Contrast detect vs phase detect AF

[Siran]

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Hi all,

I’ve been trying to take photos over the Xmas break of my children and although I love the feel of the Leica SL2 and the look of the images, I continuously struggle with the AF - I have said as much in other forum discussions too and hope firmware will make the camera more useable (at least for someone of my amateur skills 😊)

One thing I don’t understand is why Leica have gone down the contrast AF route rather than phase detect. What is the difference and is contrast AF meant to be somehow superior? From what I read phase detect is meant to be better so don’t understand Leica’s strategy. 

Thanks

 

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[frame-it]

Just now, Siran said:

 What is the difference and is contrast AF meant to be somehow superior? From what I read phase detect is meant to be better so don’t understand Leica’s strategy. 

Thanks

 

 

 

and

 

my favorite AF-C video

 

[Tp2000]

As the "owner"  of small people who I spend a lot of time taking photos of, the other issue you'll be facing with the SL2 is as they move, even with IS, the high resolution sensor will pick up more of that motion which manifests itself as blur or out of focus.

 

I really struggled with the AF on the SL2 if I'm honest. I grew up shooting sport on manual focus film cameras, then moves to digital canon cameras with AF which even in the early 00s was night and day better in terms of how predictable it was when shooting moving subjects. I find now going back to my manual focussing techniques of prefocussing or follow focussing works best, but still won't solve the high res/moving subject blur issue. 

That said, keep persevering and trying the different AF settings on the SL2. Quite often a high frame rate helps, although I did find that created secondary issues with lots of large files to work through. 

 

[Siran]

1 hour ago, frame-it said:

 

 

and

 

my favorite AF-C video

 

Thank you. 

In the first video the presenter explains phase detect as being the better option for moving subjects whereas contrast detect being better for more stationary subjects as it’s more accurate - I’m guessing that’s why Leica chose this option. 

However my feeling is the Leica SL2 has the potential to be used for more than just stationary subjects - it certainly has the build quality for it. Just hope firmware is going to help and push the camera to be more useable for, in my case, highly active little people.

[Siran]

1 hour ago, Tp2000 said:

As the "owner"  of small people who I spend a lot of time taking photos of, the other issue you'll be facing with the SL2 is as they move, even with IS, the high resolution sensor will pick up more of that motion which manifests itself as blur or out of focus.

 

I really struggled with the AF on the SL2 if I'm honest. I grew up shooting sport on manual focus film cameras, then moves to digital canon cameras with AF which even in the early 00s was night and day better in terms of how predictable it was when shooting moving subjects. I find now going back to my manual focussing techniques of prefocussing or follow focussing works best, but still won't solve the high res/moving subject blur issue. 

That said, keep persevering and trying the different AF settings on the SL2. Quite often a high frame rate helps, although I did find that created secondary issues with lots of large files to work through. 

 

Thank you. Nice to know I’m not alone. I’m not going to lie, I’m disappointed in the Leica SL2 and expect more from the company who promote premium quality. But I’m happy to persevere. I used to own a Sony A7R3 and felt that I wasn’t really doing anything and the camera was doing all the work (although the eye AF was amazing and I do miss it, but I felt like I was cheating 😀)

Would you be able to share any af settings you use for when trying to capture images of your small people? Do you adjust the Children/pets af setting or use the default settings?

[Timkr]

I own the SL2 and love shooting it as well.  However, I learned quickly it is basically useless for small, moving kids.  Picked up an a9; a world of difference.  Going to keep both.

[BernardC]

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1 hour ago, Siran said:

What is the difference and is contrast AF meant to be somehow superior?

Summary: Phase-detect = speed; contrast-detect = accuracy.

Good to know: Mirrorless phase-detect cameras try to switch to contrast-detect for fine focusing, when time allows. This would be the best of both worlds, except that sensors with phase-detect elements often exhibit banding issues. Phase-detect SLR cameras have sensors in the mirror box, so they don't have that problem.

In-Depth: Phase detect works by using two sensors: one in front of the image plane, and one behind. That's how it finds the direction of focus really fast (unless the lens is severely de-focused). The distance between these two sensors determines the minimum aperture that the sensors can work with. Basically, if the distance is within the depth of field for a certain lens, the camera can't tell front from back anymore. That's why you often hear about cameras that can't AF with slower (5.6 or 8.0) lenses. What you don't hear as often, but is equally true, is that phase sensors have a maximum aperture. For instance, a phase sensor might only "see" f:2.8, even if you mount an f:1.4 lens. What happens in this case? The CPU splits the difference, hoping to get something in focus. That's why you read about lenses that front-focus or back-focus on a particular body: the CPU's default guess is wrong, and their need correction tables to tell it things like "at 28mm and 1 meter, focus two nudges ahead of your best guess; however at 5 meters and 70mm, focus three nudges behind your best guess."

