Backfocus and Filters

[AlistairW]

Hello,

When calculating backfocus, I am sure I was told to add 2mm for the filters. If this is correct, why is it necessary ? Light is simply passing through glass, so why add 2mm ?

Thanks

Alistair

[Davey-T]

This is the QSI take on it, made my brain hurt 

Only matters if you're using  flattener / reducer

Dave

http://qsimaging.com/downloads/QSI-500-600-Series-Back-Focus.pdf

[AlistairW]

Thanks for that - i do use a FF/FR and Baader - so the table (i think) suggests 2.1mm. So thats good news.

Not sure i get the physics/optics though. I guess the path length for the light is increase by 2mm - but i presume this to account for refraction in the glass ??

[AlistairW]

Actually i think that should be 2.1/3 or 0.7mm. But why is it a negitive distance according to the table ? - Confussed.

[AlistairW]

Reading a bit more - this seems to be to do with refractive subtraction. Hence i should be subtracting 0.7mm from any backfocus calculations - ( i think adding 2mm now seems completly wrong )

[Davey-T]

33 minutes ago, AlistairW said:

Reading a bit more - this seems to be to do with refractive subtraction. Hence i should be subtracting 0.7mm from any backfocus calculations - ( i think adding 2mm now seems completly wrong )

Told you it would make your brain hurt, there's a long confusing thread on the subject somewhere.

Dave

 

[Davey-T]

Snippet from Ollies post on the Tec 140.

Dave

 

[AlistairW]

Wow - this really did make my head hurt - whether to add or subtract is dependent on your view point of preserving optical path, or the 'geometric' chip to lens distance. Good cases it seems for both sides.