Filters for Black and White.
The Web is awash with advice for young photographers and not all of it is good advice. Some of it has been around for a long while and myths become perpetuated by people who don’t think carefully about the technique, and/or don’t bother to test the theory. One such myth is that you can get a reliable exposure by using a cameras TTL meter reading with a filter attached. This might be true if you have a neutral density filter or a polariser, but it is certainly not true for coloured filters. The meter does not respond to all of the colours of the spectrum in exactly the same way as the film. Another problem is that the filter will lighten its own colour and darken its opposite, so your exposure could be affected quite strongly by the colours in the scene.
All filters reduce the amount of light passing through, and this must be compensated for. Generally, the stronger the colour, the more correction is needed. Filter manufacturers sometimes put a number on the edge of the filter, but some don’t. Kokin for instance, give no information about how much to correct for. The usual method of informing the user of the exposure correction needed, is to give a filter factor. This is a number that your exposure should be multiplied by, to arrive at the correct exposure. Why this method was universally adopted mistifies me, why not just give the correction in stops? that would be much more useful out in the field. Who has time to multiply 1/250th of a second by 1.5? Why not just write; 1/2 stop adjustment?
The following conversion chart gives filter factors and the actual exposure compensation needed in stops.
If you have bought your filters from a well known company, there should be information provided to indicate the correction needed. This can easily be written on a sticker on the filter case for reference when out shooting. Of course, some people acquire filters from all sorts of places and from a range of makers. The exposure correction, if not printed on the filter ring is then a mystery. Finding the correct advice is sometimes very difficult, even when looking at information online. Not all filters are the same density/depth of colour. For instance, I have three reds, two different orange filters, and three different green filters. See scanned sections below. They can’t all be the same in use.
FILTER COLOUR COMPARISONS
A quick search for information about how much exposure difference there is with filters will throw up a lot of forum threads with conflicting advice. On the forums there are comments to the effect of; ‘I do it my way and it works for me’, or ‘filter factors are basically useless’. which are misleading and hardly scientific. This type of advice is way off the mark and is bad for beginners.
In photography there are many areas where exposure and density can go astray; There is reciprocity correction, filter factors, bellows extension, and temperature and dilution when processing. Reliable, accurate information is needed to avoid wasted film and ruined shots. Each photographer has different lighting scenarios, film choice, and tonal preferences, so I would suggest testing around the manufacturers stated exposure corrections and making notes about your preferred result to establish how near the mark your filter factors are in your work.
Most photographers don’t bother with testing, they see it as a boring waste of time and film, so they carry on making the same mistakes, which is wasteful and frustrating.
A simple test could be set up to establish the correct exposure compensation for your own camera meter, filters and film of choice (There will be slight differences in spectral response with different films, but not too significant). A 36 exposure film is long enough to test 4 different exposures for each of 6 filters. This test will give you the information you need to avoid incorrect exposures in the future, so it is cost effective.
Shot 1. take one shot at the metered setting. Shot 2. Meter through the filter and take one shot at that setting. Shot 3. Open up half a stop and take a shot. Shot 4. Open up another half stop (one stop over the reading through the filter) and take a shot. Make notes of your exposures and corrections, and of the manufacturers filter factor. Process the film and make a contact sheet so that the unfiltered frames look correct. Look for the filtered frames that look as close as possible to those exposures and make a note of the amount of correction. Your results will probably suggest an extra half stop for readings through a yellow, or yellow green, and one extra stop when metering through an orange or red filter.
Personally, I prefer to meter the scene with a hand held meter, even if the meter in my camera is very good. I then apply a correction, depending on the filter factor. The meter I use, has the facility to set a correction before you take a reading, so that you can just forget about it. See picture.
LUNASIX WITH FILTER CORRECTION.
Looking at the scale on the front of this Lunasix meter you can see the red mark. This indicates that filter correction has been applied and reminds you to set it back to normal when you have finished. Here I have applied three stops correction. Follow the black rim leftwards from the red marker and you should see a small white mark aligned with the number 3. You must remember to reset the meter back to normal when you stop shooting with that filter.
I hope that information clears things up a bit. If you have enjoyed this post and the information here and elsewhere on my blog, would you consider a small donation via Ko-fi please? You can send as little as £3.00, or more if you are feeling generous. This money goes towards materials used for the tests and printing for these articles. The link is; Ko-fi.com/andrewsandersonphotography