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Consider that the camera is in shutter priority and the exposure time is set to bulb mode.

Now the camera doesn't know how long the shutter is going to be open in advance and so how does the camera control the aperture and ISO, in this situation?

How the camera determines the other values if in bulb mode the shutter is open for 5 sec or 30 sec?

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Is there a specific camera you're interested in that allows this combination of settings? On the Pentax K-5, for example, Bulb is a separate mode and cannot be selected at the same time as shutter priority. Normally bulb exposures are done with the aperture and ISO set manually. –  coneslayer Jul 13 '12 at 15:35

3 Answers 3

up vote 3 down vote accepted

Short: In most cases "BULB" is a speed setting accessible only in MANUAL mode so you will have full control of aperture and ISO settings.

Longer:

"Bulb" mode is the ultimate manual mode.
Bulb is accessible in Manual mode and MAY be accessible in Shutter-speed priority mode. It COULD have a setting of its own but is usually at the low end of the shutter speed range after the slowest speed that is settable.

In Manual mode the exposure is not metered so the camera does not need to "know" what the exposure is. ISO will be what you set it to. If you have an auto-ISO feature the camera maker may decide to make it track in some way to try and adjust for likely exposure but, as the camera cannot read your mind, setting it to the best quality ISO setting within the auto range probably makes most sense.

It is unlikely that you can, BUT IF you can access Bulb setting in Shutter priority mode (on eg my Sony A77 you can't) as you move down through speeds: 1/8, 1/4. 1/2, 1, 2, 4, 8, ... the aperture will increase to correct the exposure. If you have an auto-ISO feature the ISO may also adjust. If the speed/aperture/ISO settings "top out" they will remain at maximum value. When you reach bulb mode "all bets are off", the camera again cannot read your mind and each manufacturer will have made their own decision.

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do you know any camera that has BULB mode under Shutter Priority mode? –  akram Jul 13 '12 at 15:53
    
@AkramMellice I'm guessing some cameras that allow for a remote shutter release would let you do it, but I would think it would just override it into manual mode. –  dpollitt Jul 13 '12 at 16:38
    
@Pointy, the Bulb setting being discussed here isn't for flashbulbs, but for a shutter setting that holds the shutter open for as long as the release button is held down. (The name comes from an old-fashioned release mechanism consisting of a rubber bulb on a length of tube; you would squeeze the bulb and hold the shutter open by the air pressure in the tube.) –  coneslayer Jul 13 '12 at 18:37
    
Ah. Durr. Thanks for the edification. –  Pointy Jul 13 '12 at 18:46
    
@dpollitt mmmm well this would be a bad design from the camera manufacture (and ofcourse this is my opinion :-)) cause it makes perfect sense to either make it a separate mode or in manual mode (cause it's manual) –  akram Jul 14 '12 at 1:53

It does not.

Once in BULB, no automatic exposure parameter applies. Bulb is found in Manual mode on the vast majority of cameras. On most others it is a selectable shutter-speed in Manual mode.

When you can select it in another mode, then the camera uses defaults. Actually, on the two cameras I know that accept Bulb in shutter-priority, the exposure time is 30s.

Automatic ISO always uses the camera's base sensitivity which is often 100 or 200 but not necessarily the lowest. This is usually specified in the user manual.

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Most of the cameras that I know have separate Bulb mode and shutter priority, the Bulb can be in the manual mode and you can get it by increasing the shutter speed till you get bulb (Like in some Canon models). But it's not reasonable to make it attached with the shutter priority mode because the camera wouldn't know in advance how long you are going to open the shutter! The camera won't meter properly then and adjust the aperture and ISO?

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