You first have to ask, is anything past the base iso of 50, really real? This question has come up over and over, and Phase One, or a dealer has never really addressed it.
CCD's don't really record iso's like CMOS, so when you push the chip by increasing the iso, as I understand it, you are not really you really are not increasing the gain like you would with CMOS, instead a marker is added to the metadata. Capture One then sees that and works some interesting processing. However again I don't think you are really taking the chip up past the base.
On Getdpi, there was an interesting post where one user felt that the base of the 260 is really iso 100, not 50. A lot was written on that issue also, but again Phase One or a dealer did not come into give an official answer. I personally can't answer this, as I always either shoot at 50 or move to 200, mostly skipping 100 iso. The OP on the Getdpi post's point was that images taken at 50 and 100 basically had the same signal noise and really did not seem any different.
I own the 260 and previously owned the 160. I had hoped that Phase One, by coming out with a new chip for the 260, would be able to pull more out of the the "higher iso" ranges of the 260, so far, I have not seen that. In my use, if you push the 260 past 200, you will be not too happy with the results. Lets say anything past 200 in non sensor plus, You can expect:
1. Way too much noise in the shadows, enough that the image is really not useable for any details
2. Considerable color saturation loss, albeit that some of this can be gained back in C1
3. Overall image sharpness falls off especially in the finer details.
iso 200 is OK and can be used, in bright light/medium light, however again shadows will suffer. If I am put in a position where I need iso 200 I will always do a 2x shoot, 50 and 200, using 200 to stop motion mainly wind.
If you move to sensor plus, and 15MP, you can easily get to 400 and maybe 800, however I don't fee 1600 is that good, remember you are now 1/4 of the full resolution of the chip. I just don't use sensor plus as to the resolution is the main reason I am shooting the back. Nice idea, but there are plenty of 16mp cameras out there that can equal the DR of the 260 at 15MP.
The 260, seems to me to have a bit better recovery on highlights, as on average I can recover 1/2 to 3/4 of a stop. Again when shooting a tech camera this is important as I don't want to be burdened with a lot of attempts. The tech solution does not currently offer 1/3 shutter speed settings, to you again are a lot more limited in the available shutter speeds, just full stops.
Color and details at 50 never disappoint me, it's where the camera likes to be shot. However even at iso 50, I have found that 1/2 of a second can make a huge difference in the noise again in the shadows. I noticed this on a shoot last month where I found a big difference in noise between 1 sec and 1/2 a second at iso 50.
Look back to the Digital Transitions post in this forum from back in late Feb, early March, where a series of testing was done indoors, with a 250, 260 and 280. The 250 basically can be taken to 1600 and still produce a full resolution image with very low noise. You also have the ability to pull up the shadows on the 250 as much as 1.5 to 2 stops and still have excellent details, this just will not be the case on a 260 or 280 for that matter.
Phase One, may still come out with a firmware update, (I am now starting to doubt it) that will show more improvement for 260 images. However with CMOS as the primary goal now, I am not sure that too much R&D is being done to the CCD chips anymore. In the past, when there was not any CMOS to show, Phase did pull some amazing rabbits from their R&D hat on various CCD chips.
I am a landscape shooter, my website, are show below, PM me if you would like some images taken with a 260 in good light at 50 and 200. I would be more than happy to send you some.
I also would strongly state, find a dealer, and work with them on this decision. It's a huge purchase. The values of CCD backs are falling, and if and when a full frame CMOS is delivered by Phase One, most likely 2015, the value of CCD's will fall further. Remember before the IQ250, there really was no other game, just CCD if you were looking for a MFD back. and the values held up well.
Sincerely
Paul