The upside to A-mount popularity going down

Started 3 months ago | Discussions thread
OP ProfHankD Veteran Member • Posts: 7,860
Re: Some details about focus micro adjustment

SQLGuy wrote:

ProfHankD wrote:

SQLGuy wrote:

The second is, I think, a difference in how red/infrared vs mid-band visible light is focused through a particular lens. That's my theory, at least, of why it's different from lens to lens. In practice, I can guarantee you that it is. For instance, on my A900, the Tamron 70-300 USD gets -14 while the Minolta 100-300 APO gets +8.

It is possible that you'd need to correct if it's using wavelengths that far out for PDAF, but I don't think it is. If your PDAF is really correctly in line with the sensor, you should NOT need any correction unless you're using things like NIR filters. It sounds like your A900 is simply out of alignment, and the A900 is a traditional SLR, so it does have that extra complex PDAF optical path... and it also has the micro adjust support so misalignment can be claimed to be your problem, not Sony's.

No. I see the same differences in similar lenses on the LA-EA4 as well.

Really? Your Tamron get -14 and Minolta APO gets +8 there too? Well, I guess I get the same tweaks too... but mine all seem to be approximately 0.

Please expand on your theory that would explain front focusing with one lens and back focusing with another lens being misalignment in the A900 body.

Like I said, the correction is applied in the lens control domain, NOT by specifying a direct shift to correct for misalignment. It's not generally a linear relationship. Thus, the camera body needs a model of how lens focus control changes rear focus distance, and I believe the micro adjustment is just a factor applied to that model. I doubt it even has a model for the Tamron, but it will ID it as something and use that model (I have a very vague memory that no ID might default to the 50mm f/1.7 model). What does the EXIF say the Tamron is?

So, not only is it possible for different lenses to disagree about correction offsets, but even a single lens might back focus at some distances and front focus at others.

If you're right and it's using NIR for the PDAF, then a lot of lenses are going to be in a lot of trouble. I would not be surprised to see some NIR sensitivity, but I'd be really surprised if it used just NIR. It also turns out that NIR focus shift is nearly always in the same direction, so that really wouldn't explain +/- either.

To be honest, I don't even want to think about this. Separate PDAF sensors are just a terrible idea that was once necessary, but technology now gives better alternatives.

Off-sensor PDAF uses a lot of infrared. That's why IR focus assist flash patterns work very well for SLRs/SLTs. They also work for the LA-EA4, but you have to jump through hoops to actually project one, since Sony actively disables them.

Edit: Rishi's article here ( states that lens/body variations in MFA are because of how some lens aberrations interact with PDAF.

Quoting Rishi:

"Manufacturers of DSLR bodies and lenses do a lot of calibrations to make sure that this isn’t an issue, calibrating every AF point at the factory, writing look-up tables into lenses, and more."

As I said, using the wrong table is a big problem. Again, what does the Tamron ID as?

After all, remember that, except for a few stragglers, all Sony A mount lenses have had built-in focus motors. We're really talking about support of Minolta legacy glass -- and in some cases, 3rd-party glass.

Have you switched topics here? SLT and SLR off-sensor PDAF MFA applies to both screw drive lenses and those with in-lens motors.

No -- the different drive systems and lens info provided mean the AF system has to treat these flavors of lenses differently.

Off-sensor AF still has the advantage of being able to focus in the dark with an IR assist pattern.

IR? Well, that's a problem because NIR focus plane rarely matches visible. BTW, nothing prevents use of focus assist lights with main sensor AF.

Yes, the hot mirror over the sensor prevents the use of IR focus assist patterns. And Sony's firmware disabling focus assist lamps in SLR/SLT flashes, like the 5600HS(D), also blocks such use.

Again, I don't see use of out-of-band light for focus as a win. Yeah, as a less-obtrusive way of getting texture on a textureless object it beats nothing, but these lenses are not well behaved in their handling of NIR. (BTW, you do mean NIR -- silicon sensors don't see below NIR.)

It also, I think, still has advantages for cross-type sensels.

In Sony's implementation, it does. However, there's no reason masked or dual pixels (really the degenerate case of plenoptics) can't support various different angles, and some such sensors have been built.

I would be interested to know who's making cross-type OSPDAF. AFAIK, only Olympus has implemented vertical OSPDAF sensels, and nobody has done 45 degree ones.

I don't think it's generally been considered worthwhile for production cameras.

You'd think dual pixels (as used by Canon, and also by Sony in their cell-phone sensors) would work better because it gives even more data points, but at least the Canon dual pixels are less selective and "leaky" enough (as I measured with the 5D IV dual-pixel raws) that they actually produce significantly poorer phase information without a fancier algorithm... which is probably why it took a while for Canon's dual pixels to approach the speed of Sony's masked pixel AF.

Not really.

Not really what? Canon's 5D IV dual-pixel raws certainly proved some very disappointing things about their dual-pixel implementation (although it doesn't reveal their phase detect algorithm using them).

I've done my own tests which were pretty clear about the advantages of the larger, dedicated, off-sensor sensels. Though, again, OSPDAF is improving here. I'm not sure we've yet seen really objective numbers comparing low-light AF ability between off and on sensor systems.

Not something I care about. Sony's A99II claims focus EV-4 to 18 with an f/2.8 lens (the best case), while the A7RII is EV-2 to 20 at ISO100 with an f/2.0 lens... so it sounds like the AF range is shifted, but not larger. Catch is, lenses faster bring you down further with main sensor, so it'd be the same with f/1.0... or if you turn-up the ISO a couple of stops.

However, the sensels in a separate PD sensor can be far more selective about the ray angles they admit. This gives higher accuracy with the designed aperture, but makes them lousy for faster lenses because they basically ignore the other rays. Minolta/Sony has always had at least one AF point tuned for f/8 -- primarily to support the 500mm f/8 AF Reflex lens. Other points I think are tuned for f/2.8 or f/5.6. This is why AF for really fast lenses often misses: it's essentially focusing with f/2.8 DoF even if your lens is f/1.4. My understanding is this is why Minolta didn't make f/1.2 lenses for AF mount: they couldn't afford to have PD detectors that would be useful only for super-fast lenses.

Also, Sony doesn't just do PD on the main sensor, but combines it as a hybrid with CD, which is a very smart pairing.

Honestly, I think the correct longer-term answer is depth from defocus (DFD), which doesn't require any special pixels -- just fast readout. The DFD in MFT bodies is multi-shot DFD, using multiple shots to avoid much more complex computation, but single-shot DFD is feasible with accuracy better than PDAF. Again, the Sony A1 sensor tech opens the door to a lot of potential improvements -- not just eliminating the mechanical shutter, but also things like moving to a really great DFD implementation that has no artifacting in the image.

It seems to be giving Pentax/Ricoh a hard time on their full frame sensors.

Huh? Pentax K-1 II is a traditional DSLR with separate PD sensor. I believe they proudly use CD autofocus for video, which is exactly the wrong thing to use it for.

I honestly don't think the K-1 II does qualitatively better than my A7RII with an LA-EA4, although in theory it has twice as many points. Honestly, none of the separate PD units really have enough points to do better than point-focus-repoint-shoot.

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