Filter Factors, etc.

Ok, I get screwed up all the time with terminology and old stuff. On

the side of a filter there is a 3X - 1.5. If I understand

correctly, the 3x is an older Kodak system for rating filters and is

called a "filter factor". The 1.5 is a newer system and is a

compensation factor - in stops? But, I've been told it equates to a

1 2/3 stop compensation requirement. So, I'm confluxelmated. What

is the 3X really called? What is the 1.5 called? Is there a

difference between the 1.5 and the actual stops of compensation

required? And, what the heck is a Ratton system number? Is there a

different name for the K2 kind of designations? Any help would

help. :-)

Filter factors are based on simple fractions.a 2X factor means that twice as much exposure is needed,or 1 stop.The following are common f stop compensations for filter factors.

 

FF 1.2= 1/3 stop, FF 1.5=2/3 stop, FF 2=1 stop, FF 2.5=1 1/3 stop,FF 3 =1 2/3 stop, FF 4=2 stops.

 

 

" Wratten" designations are old school, and not commonly used much any more.The common wratten numbers were medium yellow=K2, orange=G, red =A, blue= C5.Im not sure what the meaning of these codes were?the only still relevant used wratten numbers are the 80,81,82 series of filters such as an 81B etc.

The 3Xis the filter factor and can be divided into your ASA to get your new ASA. ie 400

speed film with a 3x filter gives a new film speed close to 120. Change the setting on you

hand held meter and take your readings.

3X is the exposure factor needed to compenstate for the the filter. Use this factor to change the time, e.g., 1/8 second should become 3/8 seconds, which you might round to 1/2 second.

 

The exact conversion of 3X into stops (powers of 2) is 1.58 stops. Some might round this to 1.5, as written on the filter that you have, while others might round to 1 and 2/3 stops, as you found elsewhere and Steve gave in his table. It doesn't matter which -- a 1/6 stop change in exposure is photographically insignifcant and probably below the accuracy of the equipment. This is the easier number to use if compenstating for the filter by changing the aperture (f-number). For example, open up from f32 to between f16 and f22.

Filter factors relate to the amount of light the filter absorbs which is also the exact same amount you need to 'open up' to compensate for the filters absorption. Say your filter has a filter factor of 2X, you then need to open up one stop to compensate for the filters absorption,.......said another way, you need twice the light you have, OR you need to open up a stop on the lens to compensate for the filter that has a filter factor or 2X.

 

When you see a filter factor of 2x, think TWICE/double the light, twice the light also equals one stop,............say for instance you're shooting with strobes, you slap on a filter w/a filter factor of 2X, to maintain the exposure you had before you put on the filtor, you EITHER need to increase the light so it is double what it was, or you need to open up to the next stop on your lens(making the hole bigger at the end of the lens is the same as increasing light intesity, but that gets into E.V. and that's another story).

 

Considering filters which absorb more light, the filter factor increases, there's a progression and with that progression you have to increase the intensity of your lights OR in place of that open up the stop on your lens. The filter we discussed above with a filter factor of 2X, needs twice the light,...........a filter w/a filter factor of 4X needs 4 times the light to compensate for the light it absorbs,...........a filter w/a filter factor of 8x needs 8 times the light to compensate for the light it absorbs.

 

What gets folks into trouble, and me for quite a long time when I first tried to understand all this is that filter factors are NOT stops, when you think about filter factors think about the amount of light you need to increase............to convert filter factors into stops REMEMBER F-stops are geometric NOT linear.

 

Consider 3 filters, they have filter factors of 2X,...4X....8X respectively, the filter filter(2X) needs an increase of 1 stop...........the second filter(4X) needs an increase of 2 stops...............the third filter needs an increase of 3 stops.

Every time you DOUBLE a filter factor you increase by a stop.

 

An EASIER way to figure this in your head is to keep dividing the FILTER FACTOR BY 2, 2 into a filter factor of 2X IS ONE STOP...................................2 into a filter factor of 4X is two stops....................you can divide 2 into a filter factor of 8X THREE TIMES so that's 3 stops....................you can divide 2 into a filter w/a filter factor of 16X 4 times which means an increase of 4 stops, and so forth.

