Telescope Reviews: Multicoated vs Fully Multicoated

The difference from multi-coated to fully multi-coated can be negligable regarding limiting magnitude.

Lets's start with the worst possible case (leaving out non-coated lenses).
Binoc A is FMC = fully multi-coated
Binoc B is FC = fully single layer Magnesium Flouride coated on every surface
assume 12 surfaces

the FC lenses will reflect more light and it will not pass thru to the eye. The difference in light throuput is the amount of reflected light, which is the difference between reflected by FMC (0.25%) and reflected by FCMgF (1.5%)compounded for the number of surfaces.
(1+(.015-.0025)) ^ 12 = 16% difference in light throuput.

Now this you might be able to see, it represents a difference of about 0.15 magnitudes. However, it is not very likely you would be able to detect this in handheld use, *BLEEP* handheld use lowers your ability to reach limiting magnitude by anywhere from 0.5 to a full magnitude.

But more often than not, what we find, for example in a comparison of FMC to MC such as in the case of the Nikon Action Extreme, is that only the prisms are FC rather than FMC, and all other surfaces are FMC. So this may represents only 4 surfaces. So, the difference becomes
(1+(.015-.0025)) ^ 4 = 5% difference in light throuput. This represents a difference of only 0.05magnitudes

This is nearly impossible to notice. Even the most practiced variable star observers have some difficulty identifying differences of less than 0.2 mag, and many cannot identify a difference of 0.1mag. You would be able to test on a deep magnitude star field and document the differences, but the differences would be at the 5%-10% vision level, meaning objects that faint would be observed only by averted vision for 10% of the time.

Glass figure is probably the predominat indicator of color cast. Glass can be figured to be apochromatic or achromatic. Almost all binoculars are achromats. However, achromatic correction can be figured towards the red end of the spectrum, or towards the blue end of the spectrum. This figure may add factors of 10s times more influence to color cast than would the few percent difference in transmission due to coatings.

Contrast is affected by some various combinations of scatter, spherical aberration, chromatic aberration, etc. Lens quality determines degree of aberrations. A reduction in transmission due to coatings would have an equal effect on object and background and would not change the contrast ratio. Lens aberrations are the primary determinant of contrast.

Even a half moon, (let alone a full moon) can exhibit a reflection that is brigher than the brightest stars, so reflections of the moon are not unexpected at all, even in fully multi-coated binoculars.

edz

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