This Instructable shows how to build a simple device to allow you to return your camera to virtually exactly the same spot and orientation. Moreover the support can be built for about $20 to $30 dollars depending on options and what you can salvage. I have called the device "The OB-Pod" from Optical Bench and tripod. One of my lab friends pointed out that it was effectively a portable optical bench (thanks Anton).
So why would you need to do this?.................
Nearly all the latest forms of photography to be come popular are multi-image techniques and/or involve changing filters:
(1) HDR (High Dynamic Range)
A number of photographs are taken with different exposure settings and they are combined to produce an image with control of local contrast.
(2) Focus Stacking
A number of photographs are taken with different focus settings and they are combined to produce an image with a greater depth of field.
(3) Other Image Stacking Techniques
Image stacking has been used to reduce noise, reveal moving structures, artistic effects etc.
These first 3 are referred to as image stacking techniques. The common problem is keeping successive images aligned; this is usually remedied by careful use of a tripod and software. Image alignment software is good but you get the best results if the images are aligned as perfectly as possible in the first place. Many digital cameras have built in exposure bracketing but it is limited; to get better results you need an expensive remote controlled camera. Most of us though, try changing the camera settings by fiddling with the camera while it is on the tripod, but pressing menu buttons etc is enough to put the camera out of exact alignment. The loss of alignment problem is even worse if you want to stack images shot with different filters, changing a filter without the camera moving is really hard on a tripod. It would solve these problems if you could just take the camera off the tripod, change the settings and put it back in exactly the same place.
(4) Time Lapse
A large number of photographs are taken with equal temporal spacing. The photographs are linked together as a movie to provide an accelerated view of the action.
(5) Stop Action
Inanimate objects or images are photographed with mechanically applied motion between shots. The photographs are linked together as a movie to provide an animation.
Time Lapse and Stop Action normally require leaving a camera in one spot for a long time; this could tie up your favorite camera for a long time. Some time lapse sequences go for more than one day eg Panama Canal Miraflores locks covers a whole week from the same location. Rather than tie your camera up for a week or more it would be very useful if you can take your camera away and return it to exactly the same place later on particularly if your camera is out in the weather.
(6) IR Photography with an older SLR
It is quite popular to modify an old camera to be an IR sensitive camera by removing the internal IR blocking filter and using an external filter to block visible light. If you modify an early DSLR (pre-live-view) to be an IR camera you will not be able to see through the viewfinder to line up and focus your shot while the IR filter is on. With the OB-Pod you can line up and focus the shot without the filter; then lift the camera off the tripod; fit the filter and return the camera back to tripod with the alignment preserved.
I can even change the batteries in my camera without changing the camera alignment but this depends on your camera design.
The way the OB-Pod works is based on the principles that:
(1) 3 points define one plane (the reason tripods are stable)
(2) Any plane section of a sphere is a circle. In simple terms a ball will sit nicely and accurately in a smaller round hole.
In this case the ball is a dome nut and the round hole is found in the middle of a particular make of furniture foot designed to protect floors.