Headline - DSLRs have imprecise focusing due to their mirrors

Started Apr 5, 2014 | Discussions
(unknown member) Contributing Member • Posts: 969
Headline - DSLRs have imprecise focusing due to their mirrors
7

peevee1 wrote:

Live view on DSLR is not "just like mirrorless camera", it is awfully slow. And mirror does not give you better image quality, it only gives you imprecise focusing.

Interesting premise; DSLRs have imprecise focusing because of their mirror.This is one of the wildest claims I have yet heard. Can you back that up with some facts?

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f8 and be there

Leonard Migliore
Leonard Migliore Forum Pro • Posts: 14,811
Makes sense to me
17

Greg A A wrote:

peevee1 wrote:

Live view on DSLR is not "just like mirrorless camera", it is awfully slow. And mirror does not give you better image quality, it only gives you imprecise focusing.

Interesting premise; DSLRs have imprecise focusing because of their mirror.This is one of the wildest claims I have yet heard. Can you back that up with some facts?

Well, the phase detect autofocus assembly is at the bottom of the mirror box. It senses focus indirectly and the path length can be affected by mechanical inaccuracy. That's why the first thing you do when you have focus problems with a DSLR is to focus in live view; that's the only way to focus directly on the sensor. And it's why micro focus adjust is such a valuable feature.

So I don't see it as a wild claim at all.

-- hide signature --

Leonard Migliore

 Leonard Migliore's gear list:Leonard Migliore's gear list
Canon PowerShot G12 Sony RX100 III Nikon D300 Nikon D750 Nikon AF-S DX Nikkor 16-85mm f/3.5-5.6G ED VR +12 more
OP (unknown member) Contributing Member • Posts: 969
Real AF results vs theoretical speculation
3

Leonard Migliore wrote:

Greg A A wrote:

peevee1 wrote:

Live view on DSLR is not "just like mirrorless camera", it is awfully slow. And mirror does not give you better image quality, it only gives you imprecise focusing.

Interesting premise; DSLRs have imprecise focusing because of their mirror.This is one of the wildest claims I have yet heard. Can you back that up with some facts?

Well, the phase detect autofocus assembly is at the bottom of the mirror box. It senses focus indirectly and the path length can be affected by mechanical inaccuracy. That's why the first thing you do when you have focus problems with a DSLR is to focus in live view; that's the only way to focus directly on the sensor. And it's why micro focus adjust is such a valuable feature.

So I don't see it as a wild claim at all.

-- hide signature --

Leonard Migliore

Sometimes measured results show that a theory doesn't always take all the actual factors into account. Take a look at these measurements of focus accuracy measured wiith current cameras. This shows that phase detect can be more accurate and more consistent than contrast detect. This was the case with the D800, 5D MkII, 7D, and a split decision on the 6D.

"The contrast detect results on the 5Dmk2 and 7D show a big focus error with a reasonable amount of variability between the shots too. This just translates to out of focus shots – plain and simple" - from: http://www.reikan.co.uk/focalweb/index.php/2012/12/af-consistency-comparison-nikon-canon-phase-detect-contrast-detect/

Does anyone have actual measured AF performance data that differs from what was done here?

-- hide signature --

f8 and be there

stevo23 Forum Pro • Posts: 15,290
Let's not go down a path...
22

Greg A A wrote:

Leonard Migliore wrote:

Greg A A wrote:

peevee1 wrote:

Live view on DSLR is not "just like mirrorless camera", it is awfully slow. And mirror does not give you better image quality, it only gives you imprecise focusing.

Interesting premise; DSLRs have imprecise focusing because of their mirror.This is one of the wildest claims I have yet heard. Can you back that up with some facts?

Well, the phase detect autofocus assembly is at the bottom of the mirror box. It senses focus indirectly and the path length can be affected by mechanical inaccuracy. That's why the first thing you do when you have focus problems with a DSLR is to focus in live view; that's the only way to focus directly on the sensor. And it's why micro focus adjust is such a valuable feature.

So I don't see it as a wild claim at all.

-- hide signature --

Leonard Migliore

Sometimes measured results show that a theory doesn't always take all the actual factors into account. Take a look at these measurements of focus accuracy measured wiith current cameras. This shows that phase detect can be more accurate and more consistent than contrast detect. This was the case with the D800, 5D MkII, 7D, and a split decision on the 6D.

