DP2m Lens Distortion Problem

Started Sep 3, 2012 | Discussions
jbr Regular Member • Posts: 310
DP2m Lens Distortion Problem

I have been enjoying using the DP2m but noticed yesterday that my horizons are quite warped. Seems to be a flaw in the left hand side of the lens causing excessive barrel distortion. The right side looks OK to me. Please have a look and let me know what you think. It looks like I will have to get a replacement. The first is a landscape and the second is a photo of a photo(the top image edge is straight). Both processed with SPP 5.3.1. Joe

Sigma DP2
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SandyF Forum Pro • Posts: 14,941
Re: DP2m Lens Distortion Problem

I have probably over 3GB of shoreline photos from 17-mile Drive.... the majority of them have a horizon line....on which I've just studied the horizons. Are you positive that it's the camera, rather than your not holding the camera absolutely straight? FWIW, I find it really hard to get my horizons straight... but on my shots there is no persistent pattern, ie a curve always on the left. Although with my shots, yes the curve does tend to be more often left than right, but I'm 99% sure that's my doing, how I hold the camera usually on monopod without a level. But I have a good number of shots with a straight horizon, so the issue isn't generalized in DP2Ms.

FWIW, I really like the marked grids that the DP1 has (an option to turn on grid for shooting)... this really helps with horizons. I didn't find a similar grid option on the DP2Merrill... though granted I haven't read the manual LOL.

I'll have some shoreline shots up hopefully later today.
Best regards, Sandy
http://www.pbase.com/sandyfleischman (archival)
http://www.flickr.com/photos/sandyfleischmann (current, DP2M photos)

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mdavidp Senior Member • Posts: 1,965
Re: DP2m Lens Distortion Problem

I don't think your camera is level.
Take the same shot with the camera absolutely level.
Most of my horizon shots tilt to the left, but that is Me.
MP

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OP jbr Regular Member • Posts: 310
Re: DP2m Lens Distortion Problem

Ok, they may be tilted a little(not used to having no viewfinder), but is the amount of barrel distortion normal? Joe

Robert F. Tobler
Robert F. Tobler Senior Member • Posts: 1,149
Your Lens is perfectly OK...

If you use Photoshop, you can use the Lens Correction
Filter to correct the image. The values:

  • Remove Distortion +2.2

  • Rotation 0.65 Degrees

will make the horizon in the first of your posted images
perfectly straight.

On the second image the values are:

  • Remove Distortion +2.0

  • Rotation 0.55 Degrees

The differences in the distortion values can stem from
two sources: it can be due to the different focus value
(distortion may change due to focus distance), and
the horizon on of the sea is actually not completely
level, but has a slight curve to it.

The rotation values are due to your holding the camera
not level.

Greetings,
Robert
--
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HBowman
HBowman Senior Member • Posts: 1,235
Re: DP2m Lens Distortion Problem

The Dp2m do not have ANY kind of distortion.
--
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Charles2 Veteran Member • Posts: 4,787
Re: Your Lens is perfectly OK...

Robert F. Tobler wrote:

The differences in the distortion values can stem from
two sources: it can be due to the different focus value
(distortion may change due to focus distance), and
the horizon on of the sea is actually not completely
level, but has a slight curve to it.

It does look like a long distance to the horizon and a wide expanse, enough to show curvature of the earth. But that is merely a guess.

When a lens has barrel distortion, is it always a design issue, or can it be variation in manufacture? If the former, Sigma should fix it in firmware.

Roland Karlsson Forum Pro • Posts: 26,336
Re: DP2m Lens Distortion Problem

HBowman wrote:

The Dp2m do not have ANY kind of distortion.

Those two images have!

I would be rather surprised if different cameras have different distortion.

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Roland Karlsson Forum Pro • Posts: 26,336
Re: Your Lens is perfectly OK...

Charles2 wrote:

It does look like a long distance to the horizon and a wide expanse, enough to show curvature of the earth. But that is merely a guess.

