09-03-2010, 08:13 AM
[quote name='admin' timestamp='1283493195' post='2443']
Yes, I surely understand what you mean but this effect is also present with all glass elements - just not with a 45 degree reflection. There're reflections all over the place in there. Just imaging a near center ray which is (in part) bounced back by near-180 degrees and then reflected again. This would cause the same kind of issue you described here.
[/quote]
No, it can't. Like Popo also already pointed out, a rear element is not a flat slab of glass. Anything that would reflect internally would be spread all over the place, since it is convex. It would never leave a ghost image. Further, the angles within the rear element are much less than 45 degrees, so reflections to the same extent are much less likely.
Also, rear elements have very rarely a reflective coating applied to one of the surfaces <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt='
' />.
I have never seen any lens cause ghost lights (except when they have "protective filters", see an earlier post which illustrates the ghost lights mirrored through the optical axis). Lenses with lesser coatings (this is not about the rear element, but all elements) are less contrasty, not producing ghost images.
What occurs in the A55V/33 is really quite exclusive for those cameras. Even other cameras with beam splitters solved this problem: The Minolta E-10/E-20 DSLRs used prisms instead of a flat slab of glass
http:[img]//a.img-dpreview.com/reviews/olympuse10/images/e10cutaway.gif[/img]
and Canon used a very thin plastic reflective membrane in for instance the EOS RT.
Actually, strictly speaking Sony does not use a pellicle (which stands for membrane), but a mirror instead, a "plate beam splitter".
The 2nd surface reflections are a well known problem in the beam splitter world (used in telescopes, optical measurements with lasers, a.o.).
Some links to illustrate further:
http://www.datasheetarchive.com/datasheet-pdf/05/DSA0084689.html
[url="http://www.thorlabs.de/NewGroupPage9.cfm?ObjectGroup_ID=915"]Plate beam splitter (Sony A55)[/url]
[url="http://www.thorlabs.de/NewGroupPage9.cfm?ObjectGroup_ID=898"]Pellicle beam splitter (EOS RT)[/url]
"Eliminates Ghosting"
[url="http://www.thorlabs.de/NewGroupPage9.cfm?ObjectGroup_ID=754"]Prism beam splitter (Olympus E-10)[/url]
"Since there is only one reflecting surface, this design inherently avoids ghost images."
Yes, I surely understand what you mean but this effect is also present with all glass elements - just not with a 45 degree reflection. There're reflections all over the place in there. Just imaging a near center ray which is (in part) bounced back by near-180 degrees and then reflected again. This would cause the same kind of issue you described here.
[/quote]
No, it can't. Like Popo also already pointed out, a rear element is not a flat slab of glass. Anything that would reflect internally would be spread all over the place, since it is convex. It would never leave a ghost image. Further, the angles within the rear element are much less than 45 degrees, so reflections to the same extent are much less likely.
Also, rear elements have very rarely a reflective coating applied to one of the surfaces <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt='
' />.I have never seen any lens cause ghost lights (except when they have "protective filters", see an earlier post which illustrates the ghost lights mirrored through the optical axis). Lenses with lesser coatings (this is not about the rear element, but all elements) are less contrasty, not producing ghost images.
What occurs in the A55V/33 is really quite exclusive for those cameras. Even other cameras with beam splitters solved this problem: The Minolta E-10/E-20 DSLRs used prisms instead of a flat slab of glass
http:[img]//a.img-dpreview.com/reviews/olympuse10/images/e10cutaway.gif[/img]
and Canon used a very thin plastic reflective membrane in for instance the EOS RT.
Actually, strictly speaking Sony does not use a pellicle (which stands for membrane), but a mirror instead, a "plate beam splitter".
The 2nd surface reflections are a well known problem in the beam splitter world (used in telescopes, optical measurements with lasers, a.o.).
Some links to illustrate further:
http://www.datasheetarchive.com/datasheet-pdf/05/DSA0084689.html
[url="http://www.thorlabs.de/NewGroupPage9.cfm?ObjectGroup_ID=915"]Plate beam splitter (Sony A55)[/url]
[url="http://www.thorlabs.de/NewGroupPage9.cfm?ObjectGroup_ID=898"]Pellicle beam splitter (EOS RT)[/url]
"Eliminates Ghosting"
[url="http://www.thorlabs.de/NewGroupPage9.cfm?ObjectGroup_ID=754"]Prism beam splitter (Olympus E-10)[/url]
"Since there is only one reflecting surface, this design inherently avoids ghost images."