7184Re: [William-Optics] Re: Colour coding on the aberration graph
- Jan 23, 2006Someone has already answered the color question. I thought I would answer your question on what the graph means. The graph shows how different wavelengths focus as a function of lens radial position. The bottom of the y axis is the center of the lens. The top is the outer diameter. Restated this graph shows the spherical aberration of the lens at different colors and chromatic aberration. The eye is most sensitive to light around 550nmeter, so the graphs should be about a straight line at this wavelength in a well designed lens. The graph for the Fluorite Doublet shows the lens to be very well corrected at all but the wavelength extremes. At 435nmeters the focus point is about .125mm from the 550nmeter point. (typically the focus position is calculated at 70% of full aperture). This shift in focus will cause a blur diameter of about .02mm (.125/f-stop). So at 435nm the blur is much larger than due to diffraction(around .008mm). For visual use this is of little concern because the eye has very little sensitivity at this wavelength. One of the questions floating around has been what is the difference between an APO doublet and an APO triplet. WO address this question with the longitudinal aberration chart shown in Megrez 80 fluorite triplet. It shows the curves for a doublet and a triplet. In general the triplet can bring the focus points a the 70% of max radius, much closer together over the full visual range. Also look at the curves for the Megrez 80 II ED triplet APO. Even though it uses a lesser glass than the fluorite doublet, the color correction at the extremes is much better. The comparison between a triplet and a doublet is also shown on the WO website in the 66 ED APO section. Here spot diameters are shown. As one can see the spot diameters are much small at the wavelength extremes for the triplet. The photographic process tends to be more sensitive to the color extremes. This is why triplet are often preferred for photographic work. I hope this helps.Scott----- Original Message -----From: Timm BottoniSent: Monday, January 23, 2006 7:19 AMSubject: [William-Optics] Re: Colour coding on the aberration graphGood question Dmitri!
I want to know more than that, I also want someone to explain how to
read it and what it really means.
I want to be able to add this to the FAQs I am working on for the
group, as well as for my own understanding.
I actually went out and downloaded the Zemax demo, but even after
reading and understanding the basics, and searching Google, I still
don't understand what the charts mean, to be honest, and how to use
them to compare one scope to another.
The real software costs $2000-$4000 and then you have to pay for
training, so maybe this is one of those things that you can't learn
for free, but lets see if anyone has an explanation in our group.
Anyone? Tom? Ron? Anyone feel free to offer to explain these.
Also - Tom Trusak has posted his opinions and thoughts on doublets
vs. triplets, and I will try to includes some of that in the FAQs
since it comes up often as well. I have a few sites that are very
technical, but if anyone has an easy web site that helps explain it
feel free to post it.
The majjority of companies do NOT post these types of charts, or the
glass specs, or the color correction levels, or quality control
specifications. Perhaps WO is just more honest, or perhaps they feel
that the charts help people, or perhaps other companies just don't
want to open the kimono that far because it lets competitors see what
they are doing, and gives them a chance to try to one-up them with a
better looking chart.
Either way - if anyone has any information, let us all know.
--- In William-Optics@yahoogroups.com, "kulginoff"
> I would like to know the colour legend on the chromatic aberration
> graph for Zenith Star Fluorite Doublet on the WO web site,
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