The use of the CCD backs for longer exposures can be done and with the right technique, you can produce some very good images. There are some limits however to each of the backs, P45+ and IQ260, I have never shot the 645D, but it being CCD more than likely will have some of the same issues.
Of the Phase backs, the P45+ to me is still the best for long exposures. Phase did a very nice job on this back, and you can easily get to a 1 hour exposure. There are some considerations:
1. The P45+ long exposure must be accompanied by a corresponding dark frame exposure of the same time. So if you just exposed for 45 minutes, the back has to take another 45 minute exposure. Failure to allow this, will ruin the first exposure. So battery life becomes important. You can expect to get maybe 4 45 minute exposures (and that is a with a new 3400 milliamp battery), so 2 actual shots.
2. The actual outdoor temperature needs to be at 69 degree F or lower for full 1 hour exposures. You also don't want to use the back for a long exposure in time of high humidity.
3. The best long exposures from the P45+ are taken at 50 ISO, so for astrophotography, this can limit your capabilities depending your conditions.
4. There is no way to turn off the dark frame on the P45+.
The P45+ can do a very good job, right up to 1 hour, it's not a great back for stacking (which is what I do now) for several reasons, no intervalometer support and the dark frame requirement. But if the conditions the results can be very rewarding. The files will still hold up with good color saturation and details.
IQ260:
This back will go to 1 hour, but in my experience the results really don't compare well to the P45+. Some considerations:
1. The IQ260 needs to be in the Long exposure mode, which starts at a base ISO of 140.
2. The 260, will need to shoot a dark frame just like the P45+, however on shorter exposures, say 5 minutes to 10 minutes you might be able to get away
shooting in aerial mode, which means no dark frame will be shot.
3. Results vary, but I never saw the same clean detailed files that I saw from my P45+, at 30 45 and 60 minutes.
4. With the XF body you could stack with the 260 using the timer feature (as now Phase as added a intervalometer to the XF) but you would need to turn
off the dark frame and be in aerial mode. So you will get quite a few stuck pixels.
5. The files I took with the 260 were all 30 minute tests and none of them really had the same clarity of the P45+ images, so I quickly stopped using it for
LE work. The files lack both good saturation and showed quite a bit of noise and loss of finer details.
To me if you want a CCD back for LE work, I would still go with the P45+. Make sure you test one and a 260 if you can. The interface on the P45+ is very old and the LCD pretty much worthless (similar to the screen on the Canon 1ds MKI), so image review is not easy. You also can only tether via old style firewire. The 260 has the IQ interface, which is 100% better and will work with the XF body.
The newer CMOS backs have been mentioned, and the older 50MP in the IQ150, IQ250 and Credo 50, will out perform the IQ260 for sure and possible the P45+, but these are more expensive and to be honest, the Pentax 644Z would be the better choice, as you get a back and body. I don't know if there is a intervalometer for the 645Z, I have never seen one mentioned so stacking might be a problem. But the camera does a great job with the Sony sensor and you can turn off the dark frame on it.
You might be able to get a good price on a IQ150 or Credo50 as Phase One is now starting to make some attractive pricing offers on kits featuring these backs.
For sure try to locate a Phase One dealer in your location, and set up a demo. The dealer can provide you with a certified used back, for all of the backs mentioned if you wish to go that route and they will be your 1st point of contact for support.
This is an image taken with the P45+ @ around 50 minutes with the older 35mm Mamiya lens and AFDIII camera.
Paul C
