A6000 vs. A77ii Continuous AF - Very confused

Started May 14, 2014 | Discussions thread
abortabort Senior Member • Posts: 1,734
Re: Still the f/3.5 or smaller aperture restriction on low light continuous AF?

abortabort wrote:

tedandtricia wrote:

Evildogofdoom wrote:

tedandtricia wrote:

Anyone know if the A77 II still has the restriction on low light continuous AF that all the older bodies do (A37 A57 A77 A99)? On the old bodies, you can only do continuous AF at f/3.5 or smaller apertures, which pretty much rules out low light continuous AF (unless you apply a mechanical hack which not everyone wants to do).

Just wanted to find out if the restriction is still in place on the new A77 II body.

This is not true. The f3.5 limit is only for focusing at 12fps, on the A77. The cameras will otherwise continually focus at the maximum aperture of the fitted lens. The f3.5 limit is more a limitation of the mechanical aperture opening and closing 12x second.

Thanks. That's a helpful clarification on the photography side.

Still appears to be a limitation for video since at a minimum you'd shoot video at 24 fps.

That's for a different reason. For stills, the aperture has to keep flipping between 3.5 and the desired one. It can do that at up to 8 times per second (or, rather, it's willing to do that up to that rate). For movies, the aperture isn't filling at all. There, it is continually at one setting. The AF system, however, or at least on all the crop SLTs up to at least the A77, the aperture must sit at 3.5 because otherwise it can not see depth accurate enough to provide good continuous auto focus during video recording. I don't know if that's the same with the A99, or the A77ii. In theory, the A77ii *could* employ CDAF for video so that it would let you AF during video using apertures up to around f/11, like the mirrorless ones do. The word CDAF on any SLT including A7ii is a complete no-no, so probably most would immediately assert that no of course it doesn't do that. But... for video, in theory, it could. *IF* Sony wants to provide the same flexibility for video as what you get on something like an A6000, they could port some of that code over to the A77ii, as this is likely simply done in software (possibly assisted by hardware, but that's likely shared anyway). Anyway, if it did that, it would have probably come out at this point already. Or not... because I find that when you start talking about specific video details, it's surprising how little back you get. So this issue could simply have been overlooked up to this point.

A simple answer would simply to test it. Who's got an A77ii handy to answer this question in like 5 seconds flat?

They don't use CDAF in A-Mount for 'anything' because the lenses are not optomised for CDAF, they are often too big and heavy (glass elements) as well as their optical design (focussing groups) that they can't move correctly for CDAF to work even reasonably. If you use these lenses with CDAF you are talking often 3-4 seconds just to acquire focus on a completely stationary object, this is not useful in video, not at all with the slow and ponderous hunting back and forth.

It isn't just a matter of porting some code, it is about user experience and whether it is of use to anyone to have such a useless feature. CDAF on PDAF lenses is miserable and the main reason why LV on DSLRs has always been so hopeless and why SLTs exist in the first place. If it actually worked like you suggest there would never have been any need for SLT cameras.

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. 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.

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

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.

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

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.

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

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