All pictures are taken on a tripod with the first watch in focus, the ISO is constant at 800 and the shutter speed is varied to keep the pictures similar.
At an F-value of 1,4 the second watch is out of focus and the third is a blurry haze. The shutter speed here is 1/320
At F2,8 the second watch is more in focus and we can even start to see the softbox in the background. The shutter speed is 1/100, we have 4 times less light than the first picture because of the F-value, the picture is a little darker than the first one. I shot all the pictures in aperture priority, because I was lazy and didn't want to do so much math to calculate the shutter speed to keep the pictures equal, I now regret that. I should have shot manual as I always do and done the math. To better show the point the first picture should have been 1/400 shutter speed and the next one should be 1/100 shutter speed, to prove that we get exactly 4 times more light in F1,4 than F2,8, but it shows how important it is to shoot in manual. The lighting condition is more or less exactly the same since I used a tripod and the pictures are taken a few seconds apart, so optimally the camera should have chosen 1/400 in the first picture and 1/100 in the second.
At F5,6 we start seeing details in the third watch and our pet. The shutter speed is 1/25, so we have 4 times less light than F2,8 and 16 times less light than F1,4.
At F8 we can distinguish everything in the photo. The shutter speed is 1/13 and we have lost 50% of the light from F5,6
At F16 we see the ball in the background clearer, but now the diffraction hits in a little. The shutterspeed is now at 1/3 and we have about 128 times less light than at F1,4
A little bit about calculating light:
Each F-number in this list give you half the light of the previous F-number: 1,4 2 2,8 4 5,6 8 11 16 because each F-number has half the light gathering area as the previous F-number.
Taken from Wikipedia