The Leica S medium format SLR system gets around that issue by having a lens lineup with a very narrow range of maximum apertures: f:2.5 and f:2.8 for the main range, and f:2.0 to f:3.5 for the extremes. They can pick a phase-detect sensor that matches these apertures. Note: they also offer a slower zoom, and people often complain about focus issues with that lens...

Other systems get around the limitation by combining "fast" and "slow" phase sensors. If you spread enough of them around the image plane, you'll stand a decent chance that there will be one that is close at hand. Of course, that still leaves the issue of "how fast?" and "how slow?", which can be quite a wide range: roughly between f:1.0 and f:8.0 for recent hybrid systems. Clearly not all of these lenses will work well with phase-detect AF.

Contrast-detect, which is what Leica Panasonic and Sigma use in L-Mount cameras, is potentially much more accurate. It can find the focus point with the highest contrast, which is usually also the desired sharpest point (but not always, depending on the subject). It does this the same way that many of us did with manual focus SLRs: by rocking the focus back-and-forth around the sharpest point until it is satisfied.

Panasonic's "DFD" system adds some smarts to that very basic strategy. First-off, it knows a little bit about your specific lens, so it knows how sharp the image can get at a given distance/focus point. That means it stops focusing when it gets close to optimal sharpness. It also knows a little bit about how the image looks when it is focused in-front-of or behind the image plane. In other words, it's still guessing which way to turn the focus ring, but it's an informed guess. Another thing that it can do is to apply constant micro-corrections, essentially doing a back-and-forth at a tiny scale. That's fine for stills, but it's annoying in video where backgrounds will shimmer. You can turn that off if you want.

Conclusion: You can tell why contrast-detect appeals to Leica. It's sharper, and higher-quality (less heat-inducing, space-wasting circuitry on the sensor). Modern contrast-detect is also very fast, certainly faster than anything but a pro-level SLR from a decade ago. The downside is that it's still not "the fastest." That crown still belongs to pro-level SLRs.

What's next? Unknown to many, Leica is a world leader in time-of-flight (ToF) sensor technology. These sensors build a 3D maps of your image, telling the camera where everything is. In many ways, that is the ideal design, but it hasn't been applied to cameras yet. The big money will be in machine vision.

 

[Dr. G]

Can someone who knows far more about this than I do comment on something I was once told - that phase detect autofocus systems do not perform as well with sensors that lack an anti aliasing filter?  I have no idea if this was accurate information.  

[Siran]

2 hours ago, BernardC said:

Summary: Phase-detect = speed; contrast-detect = accuracy.

Good to know: Mirrorless phase-detect cameras try to switch to contrast-detect for fine focusing, when time allows. This would be the best of both worlds, except that sensors with phase-detect elements often exhibit banding issues. Phase-detect SLR cameras have sensors in the mirror box, so they don't have that problem.

In-Depth: Phase detect works by using two sensors: one in front of the image plane, and one behind. That's how it finds the direction of focus really fast (unless the lens is severely de-focused). The distance between these two sensors determines the minimum aperture that the sensors can work with. Basically, if the distance is within the depth of field for a certain lens, the camera can't tell front from back anymore. That's why you often hear about cameras that can't AF with slower (5.6 or 8.0) lenses. What you don't hear as often, but is equally true, is that phase sensors have a maximum aperture. For instance, a phase sensor might only "see" f:2.8, even if you mount an f:1.4 lens. What happens in this case? The CPU splits the difference, hoping to get something in focus. That's why you read about lenses that front-focus or back-focus on a particular body: the CPU's default guess is wrong, and their need correction tables to tell it things like "at 28mm and 1 meter, focus two nudges ahead of your best guess; however at 5 meters and 70mm, focus three nudges behind your best guess."

The Leica S medium format SLR system gets around that issue by having a lens lineup with a very narrow range of maximum apertures: f:2.5 and f:2.8 for the main range, and f:2.0 to f:3.5 for the extremes. They can pick a phase-detect sensor that matches these apertures. Note: they also offer a slower zoom, and people often complain about focus issues with that lens...

Other systems get around the limitation by combining "fast" and "slow" phase sensors. If you spread enough of them around the image plane, you'll stand a decent chance that there will be one that is close at hand. Of course, that still leaves the issue of "how fast?" and "how slow?", which can be quite a wide range: roughly between f:1.0 and f:8.0 for recent hybrid systems. Clearly not all of these lenses will work well with phase-detect AF.

Contrast-detect, which is what Leica Panasonic and Sigma use in L-Mount cameras, is potentially much more accurate. It can find the focus point with the highest contrast, which is usually also the desired sharpest point (but not always, depending on the subject). It does this the same way that many of us did with manual focus SLRs: by rocking the focus back-and-forth around the sharpest point until it is satisfied.

