Good initiative, and business wise I think FF is a good move for Pentax, all things considered. These are my opinions (misconceptions) on the matter, if anyone against all odds would be interested.
FF vs APS-C in general
+ less noise due to larger photo sites on sensor, regardless of tech improvements FF will always be ahead of APS-C, due to... laws of physics.
+ Larger dynamic range
+ Bigger viewfinder
+ Shorter DOF, iirc?
+ Better use of lenses if you are wide angle shooter.
+ Your lenses have the true 35mm FOV, a fast 50 is a fast 50, nothing else.
- More expensive system, at this point in time
- Generally, larger lenses
- Tele lenses are again true 35mm teles, a 300mm lens is equivalent to a 200mm on APS-C. And we all know how cheap a fast 300mm+ lens is...
FF vs APS-C for Pentax
+ Street cred. "No, don't buy a Pentax. They are not pro, they don't even have a FF upgrade path, even Sony has that".
+ Will provide that upgrade path for those users that do need FF (and those who just think they do)
+ Will make some owners of FA and old M/K/M42 lenses very happy (me included).
+ If FF really goes mainstream in a year or two, Pentax will be on the train.
- Pentax would need a whole new set of FF capable lenses.
- Would spread R&D money on both APS-C and FF lines
I think the last point is the most important one, can Pentax/Hoya afford to develop a whole new line of cameras / lenses while they sometimes have seemed to struggle with keeping up with the competition on just APS-C?
Also, I have been playing with the thought of converting to FF (Nikon/Canon), and this is what I concluded...
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Cost. A whole new system, with more expensive lenses. Might be a slightly cheaper transition if Pentax goes FF though.
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Tele lenses. I am more of a tele guy than WA. I don't want to lose the crop factor.
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Lens size. I really don't want larger lenses.
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My M42s won't work without a lens adapter on Nikon, and I don't like Canon DSLRs
. No problem with Pentax FF, I would guess.
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No SR on non-SR lenses, unless Sony. I don't want a Sony.
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Larger viewfinder. I already have the KPS 1.3x adapter for my K20D, but it is not the same thing. But maybe they could increase magnification even more? Not sure if that is possible, or if it is just a design preference to keep things small?
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Less noise. Would be really welcome, and if the difference is as big as stated in some places it would make up for loss of SR to some degree.
If Pentax released a FF body, I would probably pick one up sooner or later and keep the APS-C body for tele. But right now I think the best thing would be to focus on APS-C AF performance and getting the bodies and lenses with SDM up to the same standards as Nikon/Canon, like faster (and reliable) SDM drives, 1/8000 shutter speed, 1/250 flash sync, etc. I have heard as much praise for the D700's fast AF as for FF specific improvements from the threads with numerous Pentax ---> D700 deflectors over at DPreview.