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01-14-2007, 04:39 AM   #1
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6x7 Lenses

Hi, can anyone help here. What happens if you fit a 6x7 lens to a DSLR (istDS) with the appropriate adapter? Does the focal length change and if so by what factor( c.f 1.5 for film lenses)?

Thanks

Alan

01-14-2007, 07:06 AM   #2
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The focal length does not change. Changing the size of film or chip never changes the lens. It does crop the image. The lens is still projecting the same size circle into the camera. The smaller chip or film just is not capturing it all. A 165mm 6x7 lens will look just like a 165 35mm lens on a digital camera. Hope this helps.
thanks
barondla
01-14-2007, 08:59 AM   #3
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but you might get a sharper image as you are utilizing the center portion of a lens that is built for a lot larger image circle than a 35mm.
as barondla stated.. 100mm=100mm. it's the math.
01-14-2007, 09:14 AM   #4
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I think y'all mis-understood his question. He's just wondering what the crop factor would be if he used a 6x7 lens on a a DSLR with an aps sensor.

ie, 35mm lens has a 1.5x etc, etc.

he wants to know how much of a crop it would be if he used 6x7 lens.

he mentioned focal length, probably because it's always listed ie 50mm on a dslr is 75mm...etc.,

I found this site that explains the difference in sizes between 35mm and 67mm

might help you out a bit

01-14-2007, 10:06 AM   #5
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The crop factor would be 1.5X, same as with a 35mm format lens.
01-14-2007, 10:32 AM   #6
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Equivalent focal length-equal field of view

Not hardly! Try photonet. 'Factors' at bottom of charts.

Last edited by jfdavis58; 01-14-2007 at 10:34 AM. Reason: Clarification
01-14-2007, 10:33 AM   #7
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QuoteOriginally posted by jfdavis58 Quote
Not hardly! Try photonet.
\


oops..that was the link i was going to originally post....but I forgot to paste it before hitting submit....
01-14-2007, 11:01 AM   #8
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QuoteOriginally posted by roy Quote
but you might get a sharper image as you are utilizing the center portion of a lens that is built for a lot larger image circle than a 35mm.
as barondla stated.. 100mm=100mm. it's the math.
I am not an expert on this, but I know many would argue that the absolute image quality of medium format lenses is usually less than that of 35mm lenses, and presumably that the quality of digital-only lenses is (in general) another step up.

The reasoning is that the larger the format the greater the coverage required, so in principle the more expensive it is to achieve (no surprise there - you don't get anything for nothing).

On the other hand, in order to maintain the price of medium (or large) format lenses within reasonable (though expensive) bounds, compromises are made which mean that the resolution (in terms of line pairs per mm) is lower. Providing that the reduction in lp/mm is far less than the increase in frame size then the resulting image is still benefitting from the larger format.

It does however mean when using a lens designed for one format on a smaller format, the quality of the image may be slightly lower than would be expected from a lens designed for that format.

This however strikes me as an equivalent of pixel-peeping, and it is probably far better to judge the results and work out whether the compromise was worthwhile.

Simon


Last edited by Simon; 01-14-2007 at 05:01 PM. Reason: typo correction
01-14-2007, 12:23 PM   #9
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good reply simon. that's why i said ''might'' as i was not sure.

i don't think 'crop factor' means anything here as the actuall image on the sensor will be the same wheather a 35mm lens is used or a 6x7 lens is used. a 300mm lens in either format is still a 300mm lens..
01-14-2007, 06:28 PM   #10
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QuoteOriginally posted by jfdavis58 Quote
Not hardly! Try photonet. 'Factors' at bottom of charts.
So what is the crop factor, then? Your link doesn't refer to 6x7 lenses.
01-18-2007, 08:21 PM   #11
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After getting my K10D I spoke with customer service at Pentax about using 645 and 67 lenses. They said that the K10 was tested with the medium format lenses and there was NO problem with focus and sharpness.

A medium format lens has to be sharp over a much larger area than a 35MM or DSLR that combined with the larger format is the reason studio pros use them. Look at the ads in a fashion magazine or the portrait work of Annie Lebovitz to see just how sharp they can be. Also look at the price of a Hasselblad or any other Mdeium format lens. US$ 3000+ for some Hassy glass.

Focusing on a CCD as compared to film. What is this misinfo about the light rays have to hit the CCD straight on, where as film is tolerant of the light hitting at an angle. Go back to high school physics. light focuses at a single point at an angle. The reason there are lenses designed for Digital as opposed to film are two fold. 1. Because the CCD is smaller than than the area of film, the lens does not have to cover as large an area. Therefore it can be smaller and the design is different. 2. The CCD is very reflective compared to the surface of film so the digital lens must be coated differently to prevent internal reflections the cause of low contrast. A top notch 35MM lens will be an effective digital lens, a poor film lens will be a very poor digital lens.

Field of view ratios, image crop with non digital lenses. As stated focal length IS focal length. f/ stop IS f/stop. The field of view is reduced as follows.

67 = 60 X 70 mm 2.98 X as large as the 23.5 X15.7mm K10D CCD
645= 60 X45mm 2.55 X as large as the 23.5 X15.7mm K10D CCD
35 = 35 X 24 mm 1.53 X as large as the 23.5 X15.7mm K10D CCD

APS film and CCDs are 25.1 X 16.7 mm

So a 100mm 35mm lens has the field of view of a 153mm on the K10D
100mm 645 lens has the field of view of a 255mm on the
K10D
100mm 67 lens has the field of view of 298mm on the K10D.

REMEMBER the f/stop does not change. How about the 298 mm FOV F/2.8

Hope this helps :-)
01-18-2007, 09:17 PM   #12
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Michael,
Where do you get an adaptor for these lenses and have you tried either 67 or 645 lenses on your K10D? If you have, are there any samples on the net?

Thanks,
01-19-2007, 06:46 AM   #13
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Pentax makes or used to make them. I purchased a lens with one but the lens was bad so it went back. I'm having a hard time finding them.

These lenses really add quite a bit of weight up front.
01-19-2007, 07:03 AM   #14
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KEH has a lot of them for sale. They have a 75 F4.5 SMC (82) / LATE MEDIUM FORMAT SLR MANUAL FOCUS WIDE ANGLE LENS listed for $189. This could be a very interesting option depending on the cost of an adaptor.

Regards,
01-31-2007, 05:51 PM   #15
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"So a 100mm 35mm lens has the field of view of a 153mm on the K10D
100mm 645 lens has the field of view of a 255mm on the
K10D
100mm 67 lens has the field of view of 298mm on the K10D."

Would it be more clear to say this way: A 100mm lens on a K10D has the same FOV as a 153mm lens on a 35mm camera. and the same as a 298mm on a 6x7 camera.

A 300mm lens from any system has the same FOV on the K10D. How big of a system the lens comes from onlhy matters as long long as the image circle it generates at the CCD is large enough to not vignette.
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