Panasonic's "DFD" system adds some smarts to that very basic strategy. First-off, it knows a little bit about your specific lens, so it knows how sharp the image can get at a given distance/focus point. That means it stops focusing when it gets close to optimal sharpness. It also knows a little bit about how the image looks when it is focused in-front-of or behind the image plane. In other words, it's still guessing which way to turn the focus ring, but it's an informed guess. Another thing that it can do is to apply constant micro-corrections, essentially doing a back-and-forth at a tiny scale. That's fine for stills, but it's annoying in video where backgrounds will shimmer. You can turn that off if you want.

Conclusion: You can tell why contrast-detect appeals to Leica. It's sharper, and higher-quality (less heat-inducing, space-wasting circuitry on the sensor). Modern contrast-detect is also very fast, certainly faster than anything but a pro-level SLR from a decade ago. The downside is that it's still not "the fastest." That crown still belongs to pro-level SLRs.

What's next? Unknown to many, Leica is a world leader in time-of-flight (ToF) sensor technology. These sensors build a 3D maps of your image, telling the camera where everything is. In many ways, that is the ideal design, but it hasn't been applied to cameras yet. The big money will be in machine vision.

 

This is really informative, thank you for replying 

[Photoworks]

Reading all the comments I have to come to the conclusion that it comes to operator error.

Obviously it is challenging having children and capturing their live. Do you really have to need the picture running around the house and have the in focus too?
If the the children AF modus is activated and your children are not in focus, perhaps you need to focus your children to hold the 1 sec exposure. Gaffer tape to a chair, feeding them candies, anything to make them stay still. It comes down to technique of AF by the operator. Don't blame Leica for that. Or think what you have and don't have.

Enjoy you quite times with the kids and share your camera with them. Even a 2 year old can hold the SL2 ( maybe without a lens). LOL

[padam]

I got the EOS R5 and I am testing the Panasonic S1 (latest firmware) with their respected 24-105mm f4 kit zoom lenses.

No comparison - the R5 AF is far better and quicker as well as more accurate and it keeps up better in low light, too (despite the higher MP count on the sensor).
If you want to believe contrast-detect AF is the way to go, do not try these new Canon or Sony mirrorless cameras.
And video AF needs no mention, the difference is even more startling.
The S1 EVF is superior to the Canon though and the screen is a bit better as well (but both are excellent), IBIS great on both, impressive considering it costs less than half as much, but the AF on all of these cameras is the main weakness, no matter what anyone claims.

Of course if you are using manual focus glass, it is better to have a sensor that uses contrast-detect, it won't have reflection issues, banding, slightly cleaner ISO, etc. that is why I am disappointed in the SL series cameras, they are just too big and heavy for M glass.
A Leica at the size of the Panasonic S5 without the cost savings on the EVF would have been perfect, but they are releasing the CL2, so they will keep their models separated...

[caissa]

Quote

The question is not so much pdaf or cdaf, but rather using AFs or AFc. AFc needs skills and experience on the SL2. AFs is usually easier for starters. Also start with f4 or f5.6 not directly with f1.4.

Not to offend anybody. I use almost always AFs.

 

[adan]

Just some clarifying points.

1) Phase-detection AF has been around for 45 years or more, as separate "AF sensors."

2) Interestingly, Leitz/Leica invented the first separate phase-detect AF systems - the Correfot system - around 1976. But they did not consider AF "competent enough" to reveal the quality and precision of Leica lenses, so they sold it to their technology partner, Minolta (remember the Leitz/Minolta CL, and the early R cameras?). Who then produced the first AF SLR, the Maxxum.

http://gmpphoto.blogspot.com/2012/03/leitz-correfot-autofocus-system.html

(Honeywell invented a non-SLR "phase-detect rangefinder" (Visitronic) with two windows ("spider-eyes"), used in the Konica C35-AF, and in a similar way, in the Contax G "AF rangefinders" from around 2000±. And many AF point-&-shoots in between.)

https://camerapedia.fandom.com/wiki/Konica_C35_AF

3) But it is only in the last decade that being able to incorporate phase-detection directly on an imaging sensor has emerged (I think the first major product to have it was the Canon 7D Mk. 2 (2014) - beloved of videographers and sports photographers).

4) Perhaps most important, the exact mechanism of achieving and manufacturing on-sensor phase-detect AF is a patentable innovation. Fuji and Sony could not simply copy what Canon did - they had to "reinvent the wheel" for themselves.

"All property is theft - except mine!"

Leica, of course, is not a sensor-maker - and the places from which they buy their sensors are not Sony (except apparently the SL2-S) or Fuji or Canon. So they also would have to reinvent it all over again to use it legally. Or pay Sony/Canon/Fuji for a license.

