[jmbower]

I'm not sure exactly what I'm doing wrong, and haven't had a lot of time to play around. I was hoping someone might be able to diagnose the issue before I drove myself nuts trying to figure it out

Here's the deal: I have a K100d (old version) and have been happily clicking away on it without any major issues. However, I haven't tried to do a lot of longer exposures...just mostly here and there for flowing water, etc.

I was out at an observatory, and decided I'd give star trails a shot....just a brief one, not the hours of exposure to get full circles, etc. After a 30 second exposure on a tripod (with SR turned off) I got what looked like a dim picture covered in red static. I didn't think much of it at the time....but the picture at the top is from last night...I tried to do a long exposure to capture the trails of bats coming ot from under a bridge. Same deal, lots of red/purple static.

Another example:


what am I doing wrong? Any hints/tips/general rules for long exposures?

Thanks!

p.s. (sorry no exif data, I'm not at home, so had to convert the raw files with picasa)

[jmbower]

As fasr as I recall, the settings were somewhat like ISO Auto (200/400), w/ 18-55 kit lens at 30-ish, f4, 30 second exposure?

[Wheatfield]

I'm not sure exactly what I'm doing wrong, and haven't had a lot of time to play around. I was hoping someone might be able to diagnose the issue before I drove myself nuts trying to figure it out

Here's the deal: I have a K100d (old version) and have been happily clicking away on it without any major issues. However, I haven't tried to do a lot of longer exposures...just mostly here and there for flowing water, etc.

I was out at an observatory, and decided I'd give star trails a shot....just a brief one, not the hours of exposure to get full circles, etc. After a 30 second exposure on a tripod (with SR turned off) I got what looked like a dim picture covered in red static. I didn't think much of it at the time....but the picture at the top is from last night...I tried to do a long exposure to capture the trails of bats coming ot from under a bridge. Same deal, lots of red/purple static.

Another example:


what am I doing wrong? Any hints/tips/general rules for long exposures?

Thanks!

p.s. (sorry no exif data, I'm not at home, so had to convert the raw files with picasa)

You are doing nothing wrong. That is sensor noise caused by heat build up during the long exposure.
A friend of mine calls it a fatal problem, as he does long exposures a lot.

[hinckc]

Hi jmbower,

I'd like to respectfully disagree with Wheatfield... DSLR's might not be as effective as film for 3-4 hour exposures, but that's not what we're discussing here...

I've done tons of 30s exposures with the K100, K10, and K20D. Some with no "noise" to speak of.. I've also heard of people on the forum here doing 30 minute!!! exposures on Pentax DSLRs with nearly noise-free images...

To my eyes, the lines you are seeing are from an under-exposed image being automatically processed for contrast. The post-process (yes, even if it's in-camera JPEG) can expose this kind of noise.

Yes, long exposures can increase noise. But even short exposures (like 30s or 1/30s) will exhibit noise if their brightness is increased by post processing software.

I'm just guessing.... Are you using Picasa? I think Picasa is brilliant software, but it's automatic exposure adjustments will do this to a nearly black frame, or anything that's very under-exposed.

Are you shooting RAW or JPEG? If RAW, what did you do to process this shot?

-Chris

[Wheatfield]

Hi jmbower,

I'd like to respectfully disagree with Wheatfield... DSLR's might not be as effective as film for 3-4 hour exposures, but that's not what we're discussing here...

I've done tons of 30s exposures with the K100, K10, and K20D. Some with no "noise" to speak of.. I've also heard of people on the forum here doing 30 minute!!! exposures on Pentax DSLRs with nearly noise-free images...

To my eyes, the lines you are seeing are from an under-exposed image being automatically processed for contrast. The post-process (yes, even if it's in-camera JPEG) can expose this kind of noise.

Yes, long exposures can increase noise. But even short exposures (like 30s or 1/30s) will exhibit noise if their brightness is increased by post processing software.

I'm just guessing.... Are you using Picasa? I think Picasa is brilliant software, but it's automatic exposure adjustments will do this to a nearly black frame, or anything that's very under-exposed.

Are you shooting RAW or JPEG? If RAW, what did you do to process this shot?

