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09-06-2010, 12:49 AM   #1
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6x7 lens on K20D....

Guys, I'm thinking about trying a 200mm or 300mm 67 format lens on my K20D via the 67-35mm adapter.

My question is what EQUIVALENT focal length will I get?

Yes, I know the lens focal length will not change on DSLR body, but I just want to know the image size I'll get compared to similar telephoto 35mm lenses.

I can't find anything on the Web yet, but my own understanding is I'll get about 400mm effective FL using 200mm 67 lens and about 600mm using the 300mm 67 lens, am I correct?

thanks for your input.

09-06-2010, 01:06 AM   #2
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QuoteOriginally posted by igor Quote
Guys, I'm thinking about trying a 200mm or 300mm 67 format lens on my K20D via the 67-35mm adapter.

My question is what EQUIVALENT focal length will I get?

Yes, I know the lens focal length will not change on DSLR body, but I just want to know the image size I'll get compared to similar telephoto 35mm lenses.

I can't find anything on the Web yet, but my own understanding is I'll get about 400mm effective FL using 200mm 67 lens and about 600mm using the 300mm 67 lens, am I correct?

thanks for your input.
The crop factor(1.5) will be the same for 67 as it would be for any other 35mm equivalent on your K20D. So a 300mm becomes a 450mm lens and so on and so forth.
09-06-2010, 08:52 AM   #3
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The 67 lens will basically behave like any other lens of the same focal length. The main difference is that the 67 lens draws a larger image around (and outside of) your sensor -- which gets lost(*).

You'll gain, however, some IQ because you only use the very centre of the image where lens correction etc. is best, even more so than with a FF lens.

(*) the extra image area gets lost, not the sensor
09-06-2010, 09:46 AM   #4
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thanks guys, I think I got the picture.

09-06-2010, 10:04 AM - 1 Like   #5
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As many times as this sort of issue comes up (although rarely with respect to 6x7 in particular), I keep getting a chance to find a clearer way of putting this. The paragraph below is about as succinct as I can currently come up with:

The crop factor is for comparing *cameras*, not for comparing *lenses*. All 300mm lenses - regardless of the format size they were designed for - produce the exact same FOV on any single camera. However, any given 300mm lens will produce a different DOF on different cameras, depending on the "crop factor".

So if you've got two different cameras of two different format sizes, then you can use the crop factor to help you figure out how a given lens will look on one compared to the other. But if you've got only a single camera - or at least, all your cameras are the same format size - then you can forget you ever heard about a crop factor. It won't be relevant to you. At most, make a mental note of it for use in situations when you are comparing lenses with someone else who does use a camera with a different format size.
09-06-2010, 10:32 AM   #6
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I was wondering this the other day, good thread
09-08-2010, 01:39 AM   #7
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I have a Carl Zeiss Jenna 180mm f/2.8 for 6x7 and it acts exactly as 270mm on my k20D
09-08-2010, 04:46 AM   #8
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thanks for the comments guys, BTW, I have the same Zeiss 180/2.8 for Nikon mount, I think it's a great lens, too bad my sample isn't very sharp wide open...

09-08-2010, 07:33 AM   #9
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QuoteOriginally posted by hiddenhallow Quote
I have a Carl Zeiss Jenna 180mm f/2.8 for 6x7 and it acts exactly as 270mm on my k20D
You mean the 180 on your K-7 behaves exactly like a 270 would behave on FF . The 180 on your K-7 will behave exactly like any other 180 on that same K-7.
09-08-2010, 05:17 PM   #10
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QuoteOriginally posted by JohnBee Quote
The crop factor(1.5) will be the same for 67 as it would be for any other 35mm equivalent on your K20D. So a 300mm becomes a 450mm lens and so on and so forth.
NO! It's still a 300mm lens! It'll behave exactly like any other 300mm lens on the K20D.
09-08-2010, 08:01 PM   #11
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QuoteOriginally posted by kristoffon Quote
NO! It's still a 300mm lens! It'll behave exactly like any other 300mm lens on the K20D.
Because all Pentax's use a slightly smaller sensor than 35mm film, the lenses have an angle of view equivalent to a lens on a film body with 1.5x the focal length. Effectively, this means that 300mm becomes 450mm on a K20D.

Having said this, I mentioned that a 6x7 lens would be subject to the exact same rule as any other lens on a K20D(no exception).

My post probably wasn't very clear about that.
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