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Shimming astro-modded Canon DSLRs for correct autofocus


michael8554

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The IR Cut Filter is 0.6mm thick, and has a Refractive Index of about 1.5

This refractive index has the effect of extending the light cone.
So when the filter is removed and not replaced with a clear glass slip or a Baader filter,
the autofocused lightrays will now be focused slightly in front of the sensor. 
However the autofocused image in the optical viewfinder will be focused, because the
mirror to optical screen distance hasn't changed.

So manual focusing with the optical finder results in an out of focus image on the sensor.
Autofocusing will result in an out of focus image on the sensor.
Visually focusing with the LiveView image from the sensor results in an in focus image on
the sensor (but will be slightly out of focus in the optical viewfinder).

If my reasoning is correct, adding shims to the sensor will not correct the error, because
these will move the sensor further away (the sensor is mounted behind the shutter
assembly)

I calculate the sensor needs to move forward (1.5 - 1) / 1.5  x 0.6  = 0.2mm.

In other words, the original shims need to be 0.2mm thinner.

A repair manual for an EOS 5D shows a list of 11 shim sizes from 0.03mm to 0.35mm.

I have experience of modding a number of EOS cameras, and for the price I think the
professional modders who shim the sensor certainly earn their money !

Michael
 

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I've discovered that newer models have "floating" sensors.

They are retained in a similar way to a Newtonian primary mirror - with springs and screws.

This means that when modding you have to be very aware of how far in the screws are screwed - get it wrong and the sensor will be tilted.

But it also means you can screw the sensor forwards to regain correct autofocus.

The Gary Honis instructions recommends a 1/12 of a turn to compensate for a 0.97mm difference (in filter thickness, not focus difference) with both filters removed, and a MC Clear filter inserted.

You do the math....

Michael

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