[TDvN57]

Does anybody know the actual FFP distance for the 645z? The only information I get is for the 645 and checking some older 645 lenses and also 6x7 lenses with the Pentax converter I come to the conclusion that the FFP for the 645z might be a bit different from the 645 film series.

Here is the information I have:

P67 FFP distance is 85mm (although I found one source quoting it as 84.94mm).
P645 FFP distance is 70.87mm

That means the P67 to P645 converter should be 85-70.87 = 14.13mm flange to flange.

I have three adapters, two measure at 13.6mm and the third at 14.15mm. Seems the first two are out of tolerance. But not so. On the 645z my P67 55-100mm cannot focus to infinity on the 14.15mm converter, but only on the two 13.6mm converters. I managed to skim the third converter from14.15mm to 13.5mm and now I can get infinity focus, only just. Another 1mm would have been better, but it seems the maximum you can skim off on the lens side without affecting the functionality of the adapter is around 0.65mm. Skimming off on the camera side is too risky and may affect the aperture controls.

If you are familiar with the P67 55-100mm lens you will know that focusing beyond about 200m becomes a challenge because the "throw" of the focusing ring gets less as you approach infinity. On a side note I am considering skimming one of the shallow converters down from 13.6mm to 13mm which will improve the focusing of the lens nearer to infinity. If you are not familiar with lens, well let me say it is a fantastic lens, a little on the blue side, but boy oh boy you have your job cut out for you to focus that thing on long distance objects. I hope to partially remedy that with the skimming efforts.

Do these observations imply that the actual FFP distance for the 645z could actually be less that the old film 645 of 70.87mm by a few mm? This is the only conclusion I can reach so far. I'm not ready to stick a measuring device (i.e. stick) into my camera to measure the distance.

Anybody with similar observations?

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Last edited by TDvN57; 01-05-2021 at 07:13 AM. Reason: Changed 200mm to 200m which was originally intended.

[TDvN57]

UPDATE:
I just cleaned some dust from my 645z's sensor and used a lollipop. Then I realized it is an excellent stick to measure the FFP distance. So I did the best I could with two hands, out of date reading glasses and calipers. My measurement comes to 67.4mm.

That is 3.47mm less than the 645 film (70.87mm). My measurement is not super accurate and I would say probably +/- 1mm. Still gives a good indication and confirms my earlier observations.

[Paul the Sunman]

70.87mm is the official Pentax 645 flange focal distance. I don't see how it could be different for film and 645D/Z, as the lenses are the same.

[ProfessorBuzz]

UPDATE:
I just cleaned some dust from my 645z's sensor and used a lollipop. Then I realized it is an excellent stick to measure the FFP distance. So I did the best I could with two hands, out of date reading glasses and calipers. My measurement comes to 67.4mm.

That is 3.47mm less than the 645 film (70.87mm). My measurement is not super accurate and I would say probably +/- 1mm. Still gives a good indication and confirms my earlier observations.

Keep in mind there is an IR/UV blocking filter in front of the sensor, and there is cover glass above the silicon sensor itself, so that will add some mm.

[kaseki]

Keep in mind there is an IR/UV blocking filter in front of the sensor, and there is cover glass above the silicon sensor itself, so that will add some mm.

And possibly this sensor is base illuminated, so the actual (thin) photodetection zone would be somewhere inside. Also, any materials between the sensor plane and the lens will have an effective thickness determined by the speed of light in the material, so that correction would have to be included.

What is the flange to flange thickness of the official Pentax adapter in the context of the values reported above? I can't look for mine at the moment.

[TDvN57]

70.87mm is the official Pentax 645 flange focal distance. I don't see how it could be different for film and 645D/Z, as the lenses are the same.

I could not find any published data about the flange focal distance for the 645D or the 645z. The published data all seem to pre-date the digital versions. Thus assuming the digital cameras have the same dimension as the film cameras would be an assumption, unless it is published somewhere.

The comments about other layers of filters etc in front of the sensor focal plane would make a difference in a measurement, I agree. Not sure if it would be 3mm of layers.

[MossyRocks]

The comments about other layers of filters etc in front of the sensor focal plane would make a difference in a measurement, I agree. Not sure if it would be 3mm of layers.

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.

[stevebrot]

I could not find any published data about the flange focal distance for the 645D or the 645z. The published data all seem to pre-date the digital versions. Thus assuming the digital cameras have the same dimension as the film cameras would be an assumption, unless it is published somewhere.

.
Ummmm...yes, the published data all seem to pre-date the digital bodies, just like the published data on the K-mount, F-mount, and M-mount. If the specification for the camera says "Lens Mount: Pentax 645A mount..."* or 645AF or 645AF2, the specification flange focal distance is 70.87mm.

If you are concerned about the effect of a variance in flange focal distance on your photos, let's put those fears to rest. The only issues would be ability to attain infinity focus or attaining exact published magnification for macro. Manual focus and auto focus will not be affected. If your adapters don't work, that is something to take up with the makers of the adapters; alternatively, perhaps it is the 6x7 lens that does not focus to infinity.


Steve

* From the manual for the original Pentax 645 film camera, page 68

[texandrews]

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.

[TDvN57]

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.

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.

[stevebrot]

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

Then the problem is likely with your P67 55-100. Do you have a 6x7 body to check it on? Alternatively, a good lens tech can check its collimation off-camera.


Steve

[TDvN57]

Then the problem is likely with your P67 55-100. Do you have a 6x7 body to check it on? Alternatively, a good lens tech can check its collimation off-camera.


Steve

That will have to wait until after Covid.

I am curious about other users of the lens, if they have similar issues.



I'm not

[kaseki]

I believe the optical thickness is n times the physical thickness, where n is the index of refraction, so 2 mm of stuff might indeed be equivalent to 3 mm of air. (n for glass varies from 1.4+ to 2.0.)

[RICHARD L.]

I am curious about other users of the lens, if they have similar issues.

Hello Theuns,

I don't seem to suffer any infinity focusing problem with my set-up (645Z + '67 to 645' Pentax adapter + 67 M 55-100 mm f/4.5 zoom). All pictures on tripod.

100 mm

75 mm

55 mm

REGARDS

[TDvN57]

Hello Theuns,

I don't seem to suffer any infinity focusing problem with my set-up (645Z + '67 to 645' Pentax adapter + 67 M 55-100 mm f/4.5 zoom). All pictures on tripod.

100 mm

75 mm

55 mm

REGARDS

Thanks Richard, next time you are there can you test to see if you can focus on the trees on the horizon at a wide aperture?

BTW, you are blessed to live in a beautiful part of the world. Cold yes, but picturesque.