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nobody

10 Feb 2023 20:25



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@valy
Da, crescand focala de doua ori, deci de la f5 la f10, campul se micsoreaza de patru ori. Greseala autorului, este si el om. Fotografic, de la f5 la f10 sunt cam 2 f stop, deci imaginea ar trebui ar trebui sa fie de patru ori mai luminoasa. Probabil nu-i chiar asa in astrofoto, dar ar fi buna o masurare pe o mostra pusa pe o nebuloasa, masura per pixel a luminantei.

Totusi, care este greseala autorului ?

F-ratio has no effect on S/N from the perfect/ideal camera. But f-ratio can affect the S/N from a real camera by varying the number of pixels used to capture the object (numPix).  This effect is often miniscule, especially for images of bright objects or long exposure deep space images (where SkyGlow dominates noise).  But for very short exposures or narrow-band images of dim objects, the pixel noise can be very significant. (This is why binning is so beneficial for very short exposures, an effect that is often erroneously attributed to increased “pixel S/N” of combined pixels.)

Thus there is an actual relationship between S/N and f-ratio, but it is not the simple characterization of the “f-ratio myth”.

Below is an illustration of the principle.  Each image is a 10-minute exposure using the same aperture (Tak CN-212) at vastly different f-ratios.  If the “f-ratio myth” was true then the f/3.9 exposure should be 10 times “better”!