Aperture being stopped down to F5.6 for focusing (why?)

[vwcabman]

Hi Folks, Can someone explain the reason that all lenses are stopped down for focusing unless you are using an aperture below this.

If you say have a 50mm F1.8 on a say D800 it will use F1.8 for focusing whatever aperture you are shooting at.

On a Z6/Z7 the same 50mm F1.8 will use F5.6 for focusing unless you are going to shoot at an aperture below that, in which case it will open the aperture further as required.

Is greater accuracy of focus not obtained at max aperture or am I missing something?

I was thinking that it would be easier to get correct focus with a wider aperture.

 

[beatboxa]

Hi Folks, Can someone explain the reason that all lenses are stopped down for focusing unless you are using an aperture below this.

If you say have a 50mm F1.8 on a say D800 it will use F1.8 for focusing whatever aperture you are shooting at.

On a Z6/Z7 the same 50mm F1.8 will use F5.6 for focusing unless you are going to shoot at an aperture below that, in which case it will open the aperture further as required.

Is greater accuracy of focus not obtained at max aperture or am I missing something?

I was thinking that it would be easier to get correct focus with a wider aperture.

See my post here in the science forum, describing how PDAF works on both DSLR's and on mirrorless cameras:

• https://www.dpreview.com/forums/post/62365840

And also, let me correct a few statements you made, because it is largely backwards:

If you are shooting a 50mm F/1.8 on a D800, it will not use F/1.8 to focus. It uses predefined phase locations (see the red & blue dots in the link above). These are usually at F/5.6 & F/8 (depending on the AF point)--but they are not the entire aperture at F/5.6 & F/8. These numbers describe the phase differential & distance between phase samples, not the area (which aperture describes).

On a Z6/Z7, it uses the actual aperture, until F/5.6. By F/8, the phase differential is not precise enough (there is not enough difference), so it has to use F/5.6. But if you are shooting at F/1.8, it will use F/1.8.

There are many implications of these differences (aside from on-sensor vs off-sensor):

• DSLRs autofocus does not benefit from lenses faster than F/5.6
• DSLRs are more prone to focus shift (inaccurate focusing)
• Mirrorless autofocus does benefit from faster lenses
• Mirrorless have very little focus shift

 

[vwcabman]

Hi Folks, Can someone explain the reason that all lenses are stopped down for focusing unless you are using an aperture below this.

If you say have a 50mm F1.8 on a say D800 it will use F1.8 for focusing whatever aperture you are shooting at.

On a Z6/Z7 the same 50mm F1.8 will use F5.6 for focusing unless you are going to shoot at an aperture below that, in which case it will open the aperture further as required.

Is greater accuracy of focus not obtained at max aperture or am I missing something?

I was thinking that it would be easier to get correct focus with a wider aperture.

See my post here in the science forum, describing how PDAF works on both DSLR's and on mirrorless cameras:

• https://www.dpreview.com/forums/post/62365840

And also, let me correct a few statements you made, because it is largely backwards:

If you are shooting a 50mm F/1.8 on a D800, it will not use F/1.8 to focus. It uses predefined phase locations (see the red & blue dots in the link above). These are usually at F/5.6 & F/8 (depending on the AF point)--but they are not the entire aperture at F/5.6 & F/8. These numbers describe the phase differential & distance between phase samples, not the area (which aperture describes).

Maybe it was the way I worded it to put it simply

Put a 50mm F/1.8 lens on a D800 the lens is wide open. Focusing is done at F1.8

why doesn't a 50mm F/1.8 lens on a Z6/7 the lens not stay wide open.

Focusing is done at F5.6 unless you have selected a wider aperture.

Is there some benefit of not having the lens wide open? on a Z6/7

 

[24Peter]

Hi Folks, Can someone explain the reason that all lenses are stopped down for focusing unless you are using an aperture below this.

If you say have a 50mm F1.8 on a say D800 it will use F1.8 for focusing whatever aperture you are shooting at.

On a Z6/Z7 the same 50mm F1.8 will use F5.6 for focusing unless you are going to shoot at an aperture below that, in which case it will open the aperture further as required.

