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12-11-2011, 03:44 PM   #16
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QuoteOriginally posted by seventhdr Quote
No. You can get reduced sensor format lenses with the same angle of view as lenses for a Full Frame camera, so the size of the sensor is not an issue. It is true that designing ultra wide angle lenses for smaller formats is more difficult. A DA 12mm lens gives the same angle of view as an 18mm FF lens.
This needs clarification. A DA 12mm lens on a APS-C body might be about the same view as an 18mm FF lens ON A FULL-FRAME BODY, but an 18mm FF lens on a APS-C body would give you the same view as a DA 18mm on that same body. So it is a bit of a stretch to say that FF is not preferable (for wide-angle) because "you can just get a wider lens for the smaller sensor". Bottom-line is that if you use the exact same lens on a FF body vs a APS-C body, you'll get a wider view with the FF camera.

12-11-2011, 03:59 PM   #17
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QuoteOriginally posted by pcarfan Quote
That's about it, eh!...also, possibly wide angle lenses being generally larger for the same F-stop and due to this size restriction they can't be made as fast as a longer lens, no?
Some 24/1.4 lenses exist. But not for Pentax probably. And they ain't cheap.

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But, I wasn't really thinking of this, just the ability to capture the same scene from the same distance with less distortion. I assumed attaching a wider lens will result in lens distortions that I thought was inherent in wide angle lens design.
AFAIK all lenses wider/shorter than the 'normal' focal length show more-or-less distortion. For 135/FF, 43mm is normal, and I see edge distortion with 35mm lenses. For APS-C, 28-30mm is normal, and I see edge distortion with 24mm lenses.

This is the same problem mapmakers have been dealing with for centuries: how to project a thick 3D world onto a flat 2D plane. Some of their tricks can be used. A map can be constructed from a series of parallel projections; the photographic version is to stitch-together a series of shots into a pano -- maximum coverage with minimum distortion. Other tricks: just fudge and adjust; throw away the distorted edges; or accept the distortion with a tight smile.

Yes, I'd like a FF dSLR, so when I mount my 12-16-21-24mm lenses, they are WIDE, same as on my K1000. But I can get the same coverage on APS-C with my Tamron 10-24 and DA10-17. And no matter which format body, I'll still get some edge distortion with lenses shorter than 'normal'. That's life.
12-11-2011, 04:30 PM   #18
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QuoteOriginally posted by demp10 Quote
Using a lens profile you can correct for lens distortion (and other things) with a push of a button, so even if it is present it is not an issue.

Profiles exist for all modern Pentax lenses and you can create your own for older ones. If you are using Photoshop or Lightroom, they are already there.
But you lose some image in the edges when you do that, or pixels get stretched quite a bit, doesn't it?
12-11-2011, 09:44 PM   #19
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QuoteOriginally posted by pcarfan Quote
But you lose some image in the edges when you do that, or pixels get stretched quite a bit, doesn't it?
Actually both. Depending on the type of the correction you apply, some pixels are stretched while others compress. The compressed ones will leave blank areas while the stretched ones will have artifacts due to lower resolution. You can fill-in some blank areas with clonning (especially if it is the sky) but eventually you will have to crop the image accordingly.

You can shoot a bit wider to start with, so when you crop, the areas of interest will be within the cropped image.

It is not a technique you want to use all the time, but for occational use it can make a big difference.

12-12-2011, 05:19 AM   #20
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I am not totally clear. Is the issue one of distortion (straight lines become curved) or is it one where straight lines begin to lean?

Distortion is a feature of individual lenses. Some have lots and some have little. Nikon 14-24 f2.8 a very highly praised zoom, has 4 percent distortion at the wide end on full frame. The Sigma 8-16mm has 2.9 percent. Both are significant and both are probably correctable with software. The Pentax 12-24 has minimal distortion and is almost prime-like in that area.
12-12-2011, 07:02 AM   #21
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QuoteOriginally posted by vonBaloney Quote
Bottom-line is that if you use the exact same lens on a FF body vs a APS-C body, you'll get a wider view with the FF camera.
Sure. Just use the right tool for the job.
12-12-2011, 03:02 PM   #22
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QuoteOriginally posted by Rondec Quote
I am not totally clear. Is the issue one of distortion (straight lines become curved) or is it one where straight lines begin to lean?
You must deal with both.

Every lens has some distortion (some have more, some less) due to its optical design. This distortion is fixed, usually spherical in nature, can be measured and corrected. It will be the same correction for all images taken by that lens. After the correction you will have an image without lens distortion, just like using a better more "perfect" lens.

The other type of distortion relates to the perspective of the scene. We are projecting a 3-D world to a flat 2-D sensor. If an object in the 3-D space (e.g. the face of a building) is parallel to the sensor, there will be no perspective distortion. Any other angle will introduce some and it will manifest itself as converging lines for distant objects.

The simple way to correct for perspective distortion is to tilt the sensor or the lens so the image plane is parallel to the object. This can be done before taking the picture with a shift lens or a view camera, or afterwards with software. The end result will be that parallel lines in the 3-D space will remain parallel in the image. The correction required will be different for every image depending on the actual setup and orientation of the camera relative to the 3-D world.
12-12-2011, 06:34 PM   #23
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QuoteOriginally posted by pcarfan Quote
But doesn't a shorter focal length lens have more barrel distortion?
They are more prone but were usually improved with better optical design.
When Pentax first released the K15/3.5 in 1975 it had an aspherical element to help with distortion. The K28/2.0 had floating elements to combat distortion. Each new generation of lenses improves the issue.

When I use my K15/3.5 on my film cameras, you can capture whole buildings from across the street, usually without having to tilt your camera. (The K28/2.5 shift cures this issue)

Phil.

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