"The contrast detect results on the 5Dmk2 and 7D show a big focus error with a reasonable amount of variability between the shots too. This just translates to out of focus shots – plain and simple" - from: http://www.reikan.co.uk/focalweb/index.php/2012/12/af-consistency-comparison-nikon-canon-phase-detect-contrast-detect/

Does anyone have actual measured AF performance data that differs from what was done here?

Let's stop a minute - DSLR "can" have imprecise focus due to the nature of phase detect autofocus and the mirror alignment/calibration aspect of it. But it need not be so.

Let's not start a flame war with "yes it does" vs. "no it doesn't". The answer is "yes it can" but also "it doesn't have to."

It's really that simple.

 stevo23's gear list:stevo23's gear list
Fujifilm X-Pro2 Fujifilm XF 60mm F2.4 R Macro Fujifilm XF 14mm F2.8 R Fujifilm XF 27mm F2.8 Fujifilm XF 23mm F1.4 R +3 more
OP (unknown member) Contributing Member • Posts: 969
Re: Let's not go down a path...
1

stevo23 wrote:

Greg A A wrote:

Leonard Migliore wrote:

Greg A A wrote:

peevee1 wrote:

Live view on DSLR is not "just like mirrorless camera", it is awfully slow. And mirror does not give you better image quality, it only gives you imprecise focusing.

Interesting premise; DSLRs have imprecise focusing because of their mirror.This is one of the wildest claims I have yet heard. Can you back that up with some facts?

Well, the phase detect autofocus assembly is at the bottom of the mirror box. It senses focus indirectly and the path length can be affected by mechanical inaccuracy. That's why the first thing you do when you have focus problems with a DSLR is to focus in live view; that's the only way to focus directly on the sensor. And it's why micro focus adjust is such a valuable feature.

So I don't see it as a wild claim at all.

-- hide signature --

Leonard Migliore

Sometimes measured results show that a theory doesn't always take all the actual factors into account. Take a look at these measurements of focus accuracy measured wiith current cameras. This shows that phase detect can be more accurate and more consistent than contrast detect. This was the case with the D800, 5D MkII, 7D, and a split decision on the 6D.

"The contrast detect results on the 5Dmk2 and 7D show a big focus error with a reasonable amount of variability between the shots too. This just translates to out of focus shots – plain and simple" - from: http://www.reikan.co.uk/focalweb/index.php/2012/12/af-consistency-comparison-nikon-canon-phase-detect-contrast-detect/

Does anyone have actual measured AF performance data that differs from what was done here?

Let's stop a minute - DSLR "can" have imprecise focus due to the nature of phase detect autofocus and the mirror alignment/calibration aspect of it. But it need not be so.

Let's not start a flame war with "yes it does" vs. "no it doesn't". The answer is "yes it can" but also "it doesn't have to."

It's really that simple.

I agree with you there. However, how often do mirrors become misaligned? Let's clarify the incorrect statement that "it (the mirror) only gives you imprecise focusing". Also let's correct the misconception that contrast detection is always more accurate than phase detection as this isn't always true.

-- hide signature --

f8 and be there

stevo23 Forum Pro • Posts: 15,290
Re: Let's not go down a path...

Greg A A wrote:

stevo23 wrote:

Greg A A wrote:

Leonard Migliore wrote:

Greg A A wrote:

peevee1 wrote:

Live view on DSLR is not "just like mirrorless camera", it is awfully slow. And mirror does not give you better image quality, it only gives you imprecise focusing.

Interesting premise; DSLRs have imprecise focusing because of their mirror.This is one of the wildest claims I have yet heard. Can you back that up with some facts?

Well, the phase detect autofocus assembly is at the bottom of the mirror box. It senses focus indirectly and the path length can be affected by mechanical inaccuracy. That's why the first thing you do when you have focus problems with a DSLR is to focus in live view; that's the only way to focus directly on the sensor. And it's why micro focus adjust is such a valuable feature.

So I don't see it as a wild claim at all.

-- hide signature --

Leonard Migliore

Sometimes measured results show that a theory doesn't always take all the actual factors into account. Take a look at these measurements of focus accuracy measured wiith current cameras. This shows that phase detect can be more accurate and more consistent than contrast detect. This was the case with the D800, 5D MkII, 7D, and a split decision on the 6D.

"The contrast detect results on the 5Dmk2 and 7D show a big focus error with a reasonable amount of variability between the shots too. This just translates to out of focus shots – plain and simple" - from: http://www.reikan.co.uk/focalweb/index.php/2012/12/af-consistency-comparison-nikon-canon-phase-detect-contrast-detect/

Does anyone have actual measured AF performance data that differs from what was done here?