I have never seen any curvature of earth standing at shore.

How high would you have to stand to get that kind of curve, I wonder?

When a lens has barrel distortion, is it always a design issue, or can it be variation in manufacture?

I would be surprised if it varies without the lens being faulty - and then you would see other problems.

If the former, Sigma should fix it in firmware.

No! The main idea with Foveon is pixel sharpness.

I have even made a tool for correcting distortion, but .... to use it on Foveon images I dont think I would.

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SandyF Forum Pro • Posts: 14,941
Re: DP2m Lens Distortion Problem

I do see the curve in your versions online, but wonder if that's in ALL photos (?)

FWIW I've put the first of my shoreline scenes online at flickr link below. Many more to come.
My horizons seem okay, except when I don't hold the monopod/camera level.

Incidentally I find on this DP2Merrill set on 17-Mile Drive, Carmel, CA, that auto wb seems to match reality more than sunlight wb! standard colormode. And matches up with the Canon 5DII set on daylight wb.

I have a daylight wb version online as well as JPEG straight from camera, which is in daylight wb, as I had the camera set to dwb (or sunlight wb... same thing... both terms are used in SPP)

Best regards, Sandy
http://www.pbase.com/sandyfleischman (archival)
http://www.flickr.com/photos/sandyfleischmann (current, DP2Merrill, new photos)

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MacroMD Regular Member • Posts: 176
Re: DP2m Lens Distortion Problem

Sandy,

I know this is off topic to the thread, but that image is very nice. I just love the colors of the blues/aquamarines.

SandyF Forum Pro • Posts: 14,941
Re: DP2m Lens Distortion Problem

MacroMD wrote:

Sandy,

I know this is off topic to the thread, but that image is very nice. I just love the colors of the blues/aquamarines.

Thanks, what is so lovely at 17-Mile Drive, Pebble Beach, is the variation in water color. We were fortunate to have beautiful weather this year throughout our stay in Carmel. It can be very hazy and overcast, as we've seen on past visits.
Best regards, Sandy
http://www.pbase.com/sandyfleischman (archival)
http://www.flickr.com/photos/sandyfleischmann (current, DP2Merrill photos)

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Roland Karlsson Forum Pro • Posts: 26,336
Re: DP2m Lens Distortion Problem

MacroMD wrote:

Sandy,

I know this is off topic to the thread, but that image is very nice. I just love the colors of the blues/aquamarines.

I fully agree, its very beautiful. And its very sharp.

And the water line is 100% straight. Cant find any curvature.
I downloaded one of the images from your image site, and measured.

We have yet another Sigma mystery.

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OP jbr Regular Member • Posts: 310
Re: DP2m Lens Distortion Problem

I Like the colours too and the curvature in your image posted is far less that what I am seeing(maybe a third). Joe

Roland Karlsson Forum Pro • Posts: 26,336
Re: I have measured the curvature

If you measure the curvature of the horizon from the left side to the little piece of land at the right side with the photoshop ruler tool, you get that the maximum deviation from a straight line is 4 pixels in the biggest image shown at DPReview.

And that image is approx 500 pixels center to top. So ... the distortion is 0.8%.

The image by Sandy only showed half a water line, and it was not even a horizon. So .... its impossible to measure Sandy´s images with that precision.

The close up is distorted though. Or ... is the photo really 100% flat?

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spm
spm Veteran Member • Posts: 5,528
curvature of the Earth + ?

Have you consider the curvature of the Earth and the levelness of the camera while photographing?
S
--
'Life is fragile, too short, and flys by
If you let it
So choose what you want every day
and go get it'
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adegroot Senior Member • Posts: 2,745
RE:Optical illusion

I went to full size, and made the following observations:

1. the line is not 100% horizontal, and therefore creates an optical illusion. The distance to the top of the from from the water level far left and far right is clearly off.

