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Are exposure filter factors only relevant to B&W film only?


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My understanding of filter factors (which may be totally erroneous ....) is that they were originally designed primarily for B&W film.

The popular landscape filters - yellow, orange and red block various amounts of blue light depending on the 'cut' of the filter.

 

Given that the shadow areas in landscape photos are primarily illuminated by 'blue' light, is the filter factor compensation applied so a

scene's shadow areas receive enough exposure and have some detail?

Does this mean the similar colours in the scene (yellows, oranges and reds) are therefore a little overexposed by the compensation?

I'm sure the answer would be found in a bunch of filter tests .....

Matt B
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*Everything* was originally for B&W, I guess...

If the film is panchromatic, assigning an exposure factor to a filter implies an assumption as to what fraction of the usable light the filter is going to remove; that is, an assumption that the subject is 'normal-coloured'. If it isn't, you need to bear it in mind, and devise your own exposure factor, taking the filter-maker's number as a starting point. So, if you photograph some pale blue flowers, and you use a yellow filter to bring out the clouds against the blue sky behind, you'll darken your flowers too, and since they're blue-ish, you'll need a bigger factor than for an average subject to avoid doing that.

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You raise an interesting question. I was taught that the "filter factor" is applied to compensate for the loss of exposure due to the amount of light cut by the filter. It has been said that the factor should be increased or decreased depending upon the time of day [the Kelvin of the light] and the type of filter being used. Early morning and late afternoon tend to be less blue than mid day; you can decrease the FF with blue cutting filters.
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..... is it because panchromatic B&W film is sometimes more sensitive to the blue end of the visible light spectrum and the yellow, orange and red filters 'compensate' for this?

I have used these type of filters with colour film and found the manufacturer's filter factors suggested are not always applicable.

Matt B
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Are you asking about color compensating filters for color film, or are you using "black and white" contrast filters on color film? The FF on B&W contrast filters are for panchromatic B&W film; and the spectral characteristics of B&W film is different from color . They were not intended to be used on color positive or negative film. That doesn't mean you can't use them for artistic purposes. Checking the exposure with and without the filter on a hand held exposure meter will get you in the ballpark for the amount of light the filter is with holding. But the effect will be an overall hue of color you are shooting through, but I am sure you know that since it appears you have already done this.
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The filter factor is actually a multiplier. To apply, we multiply the exposure time without filter by the filter factor, this calculates a revised shutter speed that compensates for the absorption of light energy by the filter. Technically, the same filter will have different filter factors for different films based on their sensitivity to the colors (and digital sensor also). Additionally, the filter factor will be different depending on the color temperature of the exposing light. In other words, the filter factor is different for daylight than it is for tungsten light. Generally we can ignore these facts because the negative positive system has great exposure latitude. When doing critical and or scientific photography, the fact that the filter factor can vary must be taken into account. Also, when the ISO of the film or sensor is below 100, the variability of the filter factor becomes important.

 

Let me add, while the filter factor is a multiplier to revise shutter speed, se also use it to compensate by aperture adjustment or a combination of shutter and aperture.

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Are you asking about color compensating filters for color film, or are you using "black and white" contrast filters on color film? The FF on B&W contrast filters are for panchromatic B&W film; and the spectral characteristics of B&W film is different from color . They were not intended to be used on color positive or negative film. That doesn't mean you can't use them for artistic purposes. Checking the exposure with and without the filter on a hand held exposure meter will get you in the ballpark for the amount of light the filter is with holding. But the effect will be an overall hue of color you are shooting through, but I am sure you know that since it appears you have already done this.

Thanks all for the clarifications, I wanted to know if the same FF exposures for contrast filters should apply to colour negative/slides - research for a project I'm currently exploring - some basic tests had me wondering if my exposures were wrong.

Matt B
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Common filters for black and white are deep colors, such as yellow or orange that will darken skies, and make clouds show up.

They usually have fairly large filter factors. These are not normally used with color films.

 

For color film, correction filters such as to use daylight balanced film under tungsten lighting, or vice versa, have smaller factors.

Though, since they are often used under low light conditions, even the smaller factor is significant.

This is especially important for slide films, with less exposure latitude.

 

Color correction filters are also recommended for black and white films, to get accurate gray scales for the colors, but most often enough this is ignored.

For some years now, Kodak black and white films recommend blue flash bulbs, which are color correcting filter coatings on flash bulbs.

 

Many older light meters don't cover the full visible spectrum, and so you can't depend on them to meter through filters.

CdS, for example, is mostly blue sensitive.

-- glen

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Thanks all for the clarifications, I wanted to know if the same FF exposures for contrast filters should apply to colour negative/slides

 

No, the filter factor varies, depending on the film and light source. You should ideally be getting your filter factors from the film data sheets, not from the filter maker.

 

On the film data sheets for COLOR films, generally the only filters recommended are for purposes of "correcting" the color of light sources. (Many of these corrections are outdated now, as many of the listed light sources, especially certain fluorescent lamps, are seldom used.)

 

Even for b&w film, a given filter may have 3 or 4 different factors, depending on the specific film and light source. So don't rely on a single factor supplied by the filter maker; it may be good MOST of the time - just don't blindly trust it.

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Dye-based colour filters aren't perfect; there's always some absorption in the passband, and a transition area between passband and cutoff. Therefore there needs to be some exposure compensation, even if the subject colour matches that of the filter.

 

Beside which, real world colours are rarely pure and saturated. Green grass, for example, responds far more to yellow filtration than to spectral green.

 

Filter factors are computed such that neutral grey and white parts of the subject are rendered with the same density value on panchromatic film as they would be without the filter. I.e. a density wedge exposed without any filter looks the same in the negative as with the filter + filter factor.

 

Coloured parts of the subject will obviously be rendered differently. Although why you'd want to shoot colour film through a Y2, YG, XO or red filter isn't quite clear. However, the use of CC filters to correct a colour cast or compensate for lighting variations also need a filter factor to be applied to the exposure.

 

So, in short, yes, filter factors have to be used with colour film as well.

 

WRT copying slides, there are only 3 CMY dyes used to produce the colours. Therefore in theory, you're only interested in the complementary narrow RGB parts of the spectrum that correspond to those dyes' maximum densities. A CP or CC filter's density is also given as the maximum it presents to a standard 'white' spectrum, but this may not be the same as the slide's dye spectrum.

Edited by rodeo_joe|1
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