Let's stop a minute - DSLR "can" have imprecise focus due to the nature of phase detect autofocus and the mirror alignment/calibration aspect of it. But it need not be so.

Let's not start a flame war with "yes it does" vs. "no it doesn't". The answer is "yes it can" but also "it doesn't have to."

It's really that simple.

I agree with you there. However, how often do mirrors become misaligned?

Good question. And how often do they come from the factory poorly calibrated? I doubt they change much over time, but I'm sure that the micro adjust feature is there to deal with the difference in lenses as well as poorly factory calibrated units.

Let's clarify the incorrect statement that "it (the mirror) only gives you imprecise focusing".

I thought I did.

Also let's correct the misconception that contrast detection is always more accurate than phase detection as this isn't always true.

That's making an assumption as well. I do know that micro adjust has no impact on my D600's contrast detect autofocus, but it does impact the phase detect autofocus.

I also know that lenses play a role as well since each lens that you micro-adjust in your DSLR is stored and always has it's own value - rarely are two exactly alike. And linearity of focus has an impact in a lens's focus range. This may be one source above statement.

 stevo23's gear list:stevo23's gear list
Fujifilm X-Pro2 Fujifilm XF 60mm F2.4 R Macro Fujifilm XF 14mm F2.8 R Fujifilm XF 27mm F2.8 Fujifilm XF 23mm F1.4 R +3 more
Ontario Gone
Ontario Gone Senior Member • Posts: 4,183
Re: Let's not go down a path...

Greg A A wrote:

stevo23 wrote:

Greg A A wrote:

Leonard Migliore wrote:

Greg A A wrote:

peevee1 wrote:

Live view on DSLR is not "just like mirrorless camera", it is awfully slow. And mirror does not give you better image quality, it only gives you imprecise focusing.

Interesting premise; DSLRs have imprecise focusing because of their mirror.This is one of the wildest claims I have yet heard. Can you back that up with some facts?

Well, the phase detect autofocus assembly is at the bottom of the mirror box. It senses focus indirectly and the path length can be affected by mechanical inaccuracy. That's why the first thing you do when you have focus problems with a DSLR is to focus in live view; that's the only way to focus directly on the sensor. And it's why micro focus adjust is such a valuable feature.

So I don't see it as a wild claim at all.

-- hide signature --

Leonard Migliore

Sometimes measured results show that a theory doesn't always take all the actual factors into account. Take a look at these measurements of focus accuracy measured wiith current cameras. This shows that phase detect can be more accurate and more consistent than contrast detect. This was the case with the D800, 5D MkII, 7D, and a split decision on the 6D.

"The contrast detect results on the 5Dmk2 and 7D show a big focus error with a reasonable amount of variability between the shots too. This just translates to out of focus shots – plain and simple" - from: http://www.reikan.co.uk/focalweb/index.php/2012/12/af-consistency-comparison-nikon-canon-phase-detect-contrast-detect/

Does anyone have actual measured AF performance data that differs from what was done here?

Let's stop a minute - DSLR "can" have imprecise focus due to the nature of phase detect autofocus and the mirror alignment/calibration aspect of it. But it need not be so.

Let's not start a flame war with "yes it does" vs. "no it doesn't". The answer is "yes it can" but also "it doesn't have to."

It's really that simple.

I agree with you there. However, how often do mirrors become misaligned? Let's clarify the incorrect statement that "it (the mirror) only gives you imprecise focusing". Also let's correct the misconception that contrast detection is always more accurate than phase detection as this isn't always true.

If either the secondary mirror is slightly ajar for whatever reason, the light path from subject to AF sensor will be slightly further/shorter than the light path from subject to image sensor. This is called parallax and it happens a lot more than anybody notices, perhaps all the time to a degree.

I think with sufficient DOF you won't notice it, but when the play is beyond a certain point, you notice BF/FF. This doesn't happen with mirrorless because there is no change between focusing and imaging, parallax cannot exist. This is one of the ways mirrorless are simpler and easier to manufacture. They have less moving parts, less factors that can go wrong. Below is a quote and a link, perhaps this can set things straight.

