

Article 3D Photography Stereo BaseThis is apart of a series of articles on 3D photography, the best starting point is 3D Photography  An Introduction. In this article we are looking at the distance sideways between the position of one photograph of a stereo pair was taken from the other. To produce a 3D effect we are simulating the view you would get from two eyes positioned apart. If we used a lens of about the length that would be similar to our normal view, say 50mm, and nothing was too close this could be a fixed difference apart of around 7.5cm or a little under, the often quoted distance is 65mm. Increasing it a little expands the 3D effect. When we have different lens lengths, or get closer to the subject we need to change the distance between the positions that the two images are taken. This separation distance is called the Stereo Base. The factors that affects the stereo base, (distance between the camera when taking one image and the next) are the focal length of the lens, the nearest item and the furthest item. The stereo base is larger when:
As long as its about right, you will get the desired effect, and experimenting you will find that unless you are very near something you can be quite a long way out from the theoretical stereo base and still get good results. There are a number of free software programmes, that you can download, that computes the stereo base for you, or if you decide the stereo base and another two factors it gives you the remaining one, useful for checking out ideas. Put stereo base calculator into 'Google Search' to get links to them. You can download a copy of the one I currently use at 'StereoCalc100.zip' (243KB/ZIP) (please let us know if this fails to work for you, links often change). If you are using two cameras there will be a minimum distance between them, unless you have constructed some device that allows you to have one looking up and one down, and a couple of special front silvered mirrors to allow them to look forward. This will therefore put some working limitations on what you can do when using two cameras. The base that you have between them may also limit the distance you have them apart. Some examples:
When two cameras are used we often have a limit of how close we can move them together, so often the smallest stereo base is a known variable and we have to work within the limits this produces.
You can usually work up to twice the ideal stereo base, the exception being when you get very near to items, the nearer you get the more the stereo base becomes critical. The problem of working very close to the subject As the camera moves in parallel along in front of the subject, we get the slightly different view from two eyes, but when we get closer there is a parallax shift if we are not careful, so as we get closer and closer the distance we can or need to move the camera is reduced until in macro photography it may only be a couple of millimetres. As a general rule you don't have to be concerned about this as long as the nearest items to you are 6ft (2 metres) or more away. Below this its far more important to look at the stereo base and maybe take extra images with small stereo bases. The above Anagraph image works quite well, although it's using a wide angle lens.
This one does not work as well as there is
less foreground items and they are further away, When items are a long way off Our eyes give us good 3D effects at shorter distances but as items get further away we have far greater difficulty to producing a 3D effect. When we use good binoculars, to look at something some way off, the items have a greater 3D effect than we would normally see at that range with our eyes alone. This is because the design of the binoculars are such that the centre of the lenses pointing towards the subject are further apart than the eye point that we are looking into. To get a good 3D effect therefore we need to have a larger stereo base when items are a long way off, using two tripods or a very long bar. Some people position 2 cameras many feet away, perhaps as much as 8ft apart when taking firework photos. See below how using some older techniques we can compute these distances when out and about. Some more terms There are terms that you may come across relating to shortening or extending the stereo base from what was considered normal. Normal was usually considered to be 65mm for a standard 50mm lens. I personally tend to use 75mm as the standard, increasing just slightly the 3D effect but not enough for people to notice the trick I have played. So some would call this Hyperstereo. Older techniques Historically we were taught that there was a simple formula to determine this, and that this was to divide the distance of the nearest item by 30, for a 50mm lens, and adjust the dividing factor proportionally for other focal lengths. The adjustment was simply a case of multiplying the result by the new lens length over 50. Going back to some more distant items we mentioned above, lets apply this to some examples:
Okay I cheated and chose the focal length I had so I could work this out in my head. When you divide by 120, knock off the last digit and you are dividing by 12 working in feet, so you can take the result as inches direct. So with 100ft, discard the last digit and treat as inches and we had 10 inches. You may need a bit of paper and a calculator to work in metric, to make it easier in the field. Working things out in my head, paper and calculator, I am showing my age!!! Another simpler approach A simpler version or the same concept used by some, says that the distance to the closest point must be greater than the stereo base x focal length with everything in mm. So lets say you have a 50mm lens and have a stereo base of 70mm then 50x70 = 3500mm, so 3.5 metres is the closest item you should include. Take another example, using a wide angle lens of 12mm, and closing up the stereo base to 50mm, we get 50x12=600mm, so the nearest item should be no closer than 60cm or just over 2ft away, this might for example apply when taking photographs of the architecture and furnishings of a church. We have a Look up sheet using this simplified approach, and a PDF version of the look up sheet that you can print. Out in the field without your computer There are stereo base programme versions that will run on hand held computers and many phones now access the internet and therefore an online version may also be able to be used, providing you have a mobile phone signal. Also you can get a dongle that allows you to connect a laptop computer up over the mobile network, but if you use this you have to keep a very close watch on your usage, some people have got bills for thousands of pounds when using these systems. You could also have another photographer or someone you showed how to run the programme that you could call. All this is dependent on a mobile signal being present and often it is not. With a laptop you can run the programme anywhere. For most working distances when you are away, you can simply use the older method of divide by 30, it's not as accurate but in most cases it will be near enough, providing that nothing is very near. The other solution is to consider a list of possible distances and lens lengths that you may use and to look up these, making a note of them, and select the nearest or logical one between them when you need one. I have been looking to see if I could produce a set of tables like the ones I have for depth of field for various lenses, apertures and distances and if I an Ii will update this article.
The near grass in the foreground limits the amount of depth we can generate. See Also our 3D Section for more articles and projects on this topic.




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