FWIS 2.0

[FZR1KG]

I've been doing a bit of study on this and it seems everyone is parroting someone's information they don't fully understand.

Let me explain.

I never could understand why a telescope with the same diameter as another would somehow produce a brighter image than the other because it had a different F ratio.

The F ratio.

It apparently makes photons where none existed!!!!

Anyway, so now I have a digital camera, an SLR, really nice too thank you very much.
I'm looking at lens's for it.
So here comes the F number again.
The lower the lens's F number the faster I can use the shutter speed because it produces a brighter image for the same focal length.

Sure I said to myself, its because the focal length is the same so that means that the diameter of the lens must therefore be greater.
The greater the lens diameter, the more light it gets exposed to, more light = more photons. Simple.
Makes perfect sense.

Now, lets go back to the telescope.

Company A sells a F13 telescope of 100mm diameter.
Company B sells a f4 telescope of 100mm diameter.

Company B claims it's telescope is brighter than company A's because it has a faster lens.

OK, this is where I have to call bullshit.

If both telescopes are setup to have the same magnification they will both receive the exact same amount of photons and produce an image of the exact same size.
If not, I say lets make F0.00001 telescopes and harness the power of the sun because the lens is making more photons because its "faster", all the worlds energy needs are solved.

Now, we also have focal reducers being sold for a fortune to lower the F ratio of Maks, Schmitts etc.
Sure, it'll lower it. It'll make the image brighter, but it's because it makes it smaller!!!
But magnify that image up again to what it would have been and we get less photons than we had initially because the focal reducer has reflected and absorbed some away!!!

YAY, so who just paid $300 to get less light for their telescope, hands up now!!!

If you want it to increase your FOV then sure, buy it, but it's not going to make that image of Jupiter any brighter simply because you added a lens.
What it will do is make it smaller so its going to be brighter as a result of that. But you didn't want to get a smaller Jupiter, you wanted a brighter one at the same size.

Well, too bad. You lucked out. There's no such thing as a free photon.

*cough, clears throat*

If adding a lens to a system makes the image brighter then I want to know where the hell the extra photons came from. Otherwise its smoke and mirrors, IOW, marketing and misconception spread by people who didn't verify the facts for themselves.

Formulas:

F = Focal length / Lens diameter

e.g.
My scope is 102mm in diameter and 1300mm in focal length.

So its F = 1300/102

F = 12.7

Nowhere but nowhere does the formula say:

Photons = Photons / F * K

where K is the universal constant called Riggs constant.

All it is is a ratio, not a photon multiplier.


I put this confusion down to the camera stores selling camera lens's by focal length and telescope manufactures selling telescopes by their diameter.
So naturally for the same FL on a camera a lower F will mean a bigger lens diameter, so more light. The image size will remain the same because image size is determined by the focal length.

On a telescope it doesn't work that way. They are sold by diameter, so a change in F just means a greater or a smaller magnification for a given eye piece, NOT a brighter image for the same size. Using the same eyepiece will give you a smaller image thats brighter, it won't give you a brighter image thats the same size like on a camera.

If we change the eyepiece to give the same magnification, then the image will be EXACTLY the same brightness irregardless of the F ratio of the telescope.
A ratio does not gather photons, the diameter of the lens does that.

Hopefully that clears things up.
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It's not the speed that kills, its the sudden stop

[JPax]

I'm not a physicist, but I used to be a photographer for Kodak and elsewhere, so I have a bit of practical experience with them.

Camera lenses aren't just sold by focal length, as a good photographer knows to look at aperture/lens speed too (not to be confused with shutter speed, which is a different mechanism). When it comes to camera lenses, the aperture is determined, not by gross barrel diameter, but by the diameter of the aperture leaves. The aperture leaves are adjustable and can be changed for every shot. The "lens speed" is the maximum possible diameter for the aperture (lowest f-number). Since the aperture is a mechanical assembly, the ability to open it wider in a small enclosure requires more delicate mechanisms and stricter tolerances. Thus, the faster lenses are more expensive and can open more fully to allow more light per shot, if the photographer so desires (though it will reduce depth of field).

Maybe you already know this, but I thought I'd explain it just in case.
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[FZR1KG]

I did know that.
I was making the example simpler by not introducing stopping down a lens and using its wide open aperture as the basis, since that gives the fastest exposure times and the brightest images.
Telescopes typically only have this mode as stopping down a telescope defeats the purpose of having the bigger lens.
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It's not the speed that kills, its the sudden stop

[mike alexander]

Maybe another way of saying this is that the size of the image is dictated solely by the focal length, while the brightness is dictated solely by the aperture.

A fast (say f/4) scope is going to produce a smaller image than the equivalent aperture at f/8. If, as you say, you stick a x2 Barlow on the f/4, the image size will then be equivalent to the f/8, hence the brightness will be equal (less losses in the Barlow).

The focal reducer (I have one for my f/10 that brings it down to f/6.3) is helpful if your camera has a small CCD chip and you want extended views, say star fields or nebulae. But if I want a full frame Saturn I'm going to stick my x5 TeleVue extender on, and it will get dim.

Assuming I can get seeing that will allow a x5 negative lens. Fat chance.
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The Devil offered me power, but I told him I preferred aperture.

[FZR1KG]

Thats pretty much it Mike.

The problem is when people unconditionally claim a F/4 scope is going to make exposure times shorter that a slower scope.
If in both cases you want the same sized image then both scopes will take the exact same time to expose for that frame size.

But that's not what people are claiming, they are claiming that the faster scope takes less time to expose and thats plain wrong. If I am imaging Jupiter it won't make any difference if my scope if F/2, F/4 or F/24 if I want an image of the same size, it will take the same time.

Likewise the focal reducer, if you have a longer FL scope what it will do is make your FOV wider. You'll see more. If thats what you want then its perfect. It works well.

But manufacturer's are claiming it will shorten the exposure time without qualifying that it will also make the image smaller. This means people can buy one to make faster exposures of small objects but have to use smaller eyepieces to get the same image size. Then it actually takes longer to expose because now you've added an additional lens air interface into the system.

The only difference between two telescopes of the same diameter but different F ratio is that the lower the F ratio the smaller the image and the wider the FOV for the same eyepiece FL.

Brighter doesn't enter the equation because all I need to do is get a longer FL eyepiece and I get the same thing as the faster F ratio scope.

If you're not using eyepiece projection, then it makes the image brighter because it makes the image smaller. Thats fine if that is what you were expecting. If you were expecting the same size image but shorter times, then you're out of luck.
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It's not the speed that kills, its the sudden stop

[JPax]

So much for magic dream scopes. What I want is a superultramegahuge set of binoculars. Forget prisms. I want two big off-axis reflectors setup in a rig where I can nestle my face between their rounded forms, just like a pair of big ta-tas and get lost in the view.
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