SLT light show

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David Kilpatrick
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SLT light show

Unread postby David Kilpatrick » Mon Apr 08, 2013 12:48 am

This is amazing. Try it if you own an Alpha 99. I've been trying to find out why the A99 is so bad in backlight and has been causing me problems with flare intruding over dark detail or spreading in from the edge of the image when very bright areas are present. This is what has persuaded me to re-acquire an Alpha 900 this last week.

Get a point source (I have a pen with a single LED in the end). Shine it into the open camera with SLT mirror in place.

a99-mysteryblobby.jpg
a99-mysteryblobby.jpg (87.31 KiB) Viewed 4405 times


Now that's what's shines back at the lens. RX100 snap. With the neked eye, you see the image of AF module, three tiny direct reflections of the point source in the module's lenses, and more stuff - bounding around inside the camera like crazy. Every point of light hitting the SLT mirror/AA filter is actually doing this.

You know what I want? An improved NEX 7 with GPS. I really don't want any of this stuff happening and I now know why I'm having so much trouble with the extremely difficult subjects I like to photograph.

Maybe the A900 will do just the same - AA filter - who knows. Maybe something without an AA filter will not.

David

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Re: SLT light show

Unread postby redsim74 » Mon Apr 08, 2013 2:10 am

Wow, looks like Sony stuck a disco ball in there. :(

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Greg Beetham
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Re: SLT light show

Unread postby Greg Beetham » Mon Apr 08, 2013 5:40 am

One of the things I’m struggling with is the sheer amount of magnification in the reflection, it’s probably something to do with the non-polarized light source (scatter) as it passes through the mirror and the optical properties of the micro lenses themselves reflecting back towards the mirror at an angle, but just what is causing that return angle to be so greatly increased over the inbound angle and still be able to form a layered? oof image of a tiny area of sensels spread out over the back face of the mirror? It’s like a dome shaped wide angle mirror reflection effect.
Even the AF unit seems to be magnified for some reason (size of the unit as compared to the overall size of the FF mirror).
I’m not sure how relevant such a test might actually be due to the unfocused and strength of the light source, the brightness of the light coming out of the back of a lens is very dim by comparison but presumably is also non focused at the distance in front of the image plane the mirror occupies.
What about a magnifying glass with an ND filter taped in place (dim the point source a bit) and see if focusing the magnifying glass either on the sensor through the mirror or on the mirror itself sheds any more light (so to speak) on the subject.
Greg

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Re: SLT light show

Unread postby agorabasta » Mon Apr 08, 2013 12:39 pm

Those blobs are not individual images, they are formed by multiple individual reflections coinciding at the particular direction of observing. The light coming from the source hits the individual sensels/microlenses at slightly different angles, and then the angle of view from the observer is also slightly different for those sensels; so at certain positions of observer and the source there's an apparent pattern formed by the low spatial frequency beat between the multiple reflected projections inside the sensor-topping sandwich.
And it's mostly the AA filter that's causing that as it amplifies the effect by its by-refringent nature. Also, using a light source with greater coherence length/time (e.g. LED's or lasers, especially) also greatly magnifies the effect.

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Re: SLT light show

Unread postby Dr. Harout » Mon Apr 08, 2013 9:48 pm

I am not good in those things, but I have experienced inner reflections with sensor based cameras, and have noticed that happening with mainly artificial light. Thus, no big issue for me.
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David Kilpatrick
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Re: SLT light show

Unread postby David Kilpatrick » Tue Apr 09, 2013 10:43 am

The ideal sensor would be even better than film - a dead black surface, with no glass layer or microlenses over it. Of course the effect I observed never has any obvious influence on the pictures taken, but the 'maps' the way the sensor/mirror/AF assembly all reflect image-forming light back towards the rear element of the lens. I'm sure anyone who owned something like the original 50mm Minolta macro and tried to use it on the Dynax 7D will know the light patch formed in the middle of the shot when the subject was dark and the surroundings bright. This was not caused by the lens on its own, as it's not present on film, but a result of this reflected light hitting the rear element and being focused into a vague hot spot.

When I am back home I'll carry out a similar but much more controlled test using the A55, A77, A99, NEX-5N, A700, A900, A580 and probably if I'm able to the Nikon D600 and 1 V2. The object, to observe how much light from a single point source is reflected back from the interior assemblies, and in what way.

One of the changes made between the A55 and the A77 was the angle of the mirror and position of the AF module. In the A55, reflections from the lenses of the AF module definitely cause problems in some shots. The A77 solved this. In my picture the central three 'white' circles are caused by the three lenses of the AF module in the A99.

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Re: SLT light show

Unread postby agorabasta » Tue Apr 09, 2013 12:22 pm

David Kilpatrick wrote:The ideal sensor would be even better than film - a dead black surface, with no glass layer or microlenses over it.

And they are trying to accomplish this task. Right now there seems to be a viable approach with the so-called 'light traps' being basically the pipes extending from the silicon surface up. The problem to solve is that it's a significantly 3D structure hard to make with the existing chip fab processes.

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Re: SLT light show

Unread postby Greg Beetham » Tue Apr 09, 2013 1:57 pm

agorabasta wrote:
David Kilpatrick wrote:The ideal sensor would be even better than film - a dead black surface, with no glass layer or microlenses over it.

