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Comparing lenses by coating color?
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PostPosted: Sun Jan 08, 2012 12:40 pm    Post subject: Comparing lenses by coating color? Reply with quote
Many times we use coating color to distinguish lenses type, model or even company that made it.
What interests me is how much coating color is affected by (long-term) UV exposure?


PostPosted: Sun Jan 08, 2012 1:42 pm    Post subject: Reply with quote
Why would your lenses get long term UV exposure, and what do you consider to be long term? Thinking about buying a lens that has been on display in a window shop for many years? Or removing the yellowing in a thoriated lens with a UV flashlight? Smile

I don't think UV will change the color of an anti-reflection coating, because coatings show their colored reflection because of their specific thickness. Objects that change in color due to UV light do so because their pigments or dyes break down, but this isn't applicable with AR coatings.


PostPosted: Sun Jan 08, 2012 2:20 pm    Post subject: Reply with quote
interesting topic and reply. i thought the direction might be how do different coatings effect IQ?


PostPosted: Sun Jan 08, 2012 3:48 pm    Post subject: Reply with quote
1. coatings do surprisingly little to alter IQ. That is, there is a obvious big step from uncoated to coated - but past that, you have to devise torture tests that bring out the best in one and the worst in the other coating to demonstrate a difference in a pair of images.

2. coatings are a thin layer of a mineral (usually MgFl) on top of the glass - UV will never erode them, but mechanical cleaning or long term exposure to rain might.

3. Coating colour is determined by the layer strength (which makes it a interference filter for particular bands) - single coating layers ideally have their highest transmission in the centre of the visual band (in the orange to green range), hence a blue/purple reflection. On multi-coated lenses, colour is merely an artefact of the layer ordering (i.e. which band is top), and is quite meaningless.


PostPosted: Sun Jan 08, 2012 5:45 pm    Post subject: Reply with quote
thanks sevo, thats really interesting and informative. i recently noticed a listing that desribed a lens coating as 'magenta' which made me curious if there are different color casts to coatings, and, if so, would that effect the 'look' of photos taken in non-nuetral light?


PostPosted: Sun Jan 08, 2012 6:40 pm    Post subject: Reply with quote
rbelyell wrote:
thanks sevo, thats really interesting and informative. i recently noticed a listing that desribed a lens coating as 'magenta' which made me curious if there are different color casts to coatings, and, if so, would that effect the 'look' of photos taken in non-nuetral light?


Unlikely - the coatings are effectively bandpass filters. While it is not feasible to make them pass all the visible band equally, they are not that far off, and will have maybe 20-30% deviation to the margins. Given that they act on a reflectivity that is already in the 5% to start with, single coatings will act as a maybe 1-2% tinted magenta filter - which would be well within the scope of common skylight filters. And multicoatings reduce that by another magnitude. Overall there is more reason to worry about the tints caused by some optical glass types (residual alpha-radiating isotopes will cause many rare earth glass types to yellow over the course of time).

Their properties outside the visual band are another matter - coated lenses tend to be opaque to deeper UV and IR.


PostPosted: Sun Jan 08, 2012 7:53 pm    Post subject: Reply with quote
excellent explanation that even i can understand! thank you again!


PostPosted: Sun Jan 08, 2012 8:00 pm    Post subject: Reply with quote
rbelyell wrote:
excellent explanation that even i can understand! thank you again!


+1


PostPosted: Sun Jan 08, 2012 10:20 pm    Post subject: Reply with quote
http://en.wikipedia.org/wiki/Anti-reflective_coating
Quote:

Types:
Index-matching - a refractive index between those of glass and air, each of these interfaces exhibits less reflection than the air-glass interface did. In fact, the total of the two reflections is less than that of the "naked" air-glass interface.

Single-layer interference - a single quarter-wave layer of transparent material whose refractive index is the square root of the substrate's refractive index.

Multi-layer interference - see below

Absorbing - These coatings are useful in situations where high transmission through a surface is unimportant or undesirable, but low reflectivity is required.

nano - a pattern of bumps, each roughly 200 nm high and spaced on 300 nm centers. This kind of antireflective coating works because the bumps are smaller than the wavelength of visible light, so the light sees the surface as having a continuous refractive index gradient between the air and the medium, which decreases reflection by effectively removing the air-lens interface.


http://rick_oleson.tripod.com/index-166.html
Quote:

So, what’s the difference between “single” and “multi” coating?

