A few weeks ago I wrote about the story tripod of the the photographic image; and how the photograph relied on story, technique and composition to convey the moment to the reader of the photograph.
Now I want to write a short piece on the most basic photographic tripod; and this is the tripod of the manual exposure mode of your film and or digital camera.
A camera is a lightproof box connected a hole through which a controlled amount of light gains access to a light sensitive surface producing a ‘drawing with light’, or photograph.
In all cameras light is controlled by just 3 mechanisms; and they are aperture, shutter speed and ISO. The aperture controls the intensity of the light coming through the hole or lens, the shutter controls how long the light coming through the lens is in contact with the film or the digital sensor and ISO or International Standards Organisation controls the sensitivity of the film or sensor to the light coming through the aperture and shutter.
At this point I want to say that there is no single standard exposure setting for bright sun light or late afternoon light as it where. The 3 legs of this tripod work together; aperture, shutter and ISO work together for the photographer’s good, there many exposure options whatever the light conditions are.
Exposure is based on ISO and balanced out by aperture and shutter speeds like an inverted equilateral triangle. ISO is at the bottom and aperture and shutter speed keep the balance on top. ISO in photography can be defined as the film or sensor’s sensitivity to light, ISO 100 is half as sensitive to light as ISO 200. ISO 400 is twice as sensitive to light as ISO 200, and ISO 800 is twice as sensitive as ISO 400 and so on to ridiculous heights with the new digital cameras.
The principal to remember in exposure is one of halving and doubling, ISO 400 is twice as sensitive to light as ISO 200 and so on…
When coming to grips with manual mode on your camera the first step is deciding what ISO film you want to load on your film camera or what ISO you want to set your digital camera at.
Another way of understanding exposure is seeing it as two glasses filling with water under the water tap. The size of the glass is like ISO in exposure, one glass is twice the size of the other glass so it will need twice the amount of water to be filled to the brim. The water in the filled to the brim glass is like exposure in photography. The smaller of the two glasses is like ISO 400 to ISO 200, it only needs half the amount of light to be fully exposed. When filling a glass just to the brim with water whatever the size of the glass you always depend on how wide the valve in the tap is and how long the glass is held under the tap. The valve in the water tap is like the aperture, and the time I hold the glass under the tap is like the shutter speed in the camera.
So when I want a glass filled to the brim or a photograph exposed just right, if I halve the size of the valve through which the water flows I have to double the time the said glass is under the tap. Or if I double the time time I hold the glass under the tap I have to halve the size of the valve or visa versa. It always depends on the size of the glass or the ISO of the film or the camera setting.
Firstly you set your ISO then whatever you do in your aperture setting you have to compensate with your shutter speed until you get your light meter centered on the dial in your viewfinder and visa versa if you slow down your shutter speed by 3 stops you will have to close down with your aperture by 3 stops too. If you open up the aperture by a stop you increase the intensity of the light to compensate for this increase you will have to decrease the time the film or sensor is exposed to light by a stop too or you will over expose your photograph in the end.
By practising this exercise you will now discover that there are many combinations of aperture and shutter speed that will give you a perfectly exposed picture under any given lighting situation.
In closing, no you can’t change the ISO mid film, you should stick to the given ISO.