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04-18-2017, 07:42 AM   #1
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Field of View

I have a Pentax K-S1 which I know has a cropped sensor.


Because DA lenses are digitally optimized for APS-C sensors, it is my understanding that crop factor would NOT come into play when using a DA series lens (or DA-L) on a Pentax cropped sensor camera.

If I want to see what a full frame camera sees with a 50mm lens should I purchase the DA 35mm lens or the DA 50mm lens for my K-S1 camera


Does the DA 35mm lens on a crop sensor camera "see" the same thing a standard 35mm lens sees on a full frame camera or does it see what a 52.5mm lens would see on the full frame camera?

04-18-2017, 08:16 AM   #2
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QuoteOriginally posted by bgscott Quote
I have a Pentax K-S1 which I know has a cropped sensor.


Because DA lenses are digitally optimized for APS-C sensors, it is my understanding that crop factor would NOT come into play when using a DA series lens (or DA-L) on a Pentax cropped sensor camera.

If I want to see what a full frame camera sees with a 50mm lens should I purchase the DA 35mm lens or the DA 50mm lens for my K-S1 camera


Does the DA 35mm lens on a crop sensor camera "see" the same thing a standard 35mm lens sees on a full frame camera or does it see what a 52.5mm lens would see on the full frame camera?
The physical focal length of the lens is a property of the lens that doesn't change regardless of what sensor is in the camera. The lens doesn't know how big the sensor or piece of film is. At some level, it doesn't make sense to speak of what the lens sees, only what the sensor sees with a specific lens. Thus, crop-factor doesn't change what the lens sees, it only changes how much of field of view gathered by the lens is captured by the film or sensor.

To answer your last question: an APS-C sensor with a 35 mm lens sees the same field of view as a full frame sensor with a 52.5mm lens. (Yet by some definitions a DA 35 and an FF 35 may "see" the same thing -- what matters is the sensor.)

The entire notion of "equivalent focal length" is a bit wrong, because it's not the lens that is different but the sensor.
04-18-2017, 08:19 AM   #3
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If you are wanting the same frame as what you would get on a full frame i would go for the 35 mm.

When talking about equivalence the focal length stays the same meaning a 50mm is a 50 mm. when you are trying to get the same field of view between the crop sensor and full frame you will need to divide the full frame equivalent by the crop factor. So as you say what you see on a full frame at 50 mm is what you will see with a crop sensor with a 35 mm lens. You could also go for the fa 31mm if you wanted to, which is also close to the field of view of a 50 on a full frame.


04-18-2017, 08:28 AM   #4
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This image explains it best what a given focal length results in:
Crop factor - Wikipedia

The part which is missing from the graphics is that users usually display their APSC images on same size monitors or printed on same size paper as they do FF images and watch them from the same distance. That results in the "tree-part" being displayed larger, which results in users talking about "more reach" on APSC and "more noise" and "less dynamic range" (which all are just consequences of the artificial post-shot displaying enlargement).

04-18-2017, 09:26 AM   #5
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Focal length is not the same as the Field of View of the final photo. Focal length is a lens property and is independent of camera.
DA 35mm on K-1 (full frame) will look wider than on K-S1 (APSC) because literally the edges will be cropped, cut, off the frame on the APSC camera. It simply has a smaller sensor, so the edges are cut off. This makes the field of view look more narrow. But all the other lens characteristics like minimum focus distance, bokeh character, sharpness, color rendering will still depend on the particular lens design, so there will be differences. Just the FoV will be kind of similar.
Same thing for DFA 50mm. Its a full frame lens, but it will look wider on K-1 (FF) than on APSC cameras.

DA 35mm on K-S1 will have a Field of view similar to, for example, DA 50mm on the K-1.
50mm on APSC is slightly telephoto.
50mm on FF is normal
50mm on 645 (medium format) is slightly wide angle
The only catch is that a lens for a small format might not cover the area of a bigger sensor. So some DA lenses will have black edges on FF. All Pentax lenses are FF (like F, FA, DFA series), except for some DA lenses. There is a thread "DA lenses on full frame" and some other threads as well that discuss which DA lenses work 100% on FF. All FF lenses work normally on crop sensors. You can put Medium format lenses on FF or APSC lenses, if you have an adapter, but they are really big, with slower aperture, and there will be a lot of loose light in the mirror box. This is why its generally not a good idea to mount lenses that are way too big for your format. FF and APSC are fairly close, so it doesn't matter much
QuoteOriginally posted by bgscott Quote
Because DA lenses are digitally optimized for APS-C sensors, it is my understanding that crop factor would NOT come into play when using a DA series lens (or DA-L) on a Pentax cropped sensor camera.
So, the crop factor does not depend on lens. It depends on camera.
Honestly, here is the deal. If you only shoot with one format, then don't even think about the whole crop thing. It just confuses people. The only reason crop factor is a big deal is because when photographers moved from the (Very popular) 35mm film to the first digital cameras, the digital ones were all cropped. So they needed some rule to figure out the different FoV they would get on the different camera.
04-21-2017, 09:26 PM   #6
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While all of the above complexities are true, the bottom line simple answer to your question is yes- if you want to get approximately what a full-frame camera sees with a 50mm lens, get a 35mm lens for your APS-C size sensor DSLR.

Digitally optimized for APS-C is not about focal length. It simply means the lens circle is cut smaller than for a full frame sensor so as to fit the parameters of the APS-C size, which allows for a somewhat smaller lens to be designed. It also means putting that lens on a FF body would likely result in darkness around the edges of the image because the glass is too small across. But there is no problem using a lens made for FF on an APS-C body, even though bigger across than needed- that extra lens area is simply not being used.
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