And they are trying to accomplish this task. Right now there seems to be a viable approach with the so-called 'light traps' being basically the pipes extending from the silicon surface up. The problem to solve is that it's a significantly 3D structure hard to make with the existing chip fab processes.


It will be fun cleaning those pipes with no protective glass over the top, maybe someone should invent a nanobot cleaning service to go with the micro pipes.
Greg

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bfitzgerald
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Re: SLT light show

Unread postby bfitzgerald » Tue Apr 09, 2013 6:19 pm

Only thing I had spreading was that nasty green line thing which Sony it seems were unable to re-produce! (I've send them the samples of it)
Aside from that I've not seen an issue with the mirror..assuming that is a sensor problem.

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Re: SLT light show

Unread postby sybersitizen » Sat Apr 13, 2013 11:00 pm

David Kilpatrick wrote:This is amazing. Try it if you own an Alpha 99. I've been trying to find out why the A99 is so bad in backlight and has been causing me problems with flare intruding over dark detail or spreading in from the edge of the image when very bright areas are present. This is what has persuaded me to re-acquire an Alpha 900 this last week.

I'll be pretty surprised if the A900 is any different. Why do I say that? Because all three of my digital Alphas - A55, KM5D and KM7D - show the same effect when I shine a light into them. If it wasn't an issue going back to 2005, I don't see why it's an issue now. I assume most or all digital cameras behave in a similar way.

David Kilpatrick wrote:The ideal sensor would be even better than film - a dead black surface, with no glass layer or microlenses over it... I'm sure anyone who owned something like the original 50mm Minolta macro and tried to use it on the Dynax 7D will know the light patch formed in the middle of the shot when the subject was dark and the surroundings bright. This was not caused by the lens on its own, as it's not present on film, but a result of this reflected light hitting the rear element and being focused into a vague hot spot.

The 'sensor reflection effect' is indeed present on film. I've tested for it and found it using the aforementioned original Minolta 50mm f/2.8 AF Macro. A center reflection spot can be seen, but just barely... and only at very small apertures and only when the lens is focused at the known 'trouble distance' of about 15 inches. The only reason it went undetected in normal use with film cameras was that film is so much less reflective than digital sensors, on which the effect is much more intense.

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Re: SLT light show

Unread postby David Kilpatrick » Sat Apr 13, 2013 11:35 pm

I'd expect anything with a diffractive AA filter to do something similar, but I'm not sure of the way light is further reflected by the reverse side of the SLT mirror (if at all). A900 now arrives on Monday, I hope, stuck in Parcelforce non-redelivery mechanicam since April 5th because the package should never have been sent to me.

The original reason for using the light - a single LED in the end of a promotional pen, just something I happened to have on me - was just to check the SLT mirror for clarity and absence of dust after a few problem shots. It looks absolutely as new. Many situations where the A55 was not perfect are greatly improved in both the A77 and A99 (full-on big sunsets etc). It's also quite possible that when I set up the A900 and A99 to test the type of problem I've had, the A900 will turn out to be no better. In the end its only use may be in the studio, where it's far better for prolonged viewing and setting up, and never needs GPS.

David

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Re: SLT light show

Unread postby InTheSky » Sun Apr 14, 2013 2:32 am

At first ... LED has always be for me problem with Digital Sensor, when I'm a small place where music band are playing and there is cheap LED spot, I know most of my picture will be garbage ... or only usable in B&W. Problem with led is that they are producing frequency and create strange effect, to many time I got over exposed.

I was feeling that the new SLT design will reduce this flare effect of old lenses, I still has not got the time to do the test. But I still own old Minolta lenses an I will do the test. I remember the first time I use the 50mm 1.7 and tried to understand this round hallow of light in the middle of the picture on my A700.

You will be able to stop using a A99 DAvid ?

REgards,

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Re: SLT light show

Unread postby David Kilpatrick » Sun Apr 14, 2013 11:13 am

The A99 is too useful to stop using! If the A77 high ISO performance was better, I would not need the A99. Full frame is not something I want for its own sake - very few of my pictures use differential focus and most of the time, I need more depth of field, not less. Both the D600 and the A99 produce amazing high ISO results, and A77 does not.

David

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bfitzgerald
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Re: SLT light show

Unread postby bfitzgerald » Sun Apr 14, 2013 12:14 pm

I ran across this purely by accident. Any thoughts?

http://members.shaw.ca/cwrbc-lot-users/A77ghosts-b.html

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Re: SLT light show

Unread postby David Kilpatrick » Sun Apr 14, 2013 5:14 pm

Barry, that is interesting. It confirms exactly what I found with my 70-400mm at f/5.6, but I was generally putting this down to the lens and some kind of centering issue, when I first made tests. In fact I have made very little use of the my 70-400mm G because I bought it after getting the A77, and its first tests were all on the A77 - and they also featured (you guessed?) birds, heron and cormorants, against bright water or clearly lit against shade.

I would say it has to be the SLT mirror which causes this but it's almost impossible to confirm it without getting hold of a NEX-7 again, and trying a CD focus image.

I will echo the complaint made by the writer, about the naysaying atttitude of owners on dPreview - there is this amazing sort of ostrich-head-sand culture there, where reasonable investigations into issues which matter are denigrated by people who clearly lack the same awareness or critical faculties.

With the A55 I was able to identify the position of ghost images easily, the A77 not so. A99, I have not yet done any controlled tests.

David


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