Quote:
Originally Posted by codemonkey
I've never really stopped to think too much about how telescopes work and how the combination of focal length and aperture work together to produce images of a certain brightness and field of view, so now that I am thinking about these things, I find I have questions.
Stan Moore has an interesting article on the F-ratio Myth basically saying that varying the F-ratio does nothing for exposure time, only varying the aperture makes a difference.
On the face of it, this makes sense... focal length controls FOV, aperture controls the amount of light. Bigger aperture means a bigger light bucket and thus more light.
But why does a bigger aperture mean more light from the same source? Shouldn't it mean more light, but from different sources? Take a point light source, it doesn't seem like it should matter whether you have a 80mm aperture or a 500mm, it's a point source, so it seems like you should either get it or not.
Does it work differently for point light sources and diffuse light sources? |
Hi Lee. Stan Moore's article is very misleading, because in debunking one myth he inadvertently created a new and even more insidious one. The problem seems to be that he neglected to mention the key part of his argument which should be "F-ratio makes no difference to sensitivity for a given aperture IF THE PIXEL SIZE IS ADJUSTED TO COMPENSATE FOR THE CHANGE IN FOCAL LENGTH". The bit in caps is the bit he left out and now people continue to quote the first part of the argument and assert that F-ratio makes no difference - even experienced photographers who surely must know that it clearly isn't true. Suggest that you ignore that paper.
the things that matter are the aperture, which determines how many photons get in and the angular size of the pixels, which determines how many of the available photons end up in each pixel. if you fix the focal length and pixel size, the sensitivity increases with reducing FNo as Russ' example shows. This applies to extended objects and stars for most systems (except highly undersampled ones). The main point is that you cannot consider aperture, focal length, pixel size (or FNo) on their own - you must consider them all to get the system sensitivity.
re the way a lens works, please forgive me if I have misunderstood your question, but here goes. If you hold up your hand and look at it, you get almost no information on where the light that you see came from - it could have been from many different sources in the room, but you can't tell anything much about those sources by looking at the light reflected from your hand. A similar scrambled pattern of light falls all over the aperture of a lens or mirror, but the lens or mirror has the ability to unscramble that light pattern and develop a map of the angles that the light came from and how bright it is - that is the image that you detect. Light from a point source falls all over the lens aperture and it is then transformed (unscrambled) by the lens back into a point in the focal plane. Light from an extended object also falls all over the aperture, but it is transformed into a 2D representation of the source object. If the aperture is bigger, more light from both point and extended sources gets into the system.