Once upon a time it was easy to simply remember whether a lens was a normal or telephoto, etc., because there were only two dominate formats, 355mm and MF. Nowadays, we have an increasing range of sensors with interchangeable lenses. Since no one seems to want to work with coverage angles, the crop factor is a useful rough guide. There are some useful calculators for DoF on the web. Even though the lens is the same ( and hence the size of the circle of confusion) the pixel density of the sensor means that circle has a different effect in different cameras.
On Mon, Jul 2, 2012 at 8:02 AM, <li...@thrane.name> wrote: > > 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. -- Steve Desjardins -- 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.
