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+--- Thread: Lenstip drops a bombshell (/showthread.php?tid=1277)
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Lenstip drops a bombshell - Ayoh - 01-23-2015
Quote:I was tempted to mumble stupid things about Eastern technology, but their Sojus rockets still flew when American space transporters had to stay down. So I better don't say stupid things?Too late. Poland is not Russia so not sure your insightful opinions about "eastern technology" apply anyway. Also, feel free to educate the forum by providing some topical links to a review site which provides equally comprehensive yet more robust reviews which accord with "western technology".
Lenstip drops a bombshell - Ayoh - 01-23-2015
Quote:And an expensive failure of one at that...The test may have flaws but it is a hobby article, not a journal publication so I think you are being overly critical. For one, looking at the photos of their rig the interferometer setup is placed on what looks like a glass plate, supported by a wood sheet (A ground stone table may be underneath for all we know). Rather than directly placed on the wood itself as you suggest. Also the hardness of the glass test flat should be significantly higher than the plastic frame of the sony mirror so unless there are some abrasive particles in between, I don't think introducing meaningful scratches to the flat would be that likely. Additionally, the flat and rig may also be from a decommissioned lab which they use for hobby work so I doubt they spent or intended on spending much money as you suggest, or be overly concerned about slight equipment misuse. It is possibly also written by an enthusiastic student (in which case i'm sure criticism will be appreciated). Relax and enjoy the free entertainment, your technical specialty in this field is noted and I'm sure you could do better for a nice fee.
Here they have published a new article on the sony A99 mirror:
http://www.lenstip.com/137.1-article-Everything_you_always_wanted_to_know_about_the_semi-transparent_mirror_technology_(but_were_afraid_to_ask)_Good_and_bad_sides_of_conventional_mirrors.html
They go into several areas they would have been all the wiser to stay out of... first they have a test flat, about 6" diameter I would wager, and they ruin it for lab use by placing something directly on top of it. This is a $10,000 part... Certainly I assume they will continue to use it, but no good lab would ever use it because it is no longer flat due to scratches on its surface which will perturb the fringes.
They also go on to note fringes seen in the mirror with the flat... yes they would exist! You can place two test flats on top of each other and get such fringes even if they were polished to 1/500th wave roughness. The minor changes in the airspace produce fringes regardless of how flat the surfaces are. There is spherical aberration as well as astigmatism in the fringes of the mirror placed on the flat... it is clearly at least somewhat warped and also not perfectly flat on the test flat. In essense, that observation says nothing about the roughness and it certainly is not on the order of 1um - such a high value would appear very noticeably matte/opaque and not at all "clean" or sharp as the mirror actually does. 1um roughness corresponds to ground, not polished glass.
Their (homemade!) interferometer appears to be on a sheet of plywood, thus any alignment is not scientifically valid at all. It also is not in a controlled room and vibrations as well as air currents will disturb the results.
Their reference field contains fringes and very few of them, so the interferometer is useless for precise measurement. Anything finer than 5 waves or so I would say is invalid using it.
The disagreement between horizontal and vertical tilt in their data also would invalidate their findings. Any axis of tilt should produce equal results for a flat, certainly not a difference of a factor of 2.
I would also question who and how designed their diffraction limited ultrafast lens that covers such a large field. Such a feat is not impossible, but is very difficult.
The article is useful for generalizations only, it is not scientifically accurate. Please tell anyone who shares it with you to not take it as gospel, the test is magnifiscently flawed.
Lenstip drops a bombshell - Scythels - 01-23-2015
Quick reply before I go to bed,
The article aims to be higher than a "hobby article" by using tens of thousands of dollars of optical lab equipment to (attempt to do) a very technical analysis of a component.
No matter what the interferometer is pieced together on - it is not on an air ride table. It is not isolated from the room around it, so harmonic motion will skew any results. It is also not assembled in a screwed down fashion so there is almost certainly a great deal of "tremble" in the positioning of things. No material between the plywood and the components solves the issue - the mere presence of the plywood indicates all one needs to know about the stability of the test table.
Test flats have coatings on both sides of the component with different purposes. Antireflective coating on the back to maximize transmission, and a one-way ~5% reflective coating on the front-facing side to give the interferometer a reference beam. This second coating is much more fragile than regular AR coatings and is also far more flat. Touching it will smear it on the nanometer scale and such inaccuracies are easily measurable (they are within 10s, 100s, or 1000s of the times the measurement error of the machine) but will destroy the calibration of the machine. Of course this is not a commercial interferometer so there is no calibration to begin with, but touching the flat will induce wavefront aberrations. A photographic lens is polished to about 31nm rms roughness, a reference flat is more like 1-2nm rms roughness.
This article isn't by an optics student. Perhaps a physics one, but an optics student would have access to some space to do the test proper on an air ride table or at least a breadboard.
The charge for testing the mirror as-delivered in my lab would be less than $100. $20/hr * up to 2 hours for student's time + lab operator's commission would be all. All of our equipment was either donated or is on loan/'active donation' so we really have no right to charge for equipment use.
Our "parent organization" - CeFO - would have plenty of right to charge for equipment use in any number of labs or shops across the country.
Lenstip drops a bombshell - davidmanze - 01-23-2015
Lenstip get a "serious" thumbs up from me, like this site here they are doing us photographers, amateur or professional, a great service!
Seems somewhat churlish to be anything other than grateful!
Lenstip drops a bombshell - Scythels - 01-24-2015
Their lens reviews are alright, even quite good, but this article is a disservice at best. Their conclusions are alright but nothing new, but the data they use to back their conclusions up isn't valid. It helps them build credibility with photographers because "look, science! they must be very smart and accurate" when really... their 'science' is anything but.
Lenstip drops a bombshell - JJ_SO - 01-24-2015
Quote:Too late. Poland is not Russia so not sure your insightful opinions about "eastern technology" apply anyway. Also, feel free to educate the forum by providing some topical links to a review site which provides equally comprehensive yet more robust reviews which accord with "western technology".
I already know Poland is not Russia and if you can't take a joke as a joke - I don't think it's my problem. I once read the Otus review. The one with the portrait oriented pictures in landscape orientation, the one in which it was obvious the lens wasn't even focused. More robust and western? Sure, there are a couple and I think lenstip already have enough subscribers? Don't bother about me because I don't bother about them.