Today’s Question: In the past when shooting black and white film, adding a yellow/red filter darkened an over-exposed sky. Would adding a filter to a digital image and then converting to black and white have the same result?
Tim’s Quick Answer: While there would be some effect achieved with a colored filter with a digital capture, the effect will not be the same in most cases as what you would have achieved with black and white film. Therefore, my recommendation would be to save those effects for post-processing.
More Detail: To begin with, digital sensors respond differently than black and white films. For example, most digital camera sensors employ colored filters so that each individual photo site on the sensor (which will become pixels in the final image) have a colored filter so that each is only capturing a relatively narrow range of light values from the scene. With most sensors that means some pixels are only recording red light, some are only recording green light, and some are only recording blue light.
In other words, the camera isn’t exactly capturing a color image, but isn’t exactly capturing a black and white image either. A colored filter can produce results that are different from what you would achieve with film, and that in many respects would not be in line with your intent.
In addition, the camera will attempt to compensate for the presence of the colored filter with changes to the white balance setting (temperature and tint) for the capture. So in some ways you could say the camera is trying to reverse the behavior of the colored filter.
Because of these (and other) issues, I recommend saving your black and white conversion and related adjustments for post-processing. Set an exposure in the camera that preserves detail in the highlights, and then apply adjustments to the resulting image. You can, for example, apply an adjustment to darken only the blue values in the image, without affecting the other color values.
While a colored filter might provide a “shortcut” in theory, you’ll find that in actual practice it is better to use adjustments after the capture (and even presets as a good starting point) to achieve the effect you’re familiar with based on using colored filters with black and white film photography.