Old 02-09-2010, 07:11 PM
taxman (Matt)
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Focal lengths, ratios and exposure

A month or so ago I was looking at either the Canon 200mm f2.8 or 135mm f2.0 lens, but changing priorities has put that on the back burner for a while. So now I am looking at a couple of options which are much cheaper to at least get my toes wet until I can afford the 135mm f2.0.

My question is regarding light through put as it affects exposure time. Is the f-stop the only determination? That is, if I got the 35mm f2.0, would I get a good feel for how long I need to expose?

I have some difficulty with this as the 135mm lens has an obviously larger aperture. A thought experiment with two different funnels in the rain draining into the same sized cup being analagous to a light cone hitting a CCD would suggest that the larger aperture would result in greater capture and so shorter exposures. Of course, taking that idea a little further, a steeper funnel would also result in a shorter path for each individual drop at the edges of the funnel, again resulting in the cup filling up sooner.

So is exposure length tied to both aperture and f-ratio? If that is the case, then we would end up in the curious situation where exposure length would then be a function of aperture diameter^4, an odd function indeed.

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Old 02-09-2010, 08:29 PM
Alchemy (Clive)
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F2 is f2 regardless of focal length, so same exposure required for say m8 would apply for all f2 lenses, some would just have a smaller field of view than others
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Old 03-09-2010, 12:59 PM
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Geoff45 (Geoff)
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Here is some advice from the experts--challenges a lot of the normal ideas which originate from the film age.
Craig Stark (of PHD fame)
Stan Moore (writer of CCDStack).

Also worth remembering is that the camera pixel size plays a part. You can't just say this scope is faster than that one unless you are using the same camera. If you change a camera for one with double the pixel size and use the same scope, each pixel will collect 4x as much light in the same time. It's a bit like asking which telescope magnifies more--the answer is that it depends on the eyepiece.
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Old 03-09-2010, 09:17 PM
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this topic seems to be made much more complicated than it needs to be.. but i'm not sure that i can explain it any better than anybody else.

stars don't get spread out the same way as diffuse 'deep sky' objects, so if you're talking about the limiting magnitude of stars in your exposure, that is a function of aperture, not f-ratio.

with deep sky objects, the amount of exposure required (or how deep you will get in a given time) is a function of f-ratio, but comparing 35mm with 135mm really doesn't make much sense.

say you have a widefield shot that includes M8 in the middle. with your 35mm f/2 you'll have a nice spot in the middle of your frame where all the light from M8 has condensed in one small bright spot in a relatively short exposure time, say 10 minutes. but it will have no detail/structure at that wide focal length.

if you shoot the same object at a longer focal length, then that object is spread out over a much larger area. so to get the same surface brightness (intensity) over that much larger area, requires a lot more light gathering.. thus your f2 135mm lens has a much bigger aperture to deliver the same surface brightness in the same exposure time. but you've gained a whole lot of detail/magnication and you end up with two totally different images.. so it's not apples and apples..

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