Filter factor compensation

[Jarvman]

Right, so I've just bought two new filters. An yellow/orange 4x and a red 5x. Is the filter factor really as much as 4 and 5 stops compensation? The reason I ask is that I'm loaded up with ISO 50 film and if I compensate for the red filter by a factor of 5 it goes out of my sekonic light meter's ISO range. Even with my orange filter compensated for by 4 stops it knocks it all the way down to ISO 3! Is the answer simply to use faster film? I want to get the most out of my enlargements even using 6x9 negs hence the slow film. What's the course of action? Thanks.

 

[Peterpan]

The Red filters I have are B+ W 090m Light Rd and091 Dark Red are 21/4 stop and 3stops orange i would think about 1- 1/12 stops

 

[Steve Smith]

That's not stops, it's total amount of light so a x2 filter means one stop extra, a x4 filter means two stops extra. I have never come across a filter with a x5 factor but that sounds like about 2 1/3 to 2 1/2 stops extra.

I usually use one stop extra for yellow, two stops for orange and three stops for red.


Steve.

 

[Sirius Glass]

Which yellow-orange? Which red? It makes a difference.

The factor for Red 25A is usually 8. 2^3 = 8 so the difference is 3 stops. This means ISO 50 => 8 or with Sunny 16 => f/5.6 1/50sec.
If the red factor is really 5 => 2.3 stops. This means ISO 50 => 21.5 or with Sunny 16 => f/5.6 - f/8 [1/3 stop larger aperature than f/8] 1/50sec.

I am therefore guessing the yellow-orange that if it is 4, 2^2 = 4 => 2 stops.
This means ISO 50 => 25 or with Sunny 16 => f/8 1/50sec.

Steve

 

[RobC]

filters are either specified using time factors or densities.

Time Factors are usually given as nX so 4X means the time is 4 times greater than metered. And as you know 1 stop is equivalent to doubling the time and two stops is equivalent to doubling the time twice. 2x2 = 4. So a 4X time factor is equivalent to 2 stops.

Density factors are specified using the log of density and each 0.1 of density equates to 1/3 of a stop. So a 0.1 density filter requires 1/3 stop extra exposure. A 0.3 density filter requires 1 stop extra exposure. A 0.6 filter requires 2 stops extra exposure. A 0.5 filter requires 1 2/3 stops extra exposure.

how you apply the filter factor is entirely up to you. You have the choice of adjusting your meter ISO (which I don't recommend unless you are exposing whole film with same filter on all shots), adjusting aperture, or adjusting time.

I would suggest that since you have filters specified in time factors, that you meter normally and work out aperture and time without filter, Then multiply time by filter factor and then if needs be adjust aperture and time to Equivalent Value settings. But that's upto you.

 

[jd callow]

wouldn't a 2 stop filter EI be 12.5 on iso 50 film?

 

[Ole]

Yes, and a 3-stop filter would give the equivalent of EI 6.25.

But if the film in question is EFKE 50 the filter factor for a red filter is much higher, something like 5 to 8 stops depending on the filter. If it's PanF+, the filter factor is sligthy higher than nominal - still for a red filter. Tmax 100 and FP4+ should have different filter factors too, with the Ilford film needing less exposure than the Tmax even if both are exposed at nominal speed.

And that is just for red filters.

I'm not even going to think of thinking of green and blue filters - a large whisky is a much better way of getting a headache!

 

[Jarvman]

Ah alright cheers. The film is Pan F 50 and the red filter is a red 090. So 5x is 2 and a half stops but I'll call it 3 stops then eh? So effectively ISO 6 on my light meter. I'm not likely going to make any mistakes by swapping filters and not forgetting to adjust the meter's ISO because I'm only shooting the 6 remaining shots on a roll of 6x9's on my fuji and have to get it right. I've got to have 3 final fibre based prints for my darkroom assignment on the 30th of the month. Managed to pick up both 67mm B+W filters on ebay for £20, was a good buy

 

[jd callow]

Assuming you rate Pan F @ 50 when exposed unfilterred...