There is an overwhelming amount of negative feedback on autofocus issues on such fine tools as the Canon 5DIII, Nikon D800, Pentax K5 and other digital SLR cameras and it seems like most photographers do not seem to understand that the underlying problem is not necessarily with a specific model or type of a camera, but rather with the specific way these cameras acquire focus. If you search on the Internet, you will find thousands of autofocus reports on all kinds of DSLRs dating back 10+ years. Hence, the front focus and back focus issues we see in modern cameras are not anything new – they have been there ever since the first DSLR with a phase detect sensor was created.

For phase detection autofocus to work correctly, the distance between the lens mount and the camera sensor, as well as the distance between the lens mount and the Phase Detect sensor must be identical. If there is even a slight deviation, autofocus will be incorrect. On top of this, if the angle of the secondary mirror is not precisely what it should be, it will also result in autofocus issuesLINK

Multiple mirrors, precise alignment, optical path vs AF path. It seems pretty clear to me this system is weighed down by sever limitations and as soon as on sensor AF tracking speeds up, the DSLR will cease to exist. Afterall, it is this troublesome AF system that makes a DSLR a DSLR. Without those mirrors, it would be mirrorless. This isn't a comment to flame DSLR enthusiasts, it's a call for logic here. This system is archaic and problematic. If not for superior predictive tracking it would have been gone years ago. Cameras like the EM1 and A6000 show us it won't be long.

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"Run to the light, Carol Anne. Run as fast as you can!"

 Ontario Gone's gear list:Ontario Gone's gear list
Nikon D7000 Canon EOS 70D Nikon AF-S DX Nikkor 35mm F1.8G Nikon AF-S Nikkor 50mm f/1.8G Canon EF 40mm f/2.8 STM
OP (unknown member) Contributing Member • Posts: 969
Its inconvenient when facts get in the way of a hypothesis
1

stevo23 wrote:

Greg A A wrote:

stevo23 wrote:

Greg A A wrote:

Leonard Migliore wrote:

Greg A A wrote:

peevee1 wrote:

Live view on DSLR is not "just like mirrorless camera", it is awfully slow. And mirror does not give you better image quality, it only gives you imprecise focusing.

Interesting premise; DSLRs have imprecise focusing because of their mirror.This is one of the wildest claims I have yet heard. Can you back that up with some facts?

Well, the phase detect autofocus assembly is at the bottom of the mirror box. It senses focus indirectly and the path length can be affected by mechanical inaccuracy. That's why the first thing you do when you have focus problems with a DSLR is to focus in live view; that's the only way to focus directly on the sensor. And it's why micro focus adjust is such a valuable feature.

So I don't see it as a wild claim at all.

-- hide signature --

Leonard Migliore

Sometimes measured results show that a theory doesn't always take all the actual factors into account. Take a look at these measurements of focus accuracy measured wiith current cameras. This shows that phase detect can be more accurate and more consistent than contrast detect. This was the case with the D800, 5D MkII, 7D, and a split decision on the 6D.

"The contrast detect results on the 5Dmk2 and 7D show a big focus error with a reasonable amount of variability between the shots too. This just translates to out of focus shots – plain and simple" - from: http://www.reikan.co.uk/focalweb/index.php/2012/12/af-consistency-comparison-nikon-canon-phase-detect-contrast-detect/

Does anyone have actual measured AF performance data that differs from what was done here?

Let's stop a minute - DSLR "can" have imprecise focus due to the nature of phase detect autofocus and the mirror alignment/calibration aspect of it. But it need not be so.

Let's not start a flame war with "yes it does" vs. "no it doesn't". The answer is "yes it can" but also "it doesn't have to."

It's really that simple.

I agree with you there. However, how often do mirrors become misaligned?

Good question. And how often do they come from the factory poorly calibrated? I doubt they change much over time, but I'm sure that the micro adjust feature is there to deal with the difference in lenses as well as poorly factory calibrated units.

Let's clarify the incorrect statement that "it (the mirror) only gives you imprecise focusing".

I thought I did.

Also let's correct the misconception that contrast detection is always more accurate than phase detection as this isn't always true.

That's making an assumption as well. I do know that micro adjust has no impact on my D600's contrast detect autofocus, but it does impact the phase detect autofocus.

I also know that lenses play a role as well since each lens that you micro-adjust in your DSLR is stored and always has it's own value - rarely are two exactly alike. And linearity of focus has an impact in a lens's focus range. This may be one source above statement.

You are making hypothetical claims. There's no assumption in my statement. Measured results show that contrast detection isn't always more accurate than phase detection. I didn't say that phase detection was always more accurate than contrast detection.  Clearly the mirror doesn't only give you imprecise focusing as this was proven with several cameras.

