So what is a camera?
In its simplest form, a camera is a light-tight box with a means of letting light in at one end, and a means of converting and saving the light at the other end. This was film, but is now a sensor.
Letting the light in. This is usually done with a combination of a lens & shutter. The lens will probably have a few glass elements with a DIAPHRAGM somewhere between the front and the rear element.
The diaphragm aperture (the hole in the middle) is adjustable so it can let in more or less light. Think of it as a tap which controls the volume of light: the wider the aperture the greater the flow of light.
The size of the aperture is measured in STOPS or f-numbers, f/2.8, f/8, f/16 or whatever but most digital cameras show these just as the number. The SMALLER the number the more light the diaphragm lets through. Think of them as a fraction, ie f/4 = ¼ and f/16 = 1/16. Obviously ¼ is a bigger fraction than 1/16, thus the tap is open wider at f/4 than at f/16.
I’ll quote from my notes when I was teaching:
“The f/stop is a number that, mathematically, reflects the diameter of the hole in the lens through which the light passes. The number is independent of the FOCAL LENGTH of the lens but, of course the actual diameter varies depending on the lens’ focal length.
Whole Stops are:
f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32
f/1.4 lets in exactly twice as much light as does f/2 which lets in twice as much light as f/2.8 etc.
We can do the maths if you want!
(N=f/D where N=f/stop, f= the focal length of the lens and D= the diameter of the aperture)”.
(Sorry! I did promise no science or formulae.)
The size of the aperture has other uses as well, which we will consider later.
If the aperture is the VOLUME of light coming in, then the shutter controls the DURATION that the chosen volume is allowed in for. The shutter speed is measured in fractions of a second or in whole seconds ie 1/25th, 1/250th, 1/1000th, etc. but these again are usually shown on a digital camera as whole numbers – 25, 250, 1000 etc. Again, 1/25th is a bigger fraction (of a second) than 1/500th and thus lets more light in or, strictly speaking, it lets the same volume of light in for a longer period of time. Think of it as a sluice-gate.
Nowadays, DSLR cameras all have a focal plane shutter which consists of two blinds that whizz horizontally in front of the sensor with a vertical gap between them. The wider the gap then the slower (longer) the shutter speed is deemed to be. The gap between the blinds passes across the “focal plane” and thus each segment of the sensor is exposed to the light for the same amount of time, as determined by the shutter speed set by the photographer.
The shutter speed has other roles to play, in addition to helping to determine the correct exposure, which we will consider later.
So we have a tap to control the volume and a sluice-gate to control the length of time. If the sensor is thought of as a bucket, then the photographer has to control the volume & time to totally, and exactly fill the bucket to its capacity.
If you overfill the bucket then the photo is overexposed and the highlights will be “blown” ie have no detail in them. Under-fill the bucket and you underexpose the image resulting in “blocked” shadows with no detail in them.
I have a book, “Perfect Exposure”, that is 180 pages long. Getting that bucket filled just to capacity, no more no less, is a major undertaking!
A few paragraphs ago I referred to the light being let in to the camera through a lens, but it is possible to take a photograph without having a lens attached to the camera – the photos below were taken using a “pinhole” adaptor mounted on the camera in place of the body cap – no glass involved. Pinhole photography is one of our photographic ancestors, as was the camera obscura.
Now take a look at your tripod – three legs for maximum stability. Correct exposure requires three “legs” too:
And the third leg is ISO.
ISO used to refer to the sensitivity to light (the “speed”) of analogue film. Nowadays it is a means of increasing or reducing the sensitivity of the camera’s sensor.
We’ll discuss ISO, and put all three elements of exposure together, in the next post.
Now go out and take some photos! The two below are using a pinhole instead of a lens – no glass in front of the shutter.