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04-19-2012, 05:50 PM   #1
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Film 50mm equivalent on the k7

Hi, I was wondering if anyone can help me out here. Is there a formula for this?
I've been shooting my program plus lately and I've been loving the 50 focal length on my film camera. My question is what is the equivalent of a 50mm on a k7 aps-c sensor?

Thanks

04-19-2012, 05:56 PM   #2
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That would be 35mm. Divide the film focal length by 1.5 to get the corresponding APS-C focal length.
04-19-2012, 05:56 PM   #3
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QuoteOriginally posted by kaiserz Quote
Hi, I was wondering if anyone can help me out here. Is there a formula for this?
I've been shooting my program plus lately and I've been loving the 50 focal length on my film camera. My question is what is the equivalent of a 50mm on a k7 aps-c sensor?

Thanks
Image sensor format - Wikipedia, the free encyclopedia

The ratio you're looking for is the crop factor.

Divide the film focal length by 1.53. 50/1.53 = 32.5 Most people round to either 35 or 28, but there's a 30 (sigma) and a 31 (pentax) that are closer. Remember that while the field of view is the same, the depth of field for the same aperture value is deeper/thicker.
04-19-2012, 05:56 PM   #4
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35mm will be close. (35x1.5=52.5mm)

04-19-2012, 06:03 PM   #5
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Thanks for the fast reply guys. I guess my assumption was indeed correct.
04-19-2012, 09:53 PM - 1 Like   #6
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The format-faktor depends on ratios of frame diagonals, and varies with the exact frame sizes. The diagonal (also the 'normal' focal length) of FF is ~43mm. The diagonal of nominal APS-C is ~30mm; for my K20D it's ~28mm. So compared to FF, the actual format-faktor for nominal APS-C is 1.44x and for my K20D is 1.54x. We use 1.5x as a close-enough approximation, and any lens of 28-29-30-31mm is close-enough to 'normal' on a so-called APS-C camera.

We measure the AOV (angle of view) of a lens by what it projects on a frame diagonal. A 50mm lens on APS-C has an AOV very close to a 75mm lens on FF. But for a closer approximation, including DOF, we need to apply the format-faktor to the aperture also. So a 50/1.9 on APS-C has similar AOV and DOF as a 75/2.8 on FF. Shot from the same position, their images (including perspective) will be virtually the same.

Applying this to your question: My Mir-1 37/2.8 on APS-C is closely equivalent to 55/4 on FF. A 35/1.4 on APS-C would be fairly equivalent to 50/2 on FF. To get the effect of a 50/1.4 you'd need about a 35/0.9 -- rotsa ruck!
04-20-2012, 07:47 AM   #7
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Like always, Mr. RioRico thank you for enlightening me(: and that was a good read. Now I understand why some people really aim to get their hands on a FF it's because of the DOF. 35/0.9 . . .All I can say is wow..
04-20-2012, 08:13 AM   #8
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QuoteOriginally posted by RioRico Quote

To get the effect of a 50/1.4 you'd need about a 35/0.9 -- rotsa ruck!

What about a 645D with a 90mm f2.8 ? at least that actually exists.

edit: Arrggh No it wont work either, even the 645D is a 1.36 crop of the old film format.

04-20-2012, 08:21 AM   #9
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QuoteOriginally posted by steve1307 Quote
What about a 645D with a 90mm f2.8 ? at least that actually exists.

edit: Arrggh No it wont work either, even the 645D is a 1.36 crop of the old film format.
and a 75 was the standard on film 645 (pretty much the equivalent of a 43 on 35mm)

a 75 2,8 on the 645D might come pretty close to the FOV of a 55 though
04-20-2012, 10:17 AM   #10
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QuoteOriginally posted by kaiserz Quote
35/0.9 . . .All I can say is wow..
I've actually built one of those. WHAT?!?!? It's like this -- start with these facts:

* An f-stop is the ratio of aperture opening to focal length.
* A dioptre is the inverse of a focal length (divide by 1000 for mm).
* Dioptres are additive.

Put those together using... closeup lenses! More facts:

* A Raynox DCR-250 is +8dpt.
* Its focal length is 1000/8= 125mm.
* Its smallest diameter is ~36mm.
* Its rear thread is 43mm; front is 49mm.

If I stack two 49mm +10dpt optics onto the Raynox, I get 8+10+10= +28dpt with a focal length of 1000/28= ~35mm. The ratio of aperture to focal length is 36/35 or 1.1 for an f-stop of ~0.9. Ha, a 35/0.9 lens! I put a 42-43mm step ring on the back of the Raynox, an M42-PK adapter on that, mount it on the camera, and Voila!

OK, you're wondering, what's the catch? A couple catches. Focus distance from the front element is about 30mm. And except for the Raynox, the whole is not optically corrected. So it's soft and distorted. But hay, a 35/0.9 optic for under US$80!! Wow!

[/me pours icewater over head, cools off]
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