-- hide signature --

f8 and be there

pew pew
pew pew Contributing Member • Posts: 775
Re: Headline - DSLRs have imprecise focusing due to their mirrors

Greg A A wrote:

peevee1 wrote:

Live view on DSLR is not "just like mirrorless camera", it is awfully slow. And mirror does not give you better image quality, it only gives you imprecise focusing.

Interesting premise; DSLRs have imprecise focusing because of their mirror.This is one of the wildest claims I have yet heard. Can you back that up with some facts?

-- hide signature --

f8 and be there

In general dslrs live af view is sluggish, so what he said is true.

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Sony Alpha NEX-3N Sony a6000
Ontario Gone
Ontario Gone Senior Member • Posts: 4,183
Re: Let's not go down a path...
1

stevo23 wrote:

I agree with you there. However, how often do mirrors become misaligned?

Good question. And how often do they come from the factory poorly calibrated? I doubt they change much over time, but I'm sure that the micro adjust feature is there to deal with the difference in lenses as well as poorly factory calibrated units.

My last DSLR, the last one i will ever own, had micro AF adjust. I also had a basic 75-300 zoom. At one point, i had to dial in a +3 when shooting at 75mm, a -+0 at 100mm, a +1 at 135mm, and a -7 at 300mm. To have to change the setting just to use a different FL was plain stupid.

Now not all lenses and cameras have this bad of an issue, but i decided then and there that since i could live without PDAF tracking, i would never deal with focus errors again. It's not like we MUST have AF anyway, there are loads of classics that were shot with MF. I can deal with limitations, i don't want to deal with inconsistencies.

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"Run to the light, Carol Anne. Run as fast as you can!"

 Ontario Gone's gear list:Ontario Gone's gear list
Nikon D7000 Canon EOS 70D Nikon AF-S DX Nikkor 35mm F1.8G Nikon AF-S Nikkor 50mm f/1.8G Canon EF 40mm f/2.8 STM
Ontario Gone
Ontario Gone Senior Member • Posts: 4,183
Re: Its inconvenient when facts get in the way of a hypothesis
1

Greg A A wrote:

You are making hypothetical claims. There's no assumption in my statement. Measured results show that contrast detection isn't always more accurate than phase detection. I didn't say that phase detection was always more accurate than contrast detection. Clearly the mirror doesn't only give you imprecise focusing as this was proven with several cameras.

The point is at best PDAF will break even. It will never be more accurate, it's using distance measurements to judge. CDAF for example uses the actual sharpness of the image to judge. Think about focusing with your own eyes. Why don't you ever accidentally focus in front of or behind? Because you can sense when it's the sharpest and you stop adjusting your eye's lens, CDAF works exactly the same way.

-- hide signature --

"Run to the light, Carol Anne. Run as fast as you can!"

 Ontario Gone's gear list:Ontario Gone's gear list
Nikon D7000 Canon EOS 70D Nikon AF-S DX Nikkor 35mm F1.8G Nikon AF-S Nikkor 50mm f/1.8G Canon EF 40mm f/2.8 STM
OP (unknown member) Contributing Member • Posts: 969
Curious how it always boils down to the demise of the DLSR - mirrorless insecurity
4

Ontario Gone wrote:

Greg A A wrote:

stevo23 wrote:

Greg A A wrote:

Leonard Migliore wrote:

Greg A A wrote:

peevee1 wrote:

Live view on DSLR is not "just like mirrorless camera", it is awfully slow. And mirror does not give you better image quality, it only gives you imprecise focusing.

Interesting premise; DSLRs have imprecise focusing because of their mirror.This is one of the wildest claims I have yet heard. Can you back that up with some facts?

Well, the phase detect autofocus assembly is at the bottom of the mirror box. It senses focus indirectly and the path length can be affected by mechanical inaccuracy. That's why the first thing you do when you have focus problems with a DSLR is to focus in live view; that's the only way to focus directly on the sensor. And it's why micro focus adjust is such a valuable feature.

So I don't see it as a wild claim at all.

-- hide signature --

Leonard Migliore

Sometimes measured results show that a theory doesn't always take all the actual factors into account. Take a look at these measurements of focus accuracy measured wiith current cameras. This shows that phase detect can be more accurate and more consistent than contrast detect. This was the case with the D800, 5D MkII, 7D, and a split decision on the 6D.

