Help with long exposure calilation.

[S.E.V.]

I think this would be place to ask this. Ok I read on line one way to calulate you exposure time would be to set the camera to its highest iso 1600, 3200, 6400 etc, then make sure you are in aperture priority. Select the fstop you want to shoot at and see what the camera suggests the shot to be taken at 10sec 15, 25, 30. Keep that number in mind. no count the stop difference between 4200 and 100 iso 6 stops now deduct 6stops from 30sec if that was the suggested shutter speed at 3200iso f8. Now once you set you camera to bulb mode f8, how do you calculate 6 stop from 30sec or whatever the shutter speed was suggested by the camera? I don't get how to find the stop difference on the shutter end. That is what I need help understanding how to achieve.

Here is the article I read

1) Boost your camera ISO to the highest ISO possible (typically 1600 or 3200 on most DSLRs)
2) Switch your camera to Av (Aperture Priority mode)
3) Set your lens to the desired aperture
4) Check the proposed exposure time of your camera, if you are still getting a blinking 30 then you should skip to plan B
5) Now is calculation time, let say you have your camera set to f/16 and ISO 3200 and the camera proposed a 2sec exposure time.
Set back your camera to ISO 100, from ISO 3200 to ISO 100 that’s 5 stops100 200 400 800 1600 3200 than you should reduce your shutter speed by 5 stops also that’s 2sec, 4sec, 8sec, 15sec, 30sec, 1min.
Plan B
If in step 3 you set your lens to the desired aperture and in step 4 the exposure time is still more than 30Sec, then start by gradually open your lens until the 30sec is no longer blinking.
e.g. Camera set to ISO 3200, f/16 the 30sec is still blinking we start by opening the lens aperture. The blinking stops at f/11 so that’s 1 additional stop we should compensate for.
In summary we should compensate for 5 stops in ISO difference from 3200 to 100 and 1 extra stops from f/16 to f/11. that’s a total of 6 stops. So if we got 30sec at ISO 3200 f/8. This would be 32 min at ISO 100 and f/16.

From this link

http://www.adidap.com/2006/10/25/how-to-calculate-very-long-exposures/

Any help would be great.

Thanks

Sevan

[Adrena1in]

To be honest it depends what you're trying to shoot...? For astrophotography I never try to get the camera to suggest any settings, because in darkness it can't really manage. Even with a 1000mm+ attack on the moon, the camera will still suggest far too long an exposure.

[Sorarse]

In all honesty, with a 32 minute exposure time, unless you are blessed with an extremely dark sky, the chances are that you will get a blown sky due to light pollution, as well as suffering excessive noise due to heat build up on the sensor.

[Celestron]

How about reading a few topics from Jerry Lodriguss (http://www.astropix.com/HTML/I_ASTROP/TOC_AP.HTM) website ? I think you will get more better info and the level you need to start at . He's a Pro so don't under estimate him .