This is why I hate "equivalent focal lenghts". :-) The amount of light reaching the sensor does not depend on the size of the sensor, and is simply expressed by the f-stop. The f-stop is simply defined as the focal length divided by the size of the opening, so with a 200mm f/4.0 lens the opening diamater is 50mm (this is not as evident in wide angles).
On the other hand the amount of light collected by the sensors depends on the size of the pixels, which can be seen in differences in ISO. Så if a large and a small sensor have the same pixel size each pixel will be able to collect the same ampunt of light, but the larger sensor will have room for more pixels. Thus we geet differences in respolution and/or sensor noise. DagT >Hi folks! > >A confusing question came to me: > >To have FF equivalent focal length of a lens for our APS-C sensors, we have to >apply a factor of 1.6x: 40mm f/2.8 ltd is a 64mm FF equivalent. For the depth >of field, this factor is to be applied too: 40mm 2.8 ltd is a 64mm f/4.5 FF >equivalent. > >But, what about the amount of light? As the surface divide by 1.6x1.6 and the >sensor is placed at the same distance as a FF SLR film, the light reaching the >sensor is divided by the same amount: 2.56. Are the ISO from FF, APS-C, m4/3, >P&S all the same, or are they equivalent ones to take crop factor into account? > >Concerning light, is the 40mm 2.8 ltd, a true f/2.8 opened lens, then? > >~Alban. > -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