Is greater accuracy of focus not obtained at max aperture or am I missing something?

I was thinking that it would be easier to get correct focus with a wider aperture.

https://www.dpreview.com/forums/thread/4331230

 

[beatboxa]

Hi Folks, Can someone explain the reason that all lenses are stopped down for focusing unless you are using an aperture below this.

If you say have a 50mm F1.8 on a say D800 it will use F1.8 for focusing whatever aperture you are shooting at.

On a Z6/Z7 the same 50mm F1.8 will use F5.6 for focusing unless you are going to shoot at an aperture below that, in which case it will open the aperture further as required.

Is greater accuracy of focus not obtained at max aperture or am I missing something?

I was thinking that it would be easier to get correct focus with a wider aperture.

See my post here in the science forum, describing how PDAF works on both DSLR's and on mirrorless cameras:

• https://www.dpreview.com/forums/post/62365840

And also, let me correct a few statements you made, because it is largely backwards:

If you are shooting a 50mm F/1.8 on a D800, it will not use F/1.8 to focus. It uses predefined phase locations (see the red & blue dots in the link above). These are usually at F/5.6 & F/8 (depending on the AF point)--but they are not the entire aperture at F/5.6 & F/8. These numbers describe the phase differential & distance between phase samples, not the area (which aperture describes).

Maybe it was the way I worded it to put it simply

Put a 50mm F/1.8 lens on a D800 the lens is wide open. Focusing is done at F1.8

No.

why doesn't a 50mm F/1.8 lens on a Z6/7 the lens not stay wide open.

It stays wide open if you choose to focus at F/1.8.

Focusing is done at F5.6 unless you have selected a wider aperture.

No.

Is there some benefit of not having the lens wide open? on a Z6/7

Yes.

 

[Mako2011]

Maybe it was the way I worded it to put it simply

Put a 50mm F/1.8 lens on a D800 the lens is wide open. Focusing is done at F1.8

No.

Yes it is? On a D800 focus is always done at the largest aperture possible for the lens...f 1.8 in this case...when using PDAF. With Live view CDAF focusing is done at the current aperture selected. That's different from other CDAF implementations by Nikon were focus is done at the aperture that was set when Live View was turned on vs what's currently selected. OP may be missing the Live View vs Viewfinder focusing nuances vs the Nikon Z system of focus. Apples, oranges, and cherries

--
My opinions are my own and not those of DPR or its administration. They carry no 'special' value (except to me and Lacie of course)

 

[beatboxa]

Maybe it was the way I worded it to put it simply

Put a 50mm F/1.8 lens on a D800 the lens is wide open. Focusing is done at F1.8

No.

Yes it is?

On a D800 focus is always done at the largest aperture possible for the lens...f 1.8 in this case...when using PDAF.

No, it's not. That's not how the mechanics work. The lens may be wide open, but that doesn't mean that the PDAF module gets all of the light. In fact, all of the light you can see through the OVF in a DSLR is light that the PDAF module CANNOT see.

(This is also why we don't have 100% bright + 100% magnification viewfinders on DSLRs that have autofocus).

If you do not believe me or my supporting diagrams I posted, please see this thread written by Marianne Oelund several years ago that got 105 likes, describing a DSLR's PDAF systems in detail. Unfortunately, her diagrams are no longer available, but read the text. And in particular, this part of her multi-part thread:

• https://www.dpreview.com/forums/post/54212041

And here is her conclusion:

• "In Nikon's AF systems, the separator-lens images are set just inside the f/5.6 circle. This diagram shows why lenses with maximum apertures larger than f/5.6, are not able to send more light through the separator lenses, to the AF detector, than an f/5.6 lens can."

In most recent Nikon AF systems, the center columns of AF points operate between F/8 - F/5.6, while the outer points operate at F/5.6 and cease to work if a lens' effective max aperture is only F/8 (eg. F/5.6 lens with a 1.4x TC).

With Live view CDAF focusing is done at the current aperture selected. That's different from other CDAF implementations by Nikon were focus is done at the aperture that was set when Live View was turned on vs what's currently selected. OP may be missing the Live View vs Viewfinder focusing nuances vs the Nikon Z system of focus. Apples, oranges, and cherries

Live view CDAF is correct that it will operate at the current aperture selected.

