New Sony Alpha cameras - pellicle semi-translucent mirrors

#Chri5#

#Chri5#

#Chri5#

Soldato
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http://www.dpreview.com/reviews/sonyslta55/

The designation 'SLT' stands for single lens translucent and it's the 'translucent' bit that's the key to what differentiates these new models both from conventional DSLRs and mirrorless interchangeable lens cameras. The SLTs do have mirrors, but they're mirrors that let the majority of the light pass straight through to the sensor, rather than having to swing out of the way to allow exposure. As a result they are fixed in position, always reflecting a portion of the light emerging from the back of the lens onto a phase-detection AF array housed in the top of the camera. (A newly-developed 15-point array in the case of these two cameras).
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Interesting indeed, good to see somebody doing something new, however unless I'm reading it wrong it appears that they only use the mirror for the AF, there's a EVF and LCD screen but no optical viewfinder. Interesting to see what the reviews say...
 
Yup, no OVF:

They manage this by removing the optical viewfinder entirely and only having to direct light to the phase-detection AF sensor array. For packaging reasons this has been moved from its conventional position at the base of the camera's mirror box, to its roof. Roughly 30% of the light that strikes the main mirror is reflected upwards, onto the AF sensor array. Meanwhile, in the absense of an optical finder, the main imaging sensor provides a full-time live view image via an electronic viewfinder in its place.
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Begs the question how the AF feels about low light I guess, what sort of price are they planning to put on this? It's interesting at £500ish but much beyond and it starts becoming a bit of a gamble...
 
Canon and Nikon did a pellicle design back in the 60-70's as well, though it was more for high speed (fps) use. I'd like to see a return of them with OVFs though, the lack of mirror blackout is awesome.
 
So you're losing something like 1/2-2/3 of a stop all the time? I'm not sure I want to have to consider the T-stop on a camera body!
 
Canon made several of these in the 90's. 10fps film cameras.

Eg...EOS-1N RS, EOS RT.

The problem with it is that the viewfinder sucks up light. So rather than shooting ISO100 Film in daylight, you had to use ISO160 or faster.

Once they figured how to make the mirror move fast enough (VF blackout on 1D's is 80ms) it became easier for the lens to track with the mirror down time available at these speeds and of course at these speeds the photographer can still see whats going on pretty clearly.
 
Mud said:
So you're losing something like 1/2-2/3 of a stop all the time? I'm not sure I want to have to consider the T-stop on a camera body!
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That was my initial impression too, especially in low light scenes it would be a major pain...
 
Seems like a loose loose loose situation based on old film technology.

1) You loose an optical view finder,
2) The AF Sensors get 2/3rd less light meaning lower and and less accuracy, more hunting
3) The sensor gets 1/3rd less light so anyone paying the difference between an 85 1.8 or 1.4 has thrown away a good chunk of their money
 
D.P. said:
Seems like a loose loose loose situation based on old film technology.

1) You loose an optical view finder,
2) The AF Sensors get 2/3rd less light meaning lower and and less accuracy, more hunting
3) The sensor gets 1/3rd less light so anyone paying the difference between an 85 1.8 or 1.4 has thrown away a good chunk of their money
Click to expand...

Not so sure about point 2), the mirror in a conventional SLR is already only partially silvered (i.e. the light is split between the viewfinder and the AF sensors). Your figure of 2/3rds less light to the AF sensors infers they're otherwise getting ~90-100% (if my math brain is working).
 
So far reviews are actually very positive for the A55. They are saying things like:

- Fastest AF in pretty much any Sony body
- The EVF is bigger than most APS-C OVF's out there (with a digital readout of settings)
- Nice LV screen with adjustable backlight controls
- Very impressive video AF
- The video samples provided by DPReview are nice looking.
- Low light performance at least as good as NEX-5

On the negative side:

- Poor JPEG NR reduces image quality beyond ISO400...
- Dodgy bundled software


Looks like Sony has developed that old film technology very nicely...
 
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