panorama projection

 

 

 

I originally started using panos - images composed of two or more frames - to get more resolution in images from my 3 megapixel D30 and later a 6 megapixel 10D. I have a Canon 5D now which has a pretty useful 13 megapixels. Its files are the same size as a 3-shot pano from a 10D, and at the time those 10D panos seemed totally awesome. So aren't 5D panos just over-the-top ? Actually no because with the 5D it's often the wide angle lenses that are the limiting factor on image quality. A landscape photo shot at 17mm could well include a whole mountain range in the field of view and this is just where extreme resolution is needed. With the camera in portrait orientation  I can zoom to 24mm and get an equivalent vertical angle of view to the 17mm frame; and then pan to get whatever horizontal angle of view I want. The quality from multiple 24mm frames just wipes the floor with the single 17mm frame.

I seem to be making more use of super-wide lenses, and some shots get stronger the wider you go. So I sometimes stitch 17mm frames ( this is the widest lens I use ) to get even wider.

It's also possible to pan vertically if the final image is going to be square format or portrait. You have the option in this situation to use different focus points in each frame. Typically the lower frame is the foreground with a near focus point and the upper frame is the background focused at infinity. You still need to retain depth-of-field up to the point where the next frame will cut in. However with enough frames you can in theory nail the focus on everything, without using extreme apertures like f22, which tends to make everything a bit soft due to diffraction. This technique is used in the image below.

Easter flowers

panoramic tripod heads

Although the camera and lens obviously have to move as the shot is panned, they have to do so in such as way that the viewpoint of the camera is unchanged. Unless the camera movement is precisely controlled, the foreground will appear to shift against the background and it will then be impossible to stitch the frames. It is for this purpose that pano heads are made. Each lens has point about which it can be rotated to give constant viewpoint, sometimes referred to as the nodal point, although this has different meaning in optics. I'll call it the rotation point. The pano head has a slider with distance marks to set the rotation point. You need to find the correct slider setting for each lens - it's best to make that determination at home and record the results. In theory a zoom lens might have a variable rotation point but I have found that my 17-40L has one point for the whole zoom range, at least within the accuracy that I can measure. Below is the 17-40L and 5D mounted on a Manfrotto 303 pano head.

Manfrotto 303 pano head

The Manfrotto's slider is not long enough for all my lenses. It is just about falling out of the head for the 17-40L which is quite a compact lens. Maybe the head was made with traditional prime lens designs in mind, and the behaviour of the modern zooms was not anticipated. I mount the pano head onto a Manfrotto 029 tripod head which I use to level the base for the pano; I can rotate the 029 through 90' for a vertical pano.  It's a lot of kit but it all works smoothly once it's set up. You then have confidence that your frames can be stitched when you get home. Before I started using the pano head I would spend ages trying to stitch frames before realising that the viewpoint had changed, and that is so frustrating, it really sucks. The only type of shot where you don't need a pano head is for far-field where there is in effect no foreground in frame. The shift of viewpoint will then not be noticable.

 

free hugin software

I found some useful freeware for panorama stitching called hugin, which you can download from http://hugin.sourceforge.net. It works as a GUI in tandem with the freeware written by Prof Helmut Dersch known as Panorama Tools, which has been around for some time. The instructions for downloading are on the hugin site and I won't repeat them, although I will say it took me a while to get it to run. The application complains about some missing executables and I had to copy one from a subdirectory in the installed package into the main directory in which hugin.exe lives. Also there is a note on the hugin download page about downloading the program PTStitcher as an optional extra, although I found I needed it to get the features I expect of pano software. Once over these hurdles the interface is really nice to use.

I set the output from the stitcher to layered photoshop, and it puts each of the constituent frames in a separate layer. I then decide where the seams are going to be by deleting from each layer as appropriate. There is a bug with PTStitcher when creating layered files. To workaround it you have to create copies of each layer and then delete the original layers. Do this first and save the psd file. Otherwise you'll find the delete tool doesn't work on your layers ! Don't complain, all this software is free after all.

 

panorama projection

rectilinear (left) and equirectangular projections

There are a couple of basic settings that control the way a wide angle pano looks :

projection type -  I use either rectilinear or equirectangular. Rectilinear projection is what you get from most lenses and it renders straight lines in the subject as straight in the image. The consequence of this is that outer part of the image is stretched, going off to infinity as the field of view approaches 180'. This is where the equirectangular projection comes in, it is like printing from a rolling cylinder and can be used for full 360' panos.

reference point - this is the point in the projected image that the camera appears to be pointing to ( marked by the crosshairs in the picture above ). Because the camera is changing direction between frames, it's not obvious where the plane of projection should be. For example, if you are shooting a building you normally want the camera horizontal so the verticals in the image don't converge - you tell the pano software that by setting the reference point of the image to where the camera would point if it were horizontal.

 

 

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