K-x Bulb mode write-time slow?

Hi all,

I've been messing around with Bulb mode on my new K-x (thanks to me managing to turn a 'One-for-All 6' TV remote control handset into a suitable trigger, via its JP1 socket, a PC, and some free software. It'll do until the UK sees some more stock of the genuine Pentax Remote Control F unit!)

I've noticed that whenever I take a photo in Bulb mode, there is a very long wait after I've closed the shutter, while something is happening. During this time, the camera is, to all intents and purposes, locked up. The power light stays on (blue), but no buttons elsewhere will do diddly-squat. Eventually, the 'writing to SD-card' red-light flashes; the image just taken is 'previewed' for 1 sec on the LCD; and all is well again, and I can carry on and take the next shot.

Initially, I thought something was bust. I even must've wasted a couple of good shots because I freaked out and turned the power off (losing the photo in the process), thinking that the camera had locked up due to dying batteries or something.

However, now I'm a bit more accustomed to it. The lockout pause can be in the order of minutes, after a five minute exposure - but I'm more patient now.

But I would still like to confirm this is 'normal' behaviour for the K-x. I can find no mention of it in the manual, or anything that might suggest Bulb mode involves a long wait while it's doing its thing.

And does anyone know what is the 'thing' it's doing, for so long? Just curious...

Thanks in advance.

I believe, but am not sure, that this is related the to the camera applying its noise reduction algorithm to the shot you just took. The longer the exposure the longer the processing and writing to card takes. Try the same length of exposure, but turn of the noise reduction in the menus. More experienced pentaxian's may prove me wrong though.

Dark frame subtraction. It's exposing the sensor to nothing so it can see which pixels go hot with no light. They are therefore noise so can be subtracted from the first real image. The dark frame has to be the same length as the original exposure to be of use.

HTH

C

Neil,

There is an option on the custom settings for long exposure noise reduction. What this does is when an exposure goes above a certain duration, a second 'dark' frame is taken and processed against the origional frame to effectively eliminate sensor noise. Turn this off and you have you camera ready to take the next frame straight away.

I have not noticed any difference in noise with the function turned off and I can see if being more frustrating (and battery draining) for very long exposures.

Splendid responses, guys - thank you SO much!

I had wondered if it might be to do with the Noise Reduction processing, but I couldn't work out why it was taking so long. Now I get it it. What Chris says makes perfect sense (and explains why I thought I heard a second shutter sound... I didn't mention this above, because I thought my memory was playing tricks on me!)

Custom Menu p2, item 13 Slow Shutter Speed NR is, I presume, the beastie in question. I shall turn that off for Bulb work, hereafter.

Many thanks for the tips, and quick replies! Better than the best Customer Support, you are!

Sadly, with the K7 this cannot be fully turned off, probably the same with Kx (I believe K20 is the same way).
As I understand, in ANY bulb photo over 30 seconds the frame subtraction is forced!
In some photos over the specified maximum ISO and shutter speed, this also happen. I do believe this action is what can be turned off. At least I have not yet seen the dark frame subtraction come on... but I stay away from Bulb mode over 30 seconds on that camera. I go back to my K10D for very long shutter speeds.

@Amoringello - yup, you're right. It's the same on the K-x. Just tested it to find out.

Settings:
Manual mode,
Shutter = Bulb,
Aperture = f32,
ISO200 selected,
Manual-focus (MF) set on, focus set to infinity,
11pm(ish) darkness,
IR-Remote-triggered shutter in 'one-push-to-open, one-push-to-close' drive mode.

Most importantly, Custom menu p2, item 13 (Slow Shutter Speed NR) set to mode 2 = Off.

Results:
Exposure less than 30s (even one sec less) - immediate live-mode preview for 1sec (as set), and write to SD card. Camera instantly reusable.

Exposure greater than 30s (even one sec more) - dark frame exposure of the same length, plus about another 10-15 sec (presumably to do the Noise Subtraction maths), then live-mode preview and write to SD card.

So, yeah... bit of a bummer, but not the end of the world. At least I know what it's doing and why, and I guess I can live with that. I guess I'd best have a second camera handy during meteor shower time then, eh? 15-min effective 'downtime' after a 15-min exposure could be a long time to miss out on any action, eh?

I wonder if anyone's raised this with Pentax... it would seem to be a bit silly that this behaviour is not overrideable. What do the astrophotography community make of the K-x, given this 'feature', I wonder?

Still - grateful to all for the explanations and info, though. Thanks muchly.

I think it may be something to do with CMOS sensors (although I don't pretend to understand all the technicalities) but it seems a coincidence that you can turn it off on those cameras with a CCD sensor (K10D, etc) but not those with a CMOS sensor. Any techies care to hazard a guess why that might be?

They still use K10D's...

I've no idea either why.
CMOS don't get as hot as CCD and you can shoot film with them over 30 seconds without a problem and without a dark frame.

That was my understanding too but there must be something that we've overlooked. Mind you, if I did know the answers I could be a camera designer...

It's probably Pentax decision because they found the fixed pattern noise too high in some cases.
CCD are more uniform, maybe that has something to do with it?

Dredging up a few snippets from memory, I think CMOS sensors are noisier than CCDs. Maybe it's because the amplification occurs on the chip and is less effective.

If this is so, it might explain why the CMOS cameras have low speed noise reduction permanently activated.

Exposure stacking is the usual answer. Search for Registax - it's free and lets you use multiple exposures of a single object. It has the additional feature/side effect of averaging out the noise over several exposures so you get decent inky blacks instead of a field of chroma noise.

You can use multiple exposure stacking to minimize noise for normal daylight photos, I use Hugin (which gives away that I use Linux for my editing) which lets you align multiple hand held exposures. After that I let each contribute to (1/n * 100)% of a layer.

C

That sounds plausible to me.