I just bought a pair of inexpensive High Point brand 25mm eyepieces with a 52 degree apparent field of view to put on my binoviewers. The reason I go them was the advertised 22mm eye relief and the price. They claim that these eyepieces are fully multicoated and I can see a very obvious deep green tint. They also have only 4 lenses. I also have some higher end eyepieces that have up to twice as many lenses per eyepiece. How efficient are eyepieces now days. If each lens is 99% efficient then my cheap eyepieces only lose 4% of the available light. My Pentax eyepiece would loose 7% if it has 7 lenses and almost 8% if it has 8 lenses. That is just taking .99 to the n power where n is the number of lenses. If this is a topic that has been beaten to death could you guys let me know where and what the old topic is?
Decent multicoatings pass 99.5% at 550nm, and you calculate loss at about 0.995 at each air-to-glass surface and about 1% per inch thickness of glass.
A good Plossl should have a transmission in the 97.5% range and an Ethos at roughly 93%. Some eyepiece tests show those figures are about right.
Pentax XWs have from 6 to 8 lenses depending on focal length, but Pentax says about 96% transmission at 550nm.
Assuming equal spectra of transmission, an eyepiece would have to be in the 80s to be noticeably dimmer, and some older eyepieces ARE in the 80s in terms of % transmission.
Figure 96% transmission at an air-to-glass interface for an uncoated lens, about 98-98.5% for MgF2 coatings, and about 99.5% for multicoatings.
A Nikon engineer told me they have camera lens coatings on some glass types that have a 99.95% transmission, but these coatings aren't used on every lens material, nor in
affordable astronomical optics.
Some other things that influence how one eyepiece may appear darker than another in the same scope:
--focal length. Even 1mm difference in focal length makes a noticeable difference in brightness
--spectrum of transmission. A "bluer" eyepiece, at night, will appear brighter.
--types of glass used. Lanthanum glass controls many aberrations, but it may appear darker than other glasses due to its spectrum of transmission.
--spot size. A smaller spot size would make the faintest stars more visible by not blurring the stars. A sub-factor here is that less astigmatism may make fainter stars appear, too.
--multi-coating efficiency. The number of air-to-glass surfaces can be quite large--some 4 element Konigs have 6 air-to-glass surfaces, for example--and if the multi-coating is a little less efficient, transmission can suffer.
--lens polish. Having a superb wavefront after passage leads to better focus, more light concentration, and brighter images.
--apparent field. On average, a wider field will seem to be brighter, even if it doesn't measure so. This is probably a psychological phenomenon.