Red Filter compensation

[Jaime Marin]

Im going to be using a Tiffen red #25 filter and wanted to know how much filter compensation I should put in my light meter. Does it block out -1 or -2 stops of light????

 

[Bill Burk]

#25 Red is 3 stops

 

[Jeff Kubach]

I agree, it is 3 stops.

Jeff

 

[2F/2F]

Three stops is standard for a Wratten #25A, but you should always check each filter manufacturer's recommendations in case there are slight differences from the Wratten filters.

 

[Kevin Kehler]

I often do while using my spot meter is meter a scene, then hold the filter up and and re-meter the scene, using the exact same reference points. While 3 stops is the calculated average, depending on subject matter and lighting conditions, I have sometimes used between 1.5 to 4 stops for the same filter. If in a hurry, I use 2.5 stops; if you are using a camera with TTL metering, the camera will compensate for you.

The other thing to remember is that filters do not always work equally on all areas of the a photograph - if I am taking a picture with trees against the sky and put a red filter in front of the camera, the filter will darken the sky 3 stops. However, the shade under the trees is more blue than the sky: the sky has a blue background but is lit by sunlight, which consists of a number of colours whereas the shade is lighted only by the reflection off of the blue sky (and not the sunlight) and thus has less colours in it. As such, the shade might be darkened by 3.5-4 stops; this is important when placing shadow values.

 

[Rick A]

TTL metering does not always compensate fully for a #25 wratten filter, most will only compensate up to two stops. It is best to meter a specific scene without and then with the filter to se the difference, then check the results against the manufacturers specs for the filter. Film choice also plays an important part, not all films react the same to filters.

 

[Pinholemaster]

http://www.tiffen.com/camera_filters.htm

 

[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.

 

[Jeff Kubach]

BTW if you are going to use the 25A for infared film it might not be dark enough.

Jeff

 

[Adrian Twiss]

In his book Using the View Camera Steve Simmonds outlines the method used by gordon Hutchings. He meters through the filter and then applies the appropriate factor for filter being used. In the case of a #25 it is two stops.

He expalins that Hutchings developed this system to make sure that a scene's shadow areas receive enough exposure as shadow areas are primarily illuminated by blue light. I have used this method for some time and it works for me.

 

[al5256]

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

 

[johnnywalker]

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.

 

[Rick A]

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

 

[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)?

 

[Sirius Glass]

Three stops or eight times the time [23 = 8]

Steve

 

[2F/2F]

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).

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.

 

[Jaime Marin]

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

 

[benjiboy]

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.

 

[Michael W]

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.

 

[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!

 

[johnnywalker]

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.

 

[Jaime Marin]

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.

I actually dont know I have yet to purchase it

 

[Bill Burk]

I couldn't find the two-stop recommendation on the Tiffen site within a few minutes looking.

x8 (three stops) used to be printed on the slip of paper you got with your typical filter.
-If you want to keep it simple you can use that factor and be fine.

Adrian mentioned the instructions and chart from Gordon Hutchings.
-In the specific example of the 25 Red filter, AFTER reading through the filter, you open up an ADDITIONAL two stops.

Hutchings method sounds like a good way to go.

 

[Leigh B]

Here's a table of filter factors and the corresponding f/stop correction:

Fact..Stops
1.0...0.0
1.5...0.6
2.0...1.0
2.5...1.3
3.0...1.6
3.5...1.8
4.0...2.0
4.5...2.2
5.0...2.3
5.5...2.5
6.0...2.6
6.5...2.7
7.0...2.8
7.5...2.9
8.0...3.0
8.5...3.1
9.0...3.2

This derives from the relationship between factor and stops thus: factor = 2^(stops).

So the factor for 1 stop = 2^1 = 2; the factor for 2 stops = 2^2 = 4; the factor for 3 stops = 2^3 = 8; the factor for 4 stops = 2^4 = 16.

Going the other way, stops = log(factor)/log(2).

A filter factor of 3 corresponds to a correction of about 1 2/3 stops (actually 1.585).

B+W shows a filter factor of 5 for their light red (25) filter, which is about 2 1/3 stops (2.322).

- Leigh

 

[Helinophoto]

According to this article on wikipedia, 25A has a factor of 5, which is slightly over 2 stops http://en.m.wikipedia.org/wiki/Filter_factor , i have hoya and opening up roughly 2 stops worked fine for me.