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04-23-2013, 02:47 PM   #1
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6x7 lenses on K-5

I've been considering a 6x7 adapter for astrophotography. Used 6x7 lenses are relatively inexpensive. The K5 would use the center portion of the image circle for maximum sharpness and no coma or other distortion.

I just want to be sure I understand how things work with an adapter like Pentax Adapter K for 6x7 Lens (6x7 to K mount) Reviews - Pentax K-mount Teleconverters and Adapters - Pentax Lens Reviews & Lens Database and, for example, a 6x7 300 f4 lens:
  1. The adapter puts the sensor at the 6x7 register distance, so there's no problem reaching infinity focus.
  2. I don't have to worry about any additional crop factor. I'll get the same field of view as any other 300mm K-mount (450mm equivalent in 35mm terms).
  3. The lens is physically f4, so I'll get the same exposure times as a K-mount f4.
Am I understanding this correctly? I'm least certain about #3.

04-23-2013, 03:06 PM   #2
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You're correct on all counts!
04-23-2013, 05:03 PM   #3
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I would think that the crop from an 6x7 image circle to APS-c would have to change the FOV, even more than it would change going from a FF image circle to APS-c. So the equivalent focal length (in FF-equivalent terms) would have to be much more than just 450mm.

Also, since you are cropping the center portion, you are loosing a lot of light, so you can't possibly get the same exposure times as a K-mount f/4.

Someone, please correct me.
04-24-2013, 05:00 AM   #4
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OK, after one night's sleep, I'm slowly coming around ....

We all know that a 50mm lens on APS-c looks the same, regardless of the lens' projected image circle. In other words, you can't tell if it was designed for FF or for APS-c just by looking at the resulting image, the FOV is the same. Therefore, I must be wrong about #2, and you and Ole must be right.

I haven't come to grips with #3 yet ....


Last edited by Tanzer; 04-24-2013 at 05:24 AM.
04-24-2013, 08:09 AM   #5
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For #3, here's a non-scientific summary. An APS-C sensor with a 6x7 lens only uses part of the lens' image circle. The brightness of that image circle will not change with smaller film/sensor; the wasted parts of the circle cause a decreased field of view compared to 6x7 film.

A theoretical experiment with a K-5 and typical K-mount lens: Set to manual mode and take a photo. Then put black tape over half the sensor and take a 2nd photo with the same settings. The untaped half of the sensor will not lose any brightness between photos.
04-24-2013, 08:21 AM   #6
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And after the novelty of putting 6x7 lenses on an automatic APS-C camera wears off, you'll be back to using your regular lenses.

I have a Pentax 67 camera with lots of lenses. I tried it. What you get is a big, bulky lens, manual focus and stop-down metering. Why? You can get the same results using good older 35mm format lenses and get aperture priority and don't have to deal with an adapter. A better use for them would be with a tilt/shift adapter.

Medium format film lenses, in general, did not need as much resolving power to get sharp images on the larger film size as 35mm format lenses did.

Last edited by tuco; 04-24-2013 at 08:33 AM.
04-24-2013, 08:28 AM   #7
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And all the extra unwanted light in the adaptor won't help sharpness and contrast.
04-24-2013, 11:58 AM   #8
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My original post said I'd be trying it for astrophotography. The camera's going to be on a 40 pound German Equatorial Mount so the 67 lens is the least of my big & bulky concerns. Manual focus is already a given for astrophotography. My main issue with k-mount lenses has been coma distortion with stars near the corners; some otherwise great 35mm lenses are unusable for photos of the night sky.

Since so many threads turn into a call for full frame, those same 67 lenses will give me a wider field of view on the K-3 than on the K-5 Kidding aside, if I get good astrophotography results with the K-5 + 67 lenses I might buy a 67 film body for a really wide field of view.

04-24-2013, 12:02 PM   #9
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QuoteOriginally posted by DeadJohn Quote
My original post said I'd be trying it for astrophotography. The camera's going to be on a 40 pound German Equatorial Mount so the 67 lens is the least of my big & bulky concerns. Manual focus is already a given for astrophotography. My main issue with k-mount lenses has been coma distortion with stars near the corners; some otherwise great 35mm lenses are unusable for photos of the night sky.

Since so many threads turn into a call for full frame, those same 67 lenses will give me a wider field of view on the K-3 than on the K-5 Kidding aside, if I get good astrophotography results with the K-5 + 67 lenses I might buy a 67 film body for a really wide field of view.
Good points. Do you know a member here called, Nightfly? He does a lot of wide field astro photography using a Pentax 6x7. Some of his work is really stunning.
04-24-2013, 08:16 PM   #10
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QuoteOriginally posted by tuco Quote
Good points. Do you know a member here called, Nightfly? He does a lot of wide field astro photography using a Pentax 6x7. Some of his work is really stunning.
I'll search some of his posts for relevant info. Thanks.
04-24-2013, 10:03 PM   #11
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QuoteOriginally posted by DeadJohn Quote
For #3, here's a non-scientific summary. An APS-C sensor with a 6x7 lens only uses part of the lens' image circle. The brightness of that image circle will not change with smaller film/sensor; the wasted parts of the circle cause a decreased field of view compared to 6x7 film.

A theoretical experiment with a K-5 and typical K-mount lens: Set to manual mode and take a photo. Then put black tape over half the sensor and take a 2nd photo with the same settings. The untaped half of the sensor will not lose any brightness between photos.
I agree. I failed to take into account the fact that the field of view is a function of the supported image circle. So, 6x7 lenses gather a wider field of view and more light overall, but the brightness of the cropped area is the same, and the only wasted light is outside the image.

Shame on me, for questioning Ole!
04-25-2013, 08:20 AM   #12
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QuoteOriginally posted by DeadJohn Quote
I'll search some of his posts for relevant info. Thanks.
Here is his Blog Spot and here is his Flickr Stream. He also likes to do long exposures under the full moon. Apparently the best film for astro work was Kodak's E200 and he has a stash he saved up before it was discontinued. Lately, he has been trying out a K-01 he picked up too.
04-25-2013, 06:36 PM   #13
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Hi,
i did own a Pentax-67 M*400mm/4 EDIF for 4 years and was shooting with it on my K20D, great lens, very sharp. I've never used it for taking stars photos but i've read that some used it for moon shots with very good results.
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