 Irene Troy |
Diffractive optics - Definition? I need some help understanding some lens terms: what does the term “diffractive optics” mean? The price for a telephoto with DO is substantially higher than for the same lens without DO. Is the extra cost worth it? Thanks!
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robert G. Fately |
Irene, a DO lens (and here I refer to a single lens element, not an entire assembly of elements mounted in a tube on your camera) bends light by way of a different shaping technique than a traditional convex or concave design. As you probably know, regular lenses bend light by taking advantage of the diffraction that occurs at the glass/air interface and being shaped in a smoothly ground curve to bend the light as desired. A DO lens does not have that smoothly curved surface; rather, it has a series of concentric rings carved into it (well, molded into the plastic material, actually). Each "ring" is precisely designed, and performs the diffraction required to give the final built lens the necessary performance. Think of a fresnel lens - like those 8x10 sized plastic versions that you sometimes see in the back windows of RVs, or smaller versions sometimes used as readers' aids. In essence, without getting too scientifical, the rings are designed to more sharply bend the light than a simpler smooth air/glass interface would. This means that, with the proper design, the DO optic can be flatter and lighter than its more conventional counterpart, which in turn means that the entire assembled lens (the thing you mount on your camera) can be smaller and lighter than a more conventionally designed unit of the same focal length. From what I hear (and I'm not a Canon user - Canon being the only consumer company to offer DO lenses) the DO optics are pretty darn good - on a par with the "normal" high-end lenses. So the extra cost amounts to paying for the size and weight savings - a trade-off who's worth only you can decide.
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Jon Close |
Canon's explanations: Technical Report on Canon's Multi-layer Diffractive Optical Element, http://www.usa.canon.com/html/eflenses/technology/diffractive.html and another on the introduction of their EF 70-300 f/4.5-5.6 DO USM. Worth the extra cost if more compact size and lower weight is a high priority. YMMV.
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Irene Troy |
Thanks Bob and Jon for the info. I am thinking of purchasing a prime telephoto lens for use with my Canon 5D. The price difference between the OD lens and the non-OD lens is huge. The DO lens is selling for $5300 at B&H while the non-DO is selling for $1100. Other than the DO I am not sure what the difference is between the two lenses. The DO lens is faster at 2.8, compared to 5.6 for the non-DO. Whether this is sufficient justification, coupled with the DO, for the huge jump in price will have to be thought about. Ah – to win the lottery! LOL Of-course, if I won the lottery I’d go buy myself a couple of Mark II’s and all the lenses. Thanks again. Irene
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Jon Close |
The EF 400 f/2.8L IS USM is ~$6500. The EF 400 f/4 DO IS USM is ~$5300. The EF 400 f/5.6L USM is ~$1100. Even without DO elements a high quality 400 f/4 lens would probably be over $4000. For comparison:
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Irene Troy |
Thank you, Jon for showing this to me. For some idiotic reason I completely screwed up the specs on these lenses. Taking a fifth look at B&H I see that you are, of-course, correct. For the time being I will probably stay with my Sigma zoom. It's okay, not as sharp as a Canon prime, but it does the job. I think I will save up for either of the two 400mm OD lenses. They really are incredible glass and probably worth the high money. Thanks again. Irene
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