[leekil]

I would have thought that the engineers would have retained the distance from film days, since they were careful to make the cameras backwards compatible with film era lenses---which were being produced right up to the introduction of the D, and I'm guessing in a handful of cases may still be produced.

Also, at some point they reintroduced a bunch of 645 lenses to sell as new for the digital 645s. I don't think there would be any reason to change the effective flange focal distance from the 645 film distance, since one of the points of making the digital 645s the way they did was so you could use the old film lenses on it.

[SylvainB]

I would have thought that too, until I couldn't focus my P67 55-100 lens further than abt 1km. I did a couple of landscape shots add the focus cut off In a line across the picture about halfway into the scene.

That is when I started to measure the P67 to P645 adapters and found the discrepancies. Pentax adapters.

Normally such a small difference would not make a difference in any lens, although it seems as if the 55-100 is the exception. It only just makes near infinity focus with the adapters that have less than the expected flange to flange distance. I give the dimensions at the start of the thread

I say near infinity because the lens can focus far enough for my current use in landscapes with the shallower adapters.

For my two other P67 lenses this is no problem and no concern. 67 400mm and 67 105mm.

If you put extension tubes for macro, you can focus closer but no more at infinity. So this part of your message puzzles me. If you have a problem with the shortest adapters, I don't understand. Am I completely wrong in my understanding ?

[Ed Hurst]

In my experience, the P67 55-100 lens is tricky to focus near infinity - not because of any issue with the flange distance - but just because the lens itself is tricky to focus precisely in that range (the way the throw reduces near infinity is obviously part of the reason, as any small error in focusing-ring position is magnified as result; there may also be something about the way the lens renders out of focus areas as the depth-of-field tapers off, not sure). I have found this to be true when using the lens on my 67ii's as well as the 645D/Z, so I really don't think it's a flange distance thing, at least not in my case.

I don't discount the idea that you may have some other issues going on (at the same time) due to differing adaptors, but what I'm saying is the issue exists regardless - just inherently with the use of the lens itself.

That said, if one focuses very carefully, the lens rewards one's precision with wonderful results. So this is a somewhat demanding beast, but so worth it! :-)

[RICHARD L.]

can you test to see if you can focus on the trees on the horizon at a wide aperture?

I just tried viewing through the lens with the 67 M 55-100 mm f/4.5 on tripod here at home and I can assure you infinity focus is excellent even wide-open.

Regards

P.S. Next time I go out picture-hunting, I will shoot something at infinity wide-open with this zoom to post here. Did you check if you can achieve infinity focus with other 67 lenses or is it just the 55-100 mm that seems out of whack ?

---

Last edited by RICHARD L.; 01-06-2021 at 05:27 AM.

[photoptimist]

It wouldn't surprise me if it does make that difference. The 3mm isn't much difference and with IR, UV and Bayer filters on top of the sensor that probably accounts for most of that 3mm. Add in that having that additional glass in the optical path and the effects of that and it probably does make up that difference. From what I remember reading and think I understand is that having additional glass in the optical path between the lens and film/sensor does change things up. I have found this to be the case with one of my big lenses that takes drop in filters where there is a noted difference in focus with and without a filter in the holder.

This seems like the kind of thread that could use some insight from @photoptimist as he always seems very knowledgeable on these sorts of optical performance types of things.

Thanks for the shout out!

Yes, a 3 mm difference between the official flange focal distance and the measured flange-to-filter distance on the sensor sounds well within the realm of possibilities. The range of glass filter thicknesses for digital cameras range from about 1/2 mm to 4 mm ( Sensor Stack Thickness: When Does It Matter? ). Then there's also an air gap between the filters on the front of the sensor and the chip surface. That gap is about a mm or so and is needed for the space for the microwires that connect the silicon chip to the traces inside the body of the sensor.

A photo of the 645Z cut in half ( https://upload.wikimedia.org/wikipedia/commons/6/69/Pentax_645Z_cutaway_2015_CP%2B.jpg )shows a thick stack of what looks like 3 sheets of glass or filter materials between the shutter blades and the silicon.

Finally, it's worth noting that if the 645Z flange-focal distance were longer than that on the film era 645, the effects on focus-at-infinity would be most strongly felt with film-era wide angle lenses such as the FA 35 mm lens. Just a 0.5 mm increase in the flange-focal distance would move the infinity focus of a 35 mm lens to a mere 2.5 m.

[stevebrot]

I don't understand. Am I completely wrong in my understanding ?

You are correct in your understanding. 1+1 does not seem to equal two in this conversation.


Steve

[TDvN57]

Thanks to @photoptimist for the info, and the contributions by everybody else.

I would like to clarify some of the issues, as it seems I might have been unclear in explaining my observations.

I accept that the FFP distance for the 645z being the same as the 645 film versions which is 70.87, regardless of the measurement from the flange to the front glass element of the sensor. I agree this is quite clear from the cut-in-half picture.

