For many photographers, particularly hobbyists, making an image black and white is almost arbitrary. I remember in the early days of my photography, I was the same. I would mutter: "I wonder if this would look good in black and white," and then, I'd try it. Sometimes, it would look better, but usually, it would not. I presumed it was all just down to taste, but that's not true. After years of reading around the subject and experimenting, I began to understand why it worked when it did and conversely, why it often didn't. Here are some key elements that ought to be present in black and white images, and why.
This is first on my list for good reason: it's the primary consideration. Without color, an image has markedly fewer ways of differentiating the elements that make up its composition. If you turn an image black and white when it is using color to make the subject or scene pop, the subject will fade into the rest of the image when that color is removed. For portraits, you need to ask yourself whether the subject is clearly distinct from the background without the use of colour, and this is invariably achieved by contrast.
The difference in exposure between the subject and the background doesn't have to be enormous to work; subtle but clear differences like the example above can work. The background is a darker midtone as opposed to a shadow which can be seen in Alex's beard and hair. However, his skin tone is light enough that it becomes framed by the hair, beard, and background to separate the image into layers in the same way the wide aperture does. This can also be achieved the other way around, where the background is blown out (in the below case, it's a large window) and the subject is correctly exposed, still creating enough contrast that it doesn't need color (of which there wasn't a great deal) to make the subject distinct from his surroundings.
This contrast rule can be taken to extremes with fantastic effect. For example, silhouetting people in street photography and playing with the light is commonplace and part of what makes the genre so popular.
The next few reasons that dictate whether a black and white image is preferable to it being in color are far more subjective. First up is mood. Anybody who casts even a cursory look at photography will know the power that monochrome has when it comes to mood. There're no doubts that color is highly proficient at doing this too, but for when it falls short or is simply not appropriate, black and white conversions can change the tone, so to speak. I find that the strength of this is creating a darker, more dramatic, or more somber mood.
Although black and white images are devoid of any inferences or suggestions that color can bring, they are also free of any distractions that it can bring too. There is a common theme within photographers who regularly harness the power of black and white, and it is "simplicity." For whatever reason, simplicity and more somber moods go hand-in-hand, and so, if your image is intended to be dark in mood, think about shooting with the express intention of converting it to black and white.
In a similar vein to aforementioned simplicity, details can be brought out by removing the distractions of color. An image can become very busy if a color palette isn't constrained, which is only possible with a lot of forethought and styling or luck. So, if there is a detail you want to capture in particular, black and white conversions can capture it very well. In the below image, I was torn: on the one hand, the subject had piercing blue eyes that would look great in color. On the other hand, the allure of the subject was his skin and age. The color iteration of this image I enjoy, but for me, the focus became strongly about the eyes' color. With the black and white, the light-coloured eyes are still obvious, but the texture is more prominent without being overpowered by other elements.
The fascination with black and white imagery isn't overly new, but now, it's a choice. There is something enticing about black and white photography that is tied up with the vintage style of its history before color was possible. As a result, it often just makes an image look "cool" and retro. I have wondered for years whether I would grow out of that particular eye for imagery, but I haven't and am no longer sure I will. It's always worth noting the other above "rules" when shooting an image in which you might remove the color for the sake of a vintage feel, as they still apply.
Bonus: The Rule Breaker
Every single "rule" of composition has been broken with great effect, bar none. It's always worth taking into consideration that while certain elements can be paramount to whether a black and white image works, they're not a prescription for guaranteed success. In fact, sometimes an image just works in spite of it not adhering to the rules. I find it is usually the case that color is providing some type of distraction that I don't want. For example, the image below I shot with the intention of keeping it color but then changed my mind in post, which is rare for me. The image is incredibly busy and the bright colors made it too overpowering for my tastes. Not to mention, the sunglasses and their reflections were meant to be the focus, but in color, they lost a lot of their pull.
What do you think makes or breaks a black and white image?