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Author Topic: Pentax Medium Format  (Read 3010 times)
Robert Brown
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« on: January 28, 2006, 01:14:44 AM »
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Hello-
Recently read Michaels' article on using Pentax 67 lenses on his 645 via adapter.
I have long wanted a 35mm SMC for my 645, (ebay-wise, most are just outside the budget limits and also kinda rare), and after reading Michaels' article, I'm considering a 67 45mm f4 instead (somewhat more readily available and "sometimes" a little cheaper).  I have the adapter (67 to 645); I'm aware that the 67 lenes are bigger and bulkier than the 645's (thats OK).  If I read the article right, (says a 300mm 67 lens equates to a 180mm on the 645 -- using a conversion factor of .6x), then a 45mm 67 lens would figure out to about a 28mm on the 645.  Now,  for the question:
Anyone tried this combo (45mm 67 lens on the 645 body) and did it work OK (use is primarily for landscape and nature, transparency film, always tripod mounted).  Do you get the equivilant of a 28mm and how is the distortion?  Would appreciate any info regarding this setup.  
Cheers
Bob
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bob mccarthy
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« Reply #1 on: January 28, 2006, 11:39:54 AM »
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† If I read the article right, (says a 300mm 67 lens equates to a 180mm on the 645 -- using a conversion factor of .6x), then a 45mm 67 lens would figure out to about a 28mm on the 645.† [a href=\"index.php?act=findpost&pid=56975\"][{POST_SNAPBACK}][/a]

I think your math is backwards. I haven't read the article but a 300 (or 45) is the same FL on any camera it's mounted.

The difference is the 645 is a smaller format and "crops" the inside of the image circle more than the 67 does. This always gives the "effect" of a longer focal length.

a 45 (6x7 lens) mounted on a 645 would lengthen from a "very wide" angle of view to a "wide" angle of view.

Its the identical discussion as is commonly heard in 35mm form cameras i.e. using a lens on the canon full frame (1Ds) vs using the same lens on a crop camera (20D). A 24 on the larger sensor is wider than the 24 on a APS-c (roughly 35mm equivalent).

If you want a 35mm lens for your 645 you have to buy a 35mm lens.

Hope this makes sense!

Bob
« Last Edit: January 28, 2006, 01:13:23 PM by bob mccarthy » Logged
Robert Brown
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« Reply #2 on: January 28, 2006, 12:30:58 PM »
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Hi Bob
Thanks for the reply-
That is what I thought also -- you can't change the focal length of a given lens: a 45 is a 45.  I hope Michael doesn't mind, I'm going to paste a small section of his article (its from his review of the Pentax 645NII) here to make it eisier to find.

From the essay:
Greater Magnification: When using long lenses there's the greater magnification. To convert to 35mm terms a 67 lens' focal length needs to be multiplied by .5X. A 645 image needs a factor of .6X. So a 300mm Pentax 67 lens (which is like a 150mm on a 35mm camera) become the equivalent of a 180mm lens on a 645. Not a big deal, but helpful. (Of course cropping a 67 image to 645 dimensions accomplishes the same thing, but you know what I mean.)
 
 Focus Confirmation: When used with non-autofocus lenses the 645NII provides a focus confirmation light ó even with 67 lenses. This can be very helpful in finding critical focus, especially in low light levels.
 
 Using the Sweet Spot: All lenses perform better toward the center than at their edges. By using Pentax 67 lenses on the 645 format one is using the lens' "sweet spot", or central area, which exhibits maximum image quality.
 
The Disadvantages

 Greater Magnification: While having greater magnification is helpful with long lenses it's a hindrance when using wide lenses. My widest, the Pentax 45mm f/4, becomes equivalent to roughly a 28mm instead of a 24mm equivalent lens with the 67 body.  
(end of paste).

I understand that what is changing is the size of the film format. Instead of exposing 7 cm of film, you are using only 4.5 cm, (the 6 cm height of the film is a constant) exposed by the same size image circle -- you get the "sweet spot" of that circle with the smaller film.  
At least that is what I thought I understood untill I read this article -- and then maybe its my simple mind that has trouble with anything beyond 2+2!!!  
It was the last part of article that threw me the curve (NOT out to shoot down the article or Michael in any way please!) and maybe there is a typo there at the end (should be 645 body, instead of 67).  
Anyway- you are probably right about just working towards getting a 35mm for the 645--again thanks for the info.
Cheers
Bob
 



Quote
I think your math is backwards. I haven't read the article but a 300 (or 45) is the same FL on any camera it's mounted.

