The Minefield: Advertising Hype in Binoculars
The stuff you really wish you didn't need to know about budget astronomical binoculars
So, you are after a binocular for astronomy. You have been advised by well-meaning friends, vendors, and "experts" in astronomy magazines and on internet forums that you can't go far wrong if you opt for something that has "Fully Multi-Coated" (FMC) and "BAK4 prisms" printed on the binocular's cover-plate.
This is very much not the case. Let's see why:
Well, first of all, there are at least a dozen relevant things that "Fully Multi-Coated" (FMC) and "BAK4 prisms" (or "BaK4 prisms")tells you exactly nothing about. These include, in no particular order:
- Quality of internal light-baffling.
- Type and quality of eyepieces.
- Prism housings.
- Field curvature.
- Spherical aberration.
- Crispness of focus.
- Edge distortion.
- Amount of vignetting.
- Size of fully illuminated field of view.
- Chromatic aberration.
- Mechanical build quality.
- Smoothness of focus.
- Manufacturer's quality control.
But at least it will tell you about the coatings and the prisms, won't it? Well, no, not really:
- Multi-coated optics will, if the coating is done properly, transmit more light than single-coated optics. If the coatings are unevenly applied, or if they are evenly applied but are the wrong thicknesses, all bets are off! So, how can you tell? Unevenly-applied coatings can, but do not necessarily, look "blotchy". Without test equipment, you have no way of telling if they are the right thicknesses. Ultimately you are relying on the integrity of the coating process and quality control. You are buying a binocular that is probably costing you less than two reasonably good eyepieces, maybe even less than one: exactly what level of quality control were you expecting? Read what the biggest manufacturer of imported (into Europe) binoculars says:
For years, the international markets are flooded with unbelievably low-price Chinese binoculars and some of the users have been complaining or bashing loudly about the quality control consistency of Chinese binoculars for a while. Actually, it's quite simple to improve the quality consistency: spending much more time in grinding and selecting glass, spending much more time in training the workers for assembling the binoculars, and spending much more time in the final quality check - then, a much better quality binocular will be made, however, at the trade-off of much higher production cost.
- There is no industry-wide standard for the term "Fully Multi-Coated". Let me repeat that: There is no industry-wide standard for the term "Fully Multi-Coated". It could mean that all glass-to-air surfaces of the lenses in the objectives and the eyepieces, and the transmissive faces of the prisms, all have 7 layer interference coatings (properly applied). Or it may mean that just the glass-to-air surfaces of the lenses have 2 layer interference coatings. Unless the manufacturer provides more information, you have no way of knowing.
The two 70mm binoculars in the image above were made in the same factory and ostensibly have the same broadband fully multi-coated optics. I photographed them under similar conditions. One is three times the cost of the other. Guess which is which.
- Bak4 is a glass designation used by Schott
AG, an old and respected German manufacturer of optical glass. BaK stands for BaritleichKron (German for "Barium Crown"). It is used for prisms in most (but not all!) high-end binoculars. It has a higher refractive index than BK7 glass (another Schott designation). This means that, if it is used under the conditions that exist in most common binoculars ("fast" objectives, low magnification) more light from the periphery of the field of view will pass through the prisms to the eyepieces; the edge of field will be brighter, but there will be no effect on the middle of the field. It is therefore deemed desirable, and BK7 has acquired an undeserved reputation for being "cheap": in fact, it is better than BaK4 for some applications (which is why it is used in some of the very best binoviewers). I had to take the BK7 photograph to the right from slightly above in order to make the grey "cutoffs" clearly visible at the bottom of the exit pupil.
- Although there are international standards for optical glass designation, BaK4 isn't one of them. Anyone can apply it to any glass. The international standard designation for Schott BaK4 is 569561. The first three digits tell you its refractive index (1.569) and the last three tell you its Abbé number (56.1), which indicates how much it will diperse light into its component colours; the higher the Abbé number, the less the dispersion. However, I don't see customers being willing to learn and compare international standard designation codes: "Bak4" trips off the tongue so much more easily.
