Aperture priority

[KeeFy]

If you set your ISO to auto, the camera will set the optimal ISO and Shutter speed. I don't know how Nikon's algo works, but i reckon it should be similar if not the same.

The camera will try to set the lowest ISO possible and the shutter speed at hand-holdable values untill it reaches the ceiling for ISO (which in the case of the 7D, 60D, 600D is 3200ISO).
Hand holding speeds is known as 1/focal length. IE @ 50mm the minimum hand holding speed is 1/50s

Example:

With a aperture set and fixed at f2.8 and the scene exactly the same (well as close as possible due to variation of FOV due to zooming).

17mm(crop)/28mm(FullFrame) = 1/30, ISO400
35mm(crop)/56mm(FF) = 1/50, ISO640
55mm(crop)/80mm(FF) = 1/80, ISO1000

If you were to fix your ISO and aperture to say ISO100 with aperture f2.8, your camera, the resultant shutter speed would not be able to match the handholding speeds.
For the same scene above:

17mm(crop)/28mm(FullFrame) = 1/8 , ISO100
35mm(crop)/56mm(FF) = 1/8, ISO100
55mm(crop)/80mm(FF) = 1/8, ISO100

If you were to increase the Aperture from the same scene by 3 stops (more on that later) which is f8.

Original @ 2.8:
17mm(crop)/28mm(FullFrame) = 1/30, ISO400
35mm(crop)/56mm(FF) = 1/50, ISO640
55mm(crop)/80mm(FF) = 1/80, ISO1000

@ f8
17mm(crop)/28mm(FullFrame) =1/30s, ISO2500
35mm(crop)/56mm(FF) = 1/40s, ISO3200
55mm(crop)/80mm(FF) = 1/40s, ISO3200

Due to the camera hitting the set limit for usable ISO being 3200, the camera selects the fastest shutter speed possible.




Stops:
It is a set interval.
Generally they are known as F-stop (aperture), and T-stop (Shutter speed). ISO can be occasionally call an ISO stop but there is no real name for it other than ISO.

T-stop:
A full T-stop is each number shown below:

8 seconds 4 seconds 2 seconds 1 second 1/2 second 1/4 1/8 1/15 1/30 1/60 1/125 1/250 1/500 1/1000

Each of them roughly is double the neighbour. Why i say roughly because you'll notice 1/8 to 1/15 is not exactly double.

F-Stops:
A full F-stop is shown below:

1.0 1.4 2.0 2.8 4 5.6 8 11 16 22

I know this seems a little bit confusing but all you need to remember is 1.0 , 1.4 and 11. Each doubling of the number is 2 stops. So 1, 2, 4, 8, 16,32 = exactly 2x stops. 1.4, 1.8, 5.6 = 2x. Then remember 11, 22 also double.


ISO

This one is easy, it just doubles.

100 200 400 800 1600 3200 6400 12800 25600

Each interval can also be know as a "stop of ISO" just to make life easier.



Co-relation:
Shutter speed - Aperture - ISO is known as the holy trinity to cameras and the modification of any single one aspect of it will affect another or both as shown above.

To get the EXACT same exposure.
1/30, f2.8, ISO100

If i were to change the Shutter speed to 1/60.
I can either get : 1/60, f2, ISO100 or 1/30, f2.8, ISO200.

If i were to change the Aperture to f4.
I can get either: 1/15, f4, ISO100 or 1/30, f4, ISO200

If i were to change ISO to 200
I can get either: 1/60, f2.8 ISO200 or 1/30, f4, ISO200

Hope this helps and feel free to correct me if i got anything wrong.