Affect of Eye Pupil on Binocular Aperture
Posted 12 August 2004 - 05:55 PM
Mike, I like the thought or implication of the larger exit pupil causing greater constriction in brighter light. has merit.
Typically, scope design enthusiasts will tell us that a smaller acromat can get away with a shorter focal ratio to overcome the affects of CA. The larger the objective the longer it needs to be. the BT100s are f6.2
Posted 12 August 2004 - 06:02 PM
I find myself agreeing with both of you. I think Ed is correct that CA increases with magnification in otherwise identical optical systems ( like the Fujinon 10X70 and 16X70) simply because the same color fringe is being magnified more, but only if the entire area of both objectives is actually being used. In a lighting situation where the eye is only open to say 3.5mm the 10X70 would effectively be a 35mm F:8 (assuming an F:4 objective) but the 16X70 would effectively be about a 56mm F:5 and so would show more CA than would be predicted by just the difference in magnification. I think there is a confusion in your post between increasing focal ratio with reduced effective aperture and actual focal length, which doesn't change.
Posted 12 August 2004 - 06:49 PM
Tough crowd here, eh Mike. Amicable though.
<laughing> Speaking of "tough crowds", should the title be "Affect" or "Effect"...?
Posted 12 August 2004 - 07:59 PM
"By the way in the BT100, as magnification increases, CA is nearly absent."
I agree with that statement. In an examination of CA as a function of magnitude I have found this to be true...which is opposite of what I was expecting.
Posted 12 August 2004 - 09:30 PM
Sorry, I just noticed you fixed the focal ratio/ focal length bits in your post.
Posted 13 August 2004 - 12:29 AM
>Can we agree that when it comes to achromats, that focal length
>determines amount of CA?
Yes. The "rule of thumb" for a "normal" (i.e. crown/flint) achromatic doublet
is that the focal ratio should be 3A, where A is the aperture in inches.
Another way of stating this is that the focal length should be 3*A^2. And
we wonder why we see colour in a 4" f/5 !
In this talk about CA, it is also useful to distinguish between axial CA and
lateral CA. Obviously, the latter becomes more apparent with wider FoV;
for this reason higher magnifications with lower FoV, especially if it is lower
AFoV, can show less CA. Axial CA will appear greater with higher
Posted 13 August 2004 - 04:41 AM
Effect = Result
I edited a few of my uses.
Posted 13 August 2004 - 06:35 AM
Posted 13 August 2004 - 08:19 AM
Lateral CA results from a chromatic difference of magnification for off-axis objects. It is responsible for the off-axis colour fringing in most wide field binocs. Eyepieces in particular tend to have a lot of inherent lateral colour. It is darned difficult (aka expensive) to correct!
Posted 13 August 2004 - 08:54 AM
Lateral CA, in other words, is the change in a lens's magnification for different wavelengths, resulting in fringing CA. Correct? But doesn't light need to be incident obliquely to the objectives for this type of CA to be noticable? Yes, it does seems hard to mitigate.
Posted 13 August 2004 - 10:52 AM
>Then axial CA is the CA most users typically experience and talk about.
I used to think so, but I'm not convinced -- anything off-axis (like almost
everything we observe most of the time) is going to involve lateral CA.
> I was not aware of its appropriate and specific name of "axial CA".
Also commonly known as "longitudinal CA" -- the two terms are