Editing Magnification (section)

===Photography===
The image recorded by a [[photographic film]] or [[image sensor]] is always a [[real image]] and is usually inverted. When measuring the height of an inverted image using the [[Cartesian coordinate system|cartesian]] sign convention (where the x-axis is the optical axis) the value for {{math|''h''{{sub|i}}}} will be negative, and as a result {{mvar|M}} will also be negative. However, the traditional sign convention used in photography is "[[real image|real]] is positive, [[virtual image|virtual]] is negative".<ref>{{cite book |first=Sidney F. |last=Ray |title=Applied Photographic Optics: Lenses and Optical Systems for Photography, Film, Video, Electronic and Digital Imaging |publisher=Focal Press |year=2002 |isbn=0-240-51540-4 |page=40 |url=https://books.google.com/books?id=cuzYl4hx-B8C&pg=PA40 }}</ref> Therefore, in photography: Object height and distance are always {{em|real}} and positive. When the focal length is positive the image's height, distance and magnification are {{em|real}} and positive. Only if the focal length is negative, the image's height, distance and magnification are {{em|virtual}} and negative. Therefore, the ''{{dfn|photographic magnification}}'' formulae are traditionally presented as<ref>{{cite book |last= Kingslake |first= Rudolph |date= 1992 |title= Optics in Photography |page= 32 |publisher= SPIE Optical Engineering Press |location= Bellingham, Washington |isbn= 0-8194-0763-1 }} "If a lens is thin, or if we can guess at the position of the principal planes, we can readily construct from [{{math|1=1/''d''{{sub|i}} + 1/''d''{{sub|o}} = 1/''f''}} and {{math|1= M = ''d''{{sub|i}}/''d''{{sub|o}}}}] the following simple rules that it is well to bear in mind. They refer specifically to the case of a positive lens forming a real image of a real object, all distances and the magnification being assumed to be positive quantities. If virtual images are involved, it is better to return to the original formulas, [previously stated]. The equations are [{{math|1= ''d''{{sub|o}} = ''f''(1 + 1/M)}} and {{math|1= ''d''{{sub|i}} = ''f''(1 + M)}}]."</ref>

<math display="block">\begin{align}
M &= {d_\mathrm{i} \over d_\mathrm{o}} = {h_\mathrm{i} \over h_\mathrm{o}} \\
  &= {f \over d_\mathrm{o}-f} = {d_\mathrm{i}-f \over f}
\end{align}</math>