MadSci Network: Physics

Re: Focusing a camera while wearing graduated bifocals...

Date: Thu Sep 16 17:09:40 1999
Posted By: Everett Rubel, MadSci Admin
Area of science: Physics
ID: 937485550.Ph


Thanks for the question.  When you look through the viewfinder of your camera 
what you are really looking at is a "real image" produced by the optics of the 
viewfinder.  This real image is distinct from the object being photographed.  

Since you are focusing the camera I am going to assume that you have an SLR 
camera ( if you had an autofocus camera I would imagine that there would not be 
a problem!) Light comes in from the object you are photographing and reflects 
off of a mirror that is at an angle to reflect the light upward ( this mirror 
flipping up makes part of the "clunk" sound when you take a picture.) The light 
from the object focuses on a frosted piece of glass or plastic, provided that 
the lenses of the camera is focussed properly.  On some cameras it is possible 
to look directly at this frosted screen to see the image. However, most SLRs 
have a special prism over the screen that bends the light to go out the back of 
the camera through the viewfinder lense.  This lense is usually just a form of 
magnifying lense.

The real image that you see in your viewfinder is at a distance determined 
entirely by how the viewfinder is made.  I have not tested mine but I think that 
 an image distance of six feet or 2 meters is typical.  

I don't know if the above number will help you much.  One thing you might want 
to try is to use the focus marks on the lense of the camera if you have them.  
These indicate the distance that the camera is focussed at.  Just measure off a 
known distance to some object and set the lense to that distance.  You could 
also try to set the lense to infinite focus and point it at a mountain or some 
other well defined object off in the distance.  Then look through the camera 
using various parts of your bifocals to see what part seems to work best.  You 
might even want to try not using your glasses.

Best Regards,

Everett Rubel

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