Multicoating is significantly more efficient, and slower and more expensive to achieve, than single coating, but the principles and the basic process are the same. The basic difference comes down to refractive index. A single coating must have a refractive index about halfway between those of glass and air, and it took a number of years to find a suitable material in Magnesium Fluoride. To apply multiple coatings with similar effect, they must be of materials with an even progression of refractive indices, calculated to correspond to the number of layers desired. For example, if you want to apply 5 layers, you would need 5 different materials with indices of roughly 1.1, 1.2, 1.3, 1.4 and 1.5 to span the range between air (1.0) and glass (1.6). From here on, the process is the same as with the single coating, except that each layer is made a slightly different thickness from the others, with the thicknesses selected to correspond to several different wavelengths within the visible spectrum so that the suppressive effect applies not only at the 550nm center but across the whole range. A multi-coated lens does not appear as strongly colored as a single coated lens – its general tendency is to appear black by comparison to the blue-violet single coated or silver-grey uncoated lens. This is by design: the purpose of coating is to suppress reflections, and the purpose of multi-coating is to suppress ALL reflections.


PostPosted: Sun Jan 08, 2012 11:12 pm    Post subject: Reply with quote
"A multi-coated lens does not appear as strongly colored as a single coated lens – its general tendency is to appear black by comparison to the blue-violet single coated or silver-grey uncoated lens."

Just look at CZJ MC 135mm/f3.5 Sonnar - I think it is the most colorfull lens I ever saw. On the other end of a scale are Helios-44M-3 MC and Jupiter-9 MC - they both are almost colorless.


PostPosted: Sun Jan 08, 2012 11:37 pm    Post subject: Reply with quote
The term "MC" or "Multi-Coated" can be many things when the marketing team get involved.
1 element with 7 coatings on each side, 1 element with 2 coatings, 2 elements with 1 coating each, could technically be called MC.

And I believe he means the color is not as bright, though it may have more colors than a single coated lens.
The materials used and how they are applied may affect their appearance.


PostPosted: Sun Jan 08, 2012 11:46 pm    Post subject: Reply with quote
Lightshow wrote:
The term "MC" or "Multi-Coated" can be many things when the marketing team get involved.
1 element with 7 coatings on each side, 1 element with 2 coatings, 2 elements with 1 coating each, could technically be called MC.

And I believe he means the color is not as bright, though it may have more colors than a single coated lens.
The materials used and how they are applied may affect their appearance.


I believe the lens can be called MC if at least one of its element's surface has several coating layers. Not all elements and surfaces have to be multicoated but at least one of them has to.


PostPosted: Mon Jan 09, 2012 12:10 am    Post subject: Reply with quote
I agree with Sevo's 1, 2, 3 guide.

I would add that color of the light shining on the coatings can affect the reflection color. Best to use skylight or sunlight, I think.

Also add that the coatings only benefit to IQ is an increase in contrast -- the coatings reflect stray light that otherwise would 'fog' the details in dark portions of image. Seeing more details in the darker portions is certainly a visible increase in sharpness over not being able see them at all! Wink However that is not really an increase in sharpness!

Coatings on many surfaces within a lens system can increase the minimum f/# over uncoated f/# of same physical size/formula lens system that is uncoated.

I did read someplace sometimes a coat includes some thorium, to adjust its index of refraction to pass through more of the reflections from the inner coats or some such. Maybe only a rumor.


