Panoramic/Stitched Images

Indoors, outdoors, it doesn't matter how wide an angle of  view your digital camera lens might have, in our experience there are invariably situations when it just isn't enough. There are often times when you find that you can't fit into a shot all that you want. The easy answer is to move backwards, further away, but in a lot of situations this just isn't possible. Either there just isn't the room to do so, or what you are trying to take a shot of is then obscured, in part or in whole, by something that stands between. It's particularly the case if you tend to take shots in cities, where the buildings are always big and the space between them is invariably small. But it can happen anywhere at any time.

If you have a digital SLR with interchangeable lenses then of course you do have the option of changing the lens you have for one which gives a wider view. But even here you often come upon the same problem, that as wide as you can go, it's still not enough. And of course, wide angle lenses for current DSLR's are few in number, and very expensive. Even if you do have a wide angle lens you can use, constantly changing from one lens to another is always awkward, especially on city streets. Just keeping a wide angle lens on, even a zoom type, has never for us proved possible. Whilst you might want to capture as much of a building or scene as possible, it is also the case that telephoto and close up shots are equally valuable.

However, perhaps the biggest drawback of all is that as you use a wider view to get in all you want horizontally, or vertically, the other aspect also increases. The result is that almost invariably what you are trying to capture, the main focal point of the image, becomes smaller as a total part of the shot. Cropping the image to exclude too much foreground or sky that is not required may give you a panoramic view type image, but the problem that then often arises is that the image quality is not as high as you would like, because the image has had to be increased in size more than it normally would, reducing the overall resolution. It's not a problem that's restricted just to digital cameras of course, users of film cameras have suffered in just the same way. 

In the past with film this has led to the production of special panoramic cameras, made specifically to produce panoramic images. Most of these have been designed around the use of the standard 35mm and 120 film formats. Those using 120 film use special wide angle lenses, whilst some using 35mm film have lenses that are not fixed in place on the horizontal plane but pivot about the lens nodal point as the image is shot thus taking in a wider, panoramic view. In more recent times Hasselblad have made a panoramic camera called the X-Pan which also uses 35mm film to produce panoramic shots, but like the 120 format designs this uses specially made wide angle lenses instead.

All types of panoramic camera are naturally very expensive, but for those that need them they do overcome the problem of low resolution images, producing high quality high resolution shots, especially those that use 120 format film. However, nice as they are for those that can afford them, panoramic cameras don't overcome the lack of room to shoot a scene when that particular problem raises it's head.

Stitched Images

One of the advantages of digital imaging is that there is a way of overcoming these problems, and this is by stitching images together on the computer using stitching software. It's not restricted only to those that shoot using digital, film users can also use the technique by scanning their images in and producing digital files that can be worked on, but there is no doubt that, along with the right equipment and technique, it's far easier to achieve using a digital camera with it's LCD screen for comparing taken images, than a film camera. Not only that but it can be, in comparison to using film, a lot cheaper. Although you need stitching software with which to assemble and merge the individual images used, no special camera is required to take them, and the most basic low resolution digital camera can be used if you wish. Another advantage is that even in this situation a high resolution image can result, as a stitched image is naturally composed of multiple shots.

Generally, stitching images together to provide a bigger view is regarded as the way to provide panoramic images, but is is also the means whereby an image of a scene can be produced that might otherwise be impossible. This can be achieved by stitching multiple rows of images together. Not all stitching software can do this however, which is why the general view of such software has arisen that it's only for producing panoramic shots. The ability to take and assemble multiple row images gives the user of digital cameras and stitching software another big advantage over the dedicated panoramic cameras, which of course can only provide one type of fixed format image.

In actual fact three different types of panoramic images can be produced. The flat rectilinear, sometimes referred to as a planar. The cylindrical, which is basically a series of rectilinear images with the ends joined together giving up to a 360° view. And the spherical, which can give a 360° horizontal and 180° vertical view, virtually 360° all round. If you use the best available software you can take a series of shots and decide later how they should be assembled, in other words how wide the view should be, or the perspective involved, all of which are adjustable. 

