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Friday, 23 September 2016


Very good of you to step up.

Sadly I have more corrections: the Sony 99-RollingStones camera (or whatever it's called) does not have a mirror, it's a semi-reflecting force-field. It weighs less, and won't break. The main downside is that if you touch it with wet fingers, you will probably be instantly transported to another planet or even another universe. The specific seriousness is not known, since so far nobody has come back. (On the up side, the camera goes with them, so if some come back, think of the photos they may bring!)

Now I'm confused. Is there a fixed, semi-transparent mirror in the SonySLT reflecting some of the light coming through the lens onto an autofocus system? If so, it's a pellicle mirror, whether or not it's used as part of the viewfinder system. A pellicle mirror is simply a very thin, partly transmitting, partly reflecting mirror.

One obvious advantage of the pellicle-mirror approach to AF is that the AF sensors have an uninterrupted view of the scene. That makes tracking moving objects a lot easier during a burst.

A mirrorless camera is at a disadvantage in tracking during bursts. After the image is taken, the shutter has to stay closed while the sensor data is being read out*, and on mirrorless that creates a long AF blackout. An ordinary DSLR would drop its mirrors down and start feeding the AF and viewfinder during sensor readout, while the SLT never had an interruption in the AF image to begin with.

Also, on-chip AF sensors have historically been hampered relative to a dedicated AF system. For example, cross-type AF points have yet to appear (the just-announced Olympus EM-1 Mk II claims to have them). I believe that on-chip AF sensors are slower to read out, also, but I could be wrong about that. Or about anything else.

* The need to keep the shutter closed during readout is due to the absence of global electronic shutters on affordable CMOS sensors -- you can't tell them "freeze your photon counts". You need the shutter to keep further photons from getting in until the data is collected. CCD sensors generally have global shutters, and a few exotic CMOS sensors now have them. Someday this won't be a problem, but don't hold your breath.

Three wrongs don't make a right but three lefts do.

I'd like to thank you for upholding the tradition of fact-based reality, though you are a sap for doing so, of course.

Forgot the second half of that joke -
, but anyone who drives in New Jersey knows it takes three rights to make a left.

Wikipedia thinks it's a pellicle mirror:


You might need to correct your correction, Mike. :)

I think Bill Tyler is right. Even if it doesn't work the way you thought it worked (or the way pellicle mirror cameras of the past worked), there's still a pellicle mirror in there.

The errors start dropping like peacocks on a roof.

Never admit to being wrong. I handle situations like this by saying "I may not be right, but I am not wrong."

And, why does Sony need to do this at all any more, given the existence of integrated phase-detection sensors?)

In fact the A99 mark 2 does use both "traditional PDAF" and "on sensor PDAF" for focusing.

The large pixels used in "traditional PDAF" sensing, as used in conventional DSLRs, are more sensitive, are monochromatic, and can be sensitive to near IR light. All of these features help determine the level of in-focus/out-focus and direction of focus accurately and rapidly.

In the A99 mk2 Sony are claiming they can AF at light levels down to EV -4. That's focusing on a subject illuminated by the moonlight from a gibbous moon.

As lines or crosses of pixels in the PDAF sensor are larger than on-sensor PDAF sensor they can deliver more accurate distance information. In rangefinder terms they have a "longer baseline".

There is a good explanation at this link, so I don't have to type it.


Pellicle just means membrane or film.

So if the mirror is partially reflective and made of membrane or film then it's a "pellicle mirror" and it doesn't matter what you do with the reflected light.


I'll never be able to look at another peacock quite the same way.

If I wanted alphabet soup I would have gone to SmittenKitchen.com, could we please as photographers stop with all the acronymania.

It's ok, we live in a post-facts era anyway.

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