 

The number WITH the 'X' is the filter factor, the number WITHOUT the 'X' is the stop, also a 1 2/3 stop compensation translates into a filter factor of 3.2X because remember stops are geometric NOT linear.

 

You're referring to the Kodak Wratten filters and the wratten system, Kodak came up with gelatin filters made up with colored dye, after making a filter w/colored dyes, kodak would test the filter for spectral and total transmittance which delves into all the 'macaroni' discussed above.

 

I think the best thing to remember is filter factors relate to the amount of times you need to increase the light...2X,...4X,...8X, compensate for the filters absorption, to compensate by using stops just keep DIVIDING the filter factor by 2, that gives you the number stops required. I hope this helps.

There's no response from you so I hope you're not still 'fuzzy' on this, maybe I can add this without making matters worse. When you open up your lens to the next larger stop, you DOUBLE the light,.............when you close down to the next smaller f-stop on your lens you HALVE the light, .......................you're doubling or halving the light..............you're multiplying by 2(when you open to the next larger stop),.......................or dividing by 2(when you close down to the next smaller stop.

 

You have a red filter with a filter factor of 8X..........how many stops do you need to compensate for this? Divide the 8X by 2 which equals 4X(That's one stop).....................Divide the 4X by 2 which equals 2X(that's two stops)....................Divide the 2X by 2 which equals 1X(that's three stops).................you now know you have to open 3 stops on your lens to compensate for the light the filter absorbs.

 

So you open you lens one stop(doubling the factor to 2X[twice the light])..........You then open up another stop(doubling the 2X to 4X[four times the light]).......and you open the lens another stop(doubling the 4X to 8X[eight times the light]).

 

When you get into the concept of E.V., you're choosing how you're going to compensate for a filter factor, just like doubling or halving the light by opening and/or closing to a particular f-stop on your lens, you can double or halve shutter speeds, double or halve the power on your strobe, change to a higher speed film or one half as fast, you can double or halve the distance from your lighting to your subject matter.

 

Like a lot of concepts in photography this is harder to explain than to understand, but I hope this helps you to pick up on this, good luck.

One more thing, if you want to avoid all the math, simply dial the filter factor into your meter................say for instance you're using 400ISO film, you decide to slap on a filter with a filter factor of 2X(absorbs 1 stop or half your light),...................just change your 400ISO to 200ISO on your meter.

 

A filter w/a factor of 4X(reduces the light 2 stops) means you would halve your 400ISO film twice, and set your meter to 100ISO and so forth, doing it this way with the meter means you can then forget about having to caluclate for a filter factor during a shoot when you happen to change f-stops or your lighting.

Mike:

 

Some really good answers here - I learned a lot by reading them. However, being a real dummy - I usually meter through the filter.

Thanks, guys. That's a mound full of information to process, but it looks like it will really help. I'll digest it for twenty-four hours and let you know if I have any questions. After I posted yesterday, I went to "Valentines Day" dinner with my wife and then out to a Suns game. As a result, I didn't have time to read or respond again until this morning. I'll spend the day in the darkroom and then get back here tonight. LOTS of material to read - and three (3) posts from Jonathan. Geez. I'm impressed and thankful.

I read somewhere recently that filtering through a meter was not necessarily a good idea. It was new to me, but does anybody know if there is anything in that?

 

Steve

Depending on the electronic sensor used, the response of the meter versus wavelength may not be the same as film. The meter will be calibrated to match film for typical illumination with light from the entire visible band. If you use a strong filter that passes predominantly one color, then the meter may respond differently than film. The most common filter type that is strongly different from white light is red filters. Filters such as yellow and most green filters still admit a large fraction of the visual band. This problem is unlikely except for strong filters, such as red filters, and less likely with recent meters.

 

Metering through the filter doesn't work with IR filters and IR film because this is a case where the response of the meter and the film are very different. This is intentional, since normal meters are designed for use with non-IR films.