Most panoramas can be produced in Apple's Quicktime Virtual Reality format [QTVR] and viewed using the free Quicktime player - www.apple.com/quicktime . These virtual panoramas can be used to give virtual tours around almost any location or the inside of a building, room by room. Tours such as these are becoming increasingly common on web pages. Estate agents and those selling cottage holidays are finding them very useful for generating custom. The advantage of a QTVR format panorama over a short movie file produced by rotating a video camera around 360° is that, as a series of still images, there's no camera shake or jerky panning, it can be panned and rotated at will and stopped for examination at any point of interest. Great for showing the interiors of historic buildings as floor and ceiling can be viewed as well as the walls.

All image types can of course also be printed to produce panoramic views of one kind or another, which is probably of most interest to the ordinary digital camera user.

Taking shots for stitching

Images shot with the intention of stitching them together later need to be done with some care if a successful outcome is to result, and a few basic rules need to be observed. Following the rules is fairly easy, but it helps if you understand why you need to follow them, and what can go wrong if you don't. 

The Lens Nodal Point.

One of the most important points to be aware of, and understand, is that of the lens nodal point. It is around this point that ideally the camera you are using should be pivoted when taking images. There is a very good reason for doing this, and it's to do with perspective alignment and the avoidance of parallax error

All dedicated panoramic cameras take the image in one go using a fixed focal length lens so the perspective across the frame is constant. The panoramic types that use moving lenses differ slightly in that instead of the whole film being exposed at once it's done more slowly in stages as the image from the lens passes over the film from one side to the other. A constant perspective is still maintained by ensuring the rotation of the lens is around it's nodal point. Physics tells us that the nodal point of a lens is situated along the lens axis where the refracted lens image light paths intersect, fig 1, the image a lens produces being inverted, and this varies according to focal length. 

nb. some people refer to this point as the exit pupil point, no doubt because the human eye works in the same basic manner.

Lens Nodal Point

When taking multiple images to stitch together, the procedure is basically the same as that which the panoramic cameras with rotating lenses use, except it's being done in a series of separate stages, individual images, rather than in one go. There is therefore still the need to rotate the lens used around the nodal point of the focal length used to ensure that the elements in adjacent images stay constant in their perspective relationship to each other. If this isn't done, and the parallax errors that result are too great for the stitching software to overcome, a poor stitch results. Image elements may be out of alignment, i.e. not match up, or 'ghosting' may result, a shadowy secondary image element behind the first, slightly out of alignment.

To give an example, take two poles, one in front of the other. Looking straight on you should see the first, but not the second, which is hidden by the first. If you turn to your left or right, pivoting about your centre you should still not be able to see the second pole. The same happens when a camera is pivoted about the lens nodal point. Only if you move sideways left or right will the second pole become visible, because you have changed position and the perspective has changed in relation to the two poles. 

This will also happen if a camera is not pivoted about the lens nodal point. It's view perspective changes. These perspective changes, called parallax error, mean that in adjacent shots distances between objects in a scene do not remain constant but alter, and the stitching software then has difficulty registering the images because of it. 

The further away from the nodal point that the camera is pivoted between shots, the greater the parallax errors, and the harder it will be for the stitching software to merge the images successfully and obtain the correct perspective.

The most critical situation occurs where the focused distance is minimal, or there is close foreground detail. The greater the focused distance the less critical this becomes. As the nodal point varies with the focal length of the lens finding it is not a case of mathematics, but a practical matter of trial and error adjustments.

Finding the Lens Nodal Point.

As the nodal point of any lens depends on it's focal length, the way of finding it is by moving the camera forwards and backwards until a pivot point is found where little or no parallax error can be observed. It's done purely by subjective sight assessment, and it's impossible to do without the use of equipment, a panoramic head, which we will deal with later. 

It is particularly noticeable that trying to pivot the camera around a point behind the nodal point range, generally where camera tripod mounts are situated, produces much greater parallax errors than pivoting around any point in front of the camera body.

Overlapping Images

All the shots that are going to comprise a stitch must overlap each other by at least 30%, whether it's a single row or multiple row stitch. Overlapping of shots is vital as no stitching software is able to butt join images, the software requires comparable image elements - pixels - on which to base the calculations that are used to manipulate the images for merging together and warping to the correct perspective. It's better to allow more overlap than less, anywhere between 30%-50% should ensure a decent match, see fig 2. This figure applies whether taking shots horizontally or vertically i.e. in normal or portrait orientation.

