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03-02-2010, 06:44 PM   #1
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Full Frame lens vs DA & crop factor comparison...

I should explain that im a bit of a newb to the digital SLR world, having had a small break from SLR photography...

Anyway, given i had some Pentax film cameras, a couple of okay pentax mount lenses and the fact that the K7 seemed like a good camera - here i am back iin the digital world.

One thing im getting my head around is the lens and aps-c cropping factor issues - i understand that my old 30-80 zoom equates to 45-120 for the K7 etc though...

What I need explained to me is if there is any difference between the focal lengths on the new DA lenses and the full frame lenses. Up to now, ive been steering more toward buying new FA series lens so that i have some compatibility with my old film cameras (just in case i feel like taking some slides, or my partner and i going on an old B&W film hip-n-shoot session with a film camera each). At the same time i realise the K7 will take precedence for lens buying (though imo, technology advances dictates affordable full frame DSLR coming out in the not too distant future).

My query is this:
If i put my FA50mm on the K7 - its effectively cropped to 75mm;
If i put a DA50mm on the K7 - will it appear the same effective 75mm also?

Im trying to work out a nice suite of primes/zooms, and the focal length vs crop factor FA/DA etc has been a little confusing, thanks in advance!

03-02-2010, 06:53 PM   #2
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Yeah all the lenses retain the 35mm equivilency so a DA50 and an FA50 will have the exact same FOV on your film and digital SLR's.
03-02-2010, 07:33 PM   #3
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QuoteOriginally posted by JayR Quote
My query is this:
If i put my FA50mm on the K7 - its effectively cropped to 75mm;
If i put a DA50mm on the K7 - will it appear the same effective 75mm also?
Yes to both. The field of view of a 50mm lens on an APS-C DSLR is that of the field of view if a 75mm lens on a 35mm SLR.

I hope this helps:

https://www.pentaxforums.com/forums/pentax-lens-articles/90477-crop-factor-fo...ield-view.html
03-02-2010, 08:30 PM   #4
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Yes, the FOV on both 50s is the same -- on APS-C, equivalent to 75mm on your old 35/FF, so they're short portrait teles. The DOF at any specific f-stop and shooting distance is the same. The difference is that the DA throws a smaller image circle, with severe vignetting on a 35/FF frame. If you ever expect Pentax to produce a FF dSLR that you'd buy, you'd be unhappy having stocked up on DA lenses. That's where I am, so my only DA's are 10-17, 18-55 and 18-250. Otherwise I have FA's and equivalent FF glass.

Partly this derives from my film experience with the wonderful Olympus Pen-FT, a gem of a 35/HF SLR, with frames the same size as APS-C sensors. When the OM series came out, my old glass was incompatible. I'll try not to let that happen again.

03-02-2010, 09:15 PM   #5
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Thanks, glad to know I had worked it out - all this scanning of the lens discussion threads!

I think i was just badly informed early on...

Now to get a pile more primes!!
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