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01-28-2008, 05:17 PM   #1
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why 30 sec exposure

Hi,

taking some shots at sunset last night and got down to about 10 sec exposure and I wondered (this is not a complaint just plain old curiosity), my max exposure time is 30 seconds or I can go to bulb and and sit with my finger on button (or remote lock etc) for a couple of hours if I felt like it. Obvisously the camera can take longer exposures than 30 secs, why is this limit there? I could see that faster than 1/4000 might by mechanical limits, but couldn't work this one out?

Phil

01-28-2008, 05:29 PM   #2
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Not really an expert on this, but as I understand it, sensor noise goes up as the exposure duration increases, at least in part due to heating effects. There are probably other low frequency electronic noise sources too (I seem to recall problems with "1/frequency" noise in electronic detectors from my school days) that come into play. I'm sure that you have noticed the "noise reduction" function that Pentax implements at longer exposure times, where a second "blank" frame is collected to subtract from the image frame, in an attempt to contain the long exposure noise somewhat. This effectively doubles the time it takes to record an image - a 30 second exposure actually takes 60 seconds for the camera to acquire...

Curt
01-28-2008, 06:07 PM   #3
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The time limit on the shutter is 30 seconds because they need to put a limit, and they figure the average user will never even go close to that. But if you go on B, you can expose for as long as you want (limited only by the juice in your batteries). There is also the limitations imposed by the light meter's ability to meter anything that would be dark enough to need more than 30 sec. exposure. Whatever you do, enjoy your night photography!
01-28-2008, 09:06 PM   #4
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It would be an excellent idea to get a remote or a cable release so your finger doesn't shake the camera during long exposures. Not to mention a very tired finger.

01-28-2008, 09:32 PM   #5
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After 30 seconds, it's just easier to use the B setting. Exposure times still double every stop, so with longer exposures there is more "wiggle room" -- being off a few seconds makes less difference with a 60-sec exposure than with a 10-sec exposure. There is no need for an exact 60-sec setting on the exposure dial.
01-28-2008, 09:34 PM   #6
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QuoteOriginally posted by Finn Quote
After 30 seconds, it's just easier to use the B setting. Exposure times still double every stop, so with longer exposures there is more "wiggle room" -- being off a few seconds makes less difference with a 60-sec exposure than with a 10-sec exposure. There is no need for an exact 60-sec setting on the exposure dial.

I don't know, I really wouldn't have minded a 1 min, or 2 min setting... But beyond those settings, I agree. Just get a cable and use bulb.
01-29-2008, 01:07 AM   #7
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are there any limits on length of bulb?
Because last time i took some bulb exposures (2-3 minutes, about 5 in a row) and my camera had heated up.
Any chances of damaging camera/sensor because of the long bulb exposures?
01-29-2008, 01:42 AM   #8
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Hi, I cant say I have yet had the need for longer than 30 secs (probably something I should try just for the learning exercise . Just pure idle curiosity, Thanks very much everyone for the response.

01-29-2008, 02:39 AM   #9
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QuoteOriginally posted by boroot Quote
are there any limits on length of bulb?
Until your batteries go dead, I guess. And wouldn't that be a bummer for the camera to die partway through a long exposure?

If you really want to do loooong exposures, consider purchasing an old film body which uses a mechanical shutter (operates without batteries).
01-29-2008, 04:05 AM   #10
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I haven't read any warnings from Pentax or elsewhere indicating any risk from long Bulb exposure times. From what I read, the practical limit is about 3-5 minutes. Beyond that the sensor heating increases the digital noise to an extent which defeats any gain you get from admitting more light.

I know one or two people who use their SLRs for amateur astronomy - there's actually a mount adaptor available which allows you to lock your Pentax K-series camera directly to the telescope's focus point. They use Really Long exposures - 1 hour or more - , but its always with film SLRs, not digital SLRs.
01-29-2008, 05:11 AM   #11
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And if you're really into astrophotography, you use a camera designed for it, with a cooled CCD to prevent noise build-up:

01-29-2008, 07:28 AM   #12
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QuoteOriginally posted by Duck Dodgers Quote
And if you're really into astrophotography, you use a camera designed for it, with a cooled CCD to prevent noise build-up:

And no IR filter? AA???? Peltier cooled????
01-29-2008, 09:11 AM   #13
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QuoteOriginally posted by jeffkrol Quote
And no IR filter? AA???? Peltier cooled????
Has removable IR filter. They call the cooling system "TEC" for thermo-electric cooling; I'm sure it's a Peltier chip. 36 degree (F) cooling differential.
01-29-2008, 10:15 AM   #14
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I've noticed that on a dark night I can get good exposure using the 16-45 @ f4 w/ISO100 and 30 sec exposure. If not I can still increase the ISO.. It seems to be a reasonable limit.
01-29-2008, 11:03 AM   #15
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aside from the escalating noise issue, there's another great reason to 'limit' the time (though not the short 30 seconds, for which the escalating noise is probably the best reason) and that is battery drain and loss of images before you save it.

A Film camera barely has any power drain. 1 second to one hour exposure is probably the same power drain on the battery.

D-SLRs, well, we know that's not the case. Imagine how utterly aggravating it would be to take a 1 hour exposure, and have the battery die at 56 minutes... and nothing saved to the memory card from those 56 minutes. Is an incremental write possible so you don't lose everything from that one long exposure? Possibly... That though would accelerate the battery drain even more however.

There was a decent article in a British Digital Photography magazine last month on night photography. This reason was given for taking many shorter shots; and taken one immediately after the other (so you don't have a gap in the star trail) and super imposing them in PS or a grafx program of your choice.
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