When I go out in a park what factors should I consider to determine whether the present light is too harsh or too soft?
Look at your own shadow. If you can't find your shadow then the light is as soft as it possibly can be. If you have a hard edged shadow then the light is hard. If you can make out your shadow but it's faint or the edges are not defined then you have somewhere in between (which can often give the best results).
The only factor that matters is what you are looking for visually in your shot. Harsh shadows, soft shadows, no shadows, these all can be valid to have in a photo. It depends on what you want it to look like. There is no right or wrong answer. Look at how the light falls on your subject and decide if you like it.
Soft shadows tend to have a more serene feel where as harsh shadows tend to have a more edgy feel and no shadows tends to have more of a cold feel (like a hospital exam room). They can also end up giving other feelings based on the type of shot though, so that list isn't exhaustive.
Matt Grum's answer is the golden ticket answer. That rule of thumb method will work most of the time, but eyes work differently to photographs, and sometimes our eyes adapt to the light and we can't make out the shadows as definitely as they would be represented in a photograph.
Here are a couple more tangible approaches that I use.
For digital photography its easy. Take a photograph of a person. Look at their jawline and nose. If these have harsh black tones anywhere, the light is too hard to make good use of without reflectors or an alternative diffused flash setup. The bark of a tree is a good alternative too; if there are dark shadows present that impede you from seeing detail in the bark, its probably because the light is too hard.
Film photography is a little trickier since you can't trial and error as much. Use the aforementioned techniques as well as ensuring:
- Cloud cover is present, this is often greatly reduces hard light from the sun.
- Sun is low in the sky. A high sun often causes dark harsh shadows on the photo that the eyes have adjusted to in the bright daylight.
- In development, dodging can recover some detail and soften some harsh shadows.
- Use a spot meter on your shadow to make sure it isn't TOO dark (although this will vary based on what the shadow lies on, it can quantify what Matt Grums answer covered).
If you get a bit more technical, you might need to recall things quickly where rules of thumb aren't good enough. Zoning the underexposed areas and shadows can be part of compensating for hard light. Ansel Adams once had to rush a photo without taking a light meter reading, fortunately he knew the luminosity of the moon was approximately 250 foot candles and used a water-bath development to bring out the contrast of the foreground.
Harsh light is more directional while soft light is more diffuse. In most cases this means that with a harsh light source you will experience a more contrasty scene than with soft light. So you may want to be very careful about correctly capturing highlights and shadows when shooting in harsh light. For most hi-end digital cameras underexposing is a safe bet. If your camera has a high dynamic range or dynamic range compression setting, you may want to manually set it to high. The photos will have less punch, but will be much more treatable in post.
So, apart from your taste and the requirements you have about the results you may have to consider what is too harsh for your capturing medium. Some sensors or some films (e.g. Slide film) tend to have a more limited dynamic range and difficult to expose well in harsh lighting conditions.
Simply point out a thumb and check its shadow over the palm of your other hand. If the shadow is very sharp, the light is harsh. If diffused, the light is soft.
Further, you can safely assume the light from a small source will be always hard compared to light from a huge lighting source.
Another thing would be, the far away the light source(even) if its huge, the harsher the light would be and vice-versa.