[Rondec]

The 645z will be better. Its images will tolerate post processing better and print bigger. But only you can tell if it is enough better in the right ways to make it worth your while to invest in it. It is probably the best value in medium format out there right now, but it is far from cheap...

[Kolor-Pikker]

I can print up to 20"x30" on canvas with excellent quality.

Canvas isn't a particularly demanding medium with regards to resolution as matte and glossy papers are. With the 645Z I can print 40" wide on canvas without a sweat, but I haven't really gone big on other types of surfaces yet.

[jonmarsh]

I own the 645D, K3, and the 645z. On the K3, I mostly use D* or older full frame glass, including the 31mm and 77 mm limiteds, and the 200mm macro. For the 645's, I have a selection of 645 and 67 system glass. I mostly use the K3 for documenting project builds, or when I really need a light weight camera- the combination of lens rendering, resolution, freedom from aberration in the better MF lenses, and dynamic range of the 645z put it in a rather different class for me. But then, that's also for large prints. OTOH, I find I can do things at ISO 3200 on the z that I wouldn't go beyond 800 on the K3. The K3 is a very nice ASP-C camera, and a significant step up for from the K10 I still have as backup to it- but it's not remotely in MF territory for large prints... but you may not need that, I agree the best path is likely to rent one and see for yourself. It's all about your usage needs, after all.

I just got back from Antarctica, took both the D and the z, (backup and dual usage scenarios in field without changing lenses) and got great shots with both- but the z has more flexibility for poor lighting and weather conditions. As a backup, the K3 wouldn't have been in the same class- but I did have fun with a Ricoh GR for casual shots- it's surprisingly competitive with ASP-C DSLR's under selected conditions.

[mikeSF]

I can print up to 20"x30" on canvas with excellent quality...

though you may feel they are excellent, I would suggest the same image from a 33x44 sensor contains a good bit more detail that you might really appreciate.
I have neither the K3 nor the 645Z so I cannot directly comment on your question and recommend you rent the Z and see for yourself what improvement it yields.
That said, comparing my K5 and 645D, I begin to see a marked improvement in detail with 11x14" prints and larger. Little blades of grass, tiny rivets on a bridge, hairs and eyelashes on portraits, there is more information available from the 33x44 and it is obvious.
good luck!

[BarryE]

Here's some feedback from a useful 30 min trial with the 645Z, comparing it with a K-3 for my purposes.

As a reminder my purposes = mostly landscape. Low ISO. Tripod. Controlled shooting, often after a hike. Using primes. My target print is a Giclee print on Hahnemuhle photorag 308, fully colour managed and printed at 240 ppi at 740X520 mm, ie native resolution of K-3.


** I've recently upgraded my Photoshop version to CC which moved me from the old process model to the latest and, indeed, greatest. I mention this as the data that can be pulled out of the shadows and the control of the highlights seems to extend the "effective/usable" dynamic range, which influences some of the points I raise below.


The comparison was not in ideal conditions - I had 30 mins of playing with a 50 mm and 90 mm 645 lens. Here goes:



# Not surprisingly the 645Z's noise performance at ISO 400 and above far exceeds the K-3. At base ISO (100), the noise levels were not noticeably different.


# The balance of the 645Z with both the 50 and 90mm lens is nice, however after 30 mins I knew I'd been using it. This weight difference would probably be a long-day problem for me.


# I could not tell any subjective difference in the dynamic range between the 645Z and the K-3 at low ISO when processed from RAW images.


# The 645Z resolution is outstanding. Whether I will want to go above the 740x520 mm images I am planning to use is unlikely, (unless I upscale to large canvas prints, when the weave of the canvas reduces the need for the same critical viewing). Therefore, the greater resolution is possibly not especially relevant to me.


# One key point that did occur to me when thinking (dreaming) about a 645Z upgrade is the "fear factor". If I dropped a K-3 and lens I'd be upset. If I dropped a 645Z, I'd be a tad more upset ! Yes I know I could insure it, but that is a further cost disadvantage against the 645Z.



# The K-series lenses are more upto date and there are many more options.


In summary:


For studio work and/or at high ISOs the 645Z is a far better camera. Obvious that ! However, when using K-3s for my purposes (as defined above), I think I'll stick with the K-3s.


The K-3 is smaller, lighter, suffers from a much reduced "fear factor". It is very good at low ISOs and when used in controlled manner for my purposes does not differ greatly from 645Z to a print size of 740*520 mm on quality paper.



Maybe I'll reconsider the FF when it comes out, but this experiment has made me think deeply about what I want versus what I need.


Many thanks for input to help me come to a conclusion.