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07-07-2015, 10:53 AM   #1
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so how do lens focal lengths work for apsc marketed lenses?

Is it still "cropped"? Like is my 40mm xs actually 40mm or is it a higher equivalent?

I've been wondering this for a while but never asked

07-07-2015, 10:57 AM   #2
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Both. It is 40mm because that's a property of the lens and it doesn't matter what camera it is on. But if are asking if the field of view is the same as you would see from a 40mm on a full-frame, the answer is no. (Because it would be the exact same lens in both cases, but the image circle would be cropped on the APS-C.)
07-07-2015, 10:59 AM   #3
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Focal length is a fixed property of the lens, and the format the lens is designed for doesn't influence its calculation. So yes, your 40mm is a 40mm lens.

Some P&S cameras label their focal lengths in 35mm equivalents, but that's not going to be the case with SLR lenses. TLDR: trust what is written on the lens

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07-07-2015, 10:59 AM   #4
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QuoteOriginally posted by vonBaloney Quote
Both. It is 40mm because that's a property of the lens and it doesn't matter what camera it is on. But if are asking if the field of view is the same as you would see from a 40mm on a full-frame, the answer is no. (Because it would be the exact same lens in both cases, but the image circle would be cropped on the APS-C.)
I'm asking if I took the old 40mm pancake and the xs/limited would it be the same fov?

07-07-2015, 11:09 AM   #5
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QuoteOriginally posted by ZombieArmy Quote
I'm asking if I took the old 40mm pancake and the xs/limited would it be the same fov?
Yep. The field of view of both lenses on your K-01 would be the same.
07-07-2015, 11:10 AM   #6
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QuoteOriginally posted by ZombieArmy Quote
I'm asking if I took the old 40mm pancake and the xs/limited would it be the same fov?
Yes. It will be the same field of view. 40mm is 40mm.
07-07-2015, 11:11 AM   #7
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QuoteOriginally posted by ZombieArmy Quote
I'm asking if I took the old 40mm pancake and the xs/limited would it be the same fov?
Both 40mm lenses will provide the same FOV on APS-C. The FOV will be wider on FF and narrower on APS-C, but will
be the same regardless which 40mm you use.
07-07-2015, 11:54 AM   #8
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Zombie - thanks for asking!

So what's special about the DA lenses then - just that they are light weight? One could take a DA lens and use it in place of a D FA lens on a full frame camera and you'd get the same FOV?

I remember reading somewhere that if you use lenses designed for APS-C sensors and use them in a full frame camera, you'd get vignetting. How's that?

- Confused.

07-07-2015, 12:09 PM   #9
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QuoteOriginally posted by realitarian Quote
So what's special about the DA lenses then - just that they are light weight? One could take a DA lens and use it in place of a D FA lens on a full frame camera and you'd get the same FOV?

I remember reading somewhere that if you use lenses designed for APS-C sensors and use them in a full frame camera, you'd get vignetting. How's that?
DA lenses are designed for APS-C. They are compact but might only cast an APS-C size image circle that's not large enough to fully illuminate a larger "full frame" sensor. The area outside the image circle will be darker than the rest of the image and can even appear black depending on the lens.
07-07-2015, 12:21 PM   #10
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This might help. The DA lenses are designed for APS-C sized sensors, but some lenses, like the DA 40mm Limited, DA 35mm f2.4 and DA 50mm f1.8 for example are all shown to cover the FF Image circle. (there are others too like the DA*55mm f1.4 etc)

So a 40mm lens is always a 40mm lens, you just see less of the image circle on the Pentax APS-C cameras, hence the 'crop'.



A dedicated APS-C lens such as the DA 14mm, 15mm Limited, 21mm Limited etc will only usually cover the APS-C frame outlined below, you get serious vignetting where some of the frame will be black (this shot was the DA14 on a 35mm Film camera, the MZ-5N)


07-07-2015, 02:06 PM   #11
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Yea, 40mm is 40mm, regardless of what camera it is put on. But the captured field of view depends on one more factor - size of the film plane / sensor. So you can mount that 40mm on Pentax Q, Pentax K-01, and Sony A7r and the images will look very different. The second part is the size of the image circle. A lens will work on the format that it is designed for (FF, APSC, medium format, Q, m43,..) and all smaller formats. But its image circle might not be big enough to cover a bigger camera's sensor or film. So if you put a Q lens on your K-01, most of the frame will be black, with a circle in the middle.

Just remember that focal length is not the same as field of view. So FA 200mm and DA 200mm will give you the same field of view if you put them on the same camera. Same for FA 35mm and DA 35mm, M 40mm and DA 40mm, and so on.. Its just that FA (and DFA) is guaranteed to cover FF, 35mm film; while DA is meant to be optimized for APSC and might not cover whole FF sensor. I think Nikon and Sigma designate lenses as FX and DX. Pentax uses DA and DFA now.

The only relevant differences between FA and DA is that DA has newer lens coatings, optimized for digital sensors, so they should have less ghosting and fringing on a digital sensor (since digital sensors are different from film, they are more shiny). And of course construction - older lenses have Aperture ring and distance scales, while DA lenses might not (depends on lens).

Here is a thread about which Pentax DA lenses work on FF:
https://www.pentaxforums.com/forums/54-pentax-lens-articles/31629-da-lenses-f...ts-thread.html

---------- Post added 7th Jul 2015 at 23:11 ----------

One more thing: Why make APSC only lenses at all? Well, the image circle size affects the lens size. That is why 50mm lenses for medium format tend to be bigger and slower than 50mm lenses made for 35mm film/FF. And spending extra resources, glass, on making a big image circle that doesn't even get used is pointless, might even cause loss of contrast with the extra light bouncing around the inside of the camera. That said, many DA lenses are based on FA designs and they still cover FF image circle. So in my opinion, Pentax could actually go further and make even smaller lenses with faster apertures for their crop sensor bodies, but I guess that would cost even more R&D.
And APSC exists because making big sensors is difficult and expensive. Or, used to be.

Last edited by Na Horuk; 07-07-2015 at 02:15 PM.
07-07-2015, 08:03 PM   #12
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A 40mm is a 40mm is a 40mm, no matter what mount it is (m43, apsc, FF). The only is difference is the Field of view between them.
07-07-2015, 08:23 PM   #13
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Thanks for the explanation folks.
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