2. this illusion is exaggerated by two elements:
a) the island in the middle; and

b) the faint visual element at the far left horizon level, creating the illusion of
curvature.

3. the sea water level line is 100% straight; I laid a large ruler over my monitor at the water level of your photo's original size. Straight as can be. Not one curve to be seen, not even a tiny one. Total optical illusion.

A total optical illusion. Has happened before in other photos I've seen; please trust the absolute greatness of the lens.

2nd photo: Put your camera on a tripod, put a dark cloth over your head and the back of the DP2M and recompose: make sure your camera's back is 100% parallel to the object you are shooting, and voila, your lines should be just fine. The path of the light going through the lens is to be 100% perpendicular. No camera would be that far off. You don't have a bad camera. The closer you get to an object, the more crucial it si to have this perpendicular/parallel alignment. This is elementary, my dear Watson.

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Roland Karlsson Forum Pro • Posts: 26,336
Re: curvature of the Earth + ?

spm wrote:

Have you consider the curvature of the Earth and the levelness of the camera while photographing?

This is difficult stuff. I first looked at this. Here you have all formulas necessary.

http://en.wikipedia.org/wiki/Horizon

But ... tonite I found it somewhat too much to bother with this

In this forum it is claimed that a super wide angle (78 degrees coverage) shows 1-2 % distortion due to earth's curvature at typical airliner heights i.e. 11 km.

http://boards.straightdope.com/sdmb/showthread.php?t=578392

If that is true, then you wont see any curvature even if you stand at a 100 m cliff.

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Robert F. Tobler
Robert F. Tobler Senior Member • Posts: 1,149
How to fix distortion and retain most of the Foveon sharpness...

Roland Karlsson wrote:

I have even made a tool for correcting distortion, but .... to use it on Foveon images I dont think I would.

In order to fix lens distortion while retaining most of the
famed Foveon sharpness I employ the following workflow:

  • upsize the image with either OnOne Perfect Resize (formerly known as Genuine Fractals) or Alien Skin BlowUp to 150% to 200% the original size.

  • use Photoshop Lens Correction Filter to fix any distortion (I do this for both lens and perspective distortion)

  • downscale the image with Photoshop "Bicubic (best for smooth gradients)" by the inverse of the scale up percentage. This maintains most of the Sigma sharpness without introducing too much artefacts.

My observation is, that after these steps the optimal output sharpening in my opinion requires an Unsharp Mask 100%, radius=0.4, threshold=0. Since the required radius is slightly larger than my usual values for non-modified Foveon Images (there I use an Unsharp Mask of 100%, radius=0.3, threshold=0), I conclude that although there is some sharpness lost in the process, it is not all that much.

Greetings,
Robert
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Roland Karlsson Forum Pro • Posts: 26,336
RE:Optical illusion

adegroot wrote:

3. the sea water level line is 100% straight; I laid a large ruler over my monitor at the water level of your photo's original size. Straight as can be. Not one curve to be seen, not even a tiny one. Total optical illusion.

Measuring in Photoshop on the largest image gave me 0.8% distortion, which equals 4 pixels max deviation

0.8% is a moderately good normal lens. A good one can vary from 0.3 to 1.2%.

2nd photo: Put your camera on a tripod, put a dark cloth over your head and the back of the DP2M and recompose: make sure your camera's back is 100% parallel to the object you are shooting, and voila, your lines should be just fine. The path of the light going through the lens is to be 100% perpendicular. No camera would be that far off. You don't have a bad camera. The closer you get to an object, the more crucial it si to have this perpendicular/parallel alignment. This is elementary, my dear Watson.

If its the edge of a photo and it is laying 100% flat on the table, then it does not matter how you hold the camera. The edge will always be straight in a lens without distortion. That is what my program for fixing distortion measures. I promise you - it works.

There are three possible explanations.

  1. the image did not lay flat.

  2. the lens distorts when focussing closer

  3. the edge of the photo is not straight

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