But the only reason for "when live view was turned on" vs. "what's currently selected" was because Nikon's mirror mechanism for the longest time was coupled to its spring-loaded aperture mechanism, so that the only time you could actually change aperture was during the mirror flip. Nikon eventually decoupled these mechanisms through what they called "power aperture ," which they marketed as a benefit for higher-end cameras, but was actually to just to be able to change aperture in live view for higher-end cameras like the D810 (& later).

 

[Mako2011]

Maybe it was the way I worded it to put it simply

Put a 50mm F/1.8 lens on a D800 the lens is wide open. Focusing is done at F1.8

No.

Yes it is?

On a D800 focus is always done at the largest aperture possible for the lens...f 1.8 in this case...when using PDAF.

No, it's not. That's not how the mechanics work. The lens may be wide open...,

Yes it is...that is all the OP is referring to/asking about...the aperture of the lens mounted on the camera at the time of focus... not the effective min aperture of the AF secondary microlenses of the muti-cam unit and such. Apples, Oranges, and cherries.

He is asking about the actual aperture of the lens on the camera that is used when focusing on different cameras under different conditions.

--
My opinions are my own and not those of DPR or its administration. They carry no 'special' value (except to me and Lacie of course)

 

[beatboxa]

Maybe it was the way I worded it to put it simply

Put a 50mm F/1.8 lens on a D800 the lens is wide open. Focusing is done at F1.8

No.

Yes it is?

On a D800 focus is always done at the largest aperture possible for the lens...f 1.8 in this case...when using PDAF.

No, it's not. That's not how the mechanics work. The lens may be wide open...,

Yes it is...that is all the OP is referring to/asking about...the aperture of the lens at the time of focus... not the effective min aperture of the micro-lenses of the muti-cam unit. Apples, Oranges, and cherries.

He is asking about the aperture of the lens on the camera used when focusing on different cameras.

And the rest of my sentence and my previous post answers the question, does it not? Because the focusing is not done at F/1.8. The focusing is done at F/5.6, even if the lens is set to F/1.8.

...which is the very source of the OP's confusion. And if you take only this snippet of my response while cutting off the substance as you have done, or only your response in a vacuum, you would arrive at the wrong understanding, such as that an F/1.8 prime should autofocus faster than an F/2.8 zoom on a DSLR, which is not the case. Because it doesn't use the full aperture of the lens to focus.

So to clarify again:

• https://www.dpreview.com/forums/post/63251991

 

[Lance B]

Maybe it was the way I worded it to put it simply

Put a 50mm F/1.8 lens on a D800 the lens is wide open. Focusing is done at F1.8

No.

Yes it is?

On a D800 focus is always done at the largest aperture possible for the lens...f 1.8 in this case...when using PDAF.

No, it's not. That's not how the mechanics work. The lens may be wide open...,

Yes it is...that is all the OP is referring to/asking about...the aperture of the lens at the time of focus... not the effective min aperture of the micro-lenses of the muti-cam unit. Apples, Oranges, and cherries.

He is asking about the aperture of the lens on the camera used when focusing on different cameras.

And the rest of my sentence and my previous post answers the question, does it not? Because the focusing is not done at F/1.8. The focusing is done at F/5.6, even if the lens is set to F/1.8.

...which is the very source of the OP's confusion. And if you take only this snippet of my response while cutting off the substance as you have done, or only your response in a vacuum, you would arrive at the wrong understanding, such as that an F/1.8 prime should autofocus faster than an F/2.8 zoom on a DSLR, which is not the case. Because it doesn't use the full aperture of the lens to focus.

So to clarify again:

• https://www.dpreview.com/forums/post/63251991

You are correct.

 

[Mako2011]

Maybe it was the way I worded it to put it simply

Put a 50mm F/1.8 lens on a D800 the lens is wide open. Focusing is done at F1.8

No.

Yes it is?

On a D800 focus is always done at the largest aperture possible for the lens...f 1.8 in this case...when using PDAF.

