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Old Oct 24, 2010, 7:06 AM   #1
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Default Details about exposure control

So, how are aperture, shutter speed and ISO automatically selected?

The key is a turning point defined by a shutter speed.

For example, if focal length is 14mm, the turning point is 1/60 - 1/80. From this point, if it is getting darker, both shutting speed and aperture stay unchanged but ISO increases from 200. If it is so dark that iso1600 is hit, then shutter speed starts to decrease.

Now let's see the other side of the turning point, i.e., if it is getting brighter, then ISO stays at 200 and both F# and shutter speed increase. If it is so bright that F11 is hit, then aperture stays at F11 and only shutter speed increases. If 1/2000 is hit, then F# starts to increases from F11. Makes sense, nobody wants F# to be too large.

The turning point is not constant, it depends on focal length. For FL 42mm, it is 1/80 -1/100; for FL 100-150mm, it is 1/160 - 1/200. Makes sense, because longer FL needs faster shutter speed to eliminate shake.

What if an old lens that cannot tell the camera about its focal length? The turning point is 1/100 - 1/125. Fair enough.

So, EPL1's handling of these three parameters is very smart, avoiding unnecessarily higher F# and ISO, meanwhile good handheld shooting ensured at all focal lengths. The only not so smart part is: it does not take the IBIS into account, i.e., whether it is on or off, the turning point does not move.

Go to page 101 and look at the chart, then it is easy to understand above.

P.S.: in M mode, ISO is automatically adjusted for a correct exposure. You can manually set ISO too, if you like. But, if you select M mode for movie, ISO must be set manually, no automatic option. During a movie recording in P mode, both aperture and ISO are adjusted automatically all the time in addition to shutter speed (the manual gives you a false impression that these parameters do not change after recording started). For movie, shutter speed can never go below 1/30 (because of 30 fps).
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Last edited by sdcs; Oct 24, 2010 at 3:19 PM.
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Old Oct 25, 2010, 9:32 AM   #2
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Which camera brand/model are you talking about (specifically)? The reason I'm asking is because the diagram you showed does not apply to every camera. I cave cameras that will bring the shutter speed way below the values you specified before it makes any attempt to increase the ISO (the Sony R1 is a good example). The manufacturers design the f/w based on the camera's strengths and weaknesses. If a camera can't handle high ISO well, they will write the f/w so that the camera will first open the aperture as much as possible, then knock down the shutter speed to real low values (particulaarly if they know the IS system is very effective) and only then it will increase the ISO. On the other hand, cameras such as the Kx or EPL1 will bump up the ISO first before knocking down the shutter to low low levels because the camera can handle high ISO well.

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Old Oct 25, 2010, 1:04 PM   #3
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EPL1. I've noticed since day one that P mode always does a very good job, the chart explains how.
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