[protocold]

Hey guys I would need to poke your brain.

I've got 2 copy of Vivitar Macro Focusing Teleconverters, they are identical model.

When I put them side by side, under a hanging lamp on the dinning table, the color reflection are very different.

There are multiple reflection of the hanging lamp on each lens, but on one there is a purple color but on the other there is blue...

I tried a few angles and the behavior is consistent.... what gives??

Is one better than the other? which one?

[bimjo]

It's probably the angle that the light is striking the element. Set it up again, view the reflection and then move the right one into the left ones spot and see what happens.

[mattt]

Not being an optics engineer I'm speaking out of place, but I suggest it may be due to variation in film thickness - something that could easily be a quality control issue..

[Joe Nation]

Or one could have a little oil or something on it. Give them both a good clean and look again.

[Philoslothical]

Perhaps they're made at different plants, and have slightly different coatings? I wonder if you could track that info down from the serial numbers.

[RioRico]

I have had (and sold) three Vivitar M-type 2x TCs, each with the same model number, each from obviously different plants. I've never seen a TC with a serial number so I don't think they can be tracked. Face it: If a TC isn't "name brand", there's almost no way to know who churned it out.

[pacerr]

While all the other responses are certainly valid, I'd like to suggest that what you see reflected from, and what passes through, a lens are entirely different (although possibly related) issues.

One must judge from the resulting images whether the differences are significant. And sometimes those differences are intentional -- perhaps to improve the manufacture's concept of "better than before".

H2

[protocold]

Actually I was going to say that I also have 2 Pentax-A 1.7 50mm and I also see one purple and one blue......

I also swapped the right one with the left, the one with blue still carry blue and vice versa...

[vonBaloney]

Most of the better coatings appear purple. But I've never seen blue (usually an uncoated or a single-coated lens just looks clear or a bit yellow/orange). So maybe blue is best of all. See if you can tell any difference in metering between the two. The better one will have better light transmission...

[JonPB]

I'm not well read in the subject, so please read this as the ignorant musing of an amateur:

Coatings are interesting. The idea is that light is reflected off the coating and off the glass; the coating is 1/2-wavelength thick; these reflections, being 1/2 wave apart, then cancel each other out. This (counter-intuitively, but as predicted by the laws of physics and demonstrated by measurement) increases light transmission. Problem is, the coating is only functional at wavelengths (read: colors) that respond to the thickness of that particular coating. Early, single-coated lenses would therefore emphasize the transmission of a particular color to the detriment of others. While this wasn't a big deal photographically--certainly, a huge improvement over uncoated lenses--it was less than ideal. This is why Pentax's "Super Multi Coating" was such an impressive technology: the multiple layers of coating cancelled out reflections pretty well across the spectrum.

Anyhow, with lenses, there are four things that light can do: transmit light forward (yay!), reflect light back (boo!), diffuse the light (forward or back) (in which case the lens is blurry and intolerable), or absorb it (and turn it into heat) (like smokey sunglasses or a ND filter). Ignoring the latter possibility, either light is transmitted or reflected. If you reflect less, then you transmit more. That's a 1-to-1 direct relationship.

So what you're looking at with different colors of reflections are the colors that are not being handled by the coating. A blue reflection tells you that the coatings were probably for red and violet, whereas a purple reflection might suggest red and blue coatings. Pentax seems to have always run a continuous improvement system with their coatings (for a while, I picked up a number of identical model Takumars just to compare their reflections), so that might be what you're seeing. Since this is an off-brand, however, I'd guess that you're simply looking at two different coating technologies from two different manufacturers.

And, as an aside--when you're talking about coatings that are uniformly thick at 1/2 the wavelength of visible light, it is easy to see how photographers became quite afraid of touching the elements of their lenses. For a long time that was very fragile stuff. Come to think of it, one lens might have had a layer of coating washed off by some well-intentioned previous owner...but then you'd probably see swirls in the reflections, if only around the edges.

[jeffshaddix]

Nice post Jon! Very informative.

[bdery]

I'm a senior optical designer by day, so I'll insert a few comments.

Not being an optics engineer I'm speaking out of place, but I suggest it may be due to variation in film thickness - something that could easily be a quality control issue..

It's more complicated than that, but if it is indeed a coating problem (and not a test problem) then you're mostly correct.

While all the other responses are certainly valid, I'd like to suggest that what you see reflected from, and what passes through, a lens are entirely different (although possibly related) issues.

They are completely related. It either passes through or is reflected (neglecting absorption effects, which are essentially irrelevant for visible light)

Most of the better coatings appear purple.

There is no scientific ground for that affirmation

I'm not well read in the subject, so please read this as the ignorant musing of an amateur:

You seem pretty well read to me, you gave an essentially accurate description of the phenomenons involved.

The best test to see if that's a real physical issue (and not dependant on the angle of view or other parameters) is to place the camera on a tripod, select a controlled environment with constant lighting, and take comparative shots in exactly the same conditions. The resulting pictures will tell the true story.

There is often more than one way to make a coating do one specific job. Many recipes will work, with slight variations that are usually impossible to discern. I wouldn't worry except if the resulting images are different.

[KyPainter]

I have one of these teleconverters that I took the glass out of. I also have several pieces of glass which I believe to be, but cannot guarantee to be, those that came from it. One of these pieces is larger, concave on each side, and to the eye symmetrically so. One side reflects blue, the other purple. Perhaps the difference is that this element is reversed in one of your teleconverters.

[ripit]

This is just an observation not based on any technical knowledge, but on filters as well as lenses, it seems cheap ones (like the ultra cheap and crappy Chinese ones off ebay that I bought with intentions of breaking the glass out) reflect an image almost like a mirror. I use a ceiling fan in the bedroom to test. With a cheap one, all the metal parts and the wood blades can clearly be made out, and I assume that means it is reflecting a lot of light. With better glass be it filters or lenses, there is just a ghost image where the metal and blades can barely be made out if they can bee seen at all, but a ghost image of the glass cover to the light bulbs and the light bulbs can be still be seen, though in a semi transparent manner.

Is it safe to assume that the more ghostly the image, the better the coating is?

Also, the image reflected seems to very quite a bit (having just looked at several lenses and filters). Some seem to be near true to color, while others give a single color cast image, and yet others give multiple images in different colors. Is it safe to assume that if the image is pretty true to color, the coatings are balancing colors well, regardless of how much they reflect, while ones with a single color cast, are balancing colors well but reflecting a bit more of one color, while ones that have multiple images in different colors, are probably reflecting all kinds of stuff inside the lens (poor coating)?

Most of mine have a single color cast though I have at least 1 that seems to project a true color image (a vivitar series 1 70-210 ver 3). While it has a stronger image compared to some (less effective but balanced coatings?), it certainly is passing a lot more light that a cheap Chinese filter (the vivitar image does look ghost like though it is more visible that in some of my lenses).

I assume that the original poster is asking about the different color cast wondering if one is better than the other, so would any of these observations help in determining what lenses have better coatings?

[vonBaloney]

Re: Most of the better coatings appear purple.

There is no scientific ground for that affirmation

Maybe not, if you consider the world of all possible coatings. In the real world, most camera lenses I've seen appear purplish or yellowish or basically clear. The multi-coated lenses are always the purple ones. I've seen greenish before when the coating was thin and splotchy -- either overcleaned or badly applied in the first place (and lots of flare). Can't say I've ever seen one that I'd call blue rather than purple so I don't know about that. And of course if you consider scientific lenses for special purposes and telescopes I don't know -- I'm talking about SLR camera lenses that actually exist. (I'm seen a lot less German lenses than Japanese ones -- do their coatings look different?)