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11-29-2011, 04:02 PM   #1
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Pentax 18-35 lens

I recently purchased a used Pentax 18-35 F4-5.6 SMC FAJ AL lens for my Pentax K10D camera. The shots do not seem to be wide angle. Is this a film camera lens or do I have to shoot at a particular setting?

11-29-2011, 04:11 PM - 1 Like   #2
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The mm's are are the same no matter what the lens is designed for, they just get "cropped" by the aps-c sensor. Take the width times x1,6 and you'll get what your shots it equals on a film camera.
11-29-2011, 04:18 PM - 1 Like   #3
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QuoteOriginally posted by VisualDarkness Quote
The mm's are are the same no matter what the lens is designed for, they just get "cropped" by the aps-c sensor. Take the width times x1,6 and you'll get what your shots it equals on a film camera.
On Pentax that would be 1.5, not 1.6.

The field of view of the FA J 18-35 on APS-C is equivalent to 28mm-50mm on 35mm film.
11-29-2011, 04:22 PM - 1 Like   #4
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QuoteOriginally posted by VisualDarkness Quote
The mm's are are the same no matter what the lens is designed for, they just get "cropped" by the aps-c sensor. Take the width times x1,6 and you'll get what your shots it equals on a film camera.
Quite right, except that the format-faktor for our APS-C sensors is closer to 1.5x. An 18mm lens on an APS-C (half-frame or HF) camera has the same FOV (field-of-view) as a 27mm lens on a 135/FF (full-frame) camera; and a 35mm lens on HF is FOV-equivalent to 52.5mm on FF.

So if you're used to thinking in 135/FF terms, the 18-35 is roughly equivalent to 28-50. This is in FOV terms only; DOF is still different. I hope this isn't too confusing!

EDIT: Oops, Boris beat me to it.

11-29-2011, 04:28 PM   #5
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Ooops, of course it should be 1.5.
11-29-2011, 04:52 PM   #6
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Okay. I had to take a moment for my head to stop spinning. I have some grip on the difference in FOV between film and digital. The issue I have is that the photos that I've taken look nothing like the example "with" and "without" the 18-35 lens photos I see on the web to illustrate the lenses wide angle effect.
11-29-2011, 05:19 PM - 1 Like   #7
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kfwils: The question then is were the sample photos take on film (full frame) or APS-C (half frame).

Lord, I don't want to get into a semantics and wording war here with "crop factor" or other various terms here, but the magnification doesn't change, just the amount of the scene you get on APS-C vs film receptors. So the field of view equivs change, but the "zoom" doesn't change. So, if the shots were taken on film, and you're comparing your APS-C shots to those, you're going to get a narrower view of the same distance.
11-29-2011, 05:43 PM   #8
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18-35 lens

I think I'm catching on. A photo taken with a full frame camera with a 18-35 lens would look quite different from the same shot taken with a APS-C camera using the same lens?

11-29-2011, 05:49 PM - 1 Like   #9
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QuoteOriginally posted by kfwils Quote
I think I'm catching on. A photo taken with a full frame camera with a 18-35 lens would look quite different from the same shot taken with a APS-C camera using the same lens?
The APS-C photo would look like you took the full frame photo and cut away the outer edges all around, leaving only the center 2/3 of the picture. That's why the "magnification" is the same and yet what you see isn't as wide. (Nor as tall, but nobody ever gripes about that, oddly enough).
11-29-2011, 06:04 PM   #10
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That's exactly what I see. The outer edges appear cropped. Thanks a lot for all of the responses to my inquiry. I took a trip to Wikipedia to further enlighten myself on these technical differences. I understand that a full frame camera is well suited for a wide angle lens and the APS-C is well suited for the telephoto lens with the smaller sensor.
11-29-2011, 06:29 PM   #11
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The effect is less drastic with longer focal lengths for the simple reason that when you think of it in terms of how many degrees the field of view covers, 1/3 of a small number is less than 1/3 of a large number.

Imagine a 180 degree field....you lose 30 degrees off each side.

Imagine a 6 degree field....you lose 1 degree off each side.
11-29-2011, 07:14 PM   #12
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Back in the day (circa 1975) I had an Olympus Pen-FT, a half-frame (HF) 35mm film SLR with an internal porroprism (like in binoculars) and a vertical (portrait mode) film frame. It was very much like using an APS-C dSLR turned on its side for portrait-mode shooting, but smaller, lighter, kewler. And I had a full-frame (FF) T-mount Spiratone 400/6.3 long lens with an adapter. And I thought, OH BOY IT'S LIKE A 600MM LENS! And I quickly learned that it wasn't -- it was just a 400mm lens with 1/4 clipped off from each side. That was how the smaller frame cropped the image.

To see what format.faktor does, try this: Cut a picture from a magazine. Cut a 45x60mm rectangle. That's about the size of a 645 frame. Centered in that, draw a 24x36mm rectangle. That's the size of a 135/FF frame. Centered in that, draw an 18x24mm rectangle. That's about the size of a 135/FF | APS-C frame. Now look at the picture. The picture hasn't changed. But every smaller frame sees less of it. That's how crop-sensors work. The lens projects the same image; different frames see different amounts of the projected image. That's all there is.

Old-timers here know that I despise the terms crap.factor and equivalence, and think that the marketing wonks who thought-up those terms should be painfully executed. But I'm not bitter.
11-29-2011, 07:26 PM   #13
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Even on aps-c 18mm is pretty wide, but obviously not as wide as on film.

77-45 degrees vs 100-63 degrees on film

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