Sony alpha A55

[KevinBarrett]

I think the f3.5 AF sensor is ok for f3.5 and slower, if it's an f2.8 lens it'll be stopped down to f3.5 (at least)..in Auto...that's the impression I get anyway.
Greg

When an AF sensor indicates a sensitivity in f/stops, it means that the sensor is sensitive to lenses with that f/stop or greater maximum aperture. So an f/3.5 sensor would be sensitive to lenses with max apertures of f/3.5, f/3.2, f/2.8, f/2.4, and so on...

Now in video, the aperture will be constantly closed down to a given aperture, so perhaps the camera need f/3.5 or greater aperture to deliver AF to Sony's exacting consumer product standards. Or perhaps it's to keep frame lengths short enough. Who knows? I'm still pretty ignorant about video.

[Greg Beetham]

Hmm I always thought when the A700 came out with the f2.8 AF centre focus sensor it was able too focus using f2.8 lenses fully open, (finally) whereas the KM5D, A100 couldn't, (fast lenses being the problem), although, I have too say, I never had any problem that I noticed, using any of my f2.8's on either camera.
Thing is, when either of those two cameras are confronted with an f2.8 lens or faster, DO they close them down too an aperture that's acceptable (say f3.5) while AFing so they can focus accurately? .....that, I do not know.
It may even take into account the ambient EV value for all I know, if say, you are using an f1.2 lens and the EV is down around 6 or 8 the camera may allow the lens too open up fully for AF.
Greg

[agorabasta]

They need to counter a few issues quite adverse to PDAF ability to focus during video shooting.

The first and most important is that PDAF simply cannot function if the DOF is too large - there's no sufficient phase diff in that case. That is why the PDAF needs wide apertures in the first place, and the necessity to have more light at the AF sensors is very secondary in nature.
The second is that wider apertures produce an unacceptably shallow DOF for most scenarios of amateur automatic video shooting.
The third is that a DOF too shallow makes it too hard to track a moving subject when it moves out of DOF faster than the AF adjusts the DOF position.

And then there's a consideration that there are so many cheap amateurish zooms that can do f/3.5 at the wide end. Mind also that those cheap zooms have their min relative DOF about the same as they are zoomed away, as their max aperture shrinks accordingly.

Thus they get at that magical f/3.5 or the any lens' fastest that's tighter than that.

[David Kilpatrick]

Kevin - all sensors can generally focus with all lenses, because f/2.8 sensors also have an f/5.6 (or f/7.1, or whatever) capability. What happens is that a normal f/5.6 type sensor, as found on Nikon, sees every lens faster than f/5.6 as if it was f/5.6, and any lens of f/8 for example either as impossible to focus, or reduced in focus accuracy and speed.

f/2.8 sensors see every lens faster than f/2.8 as if it was f/2.8 (there is no benefit to using an f/1.4 lens instead of f/2.. If the lens has a smaller aperture, like f/6.3, the f/2.8 sensor simply switches off and does not function - in its place, a regular f/5.6-whatever sensor does the work.

It is rumoured that the 500mm f/8 RF lens has been discontinued. This lens only worked with sensors that could focus a lens with an f/7.1 aperture, the standard stop used for bench collimation of those early AF modules. If Sony has moved towards f/5.6 sensors which JUST manage to focus longer focal length f/6.3 zooms (like Nikon) the 500mm f/8 would no longer be AF-able. I have a 500mm f/8 and will try it when my ordered A55 arrives.

If Sony has made cross sensors which have an f/3.5 - f/5.6 dual function, in place of the old f/2.8 - f/7.1 'separate' sensors in A900/850/700, they could be pretty efficient. And they would make full use of the maximum aperture of many zooms, as pointed out, which f/2.8 sensors do not.

David

[dynax800si]

David I saw samples of A55 with 500/F8 and they are just great!




And the whole series here - http://klubkm.pl/forum/showpost.php?p=1 ... tcount=173

[agorabasta]

Once again, it's not the aperture that matters; it's the relation between the focal length and aperture. At 500mm the f/8 aperture creates literally tons of phase shift for PDAF to function quite effectively. The mirror creates problems though...

[KevinBarrett]

Once again, it's not the aperture that matters; it's the relation between the focal length and aperture. At 500mm the f/8 aperture creates literally tons of phase shift for PDAF to function quite effectively. The mirror creates problems though...

What? No love for donuts?

[bakubo]

What? No love for donuts?

I remember back in the early 1970s to mid 1970s donuts were all the rage. People were using those 500mm f8 mirror lenses (mostly 500mm, but also some other lengths). I used to see lots of photos published in magazines that had the donuts!

[agorabasta]

Doughnuts are no problem with PDAF, and they may even look kinda funky, especially the bigger colourful ones.

It's the doubling of straight lines that may fool the PDAF.

[dynax800si]

http://www.darrenf.com/blog/first-ever- ... 55-slt-a55

[David Kilpatrick]

Seeding cameras to shooters like this will do a lot - Sony is looking like being about as cool as a camera brand can get. In fact, things are turning round fast. The NEX is already having that effect. I was with a dozen top UK photographers last week, and half of them knew what the NEX was (a couple asked was I using the new Panasonic- etc) but when I swung a sweep panorama round the group sitting at a table outside, they were just amazed, and amused.

Sony could become THE camera to be carrying in 2011. Nikon or Canon begin to look outdated. Like being seen with a Rolleiflex in 1970.

David

[bfitzgerald]

Possibly a tad over enthusiastic there DK
What we should really be wondering is how NEX is doing v Micro 4/3 which is it's main competitor (unless you count premium compacts too)
Canon have messed up with the 60d but it won't see them fall apart sales wise. I think Nikon will impress shortly with an updated range. And we know other makers are going to jump into this segment too. NEX might look cool but the reality of camera controls come crashing down 10 seconds after picking one up.
As for Sony and the other SLT/DSLR models we'll have to wait on that one.

Sony might be the brand for some consumers, but they've burnt a lot of bridges with serious enthusiasts.

[mike2008]

Strange that so many enthusiasts keep posting here with all the bridge-burning going on

David I wanted to ask you, you mentioned something in the review of the A55 about polarization filters perhaps working strangely on the a55, but you didn't really explain why. I looked at the optical path in the camera and the reflection from the beamsplitter looks to be close to the Brewster angle, I wonder if they use a polarization filter to help reduce reflection?

I put some calculations here:

http://forums.dpreview.com/forums/readf ... e=36120294


It's probably completely wrong, just speculation on my part, but yours was the only comment about polarization effects that I've seen so thought maybe you had some insight.

[bfitzgerald]

Strange that so many enthusiasts keep posting here with all the bridge-burning going on

There is talking and selling big difference!
I have no doubts that the lack of A700 update has lost Sony a not insignificant number of users.
The fixed mirror cameras look to be aimed towards new users or what were bridge users a bit more than enthusiasts (note the control layout etc) NEX is very much aimed to p&s users as well.

Having spoken to and seen many working photographers, very few have anything to say about Sony nor any interest in their products at all. Canikon completely dominate the sector and this trickles down to semi pro and serious hobby level too. Sony are mostly interested in volume consumer sales and not photographers.

[David Kilpatrick]

Mike, I don't have any insight. I just know my working optics (I'm lousy at theory, figures and stuff like that). Confront me with any semisilvered surface and I immediately think 'polarisation'.

One hopes that Sony's engineers have the same everyday working knowledge of how these things perform.

David