What to rate Aerochrome at when shooting with a red 25 filter?

I shot a roll of Aerochrome 400 today. Most photos were taken with a heliopan yellow 12 and rated at 400 with the filter attached. I metered at 400 without adding any compensation for the filter using the tiny light meter app on my phone. I've found this app to work well with other slide film.

I also took a few shots with a red 25 filter which I've not used before. My yellow filter has a filter factor of 3=1.5 stops of exposure. The red filter has a factor of 8=3 stops. My understanding is that you add stops and multiply factors. Given that these filters differ by 1.5 stops and the film is rated at 400 with a yellow filter was I right to just increase my exposure by 1.5 stops? i.e. If my meter read 1/250 @f16 with a yellow filter, was I right to shoot at 1/250 @ f9.5 with a red filter.

What is Aerochrome?

IR aerial photography film. If you like purples, you'll love this...

From Kodak's web of intrigue:

KODAK AEROCHROME III Infrared Film 1443 (DISCONTINUED) is an infrared-sensitive, false-color reversal film featuring medium resolving power and fine grain.

Click to expand...

See http://www.thephoblographer.com/201...-guide-to-the-confusion-of-lomochrome-purple/

Richard Mosse has a large body of work using this.

I am wondering about that red filter given the IR characteristics of the film.

Just to hinder (or add to) confusion: The new Lomochrome Purple also mentioned in the article linked by Posson du Jour is not an infrared film. It is a C-41 color film with a palette that is somewhat similar to Aerochrome, but without the infrared ability to tell clorophyll from green paint.

Arctic amateur said:

Just to hinder (or add to) confusion: The new Lomochrome Purple also mentioned in the article linked by Posson du Jour is not an infrared film. It is a C-41 color film with a palette that is somewhat similar to Aerochrome, but without the infrared ability to tell clorophyll from green paint.

Click to expand...

www.thephoblographer.com doesn't help by mentioning in the header the two films in the one sentence without differentiation. It is confusing.

This is the real deal. I've shot it with a yellow filter before and been happy with the results http://www.flickr.com/photos/28305528@N02/7866483280/in/set-72157631257606148 but wanted to experiment with the red which can also give an interesting effect.

Any thoughts on the filter compensation?

3 stops. I don't know why anyone would do this.

I'm thinking 4+ stops, maybe 5 as a safety margin. Red doesn't ring true to me with this sort of film. My thinking comes from the effect of using R72 filters in IR. I think your best action would be to bracket a few frames and run notes on each exposure, based on the standard test methodology of exposing IR film.

Mind you, the colours as they present (in Fllickr) are wild, off-the-planet — enough to make Velvia blush!

This isn't like b&w infrared film, you don't use an IR pass filter as you still need to expose the other colour layers.

With a yellow filter on, blocking out the blue light which all layers of the film are sensitive to, the film is still sensitive to green, red and infrared light. After processing the infrared is displayed as red, the red as green and the green as blue. This means that the green leaves of trees and grass which also reflect IR light show up as a mixture of red and blue (magenta).

With a red filter on both blue and green light is blocked, the film will only be picking up red and infrared light. The film will only see the IR light from the green foliage and it should end up red rather than magenta.

Anyway this is rather beside the point but was just to emphasise that. This film doesn't need an IR pass filter like an R72.

The yellow filter is standard to block the blue light otherwise all layers are overexposed. This is taken into account with the box speed of 400 iso with a yellow filter. If I use the red filter instead which has a 1.5 stop difference to the yellow one, was I right to effectively rate it at 150 iso?

The logic is OK. Right or wrong, yes or no... the only way to tell is to go ahead and expose with the red filter, bracketing along the way. I would think EI200 would have been a better choice. Bracket and discover. It might be wasteful, sure, but you're guaranteed proof when otherwise it's mostly speculation here for people who have not used nor are familiar with exposure of Aerochrome. To this end, you are the expert on it!