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02-20-2013, 09:26 PM   #1
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Report -- 645 lenses on K-01

I have an older Pentax 645N, the medium-format film camera. If my new K-01 is a brick, then the 645 is a block. A concrete block with very good lenses.

So I ordered a 645-to-K adapter from Amazon, see Amazon.com: RainbowImaging Pentax 645 P645 lens to Pentax adapter, for K-7, K-X, K-M, K200D, K100D, K10D, K110D, Ist Dl2, ISt DL, IST Ds, IST D etc: Camera & Photo . It is a VERY well-made black-coated brass part that works PERFECTLY, for all of US $53.

First, I tried a SMC Pentax-A 645 45mm F/2.8. Crop factor to Pentax APS-C -- 2.6... 120mm. Tested it against my K-01 50-200mm kit lens, set to the same angle-of-view that the 645 lens showed. Surprise! As far as a kitchen-sink image test goes, the kit lens ain't so bad!

Second test, tried my SMC Pentax-A 645 150mm F3.5. With the 2.6 crop factor, it's 390mm on the K-01. Killer telephoto, cheap! And I think this is the best outcome for 645-to-K01 use. Because all my 645 lenses become tele lenses on the K-01...so I'll end up using a regular 35mm-type Pentax lens, with its 1.5X crop factor, for normal-range manual photography. But the 645 A 150mm is a blast... see the last picture below, a DAYTIME moon pic, handheld, 100% crop, unsharpened.

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02-20-2013, 11:45 PM   #2
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Can you adjust the aperture on the 645 lenses when mounted on your DSLR with the Rainbow Imaging adapter, or are you always shooting wide open? I don't think the crop factor works quite the way that you think, your 645-45mm lens will have nearly identical FOV as a 50mm, 135 format lens on a K-01.
02-21-2013, 12:14 AM   #3
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excanonfd -- yes, you can adjust the aperture... in M manual, you see the effect on the screen, even though the screen doesn't tell you what aperture you are at (same as the user manual describes). The adapter is well-machined... precise... the lenses fit securely. Was happily surprised, I didn't expect it to work so well.

As for the crop factor -- as I understand it, if I have a 50mm 645 lens, and a 50mm 135-format lens, or a 50mm lens designed for APS-C, the FOV will be identical, as you said. Absolutely identical, maybe less a VERY slight imperceptible difference due to the nature of glass.

But! When you put a medium format lens on an APS-C camera, the camera's small sensor only 'sees' a small part of the image that the same lens throws onto 120 film. And what we call the 'crop factor' is a number that tells us what kind of a telephoto effect this will produce. The image on the APS-C sensor is exactly the same as the center part of the same image on a 645 camera.... same FOV, actually, but it LOOKS telephoto.

Let's see. Remember 4x5 Speed Graphics? You could get a 120 roll-film back, and that 4x5 'normal' Graphic lens suddenly became a mild telephoto. And years ago, I took a moldy lens off a wrecked Rolleicord, and stuck it on a Kodak Instamatic, and had a wonderful 'telephoto' Instamatic.

What do you think? Am I describing all this halfway accurately?
02-21-2013, 08:47 AM   #4
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the crop factor is just 1.5, i.e. a 300mm FF on APS-C works exactly like a 300mm 645 lens on APS-C. (I've tried.)
Some of the 645 lenses do hold up surprisingly well indeed (the A* 300mm lens in any case did).
The lenses are a bit heavy for these bodies, but I have used the Macro lens on a K-x several times.

02-21-2013, 11:21 AM   #5
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It's like Smolk says, the crop factor is constant 1.5x on an APS-c sensor whether you use a 67, or 645 or 135 format lenses of equal focal length. A 645-45mm lens don't become equivalent to 120mm on your K-01, it show an equivalent FOV of 68mm - take a look at the exif file on the images, the crop factor is 1.5x. The subject of crop factor has been covered so often and in depth in the forums, that it's best if you don't think about the crop factor at all and accept all lenses at their stated focal lengths, regardless of which format the lenses originate from.

Can you post pictures of the Rainbow Imaging adapter's innards, I would like to see how this adapter let's you work the aperture on a 645 lens because the vast majority of adapters on eBay, cannot close down the aperture and have to shoot wide open. The genuine Pentax adapter has no such limitations but if a third party adapter that cost only about 50 bucks can do the same, that is great news indeed.
02-21-2013, 03:09 PM - 1 Like   #6
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Thanks guys for the crop factor info. Will test it some more!

excanonfd -- see pix below of the adapter. You are way ahead of me on this... at my end, all I know is that in M Manual it shows what happens when you turn the Aperture ring... the screen gets lighter or darker. Hope these pix tell you what you need! The top one shows this little lever on the inside, and a view from the back. On the second one, it is rotated at 120-degree spacing, so you can see all of the inside.
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02-21-2013, 03:41 PM   #7
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Thanks for the pictures, that little protrusion inside the adapter is what enables the aperture ring - without it, no aperture control. Do you have the link to the store or was it on eBay from Rainbowimaging?
02-21-2013, 06:00 PM   #8
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Here's where I bought it -- on Amazon -- http://www.amazon.com/RainbowImaging-Pentax-adapter-K200D-K100D/dp/B0044WSQNS/ -- US $53.

Now, regarding the crop factor ... I'm still not convinced ... still think that the '1.5X' refers only to 135 lenses used on an APS-C camera ... and I still think that the crop factor for 645N lenses is 2.6X, when used on an APS-C cameras.

So! Put your bets down, gentlemen -- I will run a DEFINITIVE test tomorrow to solve this intriguing controversy! Have to wait til tomorrow, since that's when I'm getting an 135-format 28mm f/2.8 lens... stay tuned!


Last edited by jon404; 02-21-2013 at 06:06 PM.
02-22-2013, 03:33 PM   #9
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Excanonfld and Smolk -- about the 645 to K-01 crop factor -- you were right at 1.5, and I was wrong when I said 2.6. And, as you said, 1.5 is the crop factor for both 645 and 135 lenses used on the K-01.

Tested this today. The first image below shows the 18-55 zoom, set at 50, compared to a 135 50mm f/1.4. The FOV of both images is the same. The K-01's APS-C 1.5 crop factor gives a picture equal to using a 75mm lens on a 135 FF camera.

The second image shows the 50-200 zoom, set at 150, compared to a 645 A 150mm f/3.5. Again, the FOV of both images is the same. And the APS-C 1.5 crop factor gives a picture equal to 225mm on a 135 FF camera.

For us, 1.5 is the crop factor for any 645 or 135 lenses that we use. So -- as you said, "it's best if you don't think about the crop factor at all and accept all lenses at their stated focal lengths." Thanks for helping me work this through!
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