----------------------------------- nobody 02 Mai 2012 21:53 ----------------------------------- Problema e mai complexa si nu chiar asa de intuitiva. In astrofoto nu sunt asa mari diferente la rapoarte focale diferite (contrar cu fototgrafia normala, de zi). The “f-ratio Myth” There is a common misconception that the “depth” of astronomical images is primarily determined by f-ratio, regardless of aperture. This misunderstanding is often expressed by claims such as: “Reducing an f/10 SCT to f/5 will allow you to take images of equal depth in 1/4th the time.” “Binning 2x will produce the same quality (depth) in 1/4th the time.“ These assertions are incorrect and represent a profound misunderstanding of physics and information theory. These erroneous notions constitute the “f-ratio myth” (also “pixel size myth”). The historical origins of this notion are doubtlessly located in the photographic experience of varying lens f-ratio, which varies aperture and thus really does vary photon statistics of objects. Further misunderstanding arises from evaluating images based on the cosmetic appearance of the image as a whole as seen at a particular display size rather than analyzing the accuracy of imaged objects. Shrinking the image scale via “faster f-ratio” (keeping aperture constant) or larger pixels will convolve (blur) apparent graininess even though it produces no improvement of information quality (shrinking can actually impair information quality if the resulting display is significantly undersampled). There is a potential f-ratio effect but it is complicated and does not follow the geometric function of f-ratio or pixel size. Vedeti comparatie: The images are obviously very similar and if the “f-ratio myth” was true then the f/3.9 exposure should be 10 times “better”!