Filter Factors / Optical Density

Asha

Asha

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Asha
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I'm posting this in f&c simply due to the fact that this question relates specifically to Formatt Hitech filters that I'm using with b&w negative film.

I have an exposure chart ( from Formatt Hitech) showing Optical densities, Filter factors and F-Stop reduction, thus easy enough to calculate the amount of exposure compensation required for any given filter.

This is straight forward with the ND filters and Grads that I have, for example using an ND filter with an optical density of 0.9, gives a filter factor of 8 and thus an F-stop reduction of 3 ( according to the chart!)

My problem comes about with the colour filters, as unlike the ND / Grads which have their optical density stamped into the corner of the filter ( ie 0.3, 0.9 etc), the colour filters state numbers which don't appear to relate to any of the figures listed in the chart.

For example:

8 Yellow

21 Orange

58 Green

Red 2 STD


So how do I calculate the optical densities of these colour filters in order to then calculate the necessary exposure compensation?

If I've confused you all, I appologise, but I'm somewhat confused myself!:LOL:
 
pretty sure that the numbers are just the Wratten / Tiffen Filter Numbers... so, find a exposure compensation chart for Wratten filters and Robert should be you're Aunts Significant Other...

failing that, 1 stop for yellow, 2 for orange and 3 for red or green is generally "close enough for rock and roll".
 
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I've measured my colour filters with my lightmeter to check for their attenuation. I usually do it every time I use them, as I end up forgetting...
 
Wratten numbers and factors can be found here.

Metering through a filter is said to normally work, except for red filters with CdS meters which are overly sensitive to red and give a reading one stop under.

When using red or orange in evening (I don't do early mornings :)) remember that there's proportionally more red light so the factors will be slightly less.

I always use my meter and apply the filter factor I find on the filter and don't worry about the subtleties - but I'm slapdash.
 
Previous post made before Asha's. I'm slow as well as slapdash...
 
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