Some baseline numbers to start off with.

P67 FFP distance: 85.00mm
P645 FFP distance: 70.87mm

The difference is 14.13mm, which one would then expect to be the flange to flange distance of an adapter between the two systems.

What I experienced in the past.

I used the P67 55-100mm lens a couple of times on the 645z taking sweeping views of panoramas with infinity focus and absolutely no problem. An excellent lens and I am happy to use despite being difficult to focus towards infinity. My general experience with tis lens is exactly what Ed Hurst explain in this thread and elsewhere. The lens is very sensitive to focus towards infinity but the IQ makes up for the trouble and effort.

So far no problems and I am very happy with this lens and its excellent performance on the 645z.


Experience last Sunday morning's shoot

Last Sunday I went out to take pictures from a local mountain overlooking the city area to get a series of sunrise shots with the P67 55-100mm lens on the 645z. It was then that I experienced the infinity focus problem. At first I thought it was a lack of strong coffee and my mojo was left at home. The camera was on a tripod, and I used live view for focusing with peaking switched on, so I could see where the focus line was and how it moved while turning the focus ring. I tried several times but each time the focus line would stop about two thirds into the scene. There was nothing I could do to get the focus line to move beyond that point. I took a couple of test shots and confirmed the buildings at the far side of the picture were indeed out of focus. I was puzzled and stopped down the aperture as much as I could to increase the dof to mitigate the issue. I did not have any other lenses with me at the time.

Once back home I started editing and confirmed my observations out in the field. I also looked at pictures I had taken previously with the same lens and had absolutely no problem focusing to infinity.

My equipment and prior actions

I have three 6x7 lenses that I use on the 645z, and bought three Pentax P67-P645 adapters, one for each lens, so that I do not have to swap adapters and lens caps around. A tip I picked up from some of PF users.

I did however clean the lenses and adapters prior to my trip on Sunday and also removed the three adapters from the P67 lenses and cleaned the whole lot. I paid no attention to as to which adapter goes with which lens, because they are all three the same and in mint condition.


The troubleshooting

After a night's sleep and enough coffee on Monday morning I started to troubleshoot the issue. To make a long story short I eventually measured the flange to flange distance of each of the adapters. It measured as follows:

Adapter 1: 14.15mm The adapter I used on Sunday's problem shoot.
Adapter 2: 13.60mm
Adapter 3: 13.60mm

Note that one would expect the adapter to be 14.13mm plus minus allowable tolerances.

I then mounted adapter #2 (13.60mm) to the 55-100mm lens and infinity focus was possible. I was astonished that 0.55mm would make a difference in infinity focus. If you look at the lens while rotating the focus ring from infinity backwards and try to mimic the small amounts of turn you need to achieve critical focus you can see the distance the front part of the lens telescopes in and out. The slightest movement of the ring shows how little the front element moves to change the focus when near infinity.

Thus a small difference in flange to flange distance between the two adapters (14.15 less 13.60mm) being 0.55mm could indeed make a difference in the focusing, if the lens was already at the end of its focusing ability, it could affect its ability to achieve infinity.

Conclusion

The problem might be specific to the lens I have but I have now dedicated an adapter to the lens that allows infinity focus.

My other lenses have sufficient focusing abilities beyond infinity to make this a non-issue for those lenses.

I hope this clarifies my observations and experience with the adapters, which I thought might be helpful to the forum users.

Once again thanks to everybody's contributions and willingness to help me get clarity on this issue which helped me to enjoy this great lens once again.

[barondla]

Thanks to @photoptimist for the info, and the contributions by everybody else.

I would like to clarify some of the issues, as it seems I might have been unclear in explaining my observations.

I accept that the FFP distance for the 645z being the same as the 645 film versions which is 70.87, regardless of the measurement from the flange to the front glass element of the sensor. I agree this is quite clear from the cut-in-half picture.

Some baseline numbers to start off with.

P67 FFP distance: 85.00mm
P645 FFP distance: 70.87mm

The difference is 14.13mm, which one would then expect to be the flange to flange distance of an adapter between the two systems.

What I experienced in the past.

I used the P67 55-100mm lens a couple of times on the 645z taking sweeping views of panoramas with infinity focus and absolutely no problem. An excellent lens and I am happy to use despite being difficult to focus towards infinity. My general experience with tis lens is exactly what Ed Hurst explain in this thread and elsewhere. The lens is very sensitive to focus towards infinity but the IQ makes up for the trouble and effort.

So far no problems and I am very happy with this lens and its excellent performance on the 645z.


