Quote:
Originally Posted by Stonius
I'm trying to understand why *field curvature* would be related to age.
I get that eyesight deteriorates with age as does the ability of the eye to focus, and I get what you're saying about focusing halfway to the edge of the field to keep the inner and outer stars within the circle of confusion.
But correct me if I'm wrong; a curved focal plane is not fixed by shifting the focal point of the eye; instead you would have to be able to change the shape of your retina to accommodate to a curved focal plane, wouldn't you? I don't think people (young or old) are able to do this are they?
Cheers.
Markus
|
Field curvature in an eyepiece is a fixed value. But can you accommodate the focus change in your own eye necessary to focus on both the center and the edge? That is what changes with age.
When you focus on the TV across the room and then change your gaze to a book in your lap, your eye is changing focus by changing the shape of the lens in the eye.
It is this ability that changes with age.
So if you have a sightly curved focal plane and you are young, when you change your gaze from the center to the edge, your eye automatically adjusts its own focus and the image is in focus both places.
There is a limit to this, and even young eyes cannot accommodate too much focus change. Older eyes can accommodate even less, so where the younger eye may see the whole field in focus, the older eye may not.
The point is that a slightly curved focal plane in an eyepiece may not be a problem for younger observers, but may be a problem for older observers.
And, because they have more strongly curved focal planes, this issue may arise more with refractors than with reflectors.