Scales are useful. Many times a picture alone may not give adequate information about the scale, particularly absent recognisable objects or vegetation. For instance, the zircon crystals above have no scale: how long do you think they are? (answer below!)
Similarly, this outcrop photo does not have a scale either. How large are the beds in the fold?
In the geosciences, a sense of scale is particularly important. Without it, these images lose context which may be important to their interpretation.
Let’s see how you did.
The zircons are around 100-150 microns long. For context, a standard piece of copier paper is around 100 microns thick.
How about the outcrop? Here’s another picture, with a pen in the lower left for scale.
Those are two examples of ways to put scale bars in. The first is by calibration of the relation between pixels and size for a microscope. The second is by the addition of a common object.
Another way to be particularly quantitative about the scale bar is to include a scale in the photo itself, as below. Off to the left you can see the edge of a small whiteboard, which is used to write the sample name and latitude/longitude coordinates for future reference. It’s the old-fashioned way to embed metadata, and is great for when you get back from your field work and are wondering what the heck your picture is actually of.
Such emphasis on scale may seem pedantic for many field or lab photos. However, in environments where there is little available to give a sense of scale, such as the polar regions, or deserts, scale is an important thing. This is a key consideration to keep in mind when travelling to places like Heard Island, where the scale will not necessarily be apparent without additional effort to include it within the pictures.
 Gould, L. M. Cold: The Record of an Antarctic Sledge Journey. New York, Brewer, Warren & Putnam, 1931.