Bulb/long exposure information needed

Robbo

Robbo

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I'm just getting into doing really long exposures, I was wondering in dark/night time, roughly how long can you expose before the picture will become too bright?

Eg if I wanted a 20 minute exposure at night, can I do it or will I need nd filters to?

Can't really explain what I mean, does this make sense?
 
Depends on just how dark it really is. Anything like a street light or even a light polluted sky will blow out over that time period. However, bear in mind that a 20 minute exposure is only double a 10 minute exposure, which is only double a 5 minute exposure. If you get to the point of going into several minutes it's not much of an exact science.
 
depends on the type of photography you want to do i guess.

As Squawk said, if you have street lights or similar in your shot, they will blow out very quickly. Even if there are no lights in your shot but they are close in the area, you'll begin to see glowing in your shots.


If you are planning on shooting in completely dark places and want to do star trails etc, you can go very long time and still have underexposed foreground.

One thing to note is the noise from long exposure. I tried a 40-50 min startrails once, and the noise was very noticable even in the camera lcd.
 
I think a lot of it is trial and error. I was in Norway recently doing 2 minute exposures at ISO 1600 f3.5 and all that was around was starlight and the Northern Lights, but not massively bright. That could equate to 4 minutes at ISO 800, 8 minutes at ISO 400 etc, so 20 minute exposures in a very dark place would not be an issue. You should get star trailing as well at those sorts of exposures.
 
The moon makes a massive difference, it pretty much full moon at the moment so it gives lots of light, which is good for some night shots to add some blue into the sky etc, but not in your case.

you can meter your shot by taking a shorter exposure with a wider aperture/ higher iso, then calculate how many stops you are adjusting on the aperture and iso and apply it to the shutter length.

noise will likely be an issue, you have a few choices, live with it, sort it out in pp, turn the dark frame subtraction on (a pain at this length because you will have to wait twice as long to see if the picture worked), or shoot raw, then when you have the shot you want take an equal length shot of your lens cap (stop down the aperture to make sure no light gets in, in this instance it wont effect the time you shoot for) and subtract that image (of the dark noise) from the original and there you have it.
 
I don't think you can do multiple exposures on the 450D, can you, if that is what you are meaning?
 
The moon makes a difference, also so does the sun going down.
I took some 40 minute exposures, it was a case of taking a couple of minute exposures, then multiplying up the values and adding a fudge for the change in light.
 
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