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11-08-2020, 09:11 PM   #1
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Weird question re. 645 and 67 mount adaptation

Hello all,

I know this is a very weird question and I am pretty sure that the answer will be 'no such thing exists without creating it as a custom build just for you'!

I make use of the 645 1.4x converter with the 645 600mm f5.6 A lens. I also have the 67 1.4x converter, which I have used with the 67 400mm f4 EDIF lens (amongst others). I have the impression that the 67 converter is optically superior (though I can't prove it) - this is based on usage in cases where I can use either converter (i.e. when using converters with 67 lenses on the 645Z/D). I wish I could use the 67 1.4x converter with the 645 600mm lens. But that would involve having a mount adaptor that works in the opposite direction to the usual - in other words, converting the lens mount UP in size rather than down, so that I can mount a 645 lens on a 67 1.4 converter (which, of course, is then converted back down in size to be mounted on the 645Z/D). I am guessing, since there wouldn't be much call for such a device, that it doesn't exist. Unless someone knows better......?

Ed


Last edited by Ed Hurst; 11-08-2020 at 09:36 PM.
11-08-2020, 10:20 PM   #2
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Haven't seen an adapter like this. Would it even focus to infinity? Registration distance is longer when adapting 67 lenses to 645 body. This allows space for the adapter.

Wouldn't going from bigger 67 teleconverter to smaller registration 645 lens eliminate room for an adapter and make infinity focus impossible?

Thanks,
barondla
11-08-2020, 10:28 PM   #3
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QuoteOriginally posted by barondla Quote
Haven't seen an adapter like this. Would it even focus to infinity? Registration distance is longer when adapting 67 lenses to 645 body. This allows space for the adapter.

Wouldn't going from bigger 67 teleconverter to smaller registration 645 lens eliminate room for an adapter and make infinity focus impossible?

Thanks,
barondla
I don't know! Good question...
11-08-2020, 11:02 PM   #4
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Can you clarify how you use either adapter? Is this the setup?

A) Native 6x7 lens -> 1.4 6x7 TC -> 6x7/645 adapter -> 645z body

B) Native 6x7 lens -> 6x7/645 adapter -> 1.4 645 TC -> 645z body

Or is there a different arrangement?

11-08-2020, 11:59 PM   #5
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At present, I use both of those set-ups. I also use the 1.4 645 TC on 645 lenses (which, of course, requires no adaptors).

But I am looking to do the following:

C) Native 645 lens -> 645/6x7 adaptor -> 1.4 6x7 TC -> 6x7/645 adaptor -> 645Z/D body
11-09-2020, 05:15 AM - 1 Like   #6
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Ed, I can without any doubt confirm that the latest 67 grey 1,4X is optically vastly superior to the 645 1,4X converter. I have tried these instruments off and on for more than 10 years. Even the 67 2X converter is very good while the 645 2X is a dismal failure.

Regards
11-09-2020, 06:01 AM - 1 Like   #7
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Hi,

I have not seen such an adapter, and I have been eyeballing all sorts of Pentax 645 and 67 things for the past few months.

I am an engineer, but not an optical one, so I might be missing something here. That said, I don't see such an arrangement as working all that well. Seems to me that the lashup would push the lens too far forward.

I have the 645 A* 300/4 and both the 645 1.4x and 2x TCs. I have used the 2x on a couple of occasions and note it isn't all that good. About the same as my Sigma 150-600 on 135 format at 600mm. So, for both, I can be kind and say it's better than no 600mm at all.

That has me rethinking. As in switching to the 67 300mm and those TCs and just using the adapter I already have for the 645D.

And, so I think that this is the answer here....

But if there is such an adapter to use the 67 TCs with the 645 lens on the 645 body, and I.am off base in my thinking, count me in as well.

Stan
11-09-2020, 08:49 AM   #8
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The key to unraveling this is trying to determine how far the elements of the 6x7 teleconverters are spaced from the lenses optical point of focus. If they are the full flange focal distance from the rear of the lens, it should be possible to build an adapter that is the correct distance for the 645 lens, but my knowledge of optics is very rudimentary. I'm unclear if that would subsequently change the distance the light focused at after going through the teleconverter, which needs to remain unchanged to work with the 6x7 to 645 adapter.

