Photography for beginners

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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