In photographic parlance, 'TTL' means 'through-the-lens', and particularly refers to a light meter which - as you may have guessed - takes its reading through the lens of the camera to which it is integral.

The advantages of a light meter which takes its readings through the lens are that, firstly, if filters are connected to the front of the lens, the TTL meter will be able to compensate for any light they block; and secondly, a TTL meter is less liable to be interfered with by spurious light from the sides of the scene.

The alternative to a TTL meter is a Non-TTL meter, a feature common on cameras made before the 1970s, the meter most commonly mounted just above the lens. In the case of a Non-TTL meter, the photographer has to guess what kind of effect filters are having on the exposure; furthermore, the lightmeter tends to draw light from outside the photographic frame, meaning that (for example) a photograph taken indoors, in daytime, through a window to the outdoors will tend to be overexposed. Non-TTL meters are typically selenium cells which power a small indicator, the exposure calculated from a complex scale mounted on the camera's top. If the photographer is clever enough to use it, the calculator can be handy for visualising the effect of different shutter/aperture combinations, and thus non-TTL meters, in the right hands, can have a slight speed advantage.

Nowadays, of course, the vast majority of all types of cameras are capable of determining the correct exposure and setting the photographic variables accordingly. With the exception of studio flash photography, lightmeters in general are now the preserve of only the most committed photographers.

Also transistor-transistor logic . One of several types of IC (Integrated Circuit). TTL logic gates are found in most simple Dual In-line Package chips.

Other types include resistor-transistor logic (RTL), diode-transistor logic (DTL), emitter coupled logic (ECL).

TTL circuits were used in Texas Instruments' TTl74 IC series in 1962, and were used into the 90's, when the +5v rail proved too inefficient for modern computer systems. They are still used in simple, DIL chips for teaching/simple logic purposes.

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