October Tutorial: Shutter Speeds and ISO (Part II)
As you work on your assignment, you may notice, as you test various shutter speeds, that something happens. As discussed in a previous issue of Canadian Scrapbooker, you know that the shutter speed and f/stop change correlatively; that is to say, as your shutter speed increases, your f/stop decreases. Remember the glass of water/running tap analogy I used? To recap, if it takes 30 seconds to fill a glass of water with the tap barely open, and 10 seconds to fill with the tap opened wide, logic concludes that it would take 20 seconds to fill it with the tap open halfway. Shutter speed and aperture works the same way.
But there’s another aspect to photography that can affect the outcome of your photos: ISO.
ISO, thanks to www.dictionary.com, is described as “the numerical exposure index of a photographic film under the system adopted by the International Standardization Organization, used to indicate the light sensitivity of the film’s emulsion.”
Back in the days of film, you could choose films of various ISOs (or sometimes refered to as ASA (American Standards Association), depending on your subject. For example, generally ISO 100 was used for standard outdoor or indoor flash photos, while many professional sports photographers used film with ISO 800 or 1600. The higher the ISO rating, the more sensitive the film was to light. The trade off was that the resulting photos had higher contrast and grain.
In digital photography, we can use ISO as part of the equation with shutter speed and f/stop to control our image results. Using a higher ISO will increase the amount of light that will reach the image sensor, and in turn, decrease the shutter speed. You know how shutter speeds increase approximately double each time (i.e. 1/60 s., 1/125 s., 1/250 s., 1/500 s., etc…)?* Well, ISOs are the same way – and they basically double the amount of light that enters the camera with each increase. The same results occur when using a higher ISO – except what appeared as grain in film, is referred to as “noise” in digital photography.
* These are standard shutter speeds correlating to film. Many cameras today have included 1/3 stops - for example: 1/500 s., 1/400 s., 1/320 s., 1/125 s., etc.
So, to illustrate, let’s go back to Part I and look at Figure 2.
Taken with Nikon D70, 50mm f1.8 lens at f/1.8, 1 sec. ISO 200
With camera settings of F/1.8, 1 sec. and ISO 200, I was at my widest aperture for my 50mm prime lens. To achieve the same exposure, but with a faster shutter speed, my only option would be to adjust the ISO setting.
By changing my ISO from 200 to 400, I am doubling the amount of light that enters the camera. Therefore, I can change decrease my shutter speed to 1/2 sec. and achieve the same result. Remember that as more light enters the camera, the shutter does not have to stay open as long.
(Please note – I would have had an example for you, but my son decided that he had had enough of being a model for the time being… 
)
The following settings would offer the same exposure:
original setting: ISO 200, f/1.8, 1 second
option 1: ISO 400, f/1.8, 1/2 second
option 2: ISO 800, f/1.8, 1/4 second
option 3: ISO 1600, f/1.8, 1/8 second
option 4: ISO 3200, f/1.8, 1/15 second
The important thing to remember is that while the above settings result in the same exposure, the amount of noise in the image would increase with each ISO change.
Many cameras allow you to set the ISO to automatic, in which it will change the ISO, depending on the amount of light available. To avoid this, check your camera’s manual – you can change this setting to set it yourself for each photo. Just remember to change it back to ISO 100 or ISO 200 for everyday photos to avoid “noisy” photos!
So, are you ready for some Hands On Photography practice?
Your October assignment:
Using a slow shutter speed, or a combination of a wide aperture, slow shutter speed and change of ISO, capture a nighttime or low light image. Try using candlelight or even moonlight (with a long shutter speed!). Use your imagination for your subject and composition, and don’t forget – the beauty of digital is that you can try many different settings! Don’t forget to bracket your exposures for the best results.
Deadline: October 31st, 2009
Please submit your photo(s), along with camera settings, using our submission form only (also linked above) for a chance to be entered into our draw. A randomly chosen entry will be drawn on November 1st, 2009 and the winner will be posted within a few days of the draw! Please click here to review the rules regarding this draw before submitting.
Please stay tuned for more information about our October prize package!
Thanks for playing along this month, and be sure to join us on November 1st, when we will talk about having fun with light and long exposures!
Happy Clicking,
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Sue has been behind a camera for more than 25 years, working professionally as a photographer for the last ten years. While she began the hobby of scrapbooking in 2000, as a complement to her photographer career, it soon grew into a rival passion! Sue loves to enlarge one of her favourite photos and make it the focal point of her page, using her supplies to bring the focus onto her photography. She has, in recent years, turned more to scrapbooking "moments" rather than "events", choosing to leave her children more insight into her point of view of their lives.
