Contributed by CherylB
Converting a colour image to black & white (or monotone - mono for short - meaning a single colour tone) isn't just about de-saturating the image. Yes, you can do that - but is that really the best way to convert images? In short, the answer is no.
There are many ways to convert a colour image to mono and there isn't a "one size fits all" way. You have to consider the image itself, and what you are trying to achieve.
Firstly, consider your camera itself. Even the most basic point-and-shoot will probably have a setting that allows you to "take" black and white
photos. Should you use it? Again, no! All the camera is doing is applying a filter to the image in-camera and then you don't get the chance to "redeem" the best qualities of the image, because the filter that's applied is what the manufacturer has decided and it's usually a generic filter that doesn't do much more than de-saturate.
We're not going to go into the mechanics of "how" to convert images here - this topic is simply to look at the effects of different filters within image processing software (in this case, Photoshop) so that you can think about how best to convert your own image to give it maximum impact.
Okay, let's start with our colour image. This image has had levels, curves and a slight saturation "boost" applied to bring out the "best" within the colour image itself. In my case, I know that my camera also has a slight tendency to create blue images, so I also normally apply a slight reduction on the blue channel to compensate. You might think it a bit odd that I'm "boosting" the saturation before converting it to black & white ... but I want the image to look its best before doing that, and I think the image looks better with the boost!
Okay, I've chosen this image because it has some nice blue sky and clouds - different filters can have a significant effect on this type of sky as you'll see shortly.
So, the first conversion is the basic "default" conversion within PS.
In this particular image, the default conversion isn't the worst I've seen, but it does look a bit "flat" to me. I think we can do a bit better.
The two PS options here are "Lighter" (on the left) and "Darker" (right). These are just taking the "default" and making it a bit lighter or darker. The darker one is starting to look a bit better, but let's see what else we have.
Above, we have "maximum white" (left) and "maximum black" (right).
From left to right above, "red", "green" and "blue" filters. Now, what's happening here? Each filter is letting through more of their "own" colour and blocking the other colours. So, in the case of the red filter, the blue spectrum is being blocked and more of the red spectrum is coming through, making the sky appear dark. The red spectrum is being blocked in the blue filter and only the blue spectrum is coming through. There's not much difference between the default image and the one with the green filter for the simple reason there isn't much (if any!) dominant green in the image. Red filters are typically used to make skies more dramatic for the reason that they block the blue spectrum. They don't work so well on a cloudy day!
The final two images above accentuate the red and blue filter differences even more. These are the "high contrast" red (left) and blue (right) filters.
There are so many more options available for converting to black & white - this has just been a very basic look at available filters.
So, which of the above options is the best? Personally, I'd go for a different way of treating this image to bring out both the sky and the texture of the sandstone in the building. That said, of the options shown here, I think the "maximum black" is closest to the result I would be looking for. But that's my preference - yours might be different!