I am just a beginner and in the process of finding out how little I know about photography. Best way is to ask experts and learn from them. Hence the question.
I noticed that when trying to take picture facing towards setting sun - almost sunset time, camera ISO jumped to 640. Couple of minutes later, when facing other side, my back towards sun and capturing some trees snap at before sunset time, ISO was 100.
When facing sun, light is much brighter, why would camera metering system bump up the ISO?
If it matters, camera is Canon SX60hs on aperture priority mode.
If you’re using matrix metering, I expect the Canon implementation includes some proprietary algorithms and tries to be smart. Perhaps it’s ignoring the sun when it works out the exposure?
When you are facing the setting sun, foreground objects will be back-lit, ie not illuminated by the sun. Facing the other way, foreground objects will be illuminated by the sun. The overall scene when facing the sun may be less bright than the scene in the opposite direction. So, if the meter is exposing for the foreground objects, the exposure will be longer (or ISO will be higher).
I’m not a Canon user, but Auto ISO is probably on. Turn that off, and it’ll stop doing that, and should then calculate an exposure within the capabilities of just aperture and shutter speed.
What the metering system does, with any of the three components it controls, is dependent on the metering mode you select and where you point the camera. Make sure you understand all the specifics of each of your camera’s metering modes so you can aim your camera accordingly for determining exposure.
It seems that the reason is what @snibgo has told.
If you are facing towards the sun, you have to think: what do you want to be well exposed? the subject or the background? If you put in automatic, the camera “has to decide” what to do.
If it increases the ISO, the camera is trying to expose the subject, that is dark compared with the background.
In the opposite way, if the sun is in the back. Most of the times, the camera has no problem to measure.
Even simpler systems do things like take the average. If the bright sun is only in the center, it will actually cover a small amount of the picture. So the other parts need brightening up.
Or, like others said, the system tries to be smart and ignores small extreme bright spots in its calculations, because often they are not your subject.
It is a situation we’re you have to stop and think for a small moment why it is doing that. So you are not wrong, in thinking that it is bright which needs lower ISO. in this case it is so bright the camera ignores it :).