Experimenting with long exposures on the K20D. Both examples below are with the 16-50 at f/5.6-6.3 at ISO 200 (no D-range mode on). Exposure times are in the order of minutes - 15-30mins between them obviously with DFS and a medium high ISO NR setting (not that it makes a difference here)...

Results to me are OK but not to my expectations - major problem: colour noise, particularly in the blue night sky. Even PS find it difficult to remove this noise without taking away from image sharpness.

Would anyone care to comment on why such low-ISO noise surfaces itself here? Is it a known phenomenon with the K20D? Any other explanations for why this occurs?





Sorry if what I'm describing is not so evident in these resized samples, but they certainly are pertinent at 100% of the original.

[Ash]

Perhaps I posted the question in the wrong section...
But I can't seem to change the posting's sub-forum...

[ThorThum]

My only suggestion would be to lower the ISO to 100, turn NR off and shorten the exposure times to less than 10 minutes. If you want longer star trails, try stacking them in PS. This combo seems to work favorably for me, but I have a k200d. As to why it occurs, I read somewhere that the NR function seems to introduce noise on longer exposures, and something having to do with the sensor heating up more the longer the exposure time, introducing noise too.

[Ash]

High ISO NR makes more noise in long exposures? Does it even kick in at ISO 200 in long exposures?

The K20D is different from the K200D in that regard - indeed, my K10D never had such noise at the same settings as above (and even longer exposures), hence my query about the noise.

[flippedgazelle]

I always assumed that long-exposure noise was due to the heat generated by the sensor. Longer exposure -> more heat -> more noise.

[Ash]

Yes, but CCD sensors are known to generate more heat than CMOS sensors.
I'm finding the CMOS sensor producing more colour noise than the CCD sensor in long low-ISO exposures...

[Ash]

I tend not to go down to ISO 100 for star trail exposures as then I would need an aperture of bigger than f/4 for the stars to register on the sensor - hence my preference for ISO 200-250 at f/5.6

I'd like to hear from other K20D users who have done long exposures like this. Any issues with colour noise? Anything else that can be done to minimise the noise?

TIA.

[daacon]

I like the colors in #2 Ash and the star trails in #1 - we tried a couple times in our trip. The sky was simply amazaing yet we were not very successfull - it's too bad as I will remember that sky for quite some time. I think we were trying f4 - f8 and between 5 - 30 minutes (at one time we were up to almost an hour).

[Ash]

Thanks Dave.
Anyone else with a K20D wish to wager their experience with long exposures?

[Ash]

Here's an example with the exact same settings on the K10D.



For some reason, the ISO 200 long exposure result is more pleasing with considerably less noise than that of the K20D (please ignore the inadvertent flare on the right of the frame).

Would anyone concur with their own findings?

[mischivo]

Don't want noise in long exposures? Buy a film camera and some FujiFilm Provia 100F or Astia 100. Digital sensors are pitiful at long exposures. Film has no heat or electric noise to worry about.

[Ash]

Well, I've been quite impressed with my 2 hour exposures on the K10D at ISO 200. Very minimal, and if any, easily managed.

It's the K20D long exposure noise I'm intrigued about. Significantly worse, evident even on images resized for web.