Originally posted by cwood The downsides to Pentax are:
1. lack of lens availability
2. lack of FF body
3. slow SDM lenses.
In the realm of portraits and weddings those things matter a little but its not the end of the world. I use both Canon and Pentax for weddings and portraits - and nothing Pentax has available in digital can create the look for shooting a 135mm F2 lens on a FF body - or an 85mm F1.2 lens of a FF body... Still I use Pentax a lot at these events and sometimes I use it exclusively.
Some people are unaware that aperture is not equivalent between APS-c and FF and at a wedding with a busy/cluttered background those wide apertures can be really nice.
I think you mean that DOF isn't the same between aps-c and FF. Aperture is always the same because it's a mathematical relation. f/2 on a 50mm will always be 25mm in diameter, and will always let the same amount of light in, but how that 25mm render the DOF depends on the sensor size that is looking through it.
But you're right. There are many lenses in FF systems that just flatout don't exist as options for us aps-c shooters. Not only talking about canikon shooters within their own systems not have equivalents, but more so, us Pentax shooters not even have the FF lenses to begin with, let alone aps-c equivs of them. The ones you mentioned come to mind, and the 200/2, and 24/1.4 also.
Not only do we not have those lenses, but they wouldn't be the same on our cameras anyways. In order to get the equivalent look as a 24/1.4 we would need a 16/1.2; for a 85/1.2 we would need a 125/1.0 and for a 200/2 we would need a 135/1.4. These are all lenses that it is pretty safe to say will never exist. And if they did, they would be massive and extraordinarily expensive.
I'm far less attached to FF, in and of itself, than I am of certain angle of view/DOF combinations. The other advantages of FF are becoming less, with the growth in sensor tech. But the optical advantages will be hard to overcome.