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  • Hey all, just changed over the backend after 15 years I figured time to give it a bit of an update, its probably gonna be a bit weird for most of you and i am sure there is a few bugs to work out but it should kinda work the same as before... hopefully :)

rear lens filters

Peter Osinski

Peter Osinski

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When you place a filter behind the lens you change the back focus distance by 1/3 the thickness of the filter, which requires using a clear filter of the same thickness when not using a filter. the cooke 20-100 has a slot for a gell filter as do all panavision cmeras. The down side of gell filters is that when you get crud on them it is realy big in the frame, like dirt on the Neg., so they are kind of high maintenance. Some extreme wide angle lenses have a slot for a filter because it would be impractical to use a matte box.

Nick
 
So if there was a slot behind the lens for 3x3 filters dropped in from the top all I would need to do is keep a UV/clear filter in there when nothing else was in? Also if the rear filter was glass is there any risk of reflections? do you increase or decrease the back focus by 1/3 of the filter thickness?
 
Yes. Basically, you reshim the lens to it is right with a filter in and then you always use one. Yes, any time you add a piece of glass to the optical path the chance for reflections and flares is increased. I don't remember whether it is plus or minus on the back focus distance.

Nick
 
anyone know what the max travel is of the back focus adjuster on Scarlet, in mm ?
 
Ok sounds good. What do you think of the idea of having a camera that allows you to drop in 3x3 glass filters behind the lens and having the mount adjusted ahead of time for the back focus change rather than shimming the lens? That way as long as you keep the clear filter in whenever you don't need other filters you can add any lens you want without re-shimming as long as the filters used match the specifications for the filter slot
 
Wide angle lenses require very tight tolerances for the back focal distance. Filters rarely have exactly the same thickness.
Also, you can't use IR-hot mirror filters as they would cause internal reflections. Rear fitering is more common on long lenses, which can suffer from (sometimes substantial) loss of sharpness when used with front filtering. Also, long lenses are not as sensitive to back focus shift due to slightly different filter thickness.

The rule is that, if you insert a thicker filter, the lens needs to be further away from the sensor. When removing a rear filter, you need to bring the lens closer towards the sensor.

hope it helps
 
Peter,

We used to use gel filters behind the lens on ACL 16mm cameras. The variation in gel filter thickness is probably small and I dont remember there being any issues. Even glass filters should not be a problem if they are made for the purpose and tolerances kept strict. I wouldnt give up on the idea.
 
Peter Osinski said:
well my idea would be to just use tiffen 3x3 filters which are supposed to be ground to within 1/10,000 inch accuracy in thickness or something like that.
Click to expand...

The accuracy is within one filter (flatness) but not across different filters. Not sure how you are going to fit a 3x3 filter behind the lens.
 
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