A6000 vs. A77ii Continuous AF - Very confused

Started May 14, 2014 | Discussions thread
123Mike Veteran Member • Posts: 4,643
Re: Still the f/3.5 or smaller aperture restriction on low light continuous AF?

I think you're right. The A-mount mechanics don't allow for that sort of faster-than-visible homing in action.

But... the question that remains is that for video with continuous AF, is the aperture still limited to f/3.5 (or brightest than the lens allows when using a darker lens)? Also, given that AF during video works just fine when using a darker lens, why does it not let the user just try and see if it works... So, why not let the user just pick f/9, say, and let them just do some video with AF. If it's crap, then take it down a notch. Are they afraid that when that would be available, that it would lead to people complaining and bad press? Right now, being limited to f/3.5 while we *know* that it *does* work at least a bit higher, is already not so great, right?

Well one thing to keep in mind with SLTs and SLRs is that they are designed to AF with the lens wide open all the time, regardless of the shooting aperture.

No, not wide open. f/3.5 or brightest when f/3.5 isn't available. So, using a 35 f/1.8 lens for instance, it'll use f/3.5, not wide open.

Also keeping in mind that these sensors, whether on an SLR or SLT only get about 1/3 of the total light coming through the lens.

I don't think that is a problem. Plenty of light in plenty of situations.

When in video mode however the lens is kept at the actual aperture, which starves the AF sensor of light.

Yes, but it *does* work at f/5.6 for instance, when using a kit lens. So, when using a 16-50 f/2.8 lens, why not let the user pick f/5.6 then too? Why must it be f/3.5 for that lens? Then my further point is that if f/5.6 works fine after all, why not let us see if we can make due with something higher like f/9 for instance. Perhaps outside in good light this will work. The mirrorless ones give you all that control, so why no the SLTs?

While it 'may' work in certain scenarios it most likely wont work well enough often enough to be that useful, but will have the oppertunity to ruin user experience, imagine all the complaints from people who don't know better saying that their AF in video doesn't work properly.

That's an assumption. I bet we could get away with a far smaller opening than f/3.5.

Another example was Canon limiting their AF system to f8 or was it something bigger? Anyway, yes in some circumstances it may still work, but that will not be the general experience.

Outside in broad daylight?

Mirrorless models on the other hand rely on on sensor AF, which is getting 100% of the light

I don't think the light difference is significant. It's just 1/2 stop if that.

additionally it relies on both PDAF (which I explained shuts down at certain light levels) and CDAF which should work regardless of aperture (but does become less effective and will hunt more).

That is true. With a mirrorless like the A6000, it has the option to not use PDAF and use CDAF automatically - which I think it does. I can tell, because I can spot a quick hunting action under certain conditions. But there *is* a lot of room to maneuver though. Not until something like f/10 does it no longer use PDAF...

Why they chose f3.5 over anything else? This may be due to the sensitivity of the AF points, where only a few are senstive to f2.8, the other points are not as useful at f2.8. So f3.5 and higher is optimum for those AF points so you can use all of them.


At the end of the day, a gimped AF system is not going to work better than MF for video, in fact its likely to be worse. So they allow AF at 'optimum' settings and if you want to use other apertures you have to do it on your own.

I find it a lousy limitation though, and I think it is possible to do better. Also, it's known that when you rig a Minolta 50mm f/1.7 lens to be forced wide open, that the SLTs auto focus during video just fine! It means much better low light performance. Shallow DOF of course, but this could be ok. And then given that f/5.6 works fine (kit lens), and that probably it can handle at least f/8 (guessing here) without too too much trouble, I think that simply dictating f/3.5 and nothing else is just too draconian. I think that if a hacker group hacks that camera like some did with the Canons, one of the first things they'd do is fix these still things.

Not sure if the A77 II is any different, I suspect not.

I have the feeling it won't be either...

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