Aperture of focus sensor

[Hakanie]

Hi there!
I've red a lot of comments about the aperture of the focus sensor. As I understand the A700 and A900 has got a 2.8 aperture focus sensor but the rest of the Sony DSLR and KM doesn't. Does this mean that my KM5D won't focus properly if I use a lens with aperture 2.8 or 1.7?
/Hakanie

[Greg Beetham]

Hi, I have a KM5D and an A100 that I use with (among others) a KM100 f2.8 macro lens, a KM28-75 f2.8 and a KM17-35 f2.8-4 and haven't noticed any habitual focusing problems, I don't own anything faster at the moment though like a 1.4 or 1.7, I have been led to believe that one can get the odd focussing glitch with those even with a camera that has a 2.8 focus sensor.
Greg

[bossel]

Does this mean that my KM5D won't focus properly if I use a lens with aperture 2.8 or 1.7?

No problem, it will focus correctly (in general). Of course AF is not always perfect, but any lens, be it F1.4 or F2.8 or F5.6 should focus fine on any KM or Sony Alpha DSLR.

[jcoffin]

Hi there!
I've red a lot of comments about the aperture of the focus sensor. As I understand the A700 and A900 has got a 2.8 aperture focus sensor but the rest of the Sony DSLR and KM doesn't. Does this mean that my KM5D won't focus properly if I use a lens with aperture 2.8 or 1.7?
/Hakanie

It will focus just as well with faster lenses as it does with slower ones -- but not really any better. With an A700 or 900, focusing does work better with lenses that are f/2.8 or faster. With these cameras, a lens that's f/2.8 or faster (used in conjunction with the center sensor) allows the camera to focus more accurately and in lower light than it can/could with a slower lens.

[KevinBarrett]

The aperture ratings of the focus sensors has to do with the amount of light it needs to do its job. An f2.8 sensor is super accurate, but needs a lens of maximum aperture f2.8 or better in order to start (meaning f/1.4, f/1.8, and f/2 lenses will work as well). Most of the rest of the sensors, in any camera system, are f/5.6, meaning that they will work with more lenses (slower ones), but are less accurate than the center f/2.8 sensors.

These work because when a lens is focusing in normal shooting, its aperture is fully open. It only closes down to smaller apertures the moment you activate the shutter, or when a depth-of-field preview function is used.

[Javelin]

These work because when a lens is focusing in normal shooting, its aperture is fully open. It only closes down to smaller apertures the moment you activate the shutter, or when a depth-of-field preview function is used.

Thank you for that. the last mystery of life has been reveiled to me now ... ok well maybe not the last, it was just the one thing I wanted to know and always forgot to ask. I should have thought of it myself to be honest...

[Mike]

These work because when a lens is focusing in normal shooting, its aperture is fully open. It only closes down to smaller apertures the moment you activate the shutter, or when a depth-of-field preview function is used.

This is a great explination. Now, how can I apply it? Looking at the Sony specs for an a350 I don't see an aperture setting of the focus sensor. How can I find out want it is?

Mike

[David Kilpatrick]

These work because when a lens is focusing in normal shooting, its aperture is fully open. It only closes down to smaller apertures the moment you activate the shutter, or when a depth-of-field preview function is used.

This is a great explination. Now, how can I apply it? Looking at the Sony specs for an a350 I don't see an aperture setting of the focus sensor. How can I find out want it is?

Mike

It is about f/6.3 - it will focus with the 18-250mm f/6.3, so it has to be.

David

[KevinBarrett]

Thank you for that. the last mystery of life has been reveiled to me now ... ok well maybe not the last, it was just the one thing I wanted to know and always forgot to ask. I should have thought of it myself to be honest...

You're welcome, Javelin! Now if only I could remember all the questions I wanted to ask...

[drok]

Still a bit confused. So if you have a 2.8 focus sensor and you mount a lens with a minimum F5.6 aperture it won't work? How does it focus then?

[David Kilpatrick]

It works because there is another sensor there. The centre f/2.8 sensor of the A700/A900 is a strip, surrounded by a double cross shaped f/6.3 sensor. When you fit a fast lens, the additional sensor comes into play. With an ordinary slower lens, only the standard sensor does any work.

David

[drok]

Thanks David. That makes much more sense!

[KevinBarrett]

I've been curious how the 500/8 mirror lens auto-focuses...any insight there, DK?

[David Kilpatrick]

Yes, its physical aperture is wider than f/6.3 - there is a big circle in the middle, which reduces it to f/8, but this is all about optical geometry and the AF sensor sees a 'donut' just as we do. It that donut had its middle filled in with image, it would be much faster than f/8.

However, the Sony/Min AF system is extremely good by comparison with other makers. Canon has a real cut-off point at f/5.6 and lenses like the Tamron 18-250mm have to fool the cameras. You will notice that there are no Canon lenses made which go to f/6.3. Sony has a cut-off point apparently around f/7.1, but Minolta were still very cautious with their 50mm f/3.5 and limited it to 1:2 ratio, otherwise it would have been worked at f/7 at life size, and unable to focus reliably.

I have tested the 70-300mm SSM with 1.4X converter (f8) and whether it works or not depends very much on light. Indoors, no go. Outdoors in bright light, it's OK. I have also tested the 70-210mm f4 beercan with a 2X (f8 constant aperture) and this combination actually works well, in almost all lighting conditions. A similar overall combination on Canon or Nikon will not focus at all.

David

[KevinBarrett]

Of course! Thanks David. I forgot about that doughnut quality. It would seem Sony has a real edge in the AF area that's gone somewhat unheralded.