Problem solving a bad stitch
So you have taken a sequence of pictures and used some software to stitch them together. But you are not happy with the final picture... there are bits that are not aligned properly, or fuzzy areas where the software appears to have failed to 'bend' a window frame and blend it correctly between pictures.
According to my experience problems are caused by either
- mistakes when taking the pictures
- mistakes when stitching
Problems caused when taking pictures
If you take a panoramic sequence handheld it is extremely likely that the images will not align vertically. The resulting panorama will therefore be much narrower than it need be because of the missing information. Some software will allow you to save the stitched image uncropped and you can then carefully use an image editing package to retouch the image (e.g. use the rubber stamp tool in Adobe Photoshop to clone areas of the picture from elsewhere).
If you get this problem after having used a tripod it is probably because
- you did not equip yourself with a small spirit level
- you've got a spirit level but didn't really use it thoroughly
- you failed to secure the tripod properly by locking all the relevant bolts
- your tripod is insufficiently stable/sturdy to maintain a level as a heavy camera is rotated
- you inadvertently knocked the tripod as you walked around it moving the camera
Remember that a pano head adds quite substantial weight to the top of your tripod so if your tripod is a cheap and cheerful one (as my first one was) you are likely to have problems.
Various other things can cause problems in your pano:
- using auto-focus (because the field of view of the camera changes slightly with focusing and so does the no-parallax-point position)
- dramatic contrast change
- changing exposure between frames (but see below)
- changing aperture
- zooming between frames
- knocking the tripod, if used (see above)
- camera shake if you are using a slow exposure time
- overlap is in excess of 50% or less than 20% or varies greatly (but see below)
Since you are supposed to be taking a sequence of pictures as if they are one picture it goes more-or-less without saying that you'd expect to use the same settings for each frame. OK, it's not quite that clear cut. In difficult lighting situations you may have to vary the exposure slightly and then rely on the blending the overlap between shots, or correcting the colours/contrast afterwards in software.
Some software is sensitive to the amount of overlap and most expect between 30% and 50% and will not always cope with a lower or significantly higher overlap. In general you should aim for 30-50% and try to keep a fairly consistent amount of overlap between frames.
Problems caused when stitching
The most obvious thing you can get wrong during stitching is to specify the wrong settings in the software, such as telling the package the camera had a 35mm lens on it when in fact you used a 15mm fisheye. Also remember that if the sequence is less than 360 degrees you may need to expressly 'tell' the software this otherwise it will attempt to join the ends up and/or mis-judge the lens characteristics. These problems are relatively rare because most software will automatically recognise the lens used from the EXIF data embedded into your photos by your camera.
Not all lenses are supported by all software, and if you are using a fisheye lens you need to be sure the stitching software you are using supports fisheye lenses.
Linear distortion is the amount by which a lens distorts the image. It can be measured in a laboratory by taking a test picture of a grid and measuring the amount by which straight lines appear curved. No lens will give a perfect result and therefore there is no such thing as a perfect rectilinear lens, however some lenses perform much better than others. Generally zoom lenses will perform much worse than fixed focal length lenses. For example my Sigma 18-35mm zoom has a linear distortion of 1.7% at 18mm compared to 0.85% for the Sigma 14mm prime lens. Some software, such as RealViz Stitcher, PTGui and PanaVue's Image Assembler, can compensate for slight linear distortion in the lens used - but some software cannot do this so you may have to carefully edit your images before stitching.
Wide angle lenses are especially likely to suffer from light fall-off, slight vignetting and/or chromatic abberations. Generally these affect the edges of the image, and you can edit your images before stitching in the software supplied with your camera, or in an image editor such as Adobe PhotoShop. Some stitching software provide functionality to adjust for these effects - see for example RealViz Stitcher and PTGui.
If you are shooting with a wide angle lens, be sure that any filters you add to the lens do not cause vignetting. A common mistake with a circular 8mm lens is to forget to remove the collar which is fitted to some lenses and which crops the image circle to less than 180 degrees.
Scanned image size vs focal length
Remember that if you used a 35mm lens to take a sequence, went down you local store to have the pictures developed and then faithfully scanned in the prints using your flatbed scanner, the resulting digitised images are in effect taken with a different focal length. Why? Because most high street stores use mass developing machines which do not develop the full negative frame, instead they crop the image slightly during developing. The effect therefore is that a 35mm sequence is likely to be in fact 36 or 37mm. If the software allows you to, allow it to second guess your lens characteristics and try those settings.
Different software achieve stitches in different ways and some quite simply out-perform others. There's nothing you can do about this other than read all the FAQs and support information about the software you use and try to get the best from it. Or, you can try, and maybe buy some new software.
You can of course re-touch the image after stitching to eliminate blurred areas, but only to some extent and only if you have some decent image editing software such as Adobe's Photoshop (use the Rubber Stamp tool), PaintShop Pro or The GIMP.
Another thing to try is to let the software estimate your lens characteristics (assuming you have the choice anyway). It could be that you have mis-judged the settings or that due to the peculiarities of the software a focal length other than that actually used will give the best results. If this means you get a distorted image (e.g. 'stretched' slightly in one direction) you can correct this afterwards with some image editing tools. Also if the software allows you to, don't forget to try adjusting blending settings as this can dramatically affect ghosting in the stitch.
If you have a highly repetitive series of pictures (such as a concert hall in which there are many rows of seats) then the software might be unable to work out where the correct stitching point is. In these situations you need to manually align images.
Still got a problem?
If you still have a problem it is likely to be aleviated if you:
- Shoot pictures landscape not portrait
- Try a different lens or lens settings (to eliminate lens characteristics/distortion as a problem)
- Try using different stitching software (note most have trial versions)
- Increase the overlap between shots and either allow the blur to occur between shots as they are resolved or cut down the scanned images so that the width of each component is less than normal.
- Make misalignment and parallax less obvious by avoiding shots that involve objects or strong lines/edges in the foreground, close to the camera
- Use a panoramic head
If you are not already using a pano-head then that should be one of the first things to try if you can afford one. Only a pano head is capable of completely eliminating parallax effects in your images. If, despite using a pano-head you still get parallax effects it could be because you are not using your pano-head correctly. If you have a pano head take the time to find the no-parallax-point of your lens and test that it is correct using the lamp post test.