The Basics : Shutter Speed, Aperture and ISO Sensitivity
Since I got this question on the relationship of shutter speed, aperture & ISO,,,, twice in one day!… I guess this is a good time as any to go back to photographic basics for a second. If you are a seasoned photographer this article may not interest you much… however if you starting to get serious in photography, I can’t stress more of the importance of this Threesome. More than gear itself!
The right exposure
Figuring out the “right” exposure can be quite a daunting task if you don’t know what you’re doing. Back in the early days of film photography, there were no automatic exposure settings. Everything had to be through experience & an external light meter. Nowadays, we are armed with the most sophisticated auto focus & auto exposure digital cameras that some of us sometimes take it for granted. These days you can simply let the camera take full control of the situation & well… most of the time they get the job done quite well. However this limits your creativity & even with today’s technology, the digital camera essentially is dumb. It can’t tell the difference between anomalies like poorly placed lights & lamps in a room or even the direction of the sun or whatever light source you are relying upon to get your image.
I’m not saying that you should use manual settings on your camera 100% of the time. I am a big fan of the aperture priority mode & I use it 90-95% of the time & rarely resort to manual mode. The time I use manual mode will be under 3 conditions… (1) The exposure compensation meter on the camera is not enough for me to get my shot, (2) I am using a flash under low light conditions or I am attempting something creative like strobing or using creative lighting & (3) I am shooting videos & the camera has the ability to use manual mode settings.
I use aperture priority mode because aperture determines Depth of Field or DOF. This determines how much of the scene will be in focus. Typically a landscape photo would require more DOF so the photographer will dial an F-Stop of f/11 to about f/16. However if you are going for portrait shots which you would want to have smooth & buttery blur, you would want to dial an F-Stop of f/3.5 or lower. Bear in mind not all lenses enable you to dial F-Stops lower than f/3.5 so do a check on your gear. Basically I use aperture priority because I want full control on the DOF when I take my photos.
ISO sensitivity indicates the sensor or film sensitivity to light. It was referred to as ASA rating many years back. Despite the digital camera age, this term continues to be used. The term used to indicate a difference in light sensitivity is “stop”. One stop in reference to ISO means doubling the camera’s sensitivity to light. ISO200 is twice as sensitive compared to ISO100… so it is refered to as “one stop higher”. Back in the days of film photography, you will need to decide the sensitivity of the film you were going to use before you went out shooting. These days, you can change your ISO settings on the fly… hence the advantages of digital photography.
Great! Now you can pump your ISO settings to 3200 or even 6400 if your camera is capable of it & just shoot using that ISO sensitivity… right? Well… using high ISO settings isn’t without its disadvantages. The higher your ISO sensitivity, the more noise you will generate in your final image. So you will want to use the lowest possible ISO setting for the shot without compromising your other 2 exposure settings (shutter speed & aperture). It’s all about balance really. DSLR cameras have lower noise levels on high ISO compared to point & shoot digital cameras because they have a larger sensor which is more sensitive to light. The larger lenses that they use also allows in more light into the camera than for instance a small compact camera.
The “shutter” covers the sensor or film & it only opens when the shutter-release button is pressed. Like ISO, shutter speed is also measured using “stops”. For instance, 1/160s is one stop higher than 1/320s.
Great! So now if we want more light, we just open the shutter for a longer time & all your photos will be bright & shiny… right? Well…yes in a way but the question is how are you going to ensure that the shot will be steady & there won’t be any motion blur? Camera movement when you press then shutter release button will cause the camera to shake & affect the sharpness of your image. So again here there is a compromise that you need to take into consideration. Just now we talked about the lowest ISO sensitivity possible to get the shot. Now if you are hand-holding the camera, you will need the highest possible shutter speed to ensure that there is minimal camera shake.
As a guide, for handheld shooting, we require at least 1/(focal length) or shorter to avoid camera shake. This assuming the lens does not have any Image Stabilizers. On a full frame camera like the Canon EOS 5DmkII, if you are using a 50mm standard lens, you will require a 1/50s or faster to avoid camera shake. If you put that same lens on a cropped sensor body like the Canon EOS 60D or EOS 550D, you will need to multiply the crop factor to the focal length. So you will be needing at least a 1/80s or faster.
We talked a little about how aperture affects DOF at the beginning of this article. Now we are going to talk about how it affects the amount of light which comes into the camera.
Since the aperture controls the opening in which the lens allows light to come into the camera, the larger the opening… the more light gets to enter & reach the sensor or film. A large aperture is denoted using a smaller f-number. So an aperture of f/4 is two stops larger than f/8.
Remember that using a large aperture reduces DOF & vice versa. So bear in mind the trade-offs that you will need to make when deciding which aperture size to use for the shot.
Controlling your Shutter Speed, Aperture & ISO sensitivity for Exposure
With the three controls of the camera at your disposal, this will be how you will control the exposure on your camera. At a given brightness level where you are capturing the photo, there are varying degrees of settings which will enable you to capture the light. There will be a trade-off to how low an ISO you can go, how fast a shutter speed you can use & how small an aperture size that can capture enough light so to properly expose the image you are trying to capture.
For instance, in a situation under low light conditions, let say you want to freeze the motion of a running child. Normally you would use 1/160s but because you want to ensure the child is not moving in the photo, you may want to use 1/320s which is a stop slower. In order to compensate, you can either open your aperture one stop wider or use an ISO sensitivity setting of one stop faster to get the same exposure. Depending on the situation, if you are already using ISO800, you may want to avoid using ISO1600 because of the potential noise levels generated. So for that instance, you will use a larger aperture size to get your image. However if you are using a 50mm & you are already shooting wide open at f/1.8… it would be impossible for you to open the aperture any wider. So you may need to bite the bullet & use ISO1600 instead. If you are feeling rich you can always buy a 50mm f/1.2 lens so you can get an aperture of f/1.2 & get that additional one stop which you are missing. Note that such lenses will cost you an arm, so it may be best to work with the gear you have to improve your photography skills at this point in time.
If you do not have moving objects in the image you are trying to capture, using a tripod will help you shoot using the lowest ISO or smallest aperture sizes without compromising on camera shake when you use those slow shutter speeds.
The Sunny 16
If your camera’s light meter fails or if you are using a combination of some old lenses like the Nikon AIS lenses on older DSLR bodies, you will not have any exposure guide when you want to take a photo. As a Sunny 16 guideline, at f/16 your exposure of a brightly lit sunny day should be 1/(ISO rating) to keep the highlights from overpowering your camera sensor or film. If you are using ISO100, your exposure should be 1/125s which is closest to 100. A one stop reduction to 1/250s will require you to open your aperture to f/11 (from f/16).
I hope this article has been useful to new DSLR users on this topic… even though basic, but is the most important fundamental in photography.