that you are experimenting with exposure modes and understanding what it can do. You have the very basic principle right. What you need to learn about, however, are the compromises involved with a certain setting. And you do need to grips with the absolute basics of shutterspeed and aperture. The idea is to control what you capture in your frame and how you capture it.
To begin with, when you increase (or decrease) the shutterspeed, the duration of time that the sensor is exposed to light decreases (or increases). The effect of this is the ability (or inability) to freeze action (or get blurs). For a given scene, as the exposure of the sensor to light decreases, the image recorded will get darker or underexposed. In the Shutter Priority mode, the camera automates every other setting in relation to the shutterspeed set.
Thus, the camera’s meter compensates for this underexposure first by opening up the aperture. The aperture is the iris inside the lens of your camera (much like the iris of our eyes) that controls the amount of light that passes through the lens. The effect of the iris or aperture opening up (of moving to a smaller f number), is that the depth-of-field reduces. In other words, focusing becomes more critical as the areas before and after the point of focus gets more and more blurred. This can lead to beautifully blurred backgrounds in portraits, but might not work out so well when you need to shoot a landscape.
The extent to which the aperture can open up in a lens is limited. After all, the lens is a barrel and its size has some physical limitations. One the camera opens the aperture to the maximum possible, and if the image is still underexposed, it starts boosting the ISO value to amplify the signals that the sensor produces from whatever light is being captured. The side effect of this amplification is ‘noise’ (that shows up as grain, splotchy colours or even colour shifts) in your image. This is a problem that you will face with any sort of amplification, even with amplifying music. Beyond a point of amplification, you will notice distortion in the sound or a hissing sound in the silent areas.
So these are some of the compromises you make when you take the shutterspeed so high. The question to ask yourself is what kind of subjects need that high a shutterspeed to freeze the action. For instance, a person standing still for a portrait needs about 1/60 sec. A walking person needs about 1/250sec. A moving car needs about 1/1000sec. A bird in flight needs about 1/4000sec. On the other hand, a landscape (which is not likely to run off anywhere) needs a large depth-of-field (or a sharp foreground to background focus).Tags: aperture, ask the expert, portrait, shutter, shutterspeed, urban photography