The fundamental issue is heat. Having the sensor on creates heat and in digital imaging [heat = noise].
Generally speaking, all MF backs are limited to about 30 - 60 seconds maximum exposure times. Some Kodak sensors with applied software improvements made it to 30 - 60 minutes depending on ambient temperatures -- notably the Phase P25+, P30+ and P45+, P45+ going the longest reliably.
The improvements from the P series to the P+ that allowed for very, very long exposures had five components
- software processing (better guessing at what is noise or a hot pixel)
- heat sink improvements (industrial design of the back to keep heat from pooling at the sensor)
- firmware improvements involving the method/speed/timing the sensor is read-off to reduce noise introduced in the read-out process
- proprietary black calibration technology which reads out the noise present on the sensor by taking a frame with no exposure (lens closed) [note this sounds simple - but just how good the black cal technology is hugely important to long exposures]
The new P65+ and P40+ are rated at 1 minute but I think Doug has tried 2 minutes with success.
Not so much.
With a P40+ or P65+ 50 seconds at ISO100 is the longest I've ever done at normal temperatures (around 70F in that example). That's the same amount of exposure as 2 minutes at ISO50 so maybe that's where you got that idea. And to be clear the quality was mediocre - depending on the content of the image and your usage it might have been acceptable but I do not want anyone thinking they can do a 2 minute equivalent exposure with a 40+ or 65+ and expect top-quality results. Good quality ends at 1 minute at base ISO (50) with the P40+/P65+ and goes up a tiny bit each time there is a firmware release or software release.
If long exposures are part of your stand-by and you don't want to be limited creatively by the technical capability of your camera then a P45+ would be my strong suggestion. If you're not able to afford a P45+ I'd explore P30+, P21+, P25+, P20+, P45 non-plus in that order. We have a pretty good selection of demo backs right now.
Here is a chart of the longest clean exposures expected for a P45+ which varies by ambient temperature. The gist is
8 minutes at 105F (41C) [yikes thats hot!]
20 minutes at 84F (29C)
1 hour at 63F (17C)
5 hours at 19F (-7C)
Doug Peterson (e-mail Me)
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