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Sqrt2n0000 (talk | contribs) Pentax Q7 and Sigma SD are long out of production already, while Sigma's Full Frame cameras (fp and fp L) have been released. |
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== Sensor size and shading effects ==
Semiconductor image sensors can suffer from shading effects at large apertures and at the periphery of the image field, due to the geometry of the light cone projected from the exit pupil of the lens to a point, or pixel, on the sensor surface. The effects are discussed in detail by Catrysse and Wandell.<ref name=Catrysse>{{cite journal|last=Catrysse |first=Peter B. |author2=Wandell, Brian A. |title=Roadmap for CMOS image sensors: Moore meets Planck and Sommerfeld |journal=Proceedings of the International Society for Optical Engineering |volume=5678 |issue=1 |pages=1 |doi=10.1117/12.592483 |year=2005 |url=http://www.imageval.com/public/Papers/EI%205678-01%20Peter%20Catrysse.pdf |access-date=29 January 2012 |url-status=dead |archive-url=https://web.archive.org/web/20150113004959/http://www.imageval.com/public/Papers/EI%205678-01%20Peter%20Catrysse.pdf |archive-date=13 January 2015 |series=Digital Photography |bibcode=2005SPIE.5678....1C |citeseerx=10.1.1.80.1320 |s2cid=7068027 }}</ref>
In the context of this discussion the most important result from the above is that to ensure a full transfer of light energy between two coupled optical systems such as the lens' exit pupil to a pixel's photoreceptor the [[Etendue|geometrical extent]] (also known as etendue or light throughput) of the objective lens / pixel system must be smaller than or equal to the geometrical extent of the microlens / photoreceptor system. The geometrical extent of the objective lens / pixel system is given by
where {{math|
where {{math|''w''<sub>photoreceptor</sub>}} is the width of the photoreceptor and {{math|(''f''/#)<sub>microlens</sub>}} is the f-number of the microlens.
<math display="block"> {(f/\#)}_\mathrm{microlens} \le {(f/\#)}_\mathrm{objective} \times \mathit{ff}\,.</math>
Thus if shading is to be avoided the f-number of the microlens must be smaller than the f-number of the taking lens by at least a factor equal to the linear fill factor of the pixel. The f-number of the microlens is determined ultimately by the width of the pixel and its height above the silicon, which determines its focal length. In turn, this is determined by the height of the metallisation layers, also known as the 'stack height'. For a given stack height, the f-number of the microlenses will increase as pixel size reduces, and thus the objective lens f-number at which shading occurs will tend to increase.
▲:<math> G_\mathrm{pixel} \simeq \frac{w_\mathrm{photoreceptor}}{2{(f/\#)}_\mathrm{microlens}} </math>,
In order to maintain pixel counts smaller sensors will tend to have smaller pixels, while at the same time smaller objective lens f-numbers are required to maximise the amount of light projected on the sensor. To combat the effect discussed above, smaller format pixels include engineering design features to allow the reduction in f-number of their microlenses. These may include simplified pixel designs which require less metallisation, 'light pipes' built within the pixel to bring its apparent surface closer to the microlens and '[[Back-illuminated sensor|back side illumination]]' in which the wafer is thinned to expose the rear of the photodetectors and the microlens layer is placed directly on that surface, rather than the front side with its wiring layers.
▲where {{math|<var>w<sub>photoreceptor</sub></var>}} is the width of the photoreceptor and {{math|<var>(f/#)<sub>microlens</sub></var>}} is the f-number of the microlens.
▲:<math> G_\mathrm{pixel} \ge G_\mathrm{objective}</math>, therefore <math> \frac{w_\mathrm{photoreceptor}}{{(f/\#)}_\mathrm{microlens}} \ge \frac{w_\mathrm{pixel}}{{(f/\#)}_\mathrm{objective}}</math>
▲:<math> {(f/\#)}_\mathrm{microlens} \le {(f/\#)}_\mathrm{objective} \times \mathit{ff}</math>
▲Thus if shading is to be avoided the f-number of the microlens must be smaller than the f-number of the taking lens by at least a factor equal to the linear fill factor of the pixel. The f-number of the microlens is determined ultimately by the width of the pixel and its height above the silicon, which determines its focal length. In turn, this is determined by the height of the metallisation layers, also known as the 'stack height'. For a given stack height, the f-number of the microlenses will increase as pixel size reduces, and thus the objective lens f-number at which shading occurs will tend to increase. This effect has been observed in practice, as recorded in the DxOmark article 'F-stop blues'<ref>{{cite web|last=DxOmark|title=F-stop blues|url=http://www.dxomark.com/index.php/Publications/DxOMark-Insights/F-stop-blues|work=DxOMark Insights|access-date=29 January 2012}}</ref>
▲In order to maintain pixel counts smaller sensors will tend to have smaller pixels, while at the same time smaller objective lens f-numbers are required to maximise the amount of light projected on the sensor. To combat the effect discussed above, smaller format pixels include engineering design features to allow the reduction in f-number of their microlenses. These may include simplified pixel designs which require less metallisation, 'light pipes' built within the pixel to bring its apparent surface closer to the microlens and '[[Back-illuminated sensor|back side illumination]]' in which the wafer is thinned to expose the rear of the photodetectors and the microlens layer is placed directly on that surface, rather than the front side with its wiring layers. The relative effectiveness of these stratagems is discussed by [[Aptina]] in some detail.<ref>{{cite web|last=Aptina Imaging Corporation|title=An Objective Look at FSI and BSI|url=http://www.eetrend.com/files-eetrend/newproduct/201101/100029156-17249-fsi-bsi-whitepaper.pdf|work=Aptina Technology White Paper|access-date=29 January 2012}}</ref>
==Common image sensor formats==
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