Panoramas: Getting a continuous sky colour

[porter]

<p>I'm trying to shoot some panorama's, but can't get the sky to maintain one continuous colour/exposure amount throughout the stitch. I don't know what is wrong, I'm shooting in manual, white balance is set to daylight instead of auto, etc. </p>

<p><a title="Untitled by Patrick E Porter, on Flickr" href="http://www.flickr.com/photos/patrickporter/6794644859/"><img src="http://farm8.staticflickr.com/7174/6794644859_6f2725ccca_z.jpg" alt="" width="640" height="414" /></a></p>

[simon_hickie1]

<p>Shooting in manual is a good start. Are you keeping the exposure the same?</p>

[porter]

<p>I am, yes. Each image looks exposed the same too, including the sky. Not sure what is happening in stitching. Everything else is uniform, so I don't understand why there would be issues with the sky.</p>

[simon_hickie1]

<p>This is indeed rather odd. I'd have expected 'manual everything' to have done as you would expect. The foreground looks generally OK, but the last segment on the right does look brighter lower down. If you shot in RAW, has the raw converter done something odd? Also, which stitching method are you using?</p>

[jim_momary]

<p>Are you using a polarizer? That will have an adverse effect in something like this.</p>

<p>Jim</p>

[porter]

<p>Shot in RAW and converted with Canon's software. All the images when viewed alone look to be the same exposure, so maybe the stitching software is doing something weird. I don't remember the name of the software (am at work now), but it came with my Panasonic LX-3. I think there is another stitching program on my computer that came bundled with the rest of the Canon stuff, maybe I'll try that.</p>

<p>Only filter I had on was a UV. Perhaps I should take it off just to see if that had an adverse effect.</p>

<p>Hey, maybe it is a bit of glare on the glass/filter? I'll put the hood on (it wasn't on at the time), take the filter off, and try again.</p>

<p>If it helps, this is with a 5D2 and 28mm f/1.8 lens @ f11</p>

[porter]

<p>Oh, and I took the shots using live view. Don't know if that might make a difference... I wear a pretty heavy prescription, making the optical viewfinder quite uncomfortable and difficult to use.</p>

[danield]

<p>Whenever I see something like this I tend to blame it on left-over lens vignetting which can still be visible at small apertures. Clear blue skies make it more apparent. Here it seems that the image is brighter to the left, but I suspect that may just be an impression due to how the stitching is done (left side is covered by another image).</p>

<p>I would check the original photos and check the pixel values in the sky in the specific areas. Also take a shot of a blank target with the lens stepped down and increase contrast to evaluate whether it's just vignetting or something else with the lens.</p>

<p>It may not be something you can fix in the camera - exposure times are never 100% precise anyway. Generally good stitching software have blending routines that deal with this kind of issue. Post-processing in Photoshop can get rid of the problem easily too.</p>

[porter]

<p>Hmm, vignetting may be an issue. Good point. Perhaps I took too many images? I shot about 8 images in portrait orientation to get this, so there was significant overlap. Maybe I took more on the left side so it registered more of the dark vignetting, and less on the right side so it appears brighter...</p>

[howard_m]

<p>I'd suggest downloading and 'trialing' better stitching software like PTgui or Autopano Pro</p>

[simon_hickie1]

<p>I've had another thought. In Photoshop or similar, open two overlapping images and on the same point in the sky check the RGB values. In theory, they should be identical. If not, that would point to camera / lens / filter / hood issues. If they are, then that suggests a stitching software problem.</p>

[howard_m]

<p>Do you perhaps have autoISO enabled? Or D-lighting or any other 'auto' garbage?</p>

[porter]

<p>No auto anything enabled. That's why I was flabergasted to see such variance in the stitch.</p>

<p>I'll try some different software and perhaps see if there is a difference between taking tons of shots and just a few.</p>

[stephen_hipperson]

<p>An off the wall thought - the sun seems to be behind you, I'm guessin you're in the shade so direct light across the front element shouldn't be a problem, but how about reflected light getting onto the flat face of the filter? This will give the appearance of slightly lower contrast, which in turn means wishy washy colours.</p>

[jcoffin]

<p>This is common with less capable stitching software. Most newer software can/will adjust the brightness to eliminate the problem.</p>

<p>As to the cause, it's pretty simple: especially in areas like the sky, even extremely small changes in light level are easy to see. Most cameras provide exposure consistency around 1/4th of a stop or so, but when you put shots side by side like this, 1/10th of a stop is easily visible.</p>

<p>Most stitching software compensates precisely because it's essentially impossible to get a perfect match directly from the camera. If people insisted, camera companies probably could build shutters and diaphragms that would be more accurate and consistent, but it would almost certainly add a great deal to the price of a camera/lens, while providing no benefit at all to most people for most ordinary shooting.</p>

[JDMvW]

<p>Adobe Photoshop CS 5.5, FWIW, does the panorama combination with vastly improved adjustment of sky tones to virtually eliminate the problem of different tones.</p>

<p>I have gone back and redone all my old panoramas done with earlier or other software.</p>

[jcuknz]

<p>I would suggest that you stitch manually so that you have each frame on a separate layer and can adjust each to match the rest in your editing programme. Using a polarising filter will emphasise the problem as I learnt the hard way :-)<br>

You cannot take too many frames and the last big one I did had fifteen and I've done more on occasions.<br>

I only use a stitch programme when in a hurry or it is a dead simple stitch, even then I probably wouldn't these days . If you need to register one layer with another, from hand holding which is my norm, you register two frames at a time by reducing the density of the upper layer to help you see the lower layer through it. Then with those registered bring in the next layer etc. You need to organise the bottom layer to have a canvas size to accomodate the completed stitch. With the large camera files we have these days I frequently reduce each file to half size to help the computer handle the large file I am creating.<br>

So apart from not using a polariser I know I can fix what the camera gives me, I even fixed that problem by cloning sky I wanted but it was a hell of a job :-)<br>

On a 360 degree pano with a polarising filter the sky turns to white twice in the circle.</p>

[JDMvW]

<p>BTW, if you take a broad panorama with a single, very wide lens and look at it, you will notice that the sky is NOT uniform in tone in all directions. Light from the sun is reflecting off water vapor and dust in the air. A polarizing filter may accentuate this by eliminating some reflections at right angles, but sky variations are present naturally.</p><div></div>

[jcuknz]

<p>If you are going to shoot panos then don't use a wide angle lens, not saying it cannot be done but it makes the job harder.</p>