aberration
An aberration is something that deviates from the normal way, i.e.
a defect. In photography this is normally colour fringing, c.f.
chromatic aberation.
achromatic
Short for
achromatic lens. This is one that has been designed to be free from
chromatic aberation, (q.v.). It can be acheived by
putting two lenses together (a doublet) made from glasses with different
refractive indexes.
ACR (Adobe Camera Raw)
The software that Adobe uses to process RAW files from a digital
camera. You can open a file in the ACR software from within
Photoshop, Elements or Bridge. After raw processing you can pass the
file to Photoshop to open for editing. A big advantage of RAW is
that in ACR processing no changes are made to the original raw file, all
processing steps are recorded in separate XML meta file.
If you have an older version of Photoshop and a modern camera which
is not supported by your ACR version, you may be able to get an update to your
photoshop version for newer cameras, or you can use the Adobe DNG converter,
see Mike Fuller's
note 5.
Aperture
The width of the lens opening that is letting light through to the
sensor (or film). c.f. f/stop . The width
is measured in millimeters but then this is divided by the focal length and
expressed as a ratio. Because this is a ratio, the larger the number the
smaller the hole] like a fraction, the larger the number the smaller the
fraction, i.e. 1/4 is smaller than 1/2.
aspect ratio
This is the ratio in an image or picture between the horizontal
dimension (denominator) and the vertical dimension (divisor). It started
with cinema and the was used in television. Intially many formats were
used but were later standardised to 4/3 or more usually 4x3 which could show
4x3, 3x2 and square with minimal blank borders. Cinema branched out into
wide screen, with varyiing aspect ratios, video has now adopted 16x9 as the
new format andphotography clubs are torn between the old aspect ratio of
4x3 (acceptable - just) for portraits and 16x9 which is poor for
portraits. The trouble is that 16x9 projectors are cheaper than 4x3
ones. It has become rather a mess.
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