"The contrast detect results on the 5Dmk2 and 7D show a big focus error with a reasonable amount of variability between the shots too. This just translates to out of focus shots – plain and simple" - from: http://www.reikan.co.uk/focalweb/index.php/2012/12/af-consistency-comparison-nikon-canon-phase-detect-contrast-detect/

Does anyone have actual measured AF performance data that differs from what was done here?

Let's stop a minute - DSLR "can" have imprecise focus due to the nature of phase detect autofocus and the mirror alignment/calibration aspect of it. But it need not be so.

Let's not start a flame war with "yes it does" vs. "no it doesn't". The answer is "yes it can" but also "it doesn't have to."

It's really that simple.

I agree with you there. However, how often do mirrors become misaligned? Let's clarify the incorrect statement that "it (the mirror) only gives you imprecise focusing". Also let's correct the misconception that contrast detection is always more accurate than phase detection as this isn't always true.

If either the secondary mirror is slightly ajar for whatever reason, the light path from subject to AF sensor will be slightly further/shorter than the light path from subject to image sensor. This is called parallax and it happens a lot more than anybody notices, perhaps all the time to a degree.

I think with sufficient DOF you won't notice it, but when the play is beyond a certain point, you notice BF/FF. This doesn't happen with mirrorless because there is no change between focusing and imaging, parallax cannot exist. This is one of the ways mirrorless are simpler and easier to manufacture. They have less moving parts, less factors that can go wrong. Below is a quote and a link, perhaps this can set things straight.

There is an overwhelming amount of negative feedback on autofocus issues on such fine tools as the Canon 5DIII, Nikon D800, Pentax K5 and other digital SLR cameras and it seems like most photographers do not seem to understand that the underlying problem is not necessarily with a specific model or type of a camera, but rather with the specific way these cameras acquire focus. If you search on the Internet, you will find thousands of autofocus reports on all kinds of DSLRs dating back 10+ years. Hence, the front focus and back focus issues we see in modern cameras are not anything new – they have been there ever since the first DSLR with a phase detect sensor was created.

For phase detection autofocus to work correctly, the distance between the lens mount and the camera sensor, as well as the distance between the lens mount and the Phase Detect sensor must be identical. If there is even a slight deviation, autofocus will be incorrect. On top of this, if the angle of the secondary mirror is not precisely what it should be, it will also result in autofocus issues. LINK

Multiple mirrors, precise alignment, optical path vs AF path. It seems pretty clear to me this system is weighed down by sever limitations and as soon as on sensor AF tracking speeds up, the DSLR will cease to exist. Afterall, it is this troublesome AF system that makes a DSLR a DSLR. Without those mirrors, it would be mirrorless. This isn't a comment to flame DSLR enthusiasts, it's a call for logic here. This system is archaic and problematic. If not for superior predictive tracking it would have been gone years ago. Cameras like the EM1 and A6000 show us it won't be long.

-- hide signature --

"Run to the light, Carol Anne. Run as fast as you can

Mirrors and phase detection AF are proven technologies. How can it be claimed that DSLR phase detect systems don't work? My SLRs and DSLRs focus just fine as do several million others. For someone that is really concerned about AF accuracy newer cameras have AF adjust that can refine this for each lens.

EVFs are not the preferred viewfinder for the majority of camera buyers today. There is a vocal minority on this forum that claims otherwise. Rants on this forum about DSLRs and their inefective mirrors won't change what people buy.

-- hide signature --

f8 and be there

OP (unknown member) Contributing Member • Posts: 969
Re: Headline - DSLRs have imprecise focusing due to their mirrors
1

pew pew wrote:

Greg A A wrote:

peevee1 wrote:

Live view on DSLR is not "just like mirrorless camera", it is awfully slow. And mirror does not give you better image quality, it only gives you imprecise focusing.

Interesting premise; DSLRs have imprecise focusing because of their mirror.This is one of the wildest claims I have yet heard. Can you back that up with some facts?

-- hide signature --

f8 and be there

In general dslrs live af view is sluggish, so what he said is true.

If one half of the quote is correct then the entire statement must be true? Did anyone question that live view on a DSLR wasn't slow?

-- hide signature --

f8 and be there

OP (unknown member) Contributing Member • Posts: 969
Re: Its inconvenient when facts get in the way of a hypothesis
1

Ontario Gone wrote:

Greg A A wrote:

You are making hypothetical claims. There's no assumption in my statement. Measured results show that contrast detection isn't always more accurate than phase detection. I didn't say that phase detection was always more accurate than contrast detection. Clearly the mirror doesn't only give you imprecise focusing as this was proven with several cameras.