No, it's not. That's not how the mechanics work. The lens may be wide open...,

Yes it is...that is all the OP is referring to/asking about...the aperture of the lens at the time of focus... not the effective min aperture of the micro-lenses of the muti-cam unit. Apples, Oranges, and cherries.

He is asking about the aperture of the lens on the camera used when focusing on different cameras.

And the rest of my sentence and my previous post answers the question, does it not?

No as I was addressing the OP's statement "a 50mm F/1.8 lens on a D800 the lens is wide open. Focusing is done at F1.8" that you mistakenly suggested was not the case. Your post does not accurately address that. It doesn't help the OP as it doesn't first accurately help him see in terms related to "The Lens Mounted on the Camera".

Your post doesn't help answer his question as it assumes he understands the difference between PDAF, CDAF (liveview), and the Nikon Z focus system. It doesn't help to explain effective aperture in a optical chain with AF secondary microlenses used by an AF sensor...if you also suggest a 50mm F/1.8 lens on a D800 IS NOT Wide Open during PDAF focusing.

That doesn't mean you are not correct about the intricacies of the different focusing systems...you are correct ...but you are not addressing the OP's central issue. Apples and Oranges

--
My opinions are my own and not those of DPR or its administration. They carry no 'special' value (except to me and Lacie of course)

 

[beatboxa]

Maybe it was the way I worded it to put it simply

Put a 50mm F/1.8 lens on a D800 the lens is wide open. Focusing is done at F1.8

No.

Yes it is?

On a D800 focus is always done at the largest aperture possible for the lens...f 1.8 in this case...when using PDAF.

No, it's not. That's not how the mechanics work. The lens may be wide open...,

Yes it is...that is all the OP is referring to/asking about...the aperture of the lens at the time of focus... not the effective min aperture of the micro-lenses of the muti-cam unit. Apples, Oranges, and cherries.

He is asking about the aperture of the lens on the camera used when focusing on different cameras.

And the rest of my sentence and my previous post answers the question, does it not?

No as I was addressing the OP's statement "a 50mm F/1.8 lens on a D800 the lens is wide open. Focusing is done at F1.8" that you mistakenly suggested was not the case. Your post does not accurately address that. It doesn't help the OP as it doesn't first accurately help him see in terms related to "The Lens Mounted on the Camera".

Your post doesn't help answer his question as it assumes he understands the difference between PDAF, CDAF (liveview), and the Nikon Z focus system. It doesn't help to explain effective aperture in a optical chain with AF secondary microlenses used by an AF sensor...if you also suggest a 50mm F/1.8 lens on a D800 IS NOT Wide Open during PDAF focusing.

That doesn't mean you are not correct about the intricacies of the different focusing systems...you are correct ...but you are not addressing the OP's central issue. Apples and Oranges

I didn't mistakenly suggest that this was not the case. I directly addressed it in my previous post and this post--and I distinguished between the lens aperture and the samples from that aperture used for phase differences. My post doesn't assume anything because it starts out with a simplified model specifically to explain the difference between OVF-PDAF and mirrorless-PDAF, including visual diagrams.

I will also point out that your post also doesn't help to explain the difference between OVF-PDAF and the Nikon Z focus system that you just tried to call me out on (which was actually the OP's question); nor does it help to explain "effective aperture in a optical chain with AF secondary microlenses used by an AF sensor."

If I didn't help answer the question, then why didn't you address these in your post if these were glaring omissions in mine? If my response is inadequate along these topics, then why are you attacking my response instead of answering the OP's question?

 

[Jack Hogan]

Is greater accuracy of focus not obtained at max aperture or am I missing something?

In general, a current 'good' digital camera and lens are at their 'sharpest' at around f/4 +/- a couple of f-numbers. Below that aberrations degrade spatial resolution. Above that, diffraction takes over, decreasing resolution but increasing dof (depth of 'focus').

Such effects interact, so that in practice we are not interested in capturing the image at the plane of minimum defocus as much as capturing it at the plane with the least combined blur spot (the so-called circle of least confusion). The CLC plane moves around as f-number is changed because with more glass exposed aberrations get worse, creating what is called focus shift. This means that, if you have good kit, it is generally a good idea to search for the CLC and capture at the same f-number.

