K20D post shoot processing times!!!

Ahhhhhh. Thanks for that explanation. Makes a lot of sense. Well apart from the fact noise is random so how would you use a second file of just noise to cancel the noise from the original shot? I'm an experienced audio editor and I know that if you subtract noise from noise you get noise. What am I missing?


Gonna have to buy a K10D for night shots now then I guess - hmmmmpf

All else being equal (same camera, same sensor, same duration) then the noise generated by your sensor will form in the same way during both exposures - it may appear random (and you certainly couldn't use a dark frame from another camera - even if it was nominally identical) but actually it's quite predictable.

You get the best results when the darkframe is taken right after but if you need to take many exposures you can make one darkframe and then later on the PC do the darkframe extraction.
Although they don't give you that option with the newer camera's.

Some here already contacted them about it and well they have done something. They seem to have improved the auto settings, just don't know with how much though.

I think i'm beginning to understand. So I would guess it's not a bit cancellation thing but more of a kind of AI algorithm that looks at the 'dark frame' and uses that as a basis to choose what to remove from the original frame. It obviously works and that's cool. But it sure is a PITA having to wait for the length of an exposure.

Pretty much. The camera simply inverts the dark frame and sandwiches it on top of the first frame - the noise produced on each one then cancels itself out. I've done a few long exposures with the dark frame turned off (did I mention I'm a smug K10D owner..? ) and the results aren't exactly pretty! I always leave it switched on now that I've seen what the other option looks like.

You'll get used to it.

Actually i take a lot of long exposure shots and i have been all too aware about the Dark Frame exposure especially if the event being photographed is only of short duration say a Firework display which may be only 15 minutes long! It is not confined to Pentax K10D or K20D but is an established method used throughout all manufacturers. What i do find astonishing is that the K7 does process long exposure images in the same amount of time as a normal exposure! Roll on Nov 5th!!!

Regards - Pete

I've chilled out more reagrding the Dark Frame processing now I understand what is actually happening and perhaps the physics involved with CCD sensors means soem kind of noise reduction is necessary. I retract my accusation of it being 'ridiculous' and apologies if any Dark Frame Lovers were offended

But I still think an option to turn the darn thing off is a silly oversight.

For example sometimes I do a long exposure as a test shot to gauge how long an exposure is needed. i.e I'll take a 30 second test exposure in very low light to gauge the situation. following that I might need to increase to 60 seconds or decrease to 15 seconds depending upon the 30s exposure. This is an instance where turning off the dark framing would be very handy

Perhaps you should consider a K7? as it does`nt appear to use a Dark Frame (unless it can do it simultaneously with the original exposure or something like that?)
It would be interesting to find out as i have not read anything in reference to it,
As regards doing your 30 second test could you not just do a test with 15 seconds or less and use a wider aperture than you plan to use for the final shot? Say - Test at 15 secs at F11 instead of 30 secs at F16.
Best regards - pete

The K7 does use dark frame subtraction for long exposure NR and, just like the K20D, it can't be switched off I'm afraid.

The last DSLR Pentax made with the ability to switch the feature off was the K10D - so if it's really bugging you you'll have to go back a generation I'm afraid. Not that's there's anything wrong with the K10D - still love mine

Good point. never thought of that



And yeah as said the K7 also suffers. I can't afford one at the moment anyway.

I have taken Many hundreds of long exposure shots with the K7 and i can assure you that it takes noise free images and the write time is no different to a normal exposure! I took my first one on my very first outing with the Camera A 30 second shot of a Chapel interior using multiple flash during the exposure. I wrote on a post here on this Forum about how astonished i was as i had been anticipating a 30 sec wait as i had been used to the K10D and Ist DL.
I was just thinking that a handheld lightmeter might be another option for you to consider as there are many good second hand models out there.

Best Regards - Pete.

Seems to conflict with info I've read elsewhere, but I guess those others didn't actually own a K7 and were regurgitating 2nd hand info - I can't afford a K7 right now, that's why I bought the K20D, but it's good to know that when the time comes to upgrade there's a way out of the Pentax Dark Frame nightmare

Now there's a thought, but I've always found the long exposures to be a hit and miss affair and more of a creative exposure. i.e sometimes an over exposure works and sometimes an underexp works.

Woudl a lightmeter work for such long exposures though? I.e if you pointed it into near darkness would it suggest an exposure of "1h 30m"

Hi,
Like i said it`s not just a Pentax thing, the Dark frame exposure is applicable to all makes, there was a series very good article on noise reduction in Amateur Photographer last month.
I only have a very ancient Weston Meter V with an invercone myself which is pretty good but it uses a selenium cell so a more modern digital one would probably be better still. The way mine works is that the needle points to a number, you rotate the dial to correspond with that number and then choose your shutter speed/aperture from any of the ones that line up with each other. I have never used a modern one but it might be worth investigating if you do a lot of long exposure work. There are several in the Amateur Photographer Mag second hand dealers pages, I`m sure that they could advise you better than i could.
Generally these days i do pretty much as you do but if i am taking shots longer than 30 seconds i open the camera aperture so that i can get a reading then close it down to my working aperture and set my exposure multiplying it by the number of clicks. so for example if i get a reading of 30 sec at F4 then i close the aperture to F16 and multiply the exposure by 4X giving 2 minutes, (though generally i make it a little longer than that so say maybe 2 mins 30 sec)

Incidentally the thread actually made me check that my slow shutter speed noise reduction was switched on... It Was!!!

Best Regards - Pete.