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Old Apr 8, 2010, 9:11 AM   #1
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Default Distortion in the corners

First of all, I recognize this is a Canon Lens forum, and mine is a Sigma Lens mounted on a Canon. So if the moderators move this thread, I certainly understand....

My problem in the attached photos is the corners.
All of these were shot with a 5D Mark II with a Sigma 24-70 lens. The lens is 2.8 all the way thru, DG HSM model...Sigma's latest version of the 24-70.

Photos 1 & 2 were shot at 2.8 1/20th 24MM ISO 200
Photos 3 & 4 were shot at 2.8 1/25th 24MM ISO 200

If you look at the corners, (and how can you not look the bad corners? They just jump out at you)

What causes this? Is it just that I'm shooting at 24, and I'd get less distortion if I was shooting at 34MM?

Also, please resist the temptation to C & C the photo from a subject standpoint. The subject is the grass itself. I'm showing the right side of my yard, (and some of my neighbors yard, then two photos of my yard only, then a photo of my yard pointed towards my other neighbor's yard.) This lawn if now in it's 5th year, raised on Indiana clay soil. No sod was used. Just me, Scotts Fertilizers, Jerry Baker's Turf Tonics and Turf Type Tall Fescue Grass seed. Thick? Lush? You bet! Had to buy a higher horsepowered mower, a 6.75 horse lawnmower couldn't cut it. W00t! W00t!

Thanks for your insights.
FP
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Old Apr 8, 2010, 9:18 AM   #2
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Here is fine, so I won't move it.

That is common for a lens at it's widest, you see this a lot on different lenses. I get it on my Canon 24-105 at 24mm, also on my 17-70 at 17mm. It gets better when you use a long lenght. If you were shooting with a crop body rather than full frame then you wouldn't have such an issue as it uses the middle of the lens.

Some lenses perform better than others but it is just something we have to live with. No there is software that can help to fix this. DXO Optics is one, but you would be looking at $200 I believe for this software. It knows the distortion in the lenses and can automatically fix them. If I start shooting more weddings back in the UK I'm likely to get this as it is a life saver with wide group shots where you suddenly get fat distorted people at the ends.
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Old Apr 8, 2010, 9:20 AM   #3
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Oh just to add, not only will you get distortion but also a reduction in lens resolution at the edges, stopping down from f2.8 will help this and would the be the normal practise in landscape type shots.

When you shoot with this lens at f2.8 for a portrait or something then usually the edges are of an out of focus background so don't even notice.
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Old Apr 8, 2010, 9:47 AM   #4
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As it happens, SLRGear.com tested that lens on the Canon 5D, the predescessor of your 5D Mk II. (See http://www.slrgear.com/reviews/showp...ct/1236/cat/31 ) What you see in the photos is also visible in the charts. You might want to take a look.
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Old Apr 8, 2010, 7:16 PM   #5
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Apart from rectilinear distortion, vignetting and lack of sharpeness in the corners (for example, as I don't know the exact performance of thi lens), are you also aware of perspective distortion?

This is particularly evident in the first shot, top right corner. The wider you go (in this case, 24mm) the more the perspective is distorted for an ultra wideangle lens. This is not a deficiency in the lens itself, but in the nature of this type of lens. Anything on the edges and even more so in the corners, tend to look stretched, because the angle of view is so wide. In these circumstances, it's best to be aware of this effect, and avoid straight lines(such as buildings) and animate objects such as people or animals that don't look natural when the perspective is "stetched".

Another technique to getting more natural shots is to make sure the camera is level. For example, the first shot is pointing down, so the buildings appear to lean out(and if you point up, building appear to lean in)
If the camera is level (and there is no REAL barrel distortion in the lens), then these buildings will look vertical and more natural
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Old Apr 8, 2010, 9:25 PM   #6
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as dna pointed out part of the problem particularly in #1 is that the camera is pointed down.

what struck me more than anything with these was the vignette, which seems really quite pronounced.
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Old Apr 8, 2010, 9:26 PM   #7
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Quote:
Originally Posted by Hards80 View Post
what struck me more than anything with these was the vignette, which seems really quite pronounced.
I noticed that too - is there a filter on the front or is the lens hood orientation correct?
-> Both sides of the images (and not just corners) are darkened by something

Mark is also correct as landscape are rarely done @ f/2.8 for the required DOF, and according to SLRGEAR blur index graph the corners already sharpened up quite nicely by as little as f/4
-> By f/8 the corners are almost on par with the center of the lens

dnas is also spotted on as the camera is pointing down, and such a wide perspective, the distance to the grass at the center of the frame is quite a bit closer than the distances to the top corners
-> There's no way possible at f/2.8 for the corners to be in focus with such a disparate distances among them (especially the far away tops from the much closer bottoms)!!!

Last edited by NHL; Apr 8, 2010 at 9:47 PM.
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Old Apr 9, 2010, 4:07 AM   #8
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The SLRGear.com test results show severe vignetting when used with a 'Full Frame' dSLR. And I think NHL is looking at the test results for that lens when mounted on the 20D; when mounted on the 5D, at the wide end, it takes stopping down to f/8 before corner sharpness becomes reasonable.

This is a great lens on an APS-C body, but it just isn't that good on a 'Full Frame' dSLR. Vignetting is bad at any focal length or aperture, and chromatic aberration is bad at any focal length shorter than 35mm. While center sharpness is quite good at any focal length, for sharp edges and corners, you need to stop down to at least f/5.6, and at 24mm you need to stop down to f/8.
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Old Apr 9, 2010, 8:32 AM   #9
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I am amazed that the amount of knowledge that you guys carry around in your heads regarding photography. And I say this with all sincerety.

Recently one person posted some photos and seemed to be frustrated that no one commented, even though the post had many "views". One of the reasons I view a photo isn't to comment on it, but to read the comments of the vast knowledge base here at Steve's Digicams.

Thanks for looking at this issue with me.

Here's what I learned from this.
1. My Sigma 24-70 lens does not need any repairs.
2. When shooting a landscape, shooting at 24mm & 2.8 is a bad combination.
3. Shoot at maybe 34 (or use a different lens) and bump up the fstop to 5.6 or so.
4. The angle at which I point my camera can result in some fisheye effects that I don't want.

Thanks for sharing your experience with me.

FP
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Old Apr 9, 2010, 3:45 PM   #10
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Use this interactive tool: http://www.slrgear.com/reviews/zprod...ff/tloader.htm

With the right slider you can see the corner sharpness increase as the aperture move up

The bottom slider varies the corner sharpness in relation to the focal lenght...
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