The point is at best PDAF will break even. It will never be more accurate, it's using distance measurements to judge. CDAF for example uses the actual sharpness of the image to judge. Think about focusing with your own eyes. Why don't you ever accidentally focus in front of or behind? Because you can sense when it's the sharpest and you stop adjusting your eye's lens, CDAF works exactly the same way.

-- hide signature --

"Run to the light, Carol Anne. Run as fast as you can!"

The theory says one thing, measurements on real cameras show in several cases phase detect was more accurate and repeatable than contrast detect. You can theorize all you want, but actual real life measurement trump theory.

-- hide signature --

f8 and be there

Ontario Gone
Ontario Gone Senior Member • Posts: 4,183
Re: Its inconvenient when facts get in the way of a hypothesis
1

Greg A A wrote:

Ontario Gone wrote:

Greg A A wrote:

You are making hypothetical claims. There's no assumption in my statement. Measured results show that contrast detection isn't always more accurate than phase detection. I didn't say that phase detection was always more accurate than contrast detection. Clearly the mirror doesn't only give you imprecise focusing as this was proven with several cameras.

The point is at best PDAF will break even. It will never be more accurate, it's using distance measurements to judge. CDAF for example uses the actual sharpness of the image to judge. Think about focusing with your own eyes. Why don't you ever accidentally focus in front of or behind? Because you can sense when it's the sharpest and you stop adjusting your eye's lens, CDAF works exactly the same way.

-- hide signature --

"Run to the light, Carol Anne. Run as fast as you can!"

The theory says one thing, measurements on real cameras show in several cases phase detect was more accurate and repeatable than contrast detect. You can theorize all you want, but actual real life measurement trump theory.

Real life measurements are subject to a slew of difficult to control factors. For one, it's very difficult to choose exactly where the AF square locks on to, because it's not a point. If you compare a Panasonic MFT camera for example, they have "pinpoint AF", it literally is the size of a dot on the full screen composition, far easier for the user to be precise.

I find it odd that even in the face of professionals who use DSLRs you argue against a consensus understanding of technology. I wonder, do you have a link to the "several cases" you mention? Any websites displaying this? Surely if DSLR owners could silence the critics about BF/FF they would be more than willing to share the proof. I think your example of a minority shouting nonsense is backwards, most people know PDAF is subject to errors, but a select few refuse to admit it.

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EthanP99 Senior Member • Posts: 2,436
Re: Its inconvenient when facts get in the way of a hypothesis
3

Ontario Gone wrote:

Greg A A wrote:

Ontario Gone wrote:

Greg A A wrote:

You are making hypothetical claims. There's no assumption in my statement. Measured results show that contrast detection isn't always more accurate than phase detection. I didn't say that phase detection was always more accurate than contrast detection. Clearly the mirror doesn't only give you imprecise focusing as this was proven with several cameras.

The point is at best PDAF will break even. It will never be more accurate, it's using distance measurements to judge. CDAF for example uses the actual sharpness of the image to judge. Think about focusing with your own eyes. Why don't you ever accidentally focus in front of or behind? Because you can sense when it's the sharpest and you stop adjusting your eye's lens, CDAF works exactly the same way.

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"Run to the light, Carol Anne. Run as fast as you can!"

The theory says one thing, measurements on real cameras show in several cases phase detect was more accurate and repeatable than contrast detect. You can theorize all you want, but actual real life measurement trump theory.

Real life measurements are subject to a slew of difficult to control factors. For one, it's very difficult to choose exactly where the AF square locks on to, because it's not a point. If you compare a Panasonic MFT camera for example, they have "pinpoint AF", it literally is the size of a dot on the full screen composition, far easier for the user to be precise.

I find it odd that even in the face of professionals who use DSLRs you argue against a consensus understanding of technology. I wonder, do you have a link to the "several cases" you mention? Any websites displaying this? Surely if DSLR owners could silence the critics about BF/FF they would be more than willing to share the proof. I think your example of a minority shouting nonsense is backwards, most people know PDAF is subject to errors, but a select few refuse to admit it.

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"Run to the light, Carol Anne. Run as fast as you can!"

I've had plenty of cameras, mostly pro bodies and the most accurate were contrast detect for still portraiture.