- This is what you should know: the "BAK4" glass used for the prisms of Chinese binoculars is not the same as Schott BaK4 – and it's not just the upper-case letter ‘A’ that's different. In fact, it's not even Barium Crown, which is what BaK stands for! It is a phosphate crown glass with a lower refractive index and dispersion than Schott BaK4 (but higher than BK7). It also potentially has a higher "bubble count"; this is noticeable as a slight milkinesss to the image when compared with good quality glass.
- In practice, this may not be all bad. Unless you have very wide-angle binoculars, you are unlikely to notice the effect of the lower refractive index, and the lower dispersion than "real" BaK4 means that there may be less dispersion in the image (not that you are likely to be able to see it). The potentially higher bubble-count means there will be more light scatter inside the prism.
- But the story does not end here. The "BAk4" printing on the cover plate not only doesn't really tell you the quality of the glass used, it also doesn't tell you:
- If the prisms are under-sized. If they are, they will cut out some light.
- The precision with which the flat surfaces of the prism have been polished.
- Whether the prisms hypotenuses are grooved. Grooved prisms reduce spurious reflections.
- Whether the prism sides are blackened. Prevents non-image-forming light entering the prism.
- Whether the reflective surfaces of the prisms are shielded. Prevents non-image-forming light entering the prism.
- How the prisms are secured into their housings. There's an enormous difference between glue and a properly constructed prism cage.
So, that's it, is it? Well, no, not quite....
Magnification and Aperture
We all know that, say, "10×50" printed on the cover plate means that the binocular has a magnification of ×10 and an aperture of 50mm, don't we? Well, it might do. It also might not:
- You might be forgiven for thinking that the top binocular has an aperture of 50mm and a magnification of ×20. Well you'd be half right: the aperture is 50mm. But the magnification is only ×10. The lower binocular must have an aperture of 60mm and a magnification of ×30,mustn't it? In fact, if you like small exit pupils, it might be worth a punt for some high magnification astro-observing, mightn't it, especially as the vendor describes it as being "Day and Night Vision Binoculars"? Well, no, you'd be disappointed: it is really what we would call a 15×22: aperture of 22mm and magnification of ×15. Possibly not entirely useless, but hardly the sort of thing you'd reach for as a first choice for astronomy.
Why on Earth do they do this sort of thing? I wish I knew!
- But you'll be OK if you buy a well-known brand, won't you? After all, that is the other bit of advice given. Well, maybe. There are several well-known brands of 10×50 binocular (oh, they also have "Fully Multi-coated" and "Bak4 Prisms" printed on them, so they must be good, mustn't they!) that certainly have a magnification of ×10 and, if you measure the diameter of the objective lens, you will find it is indeed 50mm. So, is it a 10×50? Well, not really. But, at least they are half right: the magnification is as near as dammit ×10. Now, if you measure the exit pupil, it should be 5mm (50mm ÷10), but it isn't. It's 4.2mm. That means it's effectively a 10×42. A 50mm transmits nearly 1½ times as much light as a 42mm.
So, why do they do that? I think I know: If you stop it down to 42mm, you can get away with smaller prisms. Smaller prisms require smaller housings and less robust fixing. Also, stopping it down to 42mm increases the effective focal ratio from around f/3.5 to around f/4.2. This makes much less demand on eyepiece quality and improves colour correction and edge performance – so much easier to do this by stopping down the effective aperture than by using better quality optics!
So, that's about it. What can we conclude? Only that what is printed on a binocular cover plate does not necessarily tell you anything about the binocular. I have yet to see a Fully Multi-coated budget 10×50 with BAK4 prisms that gives images of the quality or brightness of a genuine Zeiss (Jena) 10×50 (single-layer coatings and BK7 prisms); maybe now you know why.
What can you do? As someone on a forum recently wrote with respect to budget binoculars: "The only really telling way to rate any bins is by putting them up to your head and have a look through them." Always try before you buy!