Digital B&W … … with filters?

Should I use colour filters when shooting B&W on a digital camera? The resounding answer on the internet is, no, shoot RAW and you have more flexibility in casting to B&W by mixing the colour channels or filter emulation. But is there an argument in favour of using filters?

B&W on film

Shooting B&W on film we have to learn to pre-visualise the effect of colour filters on the rendition of the image. It is often said that when shooting B&W on a digital camera we can set the mode to B&W and immediately see the image in B&W. But this does not tell you how the final image will appear after colour filtering the RAW file. Indeed the camera screen image may be positively misleading and look too bland, putting you off even taking the photograph.

Filter test

I decided to photograph a colour chart using no filter and with an orange filter. Normally I shoot with a deep yellow filter but I thought the orange would make differences in colour rendition clearer – it turned out the deep yellow and orange were virtually identical in the rendition of the blues which is the area that primarily interests me. I had tried photographing outdoors when the light was constantly changing but the results reminded me that the variation in colour temperature and contrast can have as great an effect on the image as a change in the filter – after all, what we are photographing is the light, so this isn’t surprising. So I chose a moment when the light was good (white) and steady. The unfiltered shot was metered and then I used a 1.5 stop longer exposure for the (light) orange filter I was using.

What issues are there using colour filters on B&W?

The camera’s meter is dependent on the spectrum of light in a way I don’t fully understand but the presence of a filter causes the camera to underexpose, dramatically so when using a red filter. I found the most effective method to get the correct exposure with a filter attached was to meter without a filter and then use the filter factor to adjust exposure manually, just as I would when using film.

A second issue is the potential loss of data points if one or more of the three colours is missing. This is possible if using a deep red or a deep blue filter but in practice – and especially true for yellow to orange filters – all colours are transmitted to some extent so loss of image quality is unlikely to be an issue.

Filter effect on a colour chart

Using an orange filter the impact is most marked on the blues, Row1Column3, R2C2, R3C1 and R3C6 (and slightly R2C4)

Actual image comparison

The images below show a rough comparison between as it appears on the camera monitor without a filter being used and how it appears with an orange filter in place. The filter effect is subtle but but most easily seen in the darkening of the sky and the differentiation of the clouds near the horizon.

Conclusions

If you find the presence of a filter useful for composition when shooting B&W on digital then go ahead and use it. Remember to take account of the possible misreading by the camera’s meter when the filter is in place (particularly if using auto white balance).

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