This 3rd Wednesday of the month photo enthusiasts meet at Gloucester County College for one light and bounce shoot. I arrived a few minutes late and they have already setup the room with one generic strobe fitted with a 24 inch soft box and a couple of reflectors (gold and white). Our model, Beth, was already warming up with a couple of shooters doing exposure test. Other shooters are getting ready for the shoot while sharing information their shoot experiences or just catching up on things.
After the exposure has been set, it was shared to other shooter to get a baseline for their camera setup. It was not only sharing what ISO, aperture and shutter speed for the camera but why and how these parameter combinations affect the exposure. ISO determines the sensitivity of the sensor, the higher the value the more sensitive it becomes. Aperture is the opening in the lens that controls how much light gets in to the camera’s sensor (the lower the value the bigger the opening). Shutter speed controls how long the sensor is exposed to the light.
Digital cameras have different native ISO values, as an example Sony DSLR Alpha 850 has 200 and has extended values with the minimum of 100 and the highest value of 6400. The higher the value the more sensitive the sensor becomes but the draw back is noise, however, some cameras can handle better high ISO compared to others. Let us say you have a night shot and at ISO 200 you will need a shutter speed of 1/15 second and aperture of let’s say F/4, if you are hand holding this shot you may have a blurry shot due to hand movement but if you dial your ISO to say 800 you can then use a shutter speed of 1/60 second and still the same aperture at F/4. Your shot has a better chance of not getting blurry but it may introduce a little bit of noise which you can take out later by using noise reduction software. The change from ISO 200 to 800 is an equivalent of 2 stops increase and the increase of shutter speed from 1/15 to 1/60 is an equivalent of 2 stops decrease. The change of ISO and shutter speed offset each other, hence, you are still getting the same exposure as the original.
Using the above scenario, if you don’t want to increase your ISO and you have a faster lens than F/4 to say F/1.4 then you can change the aperture to F/1.4 and the shutter speed to 1/60 second. Here, the ISO remains the same and the only thing you changes are the aperture from F/4 to F/1.4 as well as the shutter from 1/15 to 1/60 second. This will also give you the same exposure as that of ISO 200, F/4 and 1/15 shutter speed. The change of aperture from F/4 to F/1.4 gives you an increase of 2 stops while shutter speed gives you a decrease of 2 stops from 1/15 to 1/60 second. However, the exposure may be the same the composition or the outcome of the shot may be totally different because changing the aperture changes the depth of field. The depth of field is the range of focus in your picture (it shows you a range where it clear/sharp), the larger the aperture the greater the depth of field. Say F/2.8 may have a depth of field of 1 foot in front of the focus point and up to 3 feet behind the fcous point, whereas, F/8 may give you 10 feet in front of the focus point and up to 30 feet behind the focus point. This means that anything in the range are in focus or sharp and all the rest are blurred. Aperture is just one factor affecting the depth of field there are also the lens and its focal length.
Shutter speed on the other hand is motion. If you want to show motion, you need to dial a lower shutter speed like 1/2 second if you want movement captured in your shot. This shot will introduce blur on a moving subject. A higher shutter speed of say 1/500 can freeze a moving subject.
These are the three basic parameters that affect your exposure as well as composition of you shot. The values written here are just for comparison purposes only and does not reflect actual values.