I am habitually underexposing my photographs. I will chalk it up to slowness of my brain as well as a few other things. The reverse lens more or less renders the camera as a Manual mode-only device which requires making measurements and judgments with the eye. Simple experience really helps with that, but it can’t really completely account for fast-changing conditions and fast-moving subjects. I tend to err on the side of underexposure because it is easy to blow out highlights with my flash on shiny bugs, and unsightly glare spots are some of my least favorite things.
In this photo, the underexposure happened for a different reason. The spider was camped out on a titular sign at a trailhead. The flash was cut off by the sign due to the angle I took to shoot the photo resulting in significant underexposure (see the original below). The challenge here was bringing out the spider without having the photo overwhelmed by noise. I do not mind some noise or grain in a shot, probably less so than other photographers much of the time, but the severity of the underexposure here really made for a difficult balance between bringing the spider out of the shadows and having an ultra-noisy background. Ultimately, I could not bring out the outstretched legs, one of the cooler aspects of this spider, especially from this point of view, without introducing a prohibitive amount of noise. So I have another less than ideal result. Compromise, thy name is photography.