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Sante Patate Veteran Member • Posts: 5,908
Closed vs open loop focusing

Greg A A wrote:

peevee1 wrote:

Live view on DSLR is not "just like mirrorless camera", it is awfully slow. And mirror does not give you better image quality, it only gives you imprecise focusing.

Interesting premise; DSLRs have imprecise focusing because of their mirror.This is one of the wildest claims I have yet heard. Can you back that up with some facts?

CD is a closed-loop system: it has to try a change and see if contrast increases, then try again, and again ... until contrast does not increase any more.  That makes it slow, and prone to hunt.  You can make it faster by reducing the number of times it tests and checks, but that reduces accuracy.  But however many checking cycles your system has, it will always have them, so the accuracy will be consistent.

Phase detect AF is inherently faster than contrast-detect, because the PD system knows exactly where the lens elements have to be to achieve focus.  To maximise speed, PD is (AFAIK) always open-loop: the system works out where the lens elements need to go and moves them there, but it does not then check to make sure it was right - unless the user activates the AF again. (Focus tracking systems are often called closed-loop, but that is misleading: the system is constantly making new guesses, but each guess is open-loop).  Because PD AF is open-loop its performance is more variable, and it is prone to occasional large errors.

It is purely the fact that CD systems are closed loop that gives them an advantage in accuracy. PD systems could be made closed-loop, but that would defeat the purpose of AF because it would make it slower.

Martin.au
Martin.au Forum Pro • Posts: 11,909
Re: Headline - DSLRs have imprecise focusing due to their mirrors
1

I guess you don't require the focus micro adjust, found in all high end DSLRs.

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Martin.au
Martin.au Forum Pro • Posts: 11,909
Re: Closed vs open loop focusing
2

Sante Patate wrote:

Greg A A wrote:

peevee1 wrote:

Live view on DSLR is not "just like mirrorless camera", it is awfully slow. And mirror does not give you better image quality, it only gives you imprecise focusing.

Interesting premise; DSLRs have imprecise focusing because of their mirror.This is one of the wildest claims I have yet heard. Can you back that up with some facts?

CD is a closed-loop system: it has to try a change and see if contrast increases, then try again, and again ... until contrast does not increase any more. That makes it slow, and prone to hunt. You can make it faster by reducing the number of times it tests and checks, but that reduces accuracy. But however many checking cycles your system has, it will always have them, so the accuracy will be consistent.

Phase detect AF is inherently faster than contrast-detect, because the PD system knows exactly where the lens elements have to be to achieve focus. To maximise speed, PD is (AFAIK) always open-loop: the system works out where the lens elements need to go and moves them there, but it does not then check to make sure it was right - unless the user activates the AF again. (Focus tracking systems are often called closed-loop, but that is misleading: the system is constantly making new guesses, but each guess is open-loop). Because PD AF is open-loop its performance is more variable, and it is prone to occasional large errors.

It is purely the fact that CD systems are closed loop that gives them an advantage in accuracy. PD systems could be made closed-loop, but that would defeat the purpose of AF because it would make it slower.

Both are closed loop. You can test it by starting the focus on a subject, then quickly moving the subject during the focus group's movement, and seeing if it focused on the background, or on the spot where the subject was.

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Jack Hogan Veteran Member • Posts: 6,160
Interesting Link

Greg A A wrote:  "The contrast detect results on the 5Dmk2 and 7D show a big focus error with a reasonable amount of variability between the shots too. This just translates to out of focus shots – plain and simple" - from: http://www.reikan.co.uk/focalweb/index.php/2012/12/af-consistency-comparison-nikon-canon-phase-detect-contrast-detect/

That is an interesting post and quite surprising (to me).  I always thought that Contrast Detect would win hands down in a shoot out against Phase Detect.

Good alignment is the name of the game for tested and true PD.  On the other hand I would guess the following critical factors for CD:

1) Number and location of pixels used for focus evaluation;
2) Amount of time that the camera is allowed to try to attain maximum focus;
3) Intelligence of the algorithm driving the focusing system.

CD in DSLRs is usually designed to be used in focus-critical, time-not-critical applications (tripod often involved), so I am really surprised to see how poorly CD perfomed on the 5DMkII and 7D.  It did better on newer cameras, so it would be good to see results for the 5DIII and D610, for instance - which I could not find at the site.

Do we have to rethink our 'dot tune' strategy, or are newer cameras better than that?

Jack

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