However, as f-number increases, say beyond f/5.6, with decent current hardware the dominant determinant of the CLC becomes diffraction. Diffraction effectively smooths out the dof, making it tougher to determine a minimum accurately, especially given the fact that we are doing it with less light. Not to worry though, because focus shift is typically fairly constant in the diffraction regime, so might as well focus at f/5.6

Jack

 

[beatboxa]

Is greater accuracy of focus not obtained at max aperture or am I missing something?

In general, a current digital camera and lens are at their 'sharpest' at around f/4 +/- a couple of f-numbers. Below that aberrations degrade spatial resolution. Above that, diffraction takes over, decreasing resolution but increasing dof (depth of 'focus').

Such effects interact, so that in practice we are no longer interested in capturing the image at the plane of minimum defocus as much as capturing one with the least combined blur spot (the so-called circle of least confusion). The CLC plane moves around as f-number is changed because with more glass exposed aberrations get worse, creating what we call focus shift. This means that, if you have good kit, it is generally a good idea to search for the CLC and capture at the same f-number.

However, as f-number increases, say beyond f/5.6, with current hardware the dominant determinant of the CLC becomes diffraction. Diffraction effectively smooths out the dof, making it tougher to determine a minimum accurately. Not to worry though, because focus shift is typically fairly constant in the diffraction regime, so might as well focus at f/5.6

Jack

I mentioned focus shift in my post above, (and it's mentioned here again); but here's an article & diagram that describes & illustrates focus shift in more detail, for the OP's benefit:

• https://photographylife.com/what-is-focus-shift

As I described, this problem of focus accuracy from focus shift is far more prominent on DSLRs, since they focus from constant angles (=constant f-numbers), which usually do not equal the max aperture of the lens.

 

[Mako2011]

No as I was addressing the OP's statement "a 50mm F/1.8 lens on a D800 the lens is wide open. Focusing is done at F1.8" that you mistakenly suggested was not the case. Your post does not accurately address that. It doesn't help the OP as it doesn't first accurately help him see in terms related to "The Lens Mounted on the Camera".

I didn't mistakenly suggest that this was not the case.

The stark "No" repeated twice when pointed out certainly did seem an error in understanding the question since the actual answer was "yes" and you later even suggested the answer was "yes":

"The lens may be wide open,..."

I directly addressed it in my previous post and this post...

In a very round about way you did ...but it also was a bit confusing for someone who does not understand the dif between PDAF, Live-View, and Hybrid focus systems. The "No" brought inaccuracies into the mix

"Daddy, why does my tire wear faster when ride my bike faster?

It doesn't dear...Frictional instabilities can lead to the formation of new self-organized patterns (or "secondary structures") at the sliding interface, such as in-situ formed tribofilms which are utilized for the reduction of friction and wear in so-called self-lubricating materials"

 

[vwcabman]

Is greater accuracy of focus not obtained at max aperture or am I missing something?

In general, a current 'good' digital camera and lens are at their 'sharpest' at around f/4 +/- a couple of f-numbers. Below that aberrations degrade spatial resolution. Above that, diffraction takes over, decreasing resolution but increasing dof (depth of 'focus').

Such effects interact, so that in practice we are not interested in capturing the image at the plane of minimum defocus as much as capturing it at the plane with the least combined blur spot (the so-called circle of least confusion). The CLC plane moves around as f-number is changed because with more glass exposed aberrations get worse, creating what is called focus shift. This means that, if you have good kit, it is generally a good idea to search for the CLC and capture at the same f-number.

However, as f-number increases, say beyond f/5.6, with decent current hardware the dominant determinant of the CLC becomes diffraction. Diffraction effectively smooths out the dof, making it tougher to determine a minimum accurately, especially given the fact that we are doing it with less light. Not to worry though, because focus shift is typically fairly constant in the diffraction regime, so might as well focus at f/5.6

Jack

Hi Jack thanks for the explanation, Its amazing how a simple question can develop so much deep discussion. I will consider my question now answered, Thanks to all.

 

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I believe that is the reason.

maljo