Focus screen

lukey1965

lukey1965

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Paul
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Please excuse my stupidity but what, if any, are the advantages or disadvantages of using/replacing a focus screen ??
 
Screens with a different matte will help with manual focus as the perceived DoF will be reduced. Some screens will have a split prism in the centre to offer even greater potential accuracy.

Bob
 
The split prism is the one that intrigues me... Can you explain in laymans terms how it works please.
 
Interchangeable focusing screens have lost their purpose somewhat with the advent of auto focusing - even manual focusing offers focus confirmation on modern cameras.

However the old Olympus OM range had an impressive range of screens which could be used dependant upon which lens you were using, if memory serves they even had a 45 degree slit image screen (not shown in the link)

The method is shown here
 
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lukey1965 said:
The split prism is the one that intrigues me... Can you explain in laymans terms how it works please.
Click to expand...

In very laymans terms then......

Have you ever seen the film "The Dambusters"?. It showed how the aircraft height was judged by shining two lights onto the water with the lights forming a common spot when the aircraft was at the correct height. Too high and the spots were seen separately, too low and they crossed and again became two separate spots of light. Similar principal for the split image.
Two small prisms set to see two parts of the central portion of the image formed by the lens. If the lens is focussed too far or too close then the images seen through the prisms will separate or cross. Only when the lens is "at the right height above the water (focus screen) will the two parts line up.

Hope that was within the bounds of comprehension.

Bob
 
Canon Bob said:
In very laymans terms then......

Have you ever seen the film "The Dambusters"?. It showed how the aircraft height was judged by shining two lights onto the water with the lights forming a common spot when the aircraft was at the correct height. Too high and the spots were seen separately, too low and they crossed and again became two separate spots of light. Similar principal for the split image.
Two small prisms set to see two parts of the central portion of the image formed by the lens. If the lens is focussed too far or too close then the images seen through the prisms will separate or cross. Only when the lens is "at the right height above the water (focus screen) will the two parts line up.

Hope that was within the bounds of comprehension.


Bob
Click to expand...

Many thanks, that makes sense even to my dim bit of grey matter.
 
Canon Bob said:
In very laymans terms then......

Have you ever seen the film "The Dambusters"?. It showed how the aircraft height was judged by shining two lights onto the water with the lights forming a common spot when the aircraft was at the correct height. Too high and the spots were seen separately, too low and they crossed and again became two separate spots of light. Similar principal for the split image.
Two small prisms set to see two parts of the central portion of the image formed by the lens. If the lens is focussed too far or too close then the images seen through the prisms will separate or cross. Only when the lens is "at the right height above the water (focus screen) will the two parts line up.

Hope that was within the bounds of comprehension.



Bob
Click to expand...


Yip, that's it. Always easier to use if what you were focussing on had a vertical line it which made it very easy to see that focus had been obtained.
Often the split image was augmented by a small circular area around it in which any out of focus was given an exaggerated blur to make it obviously out of focus.

Demo:-
http://petapixel.com/2013/01/02/a-demo-of-split-screen-and-microprism-ring-focusing-in-old-slrs/
 
A video, or even better, using a manual focus camera with a split-image screen for about 5 seconds will make it very clear.
 
Nope, not a single one of them! And no prism...

Only have one to switch on or off.
It has 3 vertical lines, 2 horizontals and a centerline. How poor is that?! :D

Better than nothing though, so always on... :lol:
 
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