Alignment

Miss-alignment of shots, vertically, horizontally or around the lens axis can lead to difficulties not only with matching images but also to the stitched results where excessive cropping is required, and some of the required scene elements end up missing. 

Warping

All images in a sequence must be shot from the same viewpoint using the lens at the same focal length setting. This enables the software to 'warp' the images. The warping of images is the means whereby the perspective of the images is aligned to match each other. Most decent stitching software now uses the EXIF data from the images used to determine the lens focal length used and the resulting perspective, correctly applying the relevant LMF factor involved. It's important to note that different view perspectives are involved depending on whether the images used are horizontal or vertical in nature.

Exposure

The final requirement is exposure. If you are taking a series of overlapping images covering a wide perspective where the light levels might well differ, it's important to use a constant exposure setting to maintain the same level of exposure across the series. In other words the same aperture and shutter combination. The use of the basic auto-exposure mode is not recommended. It can lead to too great a variance between images. Some, but by no means all stitching software, has blending and exposure correcting features, but there is still a limit to what these can do in correcting the imbalance between adjacent images.  

The best course of action is to set the exposure manually, and use the same setting across all shots taken. If there appears to be a wide variance in lighting levels, then the brightest part of the scene to be captured should be used to set the exposure, to ensure there are no blown highlights.

To achieve a really good image many photographers try to use as small an aperture as possible to ensure good depth of field, just as you would with a landscape shot. Unless light levels are high this means of course in most cases the use of some kind of support, in the form of a monopod or tripod, is required.

Following the rules

So how do you follow these rules? By taking care when shooting hand held, and using equipment to help you where you can if you wish.  We'll take a look at using monopods, tripods and other equipment in minute but first let's take a look at taking shots hand held.

Hand held

The situation that most people will find themselves in sooner or later, is wanting to take some shots they can stitch together on the spur of the moment. For it's more than likely that unless you are very keen on producing stitch or panoramic images you won't necessarily go out with the express intention of doing so. Even if you are going into a city to take some shots it doesn't always follow that everything you will want to capture is going to be too big to fit in the frame. But it's often the case that things turn out to be much bigger than you thought, looking through a camera's viewfinder, than they do viewed with the naked eye. 

So you think, I know, I'll take some images and stitch them together later. The trouble is it's notoriously difficult when you are using a camera hand-held to keep it aligned between shots, either horizontally or vertically. It's also very easy to twist the camera around the lens axis so that subsequent shots are aligned in one plane but not the other. You only have to look at photos people often take, where the horizon ends up at an angle to the horizontal plane of the image to see the difficulty that exists.

Ideally you're looking to take a series of shots as in fig 2. But often, if you are not careful, you end up with a set looking either like fig 3. or fig 4. or sometimes both, fig 5. These show horizontal format images but the same applies equally to those taken vertically, portrait format. If your taking more than one row of shots then errors like those in 

fig 3 can be overcome, the rows above or below filling in some of gaps. But images skewed around the lens axis  like those in figs 4 & 5 lead to image mis-alignments that are difficult to correct.

To combat this the task is to ensure that you stand correctly when taking shots and move around the camera as in fig 6. This will ensure that the pivot point is reasonably close to the ideal. Making yourself the pivot point and swinging the camera around you in an arc will make things much harder for the software, although this is the most natural action to take, so you have to remind yourself not to do it. fig 7. You may sometimes get away with this if taking just a couple of shots for a Panoramic type stitch with the focused distance being at or near infinity, otherwise you will end up frustrated and disappointed at the lack of stitching success. 

Just pivoting the camera/lens around its centre point might not sound ideal after what we have written regarding the nodal point of lenses, but we have found that it seems to work reasonably well with series of shots that do not cover a wide arc. We invariably take hand held shots, even for multiple row stitching, and most of the time it works out, although not always. Some of this might be down to the software we use which we will discuss further on, or just luck, although as with anything a bit of practice always helps. Going around locally and just taking practice stitch shot sequences will soon show you what's possible and what's not, and where you might need to improve your technique to succeed. It's also useful for getting to know your software and what it's capable of. And of course with digital it costs little, just a bit of time and effort.