PostPosted: Mon Jan 09, 2012 2:43 am    Post subject: Reply with quote
visualopsins wrote:
Also add that the coatings only benefit to IQ is an increase in contrast

They also affect the color balance produced by the lens. Single-coated surfaces are optimized to suppress reflections of a certain single wavelength, shifting the overall color balance in a certain direction. Some better single-coated designs were taking this fact into consideration, applying different types of coatings to each of the lens surfaces in order to color-balance the lens as a whole (example: some 100mm designs having amber-colored front glass and blue/purple rear glass coatings). However, it is much easier to produce a completely color-balanced lens if good multi-coatings are applied to each air'glass surface. In general, MC lenses will produce higher contrast *and* truer colors, as illustrated in e.g. this thread: http://forum.mflenses.com/comparing-my-jupiter-9-85mm-f2-silver-black-and-black-mc-t42988.html


PostPosted: Mon Jan 09, 2012 2:54 am    Post subject: Re: Comparing lenses by coating color? Reply with quote
Pancolart wrote:
Many times we use coating color to distinguish lenses type, model or even company that made it.
What interests me is how much coating color is affected by (long-term) UV exposure?


Not at all, to my knowledge. Such exposure may affect the cements, perhaps.


PostPosted: Mon Jan 09, 2012 3:13 am    Post subject: Reply with quote
aoleg wrote:
visualopsins wrote:
Also add that the coatings only benefit to IQ is an increase in contrast

They also affect the color balance produced by the lens. Single-coated surfaces are optimized to suppress reflections of a certain single wavelength, shifting the overall color balance in a certain direction. Some better single-coated designs were taking this fact into consideration, applying different types of coatings to each of the lens surfaces in order to color-balance the lens as a whole (example: some 100mm designs having amber-colored front glass and blue/purple rear glass coatings). However, it is much easier to produce a completely color-balanced lens if good multi-coatings are applied to each air'glass surface. In general, MC lenses will produce higher contrast *and* truer colors, as illustrated in e.g. this thread: http://forum.mflenses.com/comparing-my-jupiter-9-85mm-f2-silver-black-and-black-mc-t42988.html


Thank you aoeleg. I have noticed these "truer colors" from MC lens; how MC from different manufacturers produce images of differing color balance. I'm not so sure the colors are "truer", however. I have wondered if color calibration equipment verifies that? I notice Asahi Super and S-M-C is warm, Nikkor is less warm, Zeiss is just about perfect, and Canon is cold, compared to each other, but which colors are "truer" when comparing subject colors to output colors (colorchecker)?


PostPosted: Mon Jan 09, 2012 7:43 am    Post subject: Reply with quote
visualopsins wrote:
Also add that the coatings only benefit to IQ is an increase in contrast -- the coatings reflect stray light that otherwise would 'fog' the details in dark portions of image.

I think that's not quite an accurate description. AR coatings on photographic lenses do not reflect only "stray" light; they simply supress reflections off the glass by destructive interference but also increase transmission through the glass by constructive interference.
http://en.wikipedia.org/wiki/Anti-reflective_coating#Interference_coatings

@Sevo: Thanks for the elaboration.


PostPosted: Mon Jan 09, 2012 1:30 pm    Post subject: Reply with quote
aoleg wrote:
visualopsins wrote:
Also add that the coatings only benefit to IQ is an increase in contrast

They also affect the color balance produced by the lens. Single-coated surfaces are optimized to suppress reflections of a certain single wavelength,


Not really. While a quarter wave plate only has one frequency where it is exactly a quarter wave length strong, so that we can define it by its centre frequency, there is quite a wide spectrum where it is almost a quarter wave strong. We can identify coatings as having a warm or cold cast, but their filter property (a side effect that is kept as low as possible) is weak enough that they do not have a more strongly chromatic tint.

It would be possible to employ the weak filter characteristics of the coatings to tune the colour balancing of a lens, but I have my doubts whether that really gets (or got) done - for one, it increases production cost as the elements would have to get a greater range of different coatings, for the other, it compromises a quality relevant parameter (anti reflection coating) for a side effect that can also be achieved in a more controllable and performance neutral way by applying one separate dedicated filter coating to one element, if it can't already be done in the glass itself.

And even if some makers use coatings for colour tuning, we won't see that by the frontal MC lens reflection colour. The colour we see is merely the complimentary to the peak frequency of the top coating layer on the front element. And the peak frequency of one layer of half a dozen in this surface and some fifty to hundred spread across all elements in the lens is very insignificant when it comes to the characteristics of the entire lens. If any, the frequent odd lens colours on early MC lenses (which have mostly vanished by now) might have been chosen for marketing purposes - having MC a different tint than single coated lenses is not really technically needed or even beneficial (the peak on a single coating sits right at the best possible point), but it helped in adding prestige to the MC lens while that still was an expensive extra...


