I recently returned from an African safari. Sometimes the animals were fairly close, and at other times, those like Cheetahs were a hundred yards away. There was no time to keep switching lenses, and it was also dusty. On one Canon, I used the 70-300 L. On the other I attached the new Tamron 150-600. I kept switching cameras. My images (even 16 x 20 canvas prints) were splendid from both lenses. So as others have said, it depends on what you are shooting and how far away you are. The bottom line is that I love both of these lenses. For animals in the wild on a safari I would give up neither. Just be sure to read the reviews and buy quality or wait until you can afford it. I also like Sigma lenses. Some are superior to comparable Canon glass at a lesser price.

Cloudy days are great. There is very little problem with exposure because there is a minimized dynamic range. Sharpness can be a problem because lighting may be very flat. White balance should not be a problem different than any other time - partly because color may be a bit muted. That will also hold for saturation in general.

Those of us in Pittsburgh shoot gray days all the time. Except that you do not get blue skies, gray can be great. You may, therefore, want to minimize the sky by shooting a bit more down - an old landscape trick.

I do not mean to be rude, but you are a typical newcomer in believing that good pictures can be made only on sunny days.

Do not be afraid of darker days. Welcome them. I dislike photographing on brilliant, sunny days. I much prefer days with a thin veil of overcast.

Foggy days, in San Francisco or elsewhere, can be a challenge, but fog can give a sense of depth or distance to the view. Yes, the result will have less contrast, the white balance will be different, exposure will be different due to lower light levels, but these are not insurmountable problems, even for a newcomer.

From a practical standpoint, some of the problems caused by weather cannot be corrected in-camera, and must be addressed in post-processing. Contrast, brightness, saturation and white balance can all be adjusted in post-processing. As a newcomer, you may be a stranger to that phase of photography. NO, NO, you do not need to rush out and buy an expensive version of Photoshop. There are several free programs which will serve you well. Picasa, FlexPhotoDB, and FastStone Image Viewer, are three that come to mind.

Remember, the more effort you put into your photography, the more satisfaction you will gain. If you are unwilling to sweat a little, you will follow a path of continual dissatisfaction. Keep pushing.