Re: Ground Glass

[P3D Dr. George A. Themelis <fj834@xxxxxxxxxxxxxxxxxxxxx>]

>2. Open the lens f/stop to the widest aperature.   This will let the most
>light in and make a brighter image.  

Also, will show all the lens' aberrations and will make focusing much easier.

>4.  While looking at the ground surface of the ground glass (yes, the image
>will be on the ground side), adjust the focus of the camera untill the image is
>its sharpest.  Use a magnifying glass or loop.
>
>5. Compare the reading on the camera's distance scale to the actual distance.

The obvious question is what do you do if the camera's distance scale does
not agree with the actual distance?  Or, what do you do if the right and
left lenses are focused at different distances?

How strong is your magnifier?  You can use a microscope eyepiece to get
something like 10X magnification, or, if you want to get really fancy, you
can buy an expensive 30X achromatic focusing magnifier (it is nice to be
able to focus the magnifier on the ground glass) made specifically for
testing camera focus.

With a good magnifier you can go one step further and actually judge the
sharpness of the lens. Also, you can pan the camera to bring the same
object from the center to the edge of the film gate and get an idea of loss
of sharpness (also, loss of light) at the edges.

George Themelis


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