Hi and welcome to my blog for April. If you've been following my blogs, so far, you will know that there are three settings which control exposure. By now you should know what ISO and Aperture are and how to use them to your benefit. I hope you've been experimenting and trying out various settings, to see the effect and getting more control of your images, as a result.
Today, I'm going to write about shutter speeds. I kept this until last, as I think it's the easiest to grasp. There are three problems that may arise from using the wrong shutter speed, one is under or over exposure, another is camera shake and the third is motion blur. I will deal with each one separately.
To begin with, we must understand what a camera shutter is and how it works. Your camera sensor is a light sensitive chip, sealed into a light tight chamber at the back of your camera. On the lens side of the chamber there is a shutter or door, which opens to let light through. The length of time this shutter/door is fully opened is your shutter speed. So, if the shutter/door is fully opened of 1/60th of a second your shutter speed is 1/60th of a second.
If you double your shutter speed, the door opens and closes twice as quickly and is therefore opened for half the time. If you half the speed, the shutter opens half as quickly and remain fully opened for twice as long.
For example, if you begin with a shutter of speed of 1/30th second, your shutter is fully opened for 1/30th of a second. We'll call the quantity of light to reach your sensor X. Then increase it to 1/60th second, the shutter moves twice as quickly and remains fully open only for 1/60th of a second, half as long as at 1/30th. This means that light has only half the time to reach your sensor and only half X reaches your sensor.
Change your shutter speed to 1/15th of a second. Now your shutter is fully open for 1/15 of a second, twice as long as at 1/30th. The amount of light reaching your sensor is now 2X.
One of the reasons we would adjust our shutter is to freeze motion. In the image above, of Charlotte dancing, I used 1/200th of a second to freeze the movement of her hair and get a sharp image. A slower shutter speed would have resulted in blurring of her hair and face. Her lower body was not moving at the same speed and would have been less blurred. As a portrait, it would not have worked.
One of the main time you would want to freeze action is at sports events and this often requires high shutter speeds but maybe not as high as you would think. For example, if the person, horse or car is coming towards you, you can use a lower shutter speed than if it was crossing your angle of view. Sometimes the action has to slow down and lower shutter speeds are needed then also.
In the image of the rally car I picked a point where the vehicle had to slow down to get my image. It was a wet, dark day and achieving high shutter speeds would have required high ISO ratings, creating a lot of digital noise. My blog on ISO explains this.
With an ISO of just 640 and F9, I was able to use a shutter of 1/500th. Had the car been going at full speed across my angle of view, I would have needed a much higher shutter speed, perhaps 1/4000th of a second and it the low light I would have needed ISO ratings of 6400 at least. I didn't want to use a very high ISO rating.
Another way of avoiding very high shutter speeds is to use a technique called panning. Set your camera on single focus. Pre-focus on a selected point of your choosing. You may need to use manual focus for this, otherwise your camera will try to refocus, if you relax your finger on the focus button. As you subject approaches, frame it and press your shutter part way down. This gives you an exposure reading. Follow your subject with your camera, keeping your subject centered in the frame and just before it reaches the point where you pre-focused, press the shutter fully and continue to follow your subject until after the image is taken. This will blur the background but, if done correctly, will give you a sharp subject.
This technique takes practice, so make sure to use it often on subjects you can afford to mess up, like your dog running about. You will need a medium shutter speed for this to work, except for something like F1 motor racing.
I used this technique to take the photo of the two galloping donkeys. As you can see the background is blurred, while the black donkey (on my point of focus) is sharp.
There are times when you may want to create motion blur, as an effect, in your image. The most common example of this is running water, water falls and similar. The achieve this effect it's always better to have a tripod or place your camera on a solid base. The danger with this is that during a long exposure (slow shutter speed) you will move your camera and get camera shake, as well as motion blur.
If you don't have a tripod, place your camera on a wall or something similar. Use a bean bag, folded jacket or similar to support you camera. This will allow you to aim your camera and if done correctly, will prevent it from tilting forward, during exposure. Use the self timer to avoid any movement caused by you pressing the shutter.
The image above, of a section of the Cascades, in Lahinch, Co. Clare was taken using 1/20th of a second. I chose to blur the water because I think it conveys the power of the falls better. I could have frozen the action, with a high shutter speed but the effect would have been completely different.
As part of the exposure triangle, your shutter speed helps control the amount of light reaching your sensor. If you choose the wrong shutter speed, you may get under or over exposure. Your camera should warn you of this.
You will see something like this on your screen or in your viewfinder
-<........0........>+. This will indicate that the exposure is correct, as the camera sees it. You may want to under or over expose your image, for various reasons(I'll discuss this in a later blog) but lets assume you want to have balanced or correct exposure.
The chart here shows you the different ways the symbol above is use to let you know if your exposure is correct. If you are using shutter priority mode, the camera will make the correction automatically, by changing one of the other settings. You only have to worry about it if you are using fully manual.
With practice you will learn how to compensate for under or over exposure by changing the ISO or aperture.
You'll experience camera shake when you use slow to medium shutter speeds, if you don't stand correctly or if you move during exposure. Holding your camera correctly is also important to avoid camera shake.
My next blog will be about how to hold your camera correctly and how to choose shutter speeds to suit your lens and subject.
Thank you for reading. I hope you find it helpful and as usual, if you have any questions, feel free to ask, using the comments section below.
Until next time, take good care of yourself and those you love.