09-03-2010, 08:16 PM
[quote name='Brightcolours' timestamp='1283456315' post='2430']
No, it is not true that that is valid for all glass elements. It is only true for all flat slabs of glass at an angle like here 45 degrees, no other glass element can cause that.
Sensor reflections are usually not a huge problem with rear elements. With modern lenses they are all coated (you can see that), but some have newer coatings than others. It was said that uncoated elements can cause the so called purple fringing around very "over exposed" areas. And newer coatings would counter that.
However, whether that is true I do not really know for sure. I do know that it is a bit hard to imagine the convex shape of the rear element to to actually let light bounce back the almost exact way it came from, but who knows.
The biggest problem with sensor reflections I already mentioned extensively, and that is the famous ghost lights mirrored through the optic axis, when one is shooting in dark conditions with a filter mounted (flat slab of glass).
And yes, I do imagine the Sony A55/33 to eliminate those (filter caused ghost light images) to quite a large extent, as the light passing through the mirror twice will lose about 1 f-stop of light. I however do not really see that as an advantage, as I do not want to use "protective" filters, I do not believe in their protective qualities (rather use a hood). I do believe in their image degrading qualities under certain conditions.
But again, what is happening in the glass mirror of the Sony A55V/33 only happens in that mirror, not in other glass elements. Unless you happen to have a beam splitter in your lens. What is so specific about the mirror is the flat slab of glass, and the angle.
The Canon EOS RT has a very thin membrane of plastic. It has been known for decades that to avoid ghost images the pellicle membrane needs to be very thin, not just in SLRs. But obviously, the Sony glass mirror is less fragile.
To visualize what happens because of the mirror thickness:
![[Image: 3E8740B8CA6D493B8F26AC3622A1E265.jpg]](http://g2.img-dpreview.com/3E8740B8CA6D493B8F26AC3622A1E265.jpg)
Theoretically it would be possible for more than one ghost light to appear.
[/quote]
Your drawing is actually incorrect. The light should be refracted, twice (once at each mirror surface, see for example [url="http://www.thorlabs.de/NewGroupPage9.cfm?ObjectGroup_ID=915"]http://www.thorlabs....ectGroup_ID=915[/url]), therefore only causing at worst two images slightly offset to each other, as the beams before and after the mirror have to be parallel. Also, the scenario as you draw it here with more than one reflection is not possible, unless they had chosen the refractive index of the reflective mirror glass deliberately so to create those reflections (IOW, a very, very high refractive index). And good coatings will reduce these reflections by 99.8 % or more.
I therefore conclude, that final proof for the cause of the reflections in those images, unless scientifically proven to be from the mirror, may well be something different. They could well be reflections of the sensor, re-reflected by the AA-filter, or even a similar thing as the black dot phenomenon, IOW, a processing problem when reading the raw data. Or something completely different for that matter, but very likely not any reflections caused by the mirror, unless they really made a terrible engineering error.
Kind regards, Wim
No, it is not true that that is valid for all glass elements. It is only true for all flat slabs of glass at an angle like here 45 degrees, no other glass element can cause that.
Sensor reflections are usually not a huge problem with rear elements. With modern lenses they are all coated (you can see that), but some have newer coatings than others. It was said that uncoated elements can cause the so called purple fringing around very "over exposed" areas. And newer coatings would counter that.
However, whether that is true I do not really know for sure. I do know that it is a bit hard to imagine the convex shape of the rear element to to actually let light bounce back the almost exact way it came from, but who knows.
The biggest problem with sensor reflections I already mentioned extensively, and that is the famous ghost lights mirrored through the optic axis, when one is shooting in dark conditions with a filter mounted (flat slab of glass).
And yes, I do imagine the Sony A55/33 to eliminate those (filter caused ghost light images) to quite a large extent, as the light passing through the mirror twice will lose about 1 f-stop of light. I however do not really see that as an advantage, as I do not want to use "protective" filters, I do not believe in their protective qualities (rather use a hood). I do believe in their image degrading qualities under certain conditions.
But again, what is happening in the glass mirror of the Sony A55V/33 only happens in that mirror, not in other glass elements. Unless you happen to have a beam splitter in your lens. What is so specific about the mirror is the flat slab of glass, and the angle.
The Canon EOS RT has a very thin membrane of plastic. It has been known for decades that to avoid ghost images the pellicle membrane needs to be very thin, not just in SLRs. But obviously, the Sony glass mirror is less fragile.
To visualize what happens because of the mirror thickness:
Theoretically it would be possible for more than one ghost light to appear.
[/quote]
Your drawing is actually incorrect. The light should be refracted, twice (once at each mirror surface, see for example [url="http://www.thorlabs.de/NewGroupPage9.cfm?ObjectGroup_ID=915"]http://www.thorlabs....ectGroup_ID=915[/url]), therefore only causing at worst two images slightly offset to each other, as the beams before and after the mirror have to be parallel. Also, the scenario as you draw it here with more than one reflection is not possible, unless they had chosen the refractive index of the reflective mirror glass deliberately so to create those reflections (IOW, a very, very high refractive index). And good coatings will reduce these reflections by 99.8 % or more.
I therefore conclude, that final proof for the cause of the reflections in those images, unless scientifically proven to be from the mirror, may well be something different. They could well be reflections of the sensor, re-reflected by the AA-filter, or even a similar thing as the black dot phenomenon, IOW, a processing problem when reading the raw data. Or something completely different for that matter, but very likely not any reflections caused by the mirror, unless they really made a terrible engineering error.
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 ....