Phase coating - how to tell?

[Malcolm Stewart]

I have a pair of reasonable quality roof prism binoculars, 8x25 and claimed to be waterproof. I bought them, new, about 3-4 years ago. How can I tell whether they have phase coating by simply looking through them?

So far, all I've noticed is that they're nowhere as easy to use, simply because their exit pupil is much smaller than my larger porro prism bins. However, they are compact and slip into my pocket easily, and thereby fulfil the reason for purchase.

Cheers,

[chartwell99]

I have a pair of reasonable quality roof prism binoculars, 8x25 and claimed to be waterproof. I bought them, new, about 3-4 years ago. How can I tell whether they have phase coating by simply looking through them?



Hold the glasses at roughly half arms length and look through the objective lenses. Non-phase coated prisms will show a faint line across the image at the equator while the phase coated models will not. The effect is easier to see with larger objective lenses, but it still should be visible with 25mm objectives.

[Malcolm Stewart]

Thanks for your quick response. I've done what you suggested, and against grey rain clouds (we're now getting the rain that should have fallen last month!), I can't see any suggestion of a line. (The bins are 8x25 compact Minolta Activas, and claim to be fully multi-coated with BAK4 prisms.)

Many thanks

[chartwell99]

Thanks for your quick response. I've done what you suggested, and against grey rain clouds (we're now getting the rain that should have fallen last month!), I can't see any suggestion of a line. (The bins are 8x25 compact Minolta Activas, and claim to be fully multi-coated with BAK4 prisms.)

Many thanks

Minolta, as you probably know, is now out of business. My recollection, however, is that the Minolta Activa is actually a reverse porro and not a roof so phase coating is unnecessary. In any event, very nice bins as I recall.

[Malcolm Stewart]

Mine are definitely a roof prism design, two parallel tubes - my partner has a pair of Minolta reversed porro bins, so I know the difference.

Before Minolta did go out of business, I bought a used pair of their 7x50 standard design porros in their Activa range. After a short period of use, I realised that they were out of collimation, so took them to the UK Minolta office in Milton Keynes, and they were replaced without query under their warranty. They treated me very well, supplying me with a totally brand new pair, which were now nitrogen filled, waterproof, with case, strap, caps etc. That was good service!

I'm sorry that Minolta have gone - the Milton Keynes operation seemed to be well run with helpful staff. At one time I had 4 X700 cameras and many lenses, and still have the SRT101 with which I started serious SLR photography in 1969.

[Alexis Powell]

...I can't see any suggestion of a line...


If you didn't see the line, you didn't look hard enough or under the right lighting--it is there in ALL roof-prism binos, regardless of whether they are phase-coated. Nevertheless, from your description of your binos it sounds like they are probably the Activa D WP XL pocket-roof, a line that was advertised as being phase-coated. I can't be certain because I don't have all the Minolta catalogs (and the "Activa" name by itself is no help, having been used on many Minolta binos), but everything you've described fits the description of that model. A final comment--I haven't looked through this particular model, but in my experience pocket-roofs in this price range are universally optically mediocre in comparison to other binos, so while the use of phase-coating is nothing to disparage, the binoculars probably suffer from other optical difficiencies that are more serious than those related to the question of phase-coating. Phase-coating is not a very good predictor of optical excellence, but its lack would limit the performance of otherwise excellent optics.
--AP

[Malcolm Stewart]

... Nevertheless, from your description of your binos it sounds like they are probably the Activa D WP XL pocket-roof, a line that was advertised as being phase-coated....
--AP

Hi Alexis,
Thanks,
I've now found the original box, and all it says is "8x25WP ACTIVA" with a waterproof symbol.
From what you've said, am I right that it's impossible to tell whether they're phase coated or not, by simply looking?

Cheers,

[Alexis Powell]

From what you've said, am I right that it's impossible to tell whether they're phase coated or not, by simply looking?

I've never seen discussion of how to detect phase-coating, so as far as I know it isn't something that can be determined in any easy way. In full-sized binos, the contrast of roofs without phase-coating is limited to the extent that the effect is easy to see in direct comparison with any decent porro. But since the contrast of pocket roofs is often poor to begin with, I'm not sure you could be as confident about being able to attribute poor contrast to its lack. My follow-up point in the previous post was meant to point out that practically speaking, the question may be moot because it is probably among the least of your optical design worries at this price point.
--AP

[hinnark]

Hi Alexis,
Thanks,
I've now found the original box, and all it says is "8x25WP ACTIVA" with a waterproof symbol.
From what you've said, am I right that it's impossible to tell whether they're phase coated or not, by simply looking?

