(This post is written under the assumption that the photographer is not shooting in full Manual Mode)
Cloudy days can be a lot brighter than you think.
Instead of looking up at a blue sky, with the light source of the sun off to the side and therefore not bothering you much, when clouds are around the light gets diffused, spreading itself a lot more evenly.
This can benefit your photography tremendously, as it effectively eliminates the contrast between shady and sunny areas. Photographing on an overcast day can be fantastic.
However, when pointing your camera up towards the clouds – for instance when photographing a bird or a leopard in a tree – what will most likely happen is your camera will read the situation as being too bright, and try to compensate by underexposing, giving you a result like this:
The elements are there; you can see the leopards, you can get a read on the situation, but a lot of the detail has been lost in the most important areas.
Granted, the result your camera gives will depend on what metering mode you are shooting on. The two most well-used are evaluative metering and spot metering.
In Evaluative metering (most common), the camera takes 90% of the frame, analyses the amount of light available and adjusts the exposure accordingly. This is why in the bright situation like the one described above, the resulting image is likely to be too dark. The sky is sending a lot of light down, and the camera compensates.
The camera was calibrated in a studio somewhere. It doesn’t know that you are trying to photograph a leopard in a tree with a lot of light behind it. As a result, you need to tell the camera that you want to keep the picture bright.
This you do by controlling the exposure dial:
Understanding that the camera will want to darken the image, you need to tell it to do the converse, by overexposing. This means shifting the exposure setting to the right on the dial above. Exposure is measured in stops, which are divided into thirds. Overexposing by one or two thirds might be enough, but you might even have to go five or six up to get the exposure you are after:
As you can see, the sky is pretty blown out, with no detail of the clouds, but photography is about compromise a lot of the time, and the leopards are the subject, so need to be the priority.
Instead of instructing your camera to overexpose, you can change metering modes to spot metering. As the name suggests, the camera reads the light from a much smaller area of the frame (only about 10%), and adjusts the exposure accordingly. This means that if you have a lot of bright sky but only a small area of leopard, you can meter off the leopard to get the correct exposure. This is less likely to work in a photo like the one above, in which the light and dark areas are a bit all over the place, but it was very effective in the photo below, which was from the same sighting:
The above photo was taken using spot metering, which exposed for the cub in the foreground. On evaluative metering, I got a result like this:
Again, the essentials are still there. You can see the leopards, and the photo is technically usable, but the detail gets a little lost in the underexposed parts.
The concepts here can get a bit muddled when you first encounter them: when it’s bright you have to tell your camera to make it brighter, and when it’s dark you have to tell your camera to make it darker. Seems a little contradictory.
There’s a simpler way to think about it; imagine you are telling your camera what the conditions are… It can’t really think for itself when things get complicated, so you have to give it some simple instructions. When it’s bright, overexpose, when it’s dark, underexpose. Does that make a bit more sense?
If anything’s not clear, please feel free to leave questions in the comments section below…