Here is a chart showing the difference between the Pentax 645, 645N, and 645NII medium-format cameras.

	645	645N	645NII
Year introduced	1984	1997	2001
Mount	Pentax 645 A	Pentax 645 AF	Pentax 645 AF
Meter range	3 - 19 EV	2 - 21 EV	2 - 21 EV
Meter pattern	center-weighted	m (6 segment) c s	m (6 segment) c s
ISO range	6 - 6400	12-6400	12-6400
Film type	120 and 220 roll film, 70mm film	120 and 220 roll film, 70mm film	120 and 220 roll film, 70mm film
No. of exposures	120 film: 15, 220 film: 32, 70mm: 90	120 film: 16, 220 film: 33, 70mm: 95	120 film: 16, 220 film: 33, 70mm: 95
Data imprint on film	No	Yes	Yes
Exposure modes	P, Tv, Av, M, B, X	P, Tv, Av, M, B, X	P, Tv, Av, M, B, X
Exposure compensation	+/-3 EV	+/- 3 EV	+/- 3.3 EV
Exposure memory lock	No	Yes	Yes
Shutter speeds (auto)	15 - 1/1000s	30 - 1/1000s	30 - 1/1000s
Shutter speeds (manual)	15 - 1/1000s	4 - 1/1000s, B	6 - 1/1000s, B
Shutter speeds (mechanical)	None	None	None
Half step speeds in M and Tv	No	No	Yes
Self timer	No	Yes	Yes
Mirror lock-up	No	No	Yes
Auto bracketing	No	Yes	Yes
Multiple exposures	Yes	Yes	Yes
Winder	Built-in, 1.5 fps	Built-in, 2 fps	Built-in, 2 fps
Built-in flash	No	No	No
TTL flash	Yes	Yes	Yes
P-TTL flash	No	No	No
Sync speed	1/60s	1/60s	1/60s
Flash exposure comp	No	No	No
Autofocus	No	Yes, (3 points)	Yes, (3 points)
Autofocus sensitivity	Not applicable	-1 - 18 EV	-1 - 18 EV
Viewfinder	0.75x, 92% (vert), 93% (hor)	0.76x, 92% (vert), 93% (hor)	0.76x, 92% (vert), 93% (hor)
Viewfinder type	Keplerian telescope	Keplerian telescope	Keplerian telescope
Diopter correction	-5 - +2	-3.5 - +1.5	-3.5 - +1.5
Exchangeable screen	Yes	Yes	Yes
Depth of field preview	Yes	Yes	Yes
Image size	41.5 x 56 mm	41.5 x 56 mm	41.5 x 56 mm
Battery	6 x AA	6 x AA	6 x AA
External battery pack	No	Yes	Yes
Size (W x H x D)	147 x 109 x 117 mm	147 x 109 x 117 mm	147 x 109 x 117 mm
Weight	1320 g	1280 g	1280 g
Comment	Accepts leaf shutter lens for flash synchronization to 1/500s. Accessories: 120 and 220 film backs, 70mm film holder	Accepts leaf shutter lens for flash synchronization to 1/500s. Accessories: 120 and 220 film backs, 70mm film holder. External battery pack	Accepts leaf shutter lens for flash synchronization to 1/500s. Accessories: 120 and 220 film backs, 70mm film holder. External battery pack

[MetaD]

Just to highlight the significant differences:

(1) Autofocus on 645N & 645NII.

(2) Single-mode knobs with green-for-auto indicators 645N & 645NII. 645 has fewer knobs, and sets modes through LCD panel and menus.

(3) Data Edge Printing on 645N & 645NII.

(4) Matrix metering, auto-bracketing, and exposure lock on 645N & 645NII.

(5) Mirror Lock-up on 645NII. (Most people say the mirror is well-damped and this isn't important).

Everything else is pretty minor. Maybe lack of self-timer on 645 matters to some.

[A-Craig]

Thanks for posting!

[revdocjim]

Thanks for the helpful info. But I have one very minor quibble. I recently found an external battery pack that would appear to be part of the original 645 system. But the chart above says the original 645 didn't have an external battery pack. As you can see in the photos below, the writing says "Pentax Remote Battery Pack 645" and the finish on the grip is identical to that of the original 645, which is distinctly different from the finish of the 645n. I tested it out on my 645n and it works fine.

[Elkind]

remote cord for 645 is a mechanical ones but for 645n/nII it's the same as for mz/zx series — cable switch F.