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DPChallenge Forums >> Tips, Tricks, and Q&A >> what does "...plus bulb mean?"
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 09/16/2003 03:58:34 AM · #1
I was looking at the specs for the new digital rebel and came across this...

Shutter speeds: 30 to 1/4,000 seconds, plus Bulb

Does that mean that I have to use the flash to get the 1/4,000 shutter speed?
 09/16/2003 04:12:45 AM · #2
bump!

I don't know either.
 09/16/2003 04:17:36 AM · #3
It means that you have a bulb setting. That is, a shutter speed where you both start and stop the opening/ closing. It's used for very long exposures. Very useful for shots of the stars.

I've found with my minolta that the amount of noise on exposures over 2 seconds makes the shot worthless unless the high grain effect is required. You need to apply gausian blur to minimise the noise or use some other noise-reduction software.
 09/16/2003 04:26:01 AM · #4
Thank you. One more question...does the bulb feature superceed the listed 30 second exposure time allowing for +30 sec. shots?
 09/16/2003 05:52:22 AM · #5
Bulb means you can keep the shutter open for as long as you hold down the shutter button, although it is generally more advisable to use cable/timer release for long exposures. So yes, with the bulb setting, you can have an exposure longer than 30 seconds.

For long exposures, allow time between shots for the camera to cool down and always take a shot of your lens cap with the same exposure time. If you're using PS you can use your lens cap shot for noise subtraction in your actual shot. I've done 15 minute exposures with little or no ambient light and the results were not completely awful.

edit: removed images

Message edited by author 2003-09-18 14:32:21.
 09/16/2003 07:11:38 AM · #6
Something I'd like to try!! ... If I had a shot with and without the lens cap on, what exactly do I have to do in photoshop to subtract the noise?

Thanks.
Dave
 09/16/2003 07:54:43 AM · #7
for noise reduction, you may want to look into getting a program called neat image. It has a rather steep learning curve, but the results are worth it.
 09/16/2003 09:06:02 AM · #8
With the D60, I've been able to do 4 minute shots without any real noise issues. The main consideration with bulb mode though is that you really need a remote/cable shutter release, otherwise you'll be pressing the shutter button and causing the camera to shake during the exposure.

//www.dpchallenge.com/image.php?IMAGE_ID=20433 was a 4 minute exposure on bulb mode.

Message edited by author 2003-09-16 09:06:14.
 09/16/2003 09:17:24 AM · #9
Originally posted by sylandrix:
Something I'd like to try!! ... If I had a shot with and without the lens cap on, what exactly do I have to do in photoshop to subtract the noise?

Thanks.
Dave


Open both shots. The "lens cap" shot is referred to as the black frame. copy the black frame to a new layer over your image. On the layers menu, make the black frame active, then select "difference" for the blending mode. If the black frame and the image were taken under the same conditions (temperature of sensor is very important) and with the same settings (ISO, exposure time) then that's all you should need to do.
The effect of this will not be dramatic in most cases, since random noise is not reduced. Only "dark current" noise is reduced with this method. One very nice attribute of the method is that it eliminates "hot" pixels =)
Random noise can be reduced by another technique, stacking multiple exposures. You can do this in Photoshop, or you can get very inexpensive software to do it automatically. Here'san example of a shareware program to do stacking.
To do this manually in Photoshop, layer two or more images. Align the images carefully. Set the blending mode to "normal", then adjust opacity. The opacity of the first image (background layer) should be 100%. Next layer up, 50%. If a third image is used, set opacity to 33%. you get the picture.
For an extended discussion of this stacking or compositing technique, go
here.
 09/16/2003 05:52:37 PM · #10
thanks all - I really appreciate everyones input. I can't wait to try it out.
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