Experience last Sunday morning's shoot

Last Sunday I went out to take pictures from a local mountain overlooking the city area to get a series of sunrise shots with the P67 55-100mm lens on the 645z. It was then that I experienced the infinity focus problem. At first I thought it was a lack of strong coffee and my mojo was left at home. The camera was on a tripod, and I used live view for focusing with peaking switched on, so I could see where the focus line was and how it moved while turning the focus ring. I tried several times but each time the focus line would stop about two thirds into the scene. There was nothing I could do to get the focus line to move beyond that point. I took a couple of test shots and confirmed the buildings at the far side of the picture were indeed out of focus. I was puzzled and stopped down the aperture as much as I could to increase the dof to mitigate the issue. I did not have any other lenses with me at the time.

Once back home I started editing and confirmed my observations out in the field. I also looked at pictures I had taken previously with the same lens and had absolutely no problem focusing to infinity.

My equipment and prior actions

I have three 6x7 lenses that I use on the 645z, and bought three Pentax P67-P645 adapters, one for each lens, so that I do not have to swap adapters and lens caps around. A tip I picked up from some of PF users.

I did however clean the lenses and adapters prior to my trip on Sunday and also removed the three adapters from the P67 lenses and cleaned the whole lot. I paid no attention to as to which adapter goes with which lens, because they are all three the same and in mint condition.


The troubleshooting

After a night's sleep and enough coffee on Monday morning I started to troubleshoot the issue. To make a long story short I eventually measured the flange to flange distance of each of the adapters. It measured as follows:

Adapter 1: 14.15mm The adapter I used on Sunday's problem shoot.
Adapter 2: 13.60mm
Adapter 3: 13.60mm

Note that one would expect the adapter to be 14.13mm plus minus allowable tolerances.

I then mounted adapter #2 (13.60mm) to the 55-100mm lens and infinity focus was possible. I was astonished that 0.55mm would make a difference in infinity focus. If you look at the lens while rotating the focus ring from infinity backwards and try to mimic the small amounts of turn you need to achieve critical focus you can see the distance the front part of the lens telescopes in and out. The slightest movement of the ring shows how little the front element moves to change the focus when near infinity.

Thus a small difference in flange to flange distance between the two adapters (14.15 less 13.60mm) being 0.55mm could indeed make a difference in the focusing, if the lens was already at the end of its focusing ability, it could affect its ability to achieve infinity.

Conclusion

The problem might be specific to the lens I have but I have now dedicated an adapter to the lens that allows infinity focus.

My other lenses have sufficient focusing abilities beyond infinity to make this a non-issue for those lenses.

I hope this clarifies my observations and experience with the adapters, which I thought might be helpful to the forum users.

Once again thanks to everybody's contributions and willingness to help me get clarity on this issue which helped me to enjoy this great lens once again.

Your lens could probably use some internal adjusting. Most older mechanical lenses have adjustments for infinity. Often there are 3 screws hidden under the rubber focusing ring. Loosening the screws allows adjustment for infinity focus. It is best for a lens to focus slightly past infinity, because lens focal lengths vary wit temperature. Re tighten the 3 screws to lock in your adjustment.

The two shorter adapters were likely built that way to give a little room for error. The lens will lose a little close up capability, who's is preferable than not focusing to infinity. Good luck.
Thanks,
barondla

[672]

I second the above idea, remove rubber ring at focus, and mess with the focus screws until you have the infinity where you like it , thats what those screws are for. Do have coffee and patience and time when you do this with a zoom lens, as you will want to check it at the wide end, middle, and tight end on a zoom lens.

Also , do your adaptors match, all same brand? All originals from Pentax? Some third party ones have been known to sag under loads like the p67 55-100mm. Sag could even create a small "tilt" effect , where the top of your image will be not quite at infinity focus but the bottom will......

[RICHARD L.]

Knowing is always empowering. Once you understand the cause of the problem you can easily correct it. Always keep a matched adapter to this particular lens and the issue disappears.

Regards

[MossyRocks]

Thanks for the shout out!

This is the sort of thing that you always seem to be well versed in and could properly answer instead of others just speculating or trying to remember something technical that they don't fully remember the details of.

[TDvN57]

I found the focusing screws under the rubber ring, started fiddling with them and decided against continuing because combined with the zoom action I realized I was adjusting something I did not understand and was not willing to potentially mess up my lens just to be able to learn how the smaller details worked. But many thanks for the sound advice.

I had already skimmed down the 14.15mm adapter to +/-13.6mm which means that any of my three adapters will now work properly on the lens. For good measure I took the third adapter that measured 13.6mm and skimmed it down to 13.0mm, which is now dedicated to this lens. Infinity and beyond works fine and near focus is at 450mm, which us still nearer than the specified 600mm.

All is well and I thank everybody for contributing for a happy ending.

It is the exact same flange distans.

[Bob L]

......and near focus is at 450mm, which us still nearer than the specified 600mm.

150mm closer than the spec is huge. The minimum focus distance (close focus) is referenced to the sensor/film plane.....is your measurement from the front of the lens, normally termed the working distance, by any chance?