Simple Panorama's

To take a sequence of shots you need to look through the viewfinder and see how much of the scene is captured. Generally for a simple panoramic two or three shots are involved. Keeping it to two is okay but taking three with more overlap is better. Take one straight ahead, then one to each side. Make a mental note of where the image elements are in the first shot so you know just how far to pivot the camera to make the subsequent shots overlap the first. 

Here is a very simple cylindrical panoramic stitch involving just two images. It's of the Roman Amphitheatre in Verona, Italy.  When we took these shots of it in 2002, using a 3mp digicam, a famous Italian film director was in the process of staging the play Anthony and Cleopatra in it, which explains the props. Originally we generated the stitch image using the Photomerge application in Photoshop Elements, but this current stitch was created using the PTGui stitching software which we now use. Verona is of course famous as the setting for the Shakespeare play Romeo and Juliet.

Roman Amphitheatre, Verona, Italy.

This panorama is a classic example of when moving further back to capture a scene just isn't possible. The digicam we were using had a 34mm [equiv] view at the wide angle end - the two images used being shot at this focal length - but moving further back only meant that other features in the square in Verona where the amphitheatre is located, trees, fountains, etc, then blocked some of it from view. There was no way we could capture it in one shot, and of course the position of the large props just begged capture from the perspective we had chosen.

The ability to use a much wider angled lens might well of resulted in the capture of the same scene in one shot, but the perspective would have been different. As it is, the props, though large, do not dominate, there's a fairly equal balance between them and the amphitheatre. A wider view would have resulted in the props being very dominant and un-balanced the scene as we wanted it, the amphitheatre then becoming just scene setting background detail.

Multiple Image Panorama's

Taking more shots or multiple rows is just an extension of this basic concept, although we have to say that taking really wide panoramic images involving many shots in each row hand held is not really a good idea. To do that successfully you really need to use some kind of camera support, and pay closer attention to the lens nodal point. 

However........ having said that, here's an image that is composed from 8 shots, 2 rows of four, all hand held. The task we had here was to capture the front entrance to this shopping mall, which is situated down a pedestrian side street. Once again lack of space to capture in one shot was the problem, along with the requirement to keep the final perspective as near to normal as possible. 

Again this is not from a flat stitch but a cylindrical one, as you might see from the curved front of the extreme right hand side building, which is flat in real life although the rest of the building is curved as in the image. This is as a result of using this format as opposed to a flat projection. Flat projections keep all straight lines straight but produce distortions (compressing the image vertically/stretching horizontally) at the edges of images that cover greater than about 120°. A cylindrical stitch produces curved lines but less distortion in other areas. It has the opposite effect of a flat planar stitch compressing horizontally/stretching vertically. 

What type you choose very much depends on the type of panorama being stitched, buildings/landscape, and the perspective you require or prefer. The best Panoramic software allows you to switch views to see the results before rendering the finished stitch. You must of course understand that there is a difference between image distortion caused by optical problems and perspective distortion caused by the width in degrees of the stitch being assembled.  In fact due to perspective distortion problems flat planar images over about 100°- 120° are not really recommended. Our software is designed not to stitch them and automatically sets cylindrical stitching, the planar option being unavailable.

There's another fault we have deliberately left in this stitch. The woman in the green coat exiting the building. Two images of her exist. This is as a result of taking sequential shots and merging them. Her position had changed because she was moving between shots. And that's another downside. You can't take action stitch shots, it just doesn't work.

Distant Panorama's

Just in case you get the idea we don't take distant panoramic stitch shots here's one of a well known building, the Sydney Opera House. The use of a panoramic view here helps to put it into perspective in relation to it's surroundings. Again this is a cylindrical stitch using just three shots taken hand held, this time from the middle of Sydney Harbour bridge. The view covered is around 160° but doesn't look it because of the distance and the large expanse of water in front which helps to hide perspective distortions.

Sydney Harbour and Opera House.

On the next page we'll deal with the subject of taking shots using equipment.

Monopods, Tripods, and the use of special Panoramic Heads.