-Chris

Quite true. On second look, the more exposed image is at least a stop underexposed, and likely more like 3 stops under..

[vievetrick]

Chris has the answer I think as well. That is what my shots look like when I try and boost the exposure with software. Try that shot again in RAW, ISO100 and tripod and I am sure you will have much better result.

[jmbower]

Thanks for the info, I'll try some testing...

but for the record:

1) This is what the shot looks like on the preview screen, shot in RAW. As soon as I took it, I could see that it was staticky.

2) These are RAW shots imported into Picasa, and saved directly to Jpeg, without any additional editing past whatever Picasa does automatically.

3) The k100d can't shoot in 100, can it? All that comes up in my menu is 200.


I'll try going back to the RAW shots and editing them view GIMP, etc tonight and see if I get a a different result. But for whatever reason, this is how it looked at the time it was shot in RAW, according to the camera's screen.

[procyon]

Underexposure.
It really makes noise very visible.
High ISO and/or long exposures need very good exposure to look good.
In these cases always check the clipping indicator.
I always try to get a few small dots of overexposure warning in the brightest details.
That way the noise really looks much less prominent.

[vizjerei]

2) These are RAW shots imported into Picasa, and saved directly to Jpeg, without any additional editing past whatever Picasa does automatically.

That's the problem, Picasa will auto level those dark photos, which will bump up and show all those noise in the picture. I have seen it does it to mine. that's why I have never use it again and moved on to Silkpix and Lightroom.

[shadeless]

i get the same results, when i turn off dark frame subtraction/noise reduction in the custom settings (sorry, i don't remember how it's called exactly).

maybe you turned it off and forgot to turn it on again ?

[emalvick]

Thanks for the info, I'll try some testing...

but for the record:

1) This is what the shot looks like on the preview screen, shot in RAW. As soon as I took it, I could see that it was staticky.

2) These are RAW shots imported into Picasa, and saved directly to Jpeg, without any additional editing past whatever Picasa does automatically.

3) The k100d can't shoot in 100, can it? All that comes up in my menu is 200.


I'll try going back to the RAW shots and editing them view GIMP, etc tonight and see if I get a a different result. But for whatever reason, this is how it looked at the time it was shot in RAW, according to the camera's screen.

1) Seeing on the screen that way still indicates underexposure. Again, look at the histogram to see where the peak is. I'm guessing it is all the way left. The LCD is just showing you an automatically processed JPG in the preview.

2) see other's suggestions. I am guessing that for that particular raw file, you might not be able to improve things much as it's the raw info that is under-exposed, and if you've clipped things on the low end, you won't be able to recover a whole lot.

3) I think the main point is to take a shot at as low as an ISO as you can. I also think that ISO 200 on the K100 is equivalent to the ISO 100 on other cameras (i.e. it's the baseline for the sensor).

[vievetrick]

By the way, you said this was a 30 second exposure correct? Am I to assume that guy in the picture stayed perfectly still that entire 30seconds. I think you may want to recheck the exposure length and not trying to pick a fight or axcuse you of anything, but I think we are back to the autoexposure issue with the software. Still puzzled though and hope you get a resolve are you sure this was at a low ISO setting?

[jmbower]

sorry, should have been more clear...the second shot was a shorter exposure (though yes, he was standing there for a while). It was just another example.

so, assuming it's an issue with underexposure, how does one properly determine exposure for long exposures that are past the set exposure times (i.e. to do long star trails, etc)?

[LeoTaylor]

The Pentax DSLRs can do long exposures once you get the hang of it. The noise level is higher than some other brands but with Noise Reduction my Pentax works well.

I do a lot of 1-4 minute exposure astrophotography with my modified K110D. Here is my latest taken this week from my light polluted location. As the subtitle describes this includes 27 one minute exposures and 33 two minute exposures. Since this location is in the thick of the Milky Way there are few dark spots but you can see they are fairly black.

AstroLeo NGC Nebulas

Look for North America in the upper left corner.

[vizjerei]

As I said, it is Picasa who did it, not your camera... try open it with Adobe Camera Raw or other Raw program, you will see a much better result.