I think someone with enough optics knowledge could clearly answer the distances question. Then you could 3d print a test adapter to verify the function using a lighter lens. Then a custom metal adapter would need to be fabricated. But a potentially easier way might be to use a helicoid adapter. This would allow you to set different offset distances between the 645 lens and the 6x7 teleconverter to determine the best distance. Then you could either get a fixed tube of that length to build the adapter from.



645 lens - 645/helicoid adapter/6x7 - 6x7 teleconverter - 6x7/645 adapter - 645z

11-09-2020, 08:58 AM   #9
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You could perhaps mount the optical unit of the 67 converter into an 645 extension tube or helicoid.

11-09-2020, 09:55 AM - 1 Like   #10
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Another option would be to use a 67 800mm EDIF lens on the Z instead of trying to use a 600 + 1.4X converter. Although the 800 EDIF is not a cheap option.
11-09-2020, 10:06 AM   #11
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QuoteOriginally posted by desertscape Quote
Another option would be to use a 67 800mm EDIF lens on the Z instead of trying to use a 600 + 1.4X converter. Although the 800 EDIF is not a cheap option.
Good point. Here's one for sale on eBay, no affiliation with the seller, nor endorsement implied.

Rare Pentax 67 M*800 f6.7 ED (IF) With Rear converter & Case Excellent! | eBay

2 hours left...

It is slightly slower than the 600 f4 plus a 1.4x, but the expected CA is lower.
11-09-2020, 03:18 PM   #12
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The 645 1.4x Rear Converter works really well on my FA 300 F5.6. Very sharp. Might post some comparison photos when time permits.
Thanks,
barondla
11-09-2020, 03:27 PM - 1 Like   #13
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I've been eyeing that 67 800mm EDIF lens for years, but not in a position to make such a purchase at the moment! Wish I were...

Of course, with that lens in hand, it can be used with its dedicated 1.4x converter and get something even longer...
11-09-2020, 04:55 PM   #14
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I've also been keeping an eye on that 800mm. My reservation is that I don't know how practical it would be. At f6.7 you are already at a disadvantage to focus it, add the 1.4 TC and you will be at f11, which from experience on other lenses is a true challenge to focus.

So I'm wondering what is the practical use? Birding or astro or perhaps the rare landscape at a far distance. Making a large panorama with a 800mm lens will be another challenge because of the difficulty of placing each frame.

Hence my reservation.

So lately I was wondering if I get the 6x7 400mm, add the 6x7 TC 2x, then I'll be at 800 f8, which is only a half stop worse than the 800mm lens at f6.7. Price, weight, ease of use and versatility makes for a strong case in my head.... Or not... :-)

I've even considered the 6x7 400mm with the 6x7 TC 1.4 as an alternative to my 645 A* 600mm, because of less weight, etc when traveling. The 600mm on a monopod has become my preferred setup for wildlife, works really well for me.

Last edited by TDvN57; 11-09-2020 at 05:01 PM. Reason: Finger trouble typos and adding stuff
11-09-2020, 05:37 PM - 1 Like   #15
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I have already the 67 400mm f4 EDIF, 67 1.4x and 2x teleconverters and the 645 1.4x and 600mm f5.6 lens. So have been able to make some of those comparisons, Theuns.

For me, the 2x converter (while "ok" in quality) doesn't make sense to use. I find using the 1.4x and then up-rezzing the resultant file is actually superior. It seems that the 1.4x converter is the sweet spot and once you get to 2x, it loses more than it gains.

So, in practice, that means for me that the 400mm f4 with 1.4x converter (which gives 560mm) is a good option. Once I want more than that, the 600 (with and without the 1.4x) comes into play. There isn't much difference in quality between the 400mm with 67 1.4x converter and the unconverted 645 600mm lens. But the 645 with1.4x converter is better than the 400mm with the 67 2x converter. BUT because I know the 67 1.4x is better than the 645 1.4x converter, I wish I could use it with the 645 600mm lens!

If I got the 800mm lens, I wouldn't use it often. But I do find a surprising number of situations in which the 645 600mm with 1.4x just isn't enough. So it would be a nice one to have. It will have to wait though, as I cannot afford any major purchases for the foreseeable future!
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