Andrew Bailey

Ilfracombe, United Kingdom

After a working lifetime spent in visual communication of one sort of another, Andrew is now happily retired and living in peace and...

More Photographic Terms and Meanings

Absolute Zero.
This refers to the lowest temperature which is theoretically possible and it is the point at which all matter is at rest. Alternatively referred to as Zero degrees Kelvin, after Lord Kelvin, the 19th century physicist, who determined it absolute zero is the Celsius equivalent of -273 degrees.

Absorption Curve.
No surface has a reflective factor of 100%, as there is always a degree of absorption of the light played upon it. A matt white surface will absorb less than a black surface but a polished or shiny white surface will absorb less than its matt or non-shiny equivalent. Usually, light so absorbed is converted into heat energy but it may also be transformed into light of a different wavelength and, therefore, a different colour. It may, as in the case of photographic material, also cause a chemical change to occur. Colour is the result of absorption of specific wavelengths of light and the refection of others. If, for example, white light is played upon a surface which absorbs wavelengths specific to green and red, the resulting colour is blue. The degree to which a surface will absorb light can be derived mathematically and represented as a graphic which is termed the absorption curve.

The short form of the Latin term Camera Obscura meaning dark chamber or dark room. Now a generic term applied to any optical instrument capable of recording an in-focus image either photochemical or digitally. The term camera obscura also refers to a drawing aid much in use during the 19th century and still used today in certain circumstances.

This is the production of a sequential series of images and their projection onto a screen or similar to create movement. The impression is one of continuous movement but this is an illusion caused by a characteristic of the human eye known as persistence of vision. If the eye registers an object which disappears suddenly, the brain will retain an image of the object for a fraction of a second after it has gone.
To make use of this phenomenon in cinematography, each image must appear before the previous one has completely disappeared because the brain cannot distinguish the gaps between separate images. To be successful, this must occur at least 16 times each second when, if the images are progressive, the effect is one of movement. Persistence of vision is fleeting and decay of the after-image is extremely rapid. So, even at 16 images per seconds, the human eye will still detect flicker. To avoid this, it is necessary to introduce an additional interruption. The use of a revolving shutter in the projector has the effect of increasing the number of impressions received each second, by the retina of the eye, to 48. In this way, replacement occurs more rapidly than decay.

Converging Verticals
If you stand in front of a tall building and look up, you will notice that the vertical lines of the building appear to converge on each other are they get further away. This might be termed vertical perspective but it is an illusion and the brain refuses to see them that way. In addition, you are not viewing the entire building at once but, rather, allowing your eye to travel up and down whilst your brain adds together all the various bits of information and creates a whole but essentially interpreted image. In effect, your brain cheats but a camera cannot do that and it will only record exactly what it ‘sees’ through its lens. What it ‘sees’ is a number of vertical lines, much like a straight road or set of railway lines turn on end. The reason why it ‘sees’ the lines like that lies in the fact that the top of the building if further away than the bottom and the farther away something is, the smaller it appears. When the brain is confronted with an image like that, it is viewing things on a much reduced scale and can no longer cheat. It registers vertical lines which converge and knows this to be incorrect through experience. If the image shows vertical lines which are parallel – something which they would not be from your angle of view – the brain is satisfied because the image appears correct – it looks right. Small format cameras do not have the facility to overcome this problem but large format cameras often do. These facilities are termed camera movements (q.v.) and they allow what is, essentially, a re-construction of the image before exposure.
For the lines of a building to appear correct, it is vital that the film plane remains in parallel to them. Take, as an example, a large studio camera which is, in essence, a bellows unit with a lens housing at one end and a film carrier at the other. Housing and carrier are both movable – up and down, side to side and forward and back. Most importantly, they can be adjusted to remain in parallel with the subject even if the camera is tilted out of the vertical.


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