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12-24-2011, 10:10 AM   #1
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Misconception regarding asp-c focal length that i need clearing up :) please :)

Hey
this is a very simple potential misconception I may or may not have. Would an FA 20mm and DA20mm (hypothetical) have the same focal length on an asp-c k'r, for example? Or would the Fa20mm be a 30mm on the k-r?
Merry Christmas,

Charles

12-24-2011, 10:13 AM - 1 Like   #2
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No, the focal length doesn't change. The field of view changes.



QuoteQuote:
The outer, red box displays what a 24×36 mm sensor would see, the inner, blue box displays what a 15×23 mm sensor would see. (The actual image circle of most lenses designed for 35 mm SLR format would extend further beyond the red box than shown in the above image.)


Crop factor - Wikipedia, the free encyclopedia
12-24-2011, 10:16 AM   #3
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QuoteOriginally posted by boriscleto Quote
No, the focal length doesn't change. The field of view changes.





Crop factor - Wikipedia, the free encyclopedia
but so the field of view would be the equivalent of a 30mm?
12-24-2011, 10:21 AM - 1 Like   #4
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if you are comparing to FF then yes, to MF not

But don't think too much about it. just put the lens on your camera, look through the viewfinder, and you know how the FOV is. And then just remember it

12-24-2011, 10:22 AM - 1 Like   #5
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QuoteOriginally posted by chaza01 Quote
but so the field of view would be the equivalent of a 30mm?
Yes. The field of view changes, not the magnification.

Crop factor - Wikipedia, the free encyclopedia

If RioRico were around he would explain it better.
12-24-2011, 10:24 AM - 1 Like   #6
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And 20mm is 20mm, whether the lens is DA or FA.

The only exception I've seen so far is from Ricoh, unfortunately, who give the 35mm equivalent focal length instead of the true focal length of their GXR "lensor" modules. But that's somewhat understandable since they use various sensor sizes in them...
12-24-2011, 11:11 AM - 3 Likes   #7
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All true above. A lens is made as a certain focal length (or zoom range). Focal length doesn't change when a lens is moved between cameras; the lens doesn't stretch not shrink. Different size frames crop different amounts of the projected image, is all, as boriscleto showed above.

We have conflicting facts: the 35mm film frame (FF/135) that is 36x24mm and is a de-facto standard because so many toggers were/are used to it, versus the smaller frames of digital cameras. If makers promoted digicams by their actual sensor size, we'd be lost in a dizzy maze of lens specifications. So, many makers label cameras and lenses based of FOV equivalence to 135/FF. And naive customers then mistake the equivalence for reality.

Example: My fave old P&S has a 7-28mm zoom that's FOV-equivalent to 34-136mm on 135/FF. But it retains the DOF of a 7-28mm. Our newest P&S has a 4.6-23mm that's FOV-equivalent to 36-130mm. Those have very different real ranges but very similar equivalents. And NEITHER behaves like an actual 35-135mm zoom on 135/FF. The shorter lenses have MUCH thicker DOF. Pictures from these will not look the same.

These equivalences are somewhat useful for comparisons, especially for we dinosaurs who grew up on 135/FF. As Macario said, your best bet is just to GET OUT THERE AND SHOOT. Learn what focal lengths do what on your camera. It's not hard. Back in the day, my colleagues and I often shot multiple formats daily, from 16mm to 4x5. We never spoke of lens equivalents. We just knew what was 'normal' for each format, and chose lenses appropriately.

NOTE: Crap.factor is figured according to lens normalcy. The 'normal' focal length is the frame diagonal. For 135/FF it's 43mm; for APS-C, it's 28-30mm, depending on the exact sensor size. For 6x9 it's 101mm. And for the Q it's about 7.7mm. The focal lengths for wide and tele and portrait all shift accordingly, with vast changes in DOF. It's just something to learn.
12-24-2011, 11:40 AM   #8
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RioRico: nice review. Especially explaining how the true focal lengths of the standard P&S explains the massive DOF. I often see 35 mm denoted as '135'. Where did the term 135 come from?

12-24-2011, 12:09 PM   #9
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QuoteOriginally posted by MSM Quote
I often see 35 mm denoted as '135'. Where did the term 135 come from?
From Kodak.

135 film - Wikipedia, the free encyclopedia
12-24-2011, 01:03 PM - 1 Like   #10
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QuoteOriginally posted by boriscleto Quote
Quite right. As an aside, I once owned a first-edition 1934 Kodak Retina, the camera for which the 135 cart was designed (or they were co-developed). I'd already been shooting for 15 years, but the Retina was my first serious camera, and it really taught me photography.

The original Retina was just a little folder with 50/3.5-22 lens, 1/500 shutter, X-sync contact, tiny viewfinder, spin-the-lens focusing, and nothing else. No rangefinder, no hot.shoe, no metering, no automation of any kind. But I learned to judge distance and angle and light and form; learned to look at a scene and automatically adjust the few controls (focus, aperture, shutter) for the exposure -- a human-powered P&S with professional results. Mine was stolen long ago, but I now have a near-clone, an inherited Voigtlander Vito II.

Another aside: A year after my Retina, I acquired 135/HF (half-frame) cams: Canon Demi-EE18 and Dial35, and the Olympus Pen-FT SLR system. (And I now have an inherited Universal Mercury II CX, a 135/HF ILC RF with 35/2.7 long-normal lens.) The 135/HF frame is 24x18mm, half the size of 135/FF (imagine that!) and almost the same as APS-C. So I've dealt with crop-frame 135 cameras for over 35 years. No big thang.
12-25-2011, 12:23 AM - 1 Like   #11
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QuoteOriginally posted by chaza01 Quote
but so the field of view would be the equivalent of a 30mm?
The simple answer is that the DA20 and FA20 have the same FOV on APSC. On an APSC camera the image will look the same (lens differences excepted of course).

Of course the two lenses will look different on a full frame since the DA21 will likely vignette badly. But there isn't currently a digital FF Pentax camera. Not yet anyway...
12-25-2011, 07:10 AM - 1 Like   #12
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The way I put it:

Crop factor is for comparing *cameras*, not for comparing *lenses*. Any two 20mm lenses have exactly the same FOV on the same camera, but any one given 20mm lens can have a different FOV depending on what camera it is mounted too.
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