Spotmatic exposure with Hoya R(25A) red filter

XA2inmypocket

XA2inmypocket

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Paul
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  • I'm going to play for the first time with infrared - at first on 35mm 400S film.
  • I'm using a Pentax SP500, loaded with a 1.35V zinc/air battery.
  • I've fitted a Hoya Red 25A filter onto the Super Takumar 55mm f/2 auto lens.

I've recently been using this Spotmatic with FirstCall 400S but without a filter, and have been getting good exposures by setting the camera exposure guide to ASA 200, as I've had a history of 400S under-exposing in other cameras.

I know that this is a daft question, but on the Spotmatic exposure system - should I drop the ASA setting down further when shooting through the red filter - or is it simply the case that the light meter is reading through the filter anyway?
 
Meters are colour sensitive. In theory they'd compensate for the filter factor, but in the case of a red filter will usually underexpose by about a stop. Test it by seeing what the exposure is, then fitting the filter and see if the exposure changes by less than the filter factor.

Do remember that light at the beginning and end of the day contains more red, so the filter factor will be less.
 
StephenM said:
Meters are colour sensitive. In theory they'd compensate for the filter factor, but in the case of a red filter will usually underexpose by about a stop. Test it by seeing what the exposure is, then fitting the filter and see if the exposure changes by less than the filter factor.

Do remember that light at the beginning and end of the day contains more red, so the filter factor will be less.
Click to expand...
Yes, of course. With filter, it needs two full stops on the aperture ring, than without. Thanks for advice.
 
Just spotted that it's IR. Bets are always off with IR, since there's no easy way of measuring how much IR is being reflected. My comments applied to normal B&W. On the bright side, there is often a good correlation between visible and IR.
 
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