Split prism focusing screen on a DSLR

[Ben johns]

How easy (or hard) would it be to put a film SLR type focus screen into a DSLR? I’ve seen them for sale but I’m sure it’s not a plug and play type of deal

 

[sk66]

Depends on the camera model... some have easily replaceable screens, others do not. The bigger issue is that they make the viewfinder darker because they work differently than the modern screen (which is really a bunch of micro relay lenses and not a ground glass screen).

 

[Canon Bob]

I've got several split prism screens for my Canon 1srs bodies and it's less than a minute to swap them. Good for macro and my Zeiss manual focus lenses. Early non-1srs bodies often required shims to calibrate the screen to the sensor and swapping has become more complex in the later non-1srs bodies.

 

[MatBin]

I've got several split prism screens for my Canon 1srs bodies and it's less than a minute to swap them. Good for macro and my Zeiss manual focus lenses. Early non-1srs bodies often required shims to calibrate the screen to the sensor and swapping has become more complex in the later non-1srs bodies.

Do you need fast lenses to ensure the "split" doesnt blank out, as they used to in film days if you had a slow lens (usually a telephoto of course).

 

[sk66]

Do you need fast lenses to ensure the "split" doesnt blank out, as they used to in film days if you had a slow lens (usually a telephoto of course).

With a DSLR, yes, even more-so than with manual focus film bodies... that's because a portion of the light is sent to the PDAF module in the bottom of the camera, and not to the viewfinder screen.

 

[Canon Bob]

Do you need fast lenses to ensure the "split" doesnt blank out, as they used to in film days if you had a slow lens (usually a telephoto of course).

The Zeiss lenses that I use are typically f/1.4-f/2 and are very much usable with the prism but I revert to Liveview for slower lenses.

With a DSLR, yes, even more-so than with manual focus film bodies... that's because a portion of the light is sent to the PDAF module in the bottom of the camera, and not to the viewfinder screen.

The transmissive mirror in AF SLR/DSLR's typically sends only 60% of the light through the focusing screen and the other 40% passes through to the sub-mirror and on to the AF sensors.

 

[sk66]

The transmissive mirror in AF SLR/DSLR's typically sends only 60% of the light through the focusing screen and the other 40% passes through to the sub-mirror and on to the AF sensors.

And each side of the split prism is only using 1/2 of the light as it converges to form a focused image (2 separate virtual images)... so maybe 30% each...

 

[Canon Bob]

And each side of the split prism is only using 1/2 of the light as it converges to form a focused image (2 separate virtual images)... so maybe 30% each...

It's feasible to calculate the light loss by spot metering a constant source with different screens (without changing the configuration)....the metering sensor being after the focusing screen.