Analysing the surfaces of the elements reveal that they initially look as good as "factory"; so to say they are as good as you could expect from an factory industrial process. The surface quality is visibly flawless, with absolutely no micro scratches visible to the eyes, even with intense examination with an inspection lamp.
One slight issue however is slight distortion when looking through each element. It is not an obvious defect, in fact you have to know its there to really see it, but it focuses around the very centre of the elements. There is a very slight change in surface curvature which results in the image exhibiting slight barrel distortion just slightly. This effect has been seen in my previous lenses, but to a much greater extent. The new polishing process has resulted in the change of surface curvature being ironed out, literally its "corner" smoothed. This means that in the final image the distortion is almost invisible, and can really only be displayed if you point the optic at a scene featuring a series of parallel lines.
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| Focusing through the lens surface onto the end of the rod. Can you see the slight distortion? It's so small I think it's pretty much impossible to capture in a photo. |
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| Here I have tilted the rod slightly. Now the distortion spot is more visible. Look in the very centre where the image of the back of the rod stretches out to the right a little. This is the problem I'm talking about. |
The presence of the error still needs an explanation though, so that it can be reduced as much as possible. From experience I know that distortion like this can only be caused by the constant curvature of the lens surface changing. Looking at a cross section of a hypothetical convex lens, you would expect to see the lens surface to have a perfectly constant radius all he way across. Although it is too small to see in real life by looking at the element i have made from the side, It is obvious to me that this is what is occurring.
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| Here is an example of stepping, simulated in CAD. It is a 0.25mm step which I calculate when programming the manual cutting sequence for concave lenses. Imagine thing stepping pattern but on a 25x smaller scale. |
We are talking about errors that are now to small to even properly observe, so I can only guess at what is going wrong. My educated guess is that I have actually reached the limits of the equipment, in this case the CNC lathe. Most CNC equipment use stepper motors to move their components, and as such there is a minimum limit to the size of an individual step. As far as I am aware, the machine I use has a rated minimum step of 0.01mm, but in reality, given hat it is a 15 year old refurbished machine, this is not a very reliable number. This tolerance is important because the profiles I am trying to cut, constant radii, have to be translated into a stair like step pattern for the machine to follow. As we are using a turning tool which cuts from one side, there is a point towards the centre of the element where the steps cannot be made any smaller, and the cutting tool effectively has to travel vertically in a straight line. This flattens out the front of the lens and could cause the discussed distortion. Remember, this is all happening on a minute scale, but the effect is enough to effect the image being transmitted through the lens.
For the purposes of my prototype, this minute error has to be accepted. As mentioned previously, the new polishing process reduces the errors affect on the image to a certain extent to in reality the error should be barely noticeable in the end.