Cheers,

Malcom,

do you use a TFT monitor? If so you can try this. Take your roof binoculars and look through one barrel´s objective at the monitor. The monitor should have a bright white background while doing so. The image you´ll see should be a perfect small white circle . Now turn the barrel slowly at least to 180°. If the image you see while doing this divides into two halves of different brightness at a given degree the binoculars you examine have probably no phase coating or its phase coating isn´t that effective. The effect differs from monitor to monitor. So try several TFT monitors if possible.

Steve

[Malcolm Stewart]

Malcom,

do you use a TFT monitor? If so you can try this. ...

Steve

Hi Steve,

Thanks for the insight.

By stipulating a TFT monitor, you're indicating that the light from them is plane polarised, I assume. I don't have a TFT but still rely on a pro grade CRT, an Eizo.

However, I have done some microscopy of crystal formation using crossed polars, and as someone who's given stereo slide shows, I do have squares of linearly polarised sheet, so I'll experiment with that. (Somewhere I also have pieces of 1/4 wave sheet - not assembled in a circularly polarised filter.)

Cheers,

[hinnark]

Hi Steve,

Thanks for the insight.

By stipulating a TFT monitor, you're indicating that the light from them is plane polarised, I assume. I don't have a TFT but still rely on a pro grade CRT, an Eizo.

However, I have done some microscopy of crystal formation using crossed polars, and as someone who's given stereo slide shows, I do have squares of linearly polarised sheet, so I'll experiment with that. (Somewhere I also have pieces of 1/4 wave sheet - not assembled in a circularly polarised filter.)

Cheers,


That sounds interesting. Keep us up informed about your results.

Steve

[Malcolm Stewart]

That sounds interesting. Keep us up informed about your results.

Steve

Hi Steve,

I've now set up two 75mm squares of high extinction polar sheet approx 20cm apart, and with their angles set to the extinguish position. (Classic microscope polar and analyser set-up for polarisation studies of optically active materials.) I've got the bins placed between the two pieces of polar.

Viewing the object glass end of my my bins when rotated between the two sheets of polar, the line between the two halves of the image is now very clearly visible, and as I rotate the bins, the darkness/lightness ratio varies. When the plane of polarisation is at 45deg, I'd say the brightness between the two halves is equal (memories of Bunsen grease spot photometer from the 1950s!), but as I haven't set up any form of goniometer, I can't tell how sensitive this is versus the angle.

Without the analyser polar and just viewing plane polarised light at a variety of angles, I can't detect any hint of the dividing line, and that's with the benefit of now knowing what I'm looking for.

Using the same polar/analyser set-up, I've also done a brief check on my 7x35 porro prisms, and they are obviously affecting the plane of polarisation, but there's no hint of the dividing line so evident with the roof prims.

Many thanks for the guidance.

[NWBirder]

If you didn't see the line, you didn't look hard enough or under the right lighting--it is there in ALL roof-prism binos, regardless of whether they are phase-coated. Nevertheless, from your description of your binos it sounds like they are probably the Activa D WP XL pocket-roof, a line that was advertised as being phase-coated. I can't be certain because I don't have all the Minolta catalogs (and the "Activa" name by itself is no help, having been used on many Minolta binos), but everything you've described fits the description of that model. A final comment--I haven't looked through this particular model, but in my experience pocket-roofs in this price range are universally optically mediocre in comparison to other binos, so while the use of phase-coating is nothing to disparage, the binoculars probably suffer from other optical difficiencies that are more serious than those related to the question of phase-coating. Phase-coating is not a very good predictor of optical excellence, but its lack would limit the performance of otherwise excellent optics.
--AP

AP, your method works for me. I first tried some inexpensive compact 10x25 binoculars, I can clearly see two line (not one) running across the field and intersect at 60 degree. Then, I tried all my phase coated binoculars, none of them has the obvious lins running through. I do noticed my Bushnell legend and Nikon Monarch have two very faint lines in the middle of field. I guess there might be some center-edge uniformity issue for phase-coating thickness. Do you guys have similar experience? Frankly, I didn't much impact on image quality froom it.

[Alexis Powell]

...I guess there might be some center-edge uniformity issue for phase-coating thickness...

The line is produced by the prism itself, not any coatings on it, hence my comment about its presence in all roof prisms.
--AP

[NWBirder]

The line is produced by the prism itself, not any coatings on it, hence my comment about its presence in all roof prisms.
--AP

I see.. Thanks.. hewww.. I thought I got some lemons.. :D

BTW, forgot to acknowledge Chartwell99 too.. thanks!