Re: [APML] Presbyopia and Fine Focus

[Ray Butler <ray.butler@nuigalway.ie>]

Hi Jim,

Although the camera focusing screen is physically in close proximity to 
your eye, optically you see the image on the screen as though it is "at 
infinity". This is because the light from the image on the screen has 
been re-collimated to parallel beams by the viewfinder eyepiece on the 
camera.

Try this test in daylight. Put on whatever specs you use to see distant 
objects clearly. Now put either a 50mm lens or a standard zoom lens on 
the camera and focus it on infinity. Hold the camera so that you can see 
through it with one eye and past it with your other eye, and view with 
both eyes open: one seeing the scene directly, the other seeing it 
through the camera. Examine a distant landscape; with a zoom lens you 
can zoom to whatever focal length gives you an image of the same 
magnification as your unaided eye; otherwise a 50mm standard lens would 
be close enough in magnification.

When you do this, you will see that both images are in focus; that is, 
viewing an object at infinity with the unaided eye is the same as 
viewing the image on the camera screen, even though it is right in front 
of your nose. This would no longer be the case if you took a screwdriver 
to the camera and removed the viewfinder eyepiece, because then you 
would have to view the focusing screen as a small very nearby object 
emitting diverging rays of light, not parallel ones.

The viewfinder eyepiece has to be set by the manufacturer to be a 
particular strength (focal length) and a particular distance from the 
focusing screen, in order to give parallel rays. If (like me) you are 
short-sighted/myopic, you cannot focus parallel rays onto your retina 
without some extra optics to slightly diverge these parallel rays before 
they enter your eyeball. So we have a choice between wearing specs or 
adding a negative diopter-correction lens onto the camera viewfinder; 
they achieve the same result. I prefer to wear glasses than use a 
diopter-correction lens.

I hope that I have explained why you should use your distant-vision 
correcting specs, not your near-vision ones, when focusing your camera 
on stars.

Ray "whose exuberant 9 month old daughter broke his better pair of specs 
the other day" Butler

-- 
Dr. Ray Butler
Lecturer, Physics Department & Computational Astrophysics Laboratory,
National University of Ireland - Galway, 
University Road, Galway, Ireland. 
Web: www.nuigalway.ie/physics/ 	Email: ray.butler-AT-nuigalway.ie 
Tel: +353-91-493788 		FAX: +353-91-494584

_______________________________________________
Astro-Photo mailing list
Astro-Photo@seds.org
http://seds.org/mailman/listinfo/astro-photo