Black anodised aluminium in IR

Many cheap extension tubes, lens adapters and lens hoods are made of aluminium and have a black anodised finish: are they always better than plastic ones? As it was revealed by the macro extension tubes comparison internal reflections can be a problem in visible light and lead to glare and low contrast. These problems are caused by a relatively small amount of visible light reflected by anodised aluminium, specially if the surface is smooth. The ribbed surface frequently used avoids especular reflections, so controlling “stars” and similar artefacts but not so much the loss of contrast due diffuse reflection.

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Two aluminium lens hoods which look almost identical under visible light. Both bought from eBay sellers. (The white slab on the left is a piece of PTFE used as white reference.

Black anodised aluminium looks black as it absorbs most visible light, although not all of it. At some angles of incidence some light is reflected and and the surface may appears shinny. In the near infrared region the problem is many times worse, as black anodised aluminium reflects a large fraction of the incident radiation. Lens hoods that look almost identical under normal illumination, may differ in being coated or not on the inside. The hood from RISE(UK) is not suitable for NIR imaging while the one from JJC is, and probably this second one is slightly better also for visible light photography.

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The same two lens hoods look quite different when under near infrared (930 nm LED) and photographed with a digital camera converted to “full-spectrum”.

Most expensive macro extension tubes, lens adapters and lens hoods, and occasionally also some cheap ones, are painted matt black on the inside, and this is one of the reasons why they perform better. Those who build telescopes as a hobby use, as an easy to find non reflective paint, black blackboard paint. An alternative is the use of black flock or black velour. Black flock with self stick backing is available but can collect dust.

As some of the cheap aluminium-made Chinese macro extension tubes and lens hoods are of good quality except for the NIR-reflective and partly VIS-reflective inner surfaces, and as equivalent items are not always available with a better finish, I have started some tests on how to improve the optical performance in the NIR waveband of a lens hood, hoping later to also improve extension tubes and lens adapters.

My first trial was with water-based black acrylic blackboard paint “Déco additif” peinture ardoise/boad paint (Lefranc & Bourgeois, Le Mans, France). I painted a lens hood of a length not available from JJC. The result was a significant improvement, but not as good as I would have liked. I will try with a third coat, but apparently this paint is not fully matt.

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The lens hoods differing only in their length, but both with the black anodised aluminium finnish. The one on the right, painted on the inside with two coats of black blackboard paint. An improvement, but far from perfect.

I end with a comparison with an original lens hood from Olympus, frequently described as grossly overpriced for a piece of plastic. Its performance is excellent both in the visible and in the near infrared.

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The same lens hood on the left and an original lens hood from Olympus on the right side. They look quite different when under near infrared (930 nm LED) and photographed with a digital camera converted to “full-spectrum”

In conclusion, when possible, it is probably best to stick to original-equipment lens hoods supplied by the manufacturers of high quality objectives (caveat: I have tested only Olympus hoods). I used the painting of the lens hood as a test, as I need screw-on lens hoods with a 52 mm thread for use together with special filters of this size, as I use them with step down rings, which prevents the use of the original lens hoods.

Update: I have now realised that Heliopan makes metal lens hoods, which I expect to be of good quality. I will special-order one if I get confirmation of its finish. For the tests with adapters and macro extension tubes, I have now ordered a special paint by Tetenal, giving less than 5% reflectance (thanks Hannu for the tip!).

Conclusion: If you use lens hoods only for visible light, you should be able to approximately gauge their reflectance by eye. If you take images beyond the visible, tests are needed, specially for NIR. I have tested these same lens hoods in ultraviolet A (365 nm LED), and they all absorb UV-A radiation quite effectively.

 

 

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