PostPosted: Mon Jan 09, 2012 2:04 pm    Post subject: Reply with quote
Sevo, what do you think about this picture:
There is a color difference between the two SMC and S-M-C version in the color.. could this also be a marketing affect?
it took this today for the equipment gallery..


PostPosted: Mon Jan 09, 2012 2:20 pm    Post subject: Reply with quote
Smoli4 wrote:
Sevo, what do you think about this picture:
There is a color difference between the two SMC and S-M-C version in the color.. could this also be a marketing affect?
it took this today for the equipment gallery..



could you expose these lenses to strong backlight? So they start to flare and show some iris ghosts and glare? I'm sure those glares are influenced by the coating color.


PostPosted: Mon Jan 09, 2012 2:21 pm    Post subject: Reply with quote
Possible, but that could also be plain product variation within the specified limits - the step from cyan to green is pretty small.


PostPosted: Mon Jan 09, 2012 2:25 pm    Post subject: Reply with quote
ok, this could be the reason... as you explained earlier it could that the green color is caused just by a slightly thicker coating.

And Cooper, what do mean by exposing the lenses to a strong backlight?


PostPosted: Mon Jan 09, 2012 6:03 pm    Post subject: Reply with quote
Smoli4 wrote:
ok, this could be the reason... as you explained earlier it could that the green color is caused just by a slightly thicker coating.

And Cooper, what do mean by exposing the lenses to a strong backlight?



Just shoot a strong, directional light source in the frame


PostPosted: Tue Jan 10, 2012 3:48 am    Post subject: Reply with quote
Sevo wrote:
Not really. While a quarter wave plate only has one frequency where it is exactly a quarter wave length strong, so that we can define it by its centre frequency, there is quite a wide spectrum where it is almost a quarter wave strong. We can identify coatings as having a warm or cold cast, but their filter property (a side effect that is kept as low as possible) is weak enough that they do not have a more strongly chromatic tint.

We can. Compare several versions of Jupiter-9 that have different coatings, and it's very easy to see the difference in colors (or do you really believe that the factory used green or brown "bottle glass" to make these lenses?) Oldest J9's have purple or magenta coatings, and have a strong warm or green cast. Newer ones have amber coatings, with neutral/cold cast. MC version is completely neutral.

Of course, we're talking extreme here, but then good lenses and great lenses are only different by a slim margin.

Sevo wrote:
It would be possible to employ the weak filter characteristics of the coatings to tune the colour balancing of a lens, but I have my doubts whether that really gets (or got) done

It's been done from the 60's with single-coated lenses, I believe. Look at some old 100mm f/2.8 lens; they'll have amber coated front group and blue/purple coated rear group. Nikkor-P 105/2.5, Tokina 105/2.8, and some other lenses of similar vintage share the same pattern. Fujinon (non-EBC) 100/2.8 has amber/blue/purple coatings going one after another; that helps the lens balance its color characteristics in addition to offering a somewhat more effective anti-reflection protection.

I agree with you regarding the MC lenses. We can't judge them by the color. Even more, some early MC lenses are no much different (by the color alone) from their SC counterparts.


PostPosted: Tue Jan 10, 2012 8:57 am    Post subject: Reply with quote
aoleg wrote:
We can. Compare several versions of Jupiter-9 that have different coatings, and it's very easy to see the difference in colors (or do you really believe that the factory used green or brown "bottle glass" to make these lenses?) Oldest J9's have purple or magenta coatings, and have a strong warm or green cast. Newer ones have amber coatings, with neutral/cold cast. MC version is completely neutral.

Orio said that the design changed between the single coated and MC version, so glass types could have changed. My CZJ Pancolar 50/1.8 MC has a green cast; how can you be sure it's because of the coatings? It could be the glass...

Contax Zeiss lenses of the same design also have very different looking coatings sometimes, but minimal or no perceivable difference in color rendition.
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