Sunday, November 28, 2010

On panoramic mounts and entropy

Still going strong in the DIY photography category (even though there has been some delay). This time it's a panoramic tripod mount!

With current software it's extremely easy to make a panoramic picture out of a set of overlapping photo's. (I use the excellent free and open source tool hugin.) There are, however, some things you should be aware of. Of course, the pictures should be taken from the same spot. The logical thing to do is to use a tripod. This gives good results for distant landscapes, but if you include near objects in the scene, you will encounter some problems stitching togenther the panorama. This is due to parallax and can be resolved by using (building!) a panoramic tripod mount like this:

(If you find the shadow on the above photo a bit weird, it's because I shot it with my ringflash, no particular reason)

Instructions and lots more after the jump!

Below is a crop of an atypical panorama shot (indoors, close proximity) that shows the effects of parallax at the bottom. The distant wall is stitched nicely, but the trays at the bottom are crooked. Note that the overlap between subsequent pictures was very high (around 2/3rd of the width), nonetheless there still is a lot of misalignment.

(and yes, that plastic container is part of my ringflash from the previous post)

The solution is to make sure you pivot the camera around the no-parallax point or nodal point. This where your lens's entrance pupil is, a sort of virtual aperture. It is situated somewhere 'inside' your lens and can easily be found experimentally as sketched below:

You basically look at a near and distant object through the viewfinder while you rotate the camera. If you rotate around the nodal point, you will notice a complete lack of parallax (this is good!). On my kitlens, the pivot is about 1cm from the end of the lens when it is at a focal length of 18mm (the nodal point is focal length dependant).

Now that we've found the no-parallax point, let's make a tripod mount that enables us to easily pivot around it!

The strobe I talked about in my previous posts came with a 'bar' to mount it on the camera. This will do perfectly for the panoramic mount:

That bar will hold the camera, which gets screwed on with the left screw in the picture. The bar will get rotated so the side that now holds the strobe sticks out in front, underneath the lens. The screw on the right to mount the strobe to the bar will get connected to a bracket that mounts to the tripod's quick release plate. This screw controls the up/down tilt of the camera.

All the pieces, with the 'bar' from the strobe and the bracket connected to the quick release plate:

I have to mount my camera pretty far back, so there is quite a bit of torsion. Therefore I added some rubber to the bracket to give the arm some extra traction.

The bar mounted on the bracket, acting as an arm to mount the camera on:

The camera is mounted with the lens facing left, so the nodal point of the lens is right above the pivot of the arm. The length of the bottom piece of the bracket is such that the lens is centered above the tripod. When the panoramic head is mounted vertically on the tripod (see first picture of blogpost), the regular left/right panning of the orgiginal tripod head underneath pivots the camera nicely around its no-parallax point.

I haven't found a nice landscape to try this mount on, but I did find another subject. It's related to this blog's title ('Highly Entropic') and I decided it could use a seperate post.

The panoramic shot you see there was made with this mount. It shows that the mount does exactly what it should do, even in a pathological environment for a panoramic shot (lots of objects that are very near: a nightmare for parallax).

Some afterthoughts: I had already feared it, and it turned out to be true; the aluminium bracket I made is a bit floppy (I did not have thicker aluminium laying around). This means I had to shoot with the self-timer and mirror lock-up. Luckily I built a self release cable to avoid having to physically touch the camera. These measures proved to be adequate to minimize motion blur. It does mean that making the shots takes quite a bit of time.

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