The difference is the 645 is a smaller format and "crops" the inside of the image circle more than the 67 does. This always gives the "effect" of a longer focal length.

a 45 (6x7 lens) mounted on a 645 would lengthen from a "very wide" angle of view to a "wide" angle of view.

Its the identical discussion as is commonly heard in 35mm form cameras i.e. using a lens on the canon full frame (1Ds) vs using the same lens on a crop camera (20D). A 24 on the larger sensor is wider than the 24 on a APS-c (roughly 35mm equivalent).

If you want a 35mm lens for your 645 you have to by a 35mm lens.

Hope this makes sense!

Bob
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bob mccarthy
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« Reply #3 on: January 28, 2006, 01:19:54 PM »
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From the essay:
Greater Magnification: When using long lenses there's the greater magnification. To convert to 35mm terms a 67 lens' focal length needs to be multiplied by .5X. A 645 image needs a factor of .6X. [a href=\"index.php?act=findpost&pid=57003\"][{POST_SNAPBACK}][/a]

The discussion is using a comparison to 35mm as the baseline. Its a little confusing as to how it stated but I read it as a given lens would give a longer "apparent" crop FL by 20% (.6/.5 = 1.20) when using a 67 lens on a 645.

Bob
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Sheldon N
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« Reply #4 on: January 28, 2006, 01:28:24 PM »
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Not that this is any answer to your question, but I am selling a Pentax 67 45mm lens on Ebay this week. It's item number 7584716276.

It's a no reserve auction and is currently selling for $1.

Sheldon
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tsjanik
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« Reply #5 on: January 28, 2006, 09:10:00 PM »
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Hi Bob:

I use the Pentax 67 and 645 cameras and, according to many people, own an unreasonable number of lenses for both.  Unless you plan on using a 67 body as well, there is no advantage to using a 67 45mm lens on the 645 compared to the 645 45mm lens.  They have identical fields of view, about equivalent to a 28mm on a 35mm camera, as you noted, are optically equivalent from my observations (although the 645 version may have slightly better flare control); however the 645 45mm is smaller, lighter, cheaper and faster than the 67 45mm.  The 645 35mm is approximately equivalent to a 20mm lens on 35mm and, as noted by Bob in his reply, if thatís what you want, thatís what you should get.  I donít think your price estimates are quite correct, on ebay a 67 45mm averages about $400, a 645 35mm is $350-$400 and the 645 45mm is about $250.  Good luck
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Robert Brown
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« Reply #6 on: January 28, 2006, 10:19:54 PM »
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Back for one last stab at this and thanks to all that replied:
Oh Dear!!! you are suggesting that there is such a thing as an unreasable number of lenes you can own for your system?  Well, what do those other people know!?!  (hehe).
After rereading the review from which I started all this for the umteenth time, I finially grasped what Bob is saying about using a baseline comparrison to 35mm format. The fog is beginning to clear.
And for what it is worth, I think I have that 45mm 67 lens bookmarked. . . . .
Thanks again guys and have a great weekend.
Cheers
Bob

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Hi Bob:

I use the Pentax 67 and 645 cameras and, according to many people, own an unreasonable number of lenses for both.  Unless you plan on using a 67 body as well, there is no advantage to using a 67 45mm lens on the 645 compared to the 645 45mm lens.  They have identical fields of view, about equivalent to a 28mm on a 35mm camera, as you noted, are optically equivalent from my observations (although the 645 version may have slightly better flare control); however the 645 45mm is smaller, lighter, cheaper and faster than the 67 45mm.  The 645 35mm is approximately equivalent to a 20mm lens on 35mm and, as noted by Bob in his reply, if thatís what you want, thatís what you should get.  I donít think your price estimates are quite correct, on ebay a 67 45mm averages about $400, a 645 35mm is $350-$400 and the 645 45mm is about $250.  Good luck
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