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?

123Mike wrote:

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.

No. The AF on SLRs and SLTs is designed to focus lenses wide open. You are talking about in video on the SLTs, yes this is the case as I explained why I think it is designed for this reason in video has a lot to do with the way the AF system is designed in general (or 'for stills'). When taking stills (for which the AF system is primarily designed) the aperture of the lens is kept wide open until the time of capture where the lens is stopped down to the capture aperture. The reason for this is to provide as much light through the lens as possible, both for the viewfinder (in SLRs, sensor in SLTs that drives LV) and for the AF module (in more recent cameras that have AF).

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.

How much light do they need to be sensitive? Considering the above, even when wide open they only get 1/3 of the light to the sensor. If they are only getting the light from f8 AND only getting 1/3 of that light directed to the AF sensor that isn't a whole heap of light, even in good light.

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?

Yes. But that is why lenses are kept wide open when they aren't faster than f3.5, it does the best it can with what it has. I can almost guarentee you that the AF sensor is not considered useable at f9, even on the 1DX etc. To be even remotely usable does not mean it is usable constantly at 24/25/30/50/60fps.

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.

Most AF systems will not work beyond f8, some f5.6 or somewhere in between, because the results are unreliable. I think Sony have gone for a simplified system that it is f3.5 or max aperture whichever is greater to cover the biggest base with a simple system. Assuming the max sensitivity of the AF sensor is f5.6 and f3.5 at either end (to work with all the AF points not only some) is the hassle of implementing something that 'kinda-sorta works' for 1.3 stops of control really worth it?

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?

Yes. Canon (like most all manufacturers) stop AF from functioning below (ie smaller MAX apertures than this - Keeping in mind shooting aperture can be smaller than this as AF functions wide open). For example using an f5.6 lens on a 2x teleconverter will stop having functional AF as you are using an f11 lens.

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.

You are thinking about how much light hits the main image sensor, I am talking about how much light is actually diverted to the AF sensor. On the SLTs this is about 30%, on mirrorless being that the main image sensor IS the AF sensor and there is no light being diverted then it receives 100% of the incoming light. As you say yourself, the amount of light being diverted (in this case to the AF sensor) is not terribly significant, which is why it is easier for that sensor to be starved of sufficient 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).

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...

Actually it simply depends on the total light hitting the sensor whether the PDAF continues to function or not. Closing the aperture can 'force' it to do this as can not having enough light in the environment. But CDAF will usually continue to function up to the point of maximum sensor level gain, however it's effectiveness diminishes as gain is applied due to signal noise.

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.

Yeah I kind of think it is lousy too, but we are likely to accept it's limitations in the field and realise where 'we' went wrong given the power allowed to us by the manufacturer (ie use at your own risk) but not many people are, especially those using ANY sort of AF in video (AF in video is patchy at best), so they will be less forgiving. Just have a look how many threads of people complaining about the tracking of AF on the a6000, seems to me it works very well if you have some clue what you are doing, but many buying the camera expect every shot to be in perfect focus every time regardless of shutter speed or any other factor, like a mindreader because it is 'supposed to have great AF, worlds best they say! Well it sucks! My daughter ran at the camera as fast as she could and all the pictures are blurry, this camera sucks'... Set at 70mm f5.6 1/10th ISO 100.

Is it over the top? Maybe. But as I say, most professional use video will not use AF at all, in which case they can use any exposure values they want, the 'idiot proof' video with AF gets locked down to attempt to get the best results possible.

The other issue as I mentioned before is that the AF sensors aren't all sensitive to larger apertures, nor smaller ones. They detect phase differences (like a split prism VF), but on larger apertures these are often 'too wide' for them to 'see' so they can't detect the difference and will hunt. Additionally they are often not 'that' sensitive to smaller differences which is what you will get with smaller apertures. So those AF points, even if they have enough light may not be able to 'see' properly the phase difference and will hunt until they can closer see the differences, hunting is not ideal in video. I would say that under testing f3.5 would be the optimum to maintain the best tracking with as little error and that is why they chose it. They allow smaller aperture values within reason but these are probably a compromise that they are willing to allow to enable as many lenses as possible to be used.

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