09-04-2010, 04:13 PM
[quote name='popo' timestamp='1283615357' post='2482']
I just played about with some calculations. Of course there are a lot of unknown variables.
If we assume the mirror is 45 degrees from vertical with the silvered surface on the outside, and it has a refractive index of 1.5, then an unwanted reflected image would be formed at a spacings multiples of about 1.07x the thickness of the mirror.
For the mirrored surface reflecting at 30% and the glass surface at 1%, each additional reflected image is about 7.2 stops down from the previous. If you assume the coating is even better at 0.1% reflection then the figure becomes 10.5 stops. With such huge losses, the reflection of the reflection is going to be so far down it is unlikely to be significant short of pointing the camera at a very bright point source.
So looking back at the sample image earlier, or more precisely the original of it, the spacing of the secondary image is about 12 pixels offset from the original, working out to be about 0.06mm. That's pretty small! Even if they used a high refractive index material and a more vertical angle the mirror would still have to be pretty thin, a fraction of a mm.
I can refine the above if anyone has more precise values for any of the variables.
[/quote]
Thank you, popo. This is exactly what I worked out schematically on a piece fo paper, without sorting through an assortment of calculations.
I think you can assume that coatings would make transmissiveness 99.7 % or better, with the current state of coatings. Furthermore, in order to make the chance of double images as small as possible, I would expect the mirror coating to be at the back of the mirror, IOW, on the surface closest to the sensor for optimal image quality. This would reduce the chance of unwanted ghosting at least by another factor of 4, or 2 stops.
Of course, the neat thing of having the mirror coating on the sensor side, is that any light reflected by the sensor itself, disappears mostly in the black void of the bottom of the mirror box, before anything can be bounced back from the other side of the mirror.
I reckon this ghosting is not caused by the mirror, but by something else altogether. It is way too bright for starters.
Oh, on the reflection off the back element of a lens (this is really a reply to another post <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' />): many digital lenses have flat or almost flat surfaces at the sensor end of the lens, IOW, it is possible that ghost images are caused by reflections of the last element surface of a lens, especially if they are only slightly bigger or smaller than the original image.
Kind regards, Wim
I just played about with some calculations. Of course there are a lot of unknown variables.
If we assume the mirror is 45 degrees from vertical with the silvered surface on the outside, and it has a refractive index of 1.5, then an unwanted reflected image would be formed at a spacings multiples of about 1.07x the thickness of the mirror.
For the mirrored surface reflecting at 30% and the glass surface at 1%, each additional reflected image is about 7.2 stops down from the previous. If you assume the coating is even better at 0.1% reflection then the figure becomes 10.5 stops. With such huge losses, the reflection of the reflection is going to be so far down it is unlikely to be significant short of pointing the camera at a very bright point source.
So looking back at the sample image earlier, or more precisely the original of it, the spacing of the secondary image is about 12 pixels offset from the original, working out to be about 0.06mm. That's pretty small! Even if they used a high refractive index material and a more vertical angle the mirror would still have to be pretty thin, a fraction of a mm.
I can refine the above if anyone has more precise values for any of the variables.
[/quote]
Thank you, popo. This is exactly what I worked out schematically on a piece fo paper, without sorting through an assortment of calculations.
I think you can assume that coatings would make transmissiveness 99.7 % or better, with the current state of coatings. Furthermore, in order to make the chance of double images as small as possible, I would expect the mirror coating to be at the back of the mirror, IOW, on the surface closest to the sensor for optimal image quality. This would reduce the chance of unwanted ghosting at least by another factor of 4, or 2 stops.
Of course, the neat thing of having the mirror coating on the sensor side, is that any light reflected by the sensor itself, disappears mostly in the black void of the bottom of the mirror box, before anything can be bounced back from the other side of the mirror.
I reckon this ghosting is not caused by the mirror, but by something else altogether. It is way too bright for starters.
Oh, on the reflection off the back element of a lens (this is really a reply to another post <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='
' />): many digital lenses have flat or almost flat surfaces at the sensor end of the lens, IOW, it is possible that ghost images are caused by reflections of the last element surface of a lens, especially if they are only slightly bigger or smaller than the original image.Kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....