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03-03-2010, 08:42 AM   #1
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old 645 lenses sharp enough for 40mp digital?

I don't remember seeing this asked, but the mention of only a few lenses like the 645 120 Macro being really sharp when used w/ the K20D makes me wonder...

03-03-2010, 09:07 AM   #2
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I'm not sure where you saw that... I suppose time will tell.

In general, though, lens resolution becomes progressively more important as the frame size is reduced. An easy way to think about this - as the camera gets smaller, so do the line pairs you need to distinguish - everything gets tinier.

So, a lens that needs to cover a 120 frame 56mm across with 2,400 lines - resolve 43 lines/mm - would need to resolve the 2,400 lines across 24 mm (35mm frame) instead, or 100 lines/mm with 35mm. This is why the obsession with resolution numbers is largely a 35mm phenomenon.

*actually, if looking at 645, the short dimension is 45mm - so the mf film lens needs 53 lines/mm, to make this a fair comparison to 35mm. But then, APS-C size sensor is yet smaller... though will the digital 645 sensor be larger than a 35mm full frame?

The Pentax APS-C has a short dimension of 15.6mm, so in the above example the lens needs to resolve 154 lines/mm.

Looks like 645D has a short dimension of 36mm, so in the above example the lens needs to resolve 67 lpmm.

Therefore I expect existing 645 lenses to be sharp enough optically, though what other issues there may be with sensors instead of film (angles of acceptance etc) who knows?

ps. I picked 100 lpmm for 35mm because of an old article in Modern Photo, where they were attempting to see if 35mm lenses could achieve medium format quality on film. A few lenses did manage that, with the best films available at the time.

Last edited by Nesster; 03-03-2010 at 10:11 AM.
03-03-2010, 12:24 PM   #3
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Thanks, Nesster. I got that impression from seeing what people posted when they mounted a 645 lens onto the K20D. The only results I've been impressed with are the ones from the 120 Macro (even looked for one for a while before I found my Kiron :-)
The LPM value makes sense...it's used to derive the max resolution you can get onto the size of a sensor before diffraction sets in as well I think.
03-03-2010, 02:58 PM   #4
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Several P645 lens are super sharp such as the FA 35mm, 120mm and 300mm. More importantly there are really no "dogs" in the available lens for us.

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