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1. Hi,
When I TTL meter from Nikon FE, Mamiya 645 and others I add +2/3 stops more over TTL metering for Hoya 25A.

2. Was it Ralph Lambrecht who recommended not taking a meter reading through the filter, but rather using the filter factor provided by the manufacturer?

I notice these factors are getting hard to find on the internet. For instance the Tiffen site doesn't give them.

3. I have all my filter factor info taped to each filter case, Color, factor, etc. no excuse for error.

4. Is a filter factor the same as stops? I didn't think so. If a filter has a factor of 3 wouldn't you open up 4 stops (1, 2, 4)?

5. Originally Posted by benjiboy
According to Tiffen the filter factor of their 25A is 3, and they recommend you open up two stops to compensate for the light loss.
Wait a minute; that makes no sense. For one, the two numbers don't match each other. If you were supposed to open up two stops, the filter factor would be 4. If the filter factor was 3, you would open up 1-2/3 stops. For another, the filter factor for the filter is 8, and you are supposed to open up 3 stops (2x2x2=8).

Originally Posted by Bruce Osgood
Is a filter factor the same as stops? I didn't think so. If a filter has a factor of 3 wouldn't you open up 4 stops (1, 2, 4)?
If a filter has a factor of 3, you'd open up 1-2/3 stops.

The way I figure it out in my head is to divide 100 percent by the filter factor; that is how much light makes it through. Then I count stops to compensate. For example, with a filter factor of 3, one third of the light makes it through. Adding one stop would half 100 percent, to compensate down for 50 percent. Adding another stop would cut 50 percent in half, to compensate down to 25 percent. Therefore you open up between one and two stops. Since the light loss is slightly more than halfway between 50 and 75 percent, 1-2/3 is close enough.

If the filter factor was 8, I'd take 1/8 of 100 percent, to get 12.5 percent. Cut 100 percent in half once to get 50 percent. Cut 50 percent in half once to get to 25 percent. Cut 25 percent in half once to get 12.5 percent. Voila. Three stops makes up for the light loss.

6. Ok to make complete sure here is what I plan on using.

Ilford delta 100 in 120 film with a the tiffen #25 red filter and my light meter

from what I see here most people agree that I should compensate for 2-3 stops in my light meter correct? To be clear I am not metering TTL

7. Originally Posted by 2F/2F
Wait a minute; that makes no sense. For one, the two numbers don't match each other. If you were supposed to open up two stops, the filter factor would be 4. If the filter factor was 3, you would open up 1-2/3 stops. For another, the filter factor for the filter is 8, and you are supposed to open up 3 stops (2x2x2=8).

If a filter has a factor of 3, you'd open up 1-2/3 stops.

The way I figure it out in my head is to divide 100 percent by the filter factor; that is how much light makes it through. Then I count stops to compensate. For example, with a filter factor of 3, one third of the light makes it through. Adding one stop would half 100 percent, to compensate down for 50 percent. Adding another stop would cut 50 percent in half, to compensate down to 25 percent. Therefore you open up between one and two stops. Since the light loss is slightly more than halfway between 50 and 75 percent, 1-2/3 is close enough.

If the filter factor was 8, I'd take 1/8 of 100 percent, to get 12.5 percent. Cut 100 percent in half once to get 50 percent. Cut 50 percent in half once to get to 25 percent. Cut 25 percent in half once to get 12.5 percent. Voila. Three stops makes up for the light loss.
I know you're quite correct in everything you write about filter factors 2F/2F, and I thought when I read it on a Tiffen filter site today it was screwy at the time but I'm just quoting what Tiffen themselves say.

8. I would give 3 stops more than the meter reading.
Does the filter have the filter factor engraved on the ring? If it says "x8" then that confirms 3 stops.

9. Maybe I got it now....

If the filter factor is 3 and the correct number of stops is 1.74, then the factors' Sq Rt equals the number of stops: Sq Rt 3 = 1.74

By jove, I think I've got it!

10. Originally Posted by Bruce Osgood
Maybe I got it now....

If the filter factor is 3 and the correct number of stops is 1.74, then the factors' Sq Rt equals the number of stops: Sq Rt 3 = 1.74

By jove, I think I've got it!
No, it's not the square root except in the case of filter factor 4, which is a 2 stop correction.
It's actually 2^s = ff, where s=stop correction and ff= filter factor. In other words, two to the power of the stop correction = the filter factor.
2 to the power of 2 = 4
And 2 to the power of 3 = 8 (2 x 2 x 2). (8 is the filter factor, 3 is the stop corrrection)
It's best to find a table with the equivilants, or make one yourself with a calculator that will do roots. Finding 2 to the power of ? = 5 (for instance) in your head ain't easy. Or 2f/2f's method will get you close.

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