Ever wondered what some of the arcane terms I use in my articles mean. This is an overview…
This is a term which tends to be used to describe a certain type of viewfinder camera made between the late 80s and late 90s. There is not strict definition, but these cameras would tend to have features such as DX film speed sensing, auto-focus and auto-exposure, motor wind and various exposure modes such as red-eye reduction, slow sync flash and so on.
In it’s simplest term ‘aperture’ refers to the size of the hole in the lens that the light enters. Basically at some point in the construction of the lens an opening restricts the size of the light transmission path. What is slightly spooky is that the size is not the actual size of the opening, it is the size of the image of the opening as it is seen through the front of the lens.
Aperture is measured relative the to focal length of the lens, with the equation N = f/EN, where N is the aperture value, f is the focal length and EN is the diameter of the pupil. Thus for a focal length of 30mm an aperture with a value of 4 would have a diameter of 7.5mm as 4=30/7.5.
As the aperture number is a function of the focal length this value of 2 is expressed as f/2. Because it is each doubling of the area that will allow double the amount of light to pass, a multiplication of about 1.41 in diameter will produce this doubling of area. So the aperture scale goes 1, 1.4, 2, 2.8, 4, 5.6, 8 and so on. What all this means is that if you know the focal length of a lens you can measure the opening and work out the aperture value. Or vice versa. And you can make reasonable job of this due to the fact it is the image of the opening on the front element; you don’t have to open up the lens if it is hidden inside.
Clean, Lubricate and Adjust. A basic camera overhaul.
The Full specification for DX (Digital indeX) consists of a unique identifier for the manufacturer and type of a film emulsion through barcodes on the canister and as a latent image on the film itself and a method for cameras and processing equipment to automatically read information about the film.
For photographers, the most important part is a method for 35mm cameras to automatically set read the film speed and pass this information to the light meter. The film canister has a series of square contacts on the back created by a pattern of non-conductive paint and bare metal. Pins in the camera body read these contacts and as well as film speed can gather information on the number of frames and exposure latitude of the film.
The contacts are arranged as in thge diagram on the right. It makes sense that from this one can also assess the possible films speeds a camera can set by examining the pins.
You can read more about DX coding on Wikipedia and there is a helpful Java applet to help you read the codes on canisters.
The film plane is the position in the camera body where the light from the lens strikes the film (or the digital sensor), so this is used in relation to the calculations for the focal length of the lens.
The focal length of a lens is the point from a certain point in the optical system to one of the principle foci (for practical purposes the image plane). In a thin lens (where the thickness is negligible compared to the focal length) this is from the centre of the lens. In a multi-element camera lens it is from the rear nodal point (which can be outside the physical structure of the lens, woooo, spooky).
Focal length affects the field of view (how much you can fit within the frame) and the depth of field (how sharp the image remains between the foreground and background). You can read more about this relationship in detail here.
An idiom for ‘Gear Acquisition Syndrome’, the tendency for photographers to acquire large quantities of equipment with the hope that this will improve their photography. There is an obvious reference to bloating and flatulence in this. It is debatable how far this is polite to apply to genuine collectors.
Guide number is a measure of the power of a flash output. It is normally given as a figure in metres or feet (sometimes with an ISO number, though if one it not given this usually means at ISO 100). The equation is GN = distance × f-number. Thus a guide number of 14 does not mean the flash correctly exposes at 14 metres; it means that it would do so for an aperture of f/1.
A more likely example would be a guide number of 14 with an aperture of 8 would have a range of 1.75m (1.75 x 8 = 14). For Cameras with an adjustable aperture knowing the guide number of the flash can allow you to set the correct aperture for distance. For simple point and shoot cameras if you know (or estimate) the aperture knowing the guide number would allow you to estimate the coverage of the flash.
Guide numbers follow the same relationships as apertures. If you do the sums you will need that to double the distance you would need to open an aperture by 2 stops, not one (as you might expect because this would halve the shutter speed). Thus, to change the equation for ISO 200 film you would multiply the distance by 1.4; double it for ISO 400 and so on.