 

[RobC]

Ah alright cheers. The film is Pan F 50 and the red filter is a red 090. So 5x is 2 and a half stops but I'll call it 3 stops then eh?

NO you haven't quite got it. 2x2x2 = 8 your factor is 5 which is only a tad over 2 stops. It somewhere between 2 stops and 2 1/3 stops so 3 stops would be at least 3/4 of a stop out. 2 1/3 stops would be closest.

 

[Bruce Osgood]

The f-Stop Factor is the Sq. Root of the X number. IE: A 4X filter will be opened 2 f-stops. A 9X filter (?) would be opened 3 f-Stops.

 

[RobC]

The f-Stop Factor is the Sq. Root of the X number. IE: A 4X filter will be opened 2 f-stops. A 9X filter (?) would be opened 3 f-Stops.

That might work as an approximation but it is factually wrong.

2X = 1 stop
4X = 2 stops
8X = 3 stops
16X = 4 stops
etc

Both Rodenstock and Schneider / B+W use the factors as I have given them. And as far as I know, so does every other filter manufacturer on the planet.
As the factor gets bigger, the more erroneous your method becomes.

 

[Jarvman]

Ok yes I see now. x8 = 3 stops so x6 = 2.5 stops making x5 = 2.25? Stupid maths! So there really is very little difference in the filter factor between the orange and the red. There can't be much harm in calling it three stops and slightly overexposing like I usually do though can there? I need to get this down to a science which will hopefully come with the 5x4 I've bought. There's a discipline I'm lacking. Perhaps I should've gone for a deeper red (x8). The bloke had one and it only sold for £6. Is there going to be very much of a marked difference in the effects caused between them. Ta Rob thanks for putting me straight.

 

[RobC]

Ok yes I see now. x8 = 3 stops so x6 = 2.5 stops making x5 = 2.25? Stupid maths! So there really is very little difference in the filter factor between the orange and the red. There can't be much harm in calling it three stops and slightly overexposing like I usually do though can there? I need to get this down to a science which will hopefully come with the 5x4 I've bought. There's a discipline I'm lacking. Perhaps I should've gone for a deeper red (x8). The bloke had one and it only sold for £6. Is there going to be very much of a marked difference in the effects caused between them. Ta Rob thanks for putting me straight.

correct there would be no more than 1/4 stop difference between the 5X and 6X filters.

Using filters is not quite that straightforward because the effect they have depends very much on the subject colour. Firstly, for black and white subjects you will find that anything not in direct sunlight will usually have a lot of blue in it and so yellow, orange and red will all darken shadow areas. Red filters have a very dramatic effect. So much so that many people find it too strong a filter for "normal" usage whereas a light yellow filter will have a more subtle effect. You have to learn how each filter will affect the result based on experience and careful judgement of each subjects colours.
For example, just throwing a red filter on will not necessarily make the sky go black. The reason is that when you look at the sky, especially the nearer you get to the horizon, the sky tends to get very much lighter and contain a lot more humidity/haze which is frequently not removed by filters. Maybe partially but not necessarily fully. On the other hand if you are at high altitude where the air is below freezing so all the humidity has frozen out of the atmosphere and the air is really dry, then the sky becomes a much deeper blue and a light yellow filter can make the sky completely black.
Alternatively you might photograph a red flower using a red filter. Place the flower on say zone 7 or 8. Result is that flower comes out almost white and really stands out against anything not red behind it.
So it really depends on what you are trying to do and your subject. Trial and error is the name of the game to find out what filter factor works for which subjects and how you can affect the contrast of different coloured subjects.

 

[Bruce Osgood]

That might work as an approximation but it is factually wrong.

2X = 1 stop
4X = 2 stops
8X = 3 stops
16X = 4 stops
etc

Both Rodenstock and Schneider / B+W use the factors as I have given them. And as far as I know, so does every other filter manufacturer on the planet.
As the factor gets bigger, the more erroneous your method becomes.

The Square Root method was told to me by a NY Daily News Photogrpaher. I like this method better and will try it.