How big of an issue is mirror slap on Pentax 6x7 (thought I was getting MLU, but...)

Oren - Ctein would probably be in a minority opinion in that respect. I've talked to him in person about it, and he did shoot primarily P67 for many years, but rarely in conjunction with long teles.
I've certainly never had a problem at 1/15th provided MLU was used, and I do expect very sharp results. I'd also want to know which specific lenses were allegedly involved. I often use 1/15th when shooting 75mm or 105mm lenses, and see no difference in sharpness from other shutter speeds.

What does come into play, on the positive side, is the relatively bigger mass of the P67 teles, which tends to counteract secondary camera vibrations. I even prefer to shoot my little Nikon FM2a using a P67 300 mm lens via adapter rather than a dedicated 35mm lens of that focal length. For that series of Nikons also had a known shutter issue with ultra-tele work. I once discussed that with a specialist.

This is not a problem, if it were Pentax would have corrected it. The same comments are made about Hasselblad, and it is not a problem with Hasselblads either.

Yep. And both were very popular pro systems; still are. But I didn't buy my first P67 until 1978, and Ctein probably did so earlier. Perhaps there was some kind of "1/15th" bug in the system earlier. I just never heard about it myself, even from people who already made their living with these Pentax cameras.

My own brother both sold and owned expensive Rollei 6X6 SLR's, noted for their extremely gentle shutters and mirrors, yet preferred to borrow my P67; and I never saw an incident of loss of sharpness in either his shots or my own which could reasonably attributed to a shutter issue. He shot up to 165mm lenses and used just a basic second-hand Tiltall tripod. At that point in time, I had sold off my 200mm tele, but didn't own a 300 yet.

The flash sync speed on the p67 is fairly low at 1/30 sec. (Unless using a leaf shutter lens.) One might mitigate vibration issues by having low ambient light on the subject so the flash can freeze any motion.

They made 90, 105, and 165 lenses in optional leaf shutters for sake of flash synch at any speed. Those were popular portraiture and fashion focal lengths. But flash fill has always been uncommon,
or downright unrealistic, among landscape photographers, who also took a liking to this camera system.

DREW WILEY said:

They made 90, 105, and 165 lenses in optional leaf shutters for sake of flash synch at any speed. Those were popular portraiture and fashion focal lengths. But flash fill has always been uncommon,
or downright unrealistic, among landscape photographers, who also took a liking to this camera system.

Click to expand...

I have the 90mm and the 165mm leaf shutter lenses, but have never seen a 105mm leaf shutter lens and didn't know one even existed until now.

I just double checked with the lens data base on the Pentax Forum. I was incorrect. Leaf shutters were only available in 90 and 165 versions.

DREW WILEY said:

I just double checked with the lens data base on the Pentax Forum. I was incorrect. Leaf shutters were only available in 90 and 165 versions.

Click to expand...

Damit Drew, I was going to start looking for a 105mm leaf shutter lens cuz it would be ideal. Shucks!

I've been using a 6x7MLU since the 1990s and have never found the slap-up to be a problem. As others have said, a tripod and cable release would alleviate any worries. I've hand-held my 6x7s making sure I load at least 200 ASA, preferably 400 ASA and an f4 or faster lens shooting outdoors in the sunlight.

If the subject and conditions allow you could alleviate slap by making even longer exposures. The short 'slap' vibration is a large percentage of the exposure at 1/8th second, but a fraction of the exposure time at 4 seconds. So buy a few ND filters, and cheaper than a wooden tripod.

dave olson said:

I've been using a 6x7MLU since the 1990s and have never found the slap-up to be a problem. As others have said, a tripod and cable release would alleviate any worries. I've hand-held my 6x7s making sure I load at least 200 ASA, preferably 400 ASA and an f4 or faster lens shooting outdoors in the sunlight.

Click to expand...

I've had my 67 system since the early 80's and I'm on your team Dave. For hand-held shooting I almost never go below a 400 speed film, but I also have some tremors in the hands, which adds to the problem. Even shooting hand-held I will usually seek out something to brace the camera against such as a tree, wall, fence post or whatever. If I have a blurry shot it's usually/almost always me and not the Pentax 67 or whatever camera I'm using. My tripod setup is a Bogen 3033 with a Kaiser large ballhead(same as the largest Kirk ballhead). I know Drew doesn't care for ballheads, but this is a huge, stable outfit that I'd put up against about anything. Of course a tripod this big gets used only for certain special occasions.

I use both a 6x7 MLU and a pre-MLU a lot (I was going to say I use them "heavily" but that seemed a little on-the-nose given the weight of these beasts!). I agree with everything already said about the consequences of the mirror slap being dependent on factors such as shutter speed and (most importantly) lens length, but I wanted to add that in most circumstances I MUCH prefer the pre-MLU body. The main problem with the MLU is accidentally triggering the lock-up, especially if you have one of these wooden ergonomic grips on the right-hand side (see attached photo showing the grip on my pre-MLU). Accidentally triggering the lock-up wastes a shot and, if you don't notice it, drains the battery. One can mitigate this problem a little by putting tape over the MLU but it doesn't stop it completely and the tape can come off.

Because the circumstances in which you actually NEED the MLU are rare, this makes the pre-MLU body my go-to 6x7. If I could only have one I would choose the pre-MLU.

Alan9940 said:

I’ve only ever owned the P67 (with MLU) and now the 67II, but my guess is that even on a good tripod you might notice some sharpness degradation at slower shutter speeds. How slow? I’d say anything below 1/60 second; at least, that’s what my own testing of this very question proved many years ago with my P67.

Click to expand...

My tests years ago (with the 75mm f/2.8 AL lens on a Pentax 67II and a matching tripod) showed marginal loss of sharpness at 1/25s.
Because the mirror moves vertically after the shutter is closed this is/was most likely due to the horizontal movement and torque of the focal plane shutter.

According to the Pentax Forum, the non-MLU version was only made during the first year, 1969. I'm surprised they still turn up for sale. Perhaps some of them got set aside by their owners once the MLU series soon arrived. Personally, I've never accidentally triggered the MLU button. And as far as alleged sharpness loss with even MLU engaged, I still contend that it's more likely due to harmonic tripod vibration instead. I certainly don't experience it. But I stopped using metal tripods a long time ago. Even garden variety aluminum tripods could probably be improved in that respect with simple closed-cell poly foam snap-on water pipe wrap.

The wooden hand grip probably helps dampen vibrations too. I don't hand shoot the P67 very often, and if I have to, I'm likely to substitute TMY400 for my usual TMX100. I do like the hand grip for more securely handling my 300 EDIF tele setup, and getting it atop the tripod safely. That would be a "crying expensive" lens to drop. Yeah, I do have a clean 300 Takumar as a backup; but the EDIF series has that extra special something.

If I anticipate handheld shooting, I'm more likely to resort to my Fuji RF. The wind has been howling so bad here near the coast that I wasn't able to do any outdoor house painting today, or even lawn mowing. Last week, I had to wait 45 minutes for just a 1/15th sec lull to trip the shutter on my 4x5. Don't get me wrong. I love our free summer "natural air conditioning". But some days, I have to seek shelter from the wind deep in the redwood forest, where exposures are always long.