Shooting B&W Film Through a Red or Orange Filter...

Doesn't the setting of the polarizer effect the loss of light? As you turn the filter, and more or less light is polarized, wouldn't the loss of light change?

Of course. That is why you use the polarizer: to block polarized light. Same with other filters: you use, say, a red or orange filter to darken the blue end of the spectrum.

If you do not want to get rid of what the filter blocks, you should not use a filter. When you 'compensate' for the light lost, you not only lighten the parts you wanted to darken, but also overexpose the parts not affected by the filter.

So use the fixed filter factor as provided by the manufacturer.

Here's a link to the transmission spectra of a few common Wratten filters.

 

You can see that none of them passes more than 90% of the incident light. So there's a small loss of light even in their passband(s).

 

However, it also has to be remembered that those graphs are only representative of equi-energy radiation. A phenomenon not normally encountered in real-life.

 

Things are complicated much more if we look at the spectrum of 'blue' sky light - Wikipedia link. It's not monochromatic, nor limited only to the blocking band of yellow, orange or red filters. It also forms a fair proportion of the light reflected back from every part of every sunlit daylight scene. So, in effect, a proportion of the total light reflected back from the scene and passing through the filter is attenuated to some degree.

 

This makes the computation of a filter 'factor' extremely complex and variable with each and with every reflective surface in the scene. Therefore the best compromise we can make is to simply use a filter factor that's been empirically arrived at. Plus rely on the broad leeway that the exposure latitude of B&W film affords.

Edited January 20, 2021 by rodeo_joe|1

Of course. That is why you use the polarizer: to block polarized light. Same with other filters: you use, say, a red or orange filter to darken the blue end of the spectrum.

If you do not want to get rid of what the filter blocks, you should not use a filter. When you 'compensate' for the light lost, you not only lighten the parts you wanted to darken, but also overexpose the parts not affected by the filter.

So use the fixed filter factor as provided by the manufacturer.

The stop factor recommended by the polarizer manufacturer might be 1 1/2 stops. But isn't that when you turn the polarizer for maximum polarizing effect. If you turn it only partially for less than full effect, isn't the stop loss less than 1 1/2? If so, how do you determine what the correct adjustment is?

The stop factor recommended by the polarizer manufacturer might be 1 1/2 stops. But isn't that when you turn the polarizer for maximum polarizing effect. If you turn it only partially for less than full effect, isn't the stop loss less than 1 1/2? If so, how do you determine what the correct adjustment is?

The fixed filter factor is for the fixed absorption of the polarizer filter. Half of all 'unpolarized' light (1 stop) plus a bit for the neutral density absorption of the material the filter is made of.

It does not and should not include compensation for the variable effect. You do not want to compensate the selective darkening of parts of the scene. That darkening is why you would want to use the filter.

So only use the fixed filter factor. Do not meter through the filter.

The fixed filter factor is for the fixed absorption of the polarizer filter. Half of all 'unpolarized' light (1 stop) plus a bit for the neutral density absorption of the material the filter is made of.

It does not and should not include compensation for the variable effect. You do not want to compensate the selective darkening of parts of the scene. That darkening is why you would want to use the filter.

So only use the fixed filter factor. Do not meter through the filter.

So when people use these filters on digital cameras, they remove the filter for each shot and set the exposure manually?

So when people use these filters on digital cameras, they remove the filter for each shot and set the exposure manually?

Probably not. They should, though. They reduce the effect of filters when they do not.

 

See earlier: allowing a meter to compensate for the reduction results in overexpose of the unaffected parts, and lightening of the bits the filter purposely darkened. A meter does not know it is behind a filter that is used to selectively darken parts of a scene. It just sees the amount of light reaching it.

 

So the only proper way to go is to go to manual mode, meter without filter, and apply the filter factor.

But yes, many will not go that way.

Edited January 20, 2021 by q.g._de_bakker

OK. Let's assume a film or sensor with an absolutely flat spectral response, and a subject such as a neutral greyscale lit by an 'even' spectrum. Then we interpose a filter - of any colour or neutral density.

 

The energy reflected from the white, or lightest part of the greyscale will be attenuated by however much the filter absorbs. If that's 1/3rd of the spectrum, as absorbed by a yellow filter, then the exposure needs to be increased by a factor of 3/2 to keep the 'white' exposure at the same level.

 

The same will happen to all of the greyscale steps. So keeping the exposure the same as without the filter will result in underexposure.

 

Moving on to a fully coloured scene will, broadly, have the same result. Since most pigments and dyes reflect light across a broad band. They aren't monochromatic reflectors whose colour is only either inside or outside of the filter passband.

 

Light can only be a positive quantity, and any subtraction from its total energy will affect exposure negatively.

OK. Let's assume a film or sensor with an absolutely flat spectral response, and a subject such as a neutral greyscale lit by an 'even' spectrum. Then we interpose a filter - of any colour or neutral density.

 

The energy reflected from the white, or lightest part of the greyscale will be attenuated by however much the filter absorbs. If that's 1/3rd of the spectrum, as absorbed by a yellow filter, then the exposure needs to be increased by a factor of 3/2 to keep the 'white' exposure at the same level.

 

The same will happen to all of the greyscale steps. So keeping the exposure the same as without the filter will result in underexposure.

 

Moving on to a fully coloured scene will, broadly, have the same result. Since most pigments and dyes reflect light across a broad band. They aren't monochromatic reflectors whose colour is only either inside or outside of the filter passband.

 

Light can only be a positive quantity, and any subtraction from its total energy will affect exposure negatively.

Or, in short, there are filter factors to be applied.

I've really enjoyed this thread and the inputs provided. As I said earlier in the thread I've been using filters (red, orange, yellow, and green) with black & white film (mainly Tri-X and Plus-X) since the mid 1970's. I started with a K2 filter that I borrowed from my dad (he'd had it since 1968) and bought (from family camera shop) some Yashica filters in red, orange, and green. When I first started using them I just metered through them and adjusted during printing. I started with a Konica Auto S2 (CDS cell on filter mount) and a couple of years later my first Minolta SRT 201.