Back focus
Space between the far surface of the lens and the image plane, when the lens is focused at infinity.
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Back Focus Back focal distance, meaning the distance from the tip of the lens' furthest rear surface to the film/image sensor surface where the image is focused.
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Leonore Hetling says: ...
* Front / back focusing are where the camera indicates an auto focus lock but the actual focus position produced is slightly in front or slightly behind the actual ideal focus position.
For example, the back focus distance (distance from the rearmost lens element to the film plane) on EOS cameras is 44mm, but on Leica rangefinder cameras it's 27.8mm.
The "S" stands for "short back focus" and indicates that the distance from the back of the lens to the camera's sensor is less than that of an EF lens.
Finally there is always the chance that your lens (usually the lens not the camera) has a front or back focusing issue. To detect this, you need can photograph something on a flat surface, like a sidewalk.
In a retrofocus design, which is advantageously applied to wideangle lenses, the back focus is designed longer than the lens' focal length to allow clearance for the movement of the reflex-mirror (No Mirror Lock up or seperate viewing accesory ...
We don't want to decrease focus on all our image, so we will duplicate our base layer to bring back focus to the image once we apply blur. To duplicate the layer simply press Ctrl/Cmd + J to jump the selected layer into a new one.
However, there were (disavowed) rumors than Samsung was developing an APS-C-sensor DSLR based on the same no-mirror, shorter back focus, hi-res EVF formula as the G1 (this, of course would require yet another lens mount), ...
The "-S" stands for "short back focus" and means that the lens design protrudes more deeply into the camera body.
Inverted telephoto lens
A lens construction which gives a short focal length with a long back focus or lens angle lenses to be produced for small format cameras, where space is required for mirrors, shutters, etc.
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Inverted telephoto lens.
Lens constructed so that the back focus (distance from rear of lens to film) is greater than the focal length of the lens. This construction allows room for mirror movement when short focus lenses are fitted to SLR cameras.
This was also set to f/11 to get sufficient front to back focus. I always use the ‘shoot to the right’ technique for exposing landscape shots, whereby you make the brightest part of the image fall just below clipping on the histogram.
However, if your Canon camera lens has both a red dot and a white square to line up the body to when mounting the lens, then it is an EF-S lens. EF-S is stands for 'short back focus'.
Film back focusing
So far this type of focusing is used exclusively in the Contax AX. In contrast to the other focusing systems there's no need to shift heavy groups of glass but the film plane in the camera itself is moved for focusing.
Depth of field decreases as focal length increases. (Telephoto lens construction: this allows a long focal length with short back focus, making for relative compactness).
(see Depth of field, Long lens , Standard lens & Wide) ...
The finder does not show exactly what the lens sees
The offset from the lens leads to easy misalignment, especially at close range
Depth of field (the degree of front to back focus) can't be previewed.
The two images are then analysed for similar light intensity patterns (peaks and valleys) and the phase difference is calculated in order to find if the object is in front focus or back focus position.
 See also: Back, Focus, Camera, Lens, Image
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