5) The key advantage to phase-detect focus sensing is that it can tell which way the focus is off (too close or too far) and thus which way to move the lens. Contrast-detect has to hunt a little to find maximum contrast. This is especially important for predictive focus-tracking with a subject moving rapidly towards or away from the camera in a series of pictures.

Panasonic has developed the Depth-from-Defocus system to get around that with a contrast-detect sensor, by analysing (quick like a bunny) the appearance of the defocused image or "bokeh" to tell if a lens is front- or back-focused (there is a difference), and speed up the focus aquisition. But it is still slower than phase-detect follow-focus.

The Sony RX10mk4 phase-detect pixels can track autofocus at 25 "still" pictures per second, thanks to the stack behind the sensor of: a buffer (to hold all the pictures) and a processor (to analyze them at 25 per second and keep predicting where the subject will be in the next 1/25th of a second.) And of course only having to move the picture data off a 1-inch, 20Mpixel sensor (small = fast when moving electrons).

Whether one "needs" the remaining speed advantage of phase-detect AF depends on whether one's assignment (paid or personal) includes fast motion towards or away from the camera - athletes (or running kids), jet fighters, race cars, etc. etc.

[Tp2000]

4 hours ago, Photoworks said:

Reading all the comments I have to come to the conclusion that it comes to operator error.

Obviously it is challenging having children and capturing their live. Do you really have to need the picture running around the house and have the in focus too?
If the the children AF modus is activated and your children are not in focus, perhaps you need to focus your children to hold the 1 sec exposure. Gaffer tape to a chair, feeding them candies, anything to make them stay still. It comes down to technique of AF by the operator. Don't blame Leica for that. Or think what you have and don't have.

Enjoy you quite times with the kids and share your camera with them. Even a 2 year old can hold the SL2 ( maybe without a lens). LOL

A 2 year old can of course hold a SL2....the question is whether your heart rate goes high enough whilst that is happening  I have a photo 

And I disagree a little with the comment about just waiting until they sit still (I do know it was a little tongue in cheek) - these little people are at their most engaging and interesting when they are bounding around showing their full character. 

Just to prove the first point though, 2 photos.....the 2nd being what happens in the split second you take your eye off what is happening  

 

 

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[frame-it]

8 hours ago, Tp2000 said:

As the "owner"  of small people who I spend a lot of time taking photos of, the other issue you'll be facing with the SL2 is as they move, even with IS, the high resolution sensor will pick up more of that motion which manifests itself as blur or out of focus.

 

 

IS is useless for moving objects..it can create an additional blurry edge around e.g people walking etc

[panoreserve]

Thanks to "BernardC" and "adan"! I appreciate your insights in this matter very much!

[P1505]

Maybe your feeling the difference more because you’ve gone from Sony to Leica. Sony can focus on anything and hold it in focus while making your bed for you. No Leica gets close, and I doubt ever will.

Ive accepted I’ll have to practice manual focus, or zone focus, if I want the luxury of using a Leica.

[tom0511]

Here is my take (father of 2 small kids who play nadball and soccer and a fast dog).

1) A camera with good! face detection beats the SL2 for sports and fast moving kids easily (for example the D500 with face detection)

2) I dont see that contrast AF is more accurate in general. However-for phase detection AF- the lenses and camera has to calibrated realley well.

3) In my experience (with SL/SL2/CL/Oly EM1II) contrast detection works quite well and fast in S-AF, but is not great (more shots out of focus than in focus) for C-AF. I assume thats why Sony pairs contrast AF with phase AF in the A9

4) I think it is still possible to shoot kids in movement with the SL2, but forget it for fast sports , a running dog , or at least do many shots and hope that some are in focus. When I got the D500 with a fast lens (70200/2.8II) it opened my eyes.

5) But if you use a camera with phase AF also expect to spend time with AF-adjustment, or if you are unlucky for sending in lenses for adjustment

6)maybe cameras with a combination of phase and contrast AF are the solution, but I prefer the Leica user interface and colors and overall IQ, and as mentioned above there are workarounds. I think the Sl2 is really ok for kids, but it is not a professional sports camera IMO

 

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[Simone_DF]

12 hours ago, tom0511 said:

5) But if you use a camera with phase AF also expect to spend time with AF-adjustment, or if you are unlucky for sending in lenses for adjustment

But the SL has a whole submenu just to adjust AF to different situations, so isn't it the same?

[Tp2000]

51 minutes ago, Simone_DF said:

But the SL has a whole submenu just to adjust AF to different situations, so isn't it the same?

Not entirely - changing AF parameters is (to some extent) a case of trial and error - whereas on a DSLR (which doesn't access focus accuracy on the sensor) the AF micro adjustment was around tuning the lens to the camera, which if that wasn't successful, results in the need for lens/camera calibration. 

 

Its one of the joys of mirrorless cameras and (whisper it) why SL series are so great with M lenses - you don't have the risk of poor focusing due to calibration between lens and rangefinder.