# Thread: Does aFOV affect magnification or brightness of clarity?

1. ## Does aFOV affect magnification or brightness of clarity?

Help me out:

I can calculate magnification and exit pupil
I understand aFOV, as "how much sky you see"
I get that there are limits for useful magnification and that it is a function of aperture.
I get that the limit of useful magnification is somewhat variable, depending on what you are looking at and what you want to see.
I also get that there are "sweet spot" exit pupil sizes depending on what you are looking at and what you want to see.

What I am confused about is how can you see more sky (higher aFOV) without changing magnification or losing brightness or some other trade-off?
There is only so much light going into the scope, so how do larger aFOV eyepieces give you more to look at without compromising what you see, how big it is, or how bright?

thanks

Daniel

2. ## Re: Does aFOV affect magnification or brightness of clarity?

Apparent field of view is NOT how much sky you actually see - that is TRUE field of view. aFOV depends only on the design of the eyepiece, it is typically around 50 degrees. So if the magnification is 50x then the true FOV is 1 degree.

3. ## Re: Does aFOV affect magnification or brightness of clarity?

Originally Posted by pikaia
Apparent field of view is NOT how much sky you actually see - that is TRUE field of view. aFOV depends only on the design of the eyepiece, it is typically around 50 degrees. So if the magnification is 50x then the true FOV is 1 degree.
Yes, sorry, that was what I meant. Thank you for clarifying that point, but my question remains: to increase aFOV, what is the trade off? It seems there is only so much light to go around, so by spreading the light it over a greater apparent field are you getting dimmer or otherwise degraded images?

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5. ## Re: Does aFOV affect magnification or brightness of clarity?

The incoming photons of light are fixed in their brightness. The AFOV will have no effect on the brightness as you are just seeing a larger field of a fixed brightness. The exit pupil has the most effect on brightness. As you magnify more the exit pupil becomes smaller, whether with large AFOV or small. Your eye only has so many rods and cones. As an image becomes larger due to magnification, it's fixed brightness is spread out over more of the rods and cones in your eye in order to perceive it as larger. The object is still just as bright as it ever was, but each of your rods and cones is receiving less photonic information as you are dividing the fixed amount of brightness over more receptors.
Hope that makes sense.

Take a toilet paper tube and look through it at an object = Large AFOV. Now look through a paper towel tube at the same object = Small AFOV. The objects apparent brightness did not change just the surrounding amount of the observed field.
JJ
Last edited by Johnny J.; 04-26-2017 at 01:15 PM.

6. ## Re: Does aFOV affect magnification or brightness of clarity?

An eyepiece designer has two ways to increase the AFOV. They can either keep the TFOV the same and increase the magnification, or they can keep the magnification the same and increase the TFOV.

The true field of view that is available in the scope depends on the focal length of the scope and the diameter of the focuser draw tube. Any eyepiece can show less than that just by masking off the part the the designer wants to exclude (and those photons are simply lost). No eyepiece can ever show more than that much TFOV.

So the eyepiece designer can decide how much of the TFOV to allow, using a field stop, and how much to magnify it, resulting in a specific AFOV.

7. ## Re: Does aFOV affect magnification or brightness of clarity?

Originally Posted by Gaff
Help me out:
...
What I am confused about is how can you see more sky (higher aFOV) without changing magnification or losing brightness or some other trade-off?
There is only so much light going into the scope, so how do larger aFOV eyepieces give you more to look at without compromising what you see, how big it is, or how bright?

thanks

Daniel
Hello Daniel,

and back to your original question.

The telescope optics (lens or mirror) gets a very wide field focussed on a nearly spherical surface, a small cut of this surface is called the focus plane in common language.
Now, the eyepiece can see, and it can transmit into the observers eye, just a section of the telescope focussed field, depending on the field lens size of the EP, and on the distance between the field lens and the telescope focussed field.
So it's an easy geometry consideration, do you get it?
Now the technical issue is, to get more apparent field of view, you need larger field lens in the EP, eventually from 1.25" towards 2", or alternatively a much shorter distance between the EP field lens and the telescope focussed field.
The scaling parameter in this game is the focus length of the EP.
However, whatever scaling you play, when you want to increase the apparent field of view, the EP design requires more lenses inside to correct the aberrations.

- Up to the apparent field of view of 50°, the sufficient number of lenses in the eyepiece is 4.
- Up to 65°, you'll need as a minimum 5 - 6 lenses in several groups.
In the ultrawide designs (82° and more) you'll need many more lenses, as already the field lens turns into a field lenses assembly doublet, or a tandem of 2 doublets. This makes the ultrawide eyepieces big, heavy, and the increasing number of the lenses "eats" light in the range of 5% up to 10%. That's also the case, where you compromize a tradeoff at the costs of the brightness, and also of the contrast view.

Hoping this helps, if this should not, then don't hesitate to ask for more,

JG
Last edited by j.gardavsky; 04-26-2017 at 02:03 PM. Reason: typo

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9. ## Re: Does aFOV affect magnification or brightness of clarity?

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