AF fine tune and focus shift for fast lenses

Started Sep 12, 2016 | Discussions
Ryan Mack Regular Member • Posts: 191
AF fine tune and focus shift for fast lenses

My understanding is that Nikon's AF system cannot take advantage of lenses faster than 2.8. Does that mean for a 1.4 lens that it is focusing based on where the lens focuses at 2.8? Specifically, it seems many 58 1.4 lenses need a significant positive fine tune adjustment at 1.4 and that lens is known to have a significant frontwards focus shift at 2.8. Is that causation or are they unconnected?

Ferguson
Ferguson Senior Member • Posts: 1,826
Re: AF fine tune and focus shift for fast lenses
1

Nikon DSLR's, when using phase detection focus (i.e. through the viewfinder focus) focus at the widest aperture of the lens.  I'm not sure of the context of not working wider than F2.8.  I know the viewfinder screens do not give an accurate DOF preview wider than about F2.8, and I suspect the AF system is designed to target F2.8 as a sweet spot, but that's guessing.

But the lens is wide open, so rather by definition it is focusing wide open.

So if you fine tune at the widest aperture it will be affected by focus shift.  Since the DOF gets deeper as you go up in f stop though, this is usually the best idea -- you have the Fine Tune nailed when you need it most.  Of you don't shoot wide open ever, though, just back/front it a bit so it lands where you want it.

My 85/1.4 shifts backwards, for example.  At about F2.8 the DOF range, centered at F1.4, is back so the very front of the DOF range is on the prior focus target.  So I have to decide if I want to shift a bit forward to compensate, and if so, I tend to get noses not eyes in focus at 1.4.  That lens is problematic in so many ways -- it also has a very different (visibly different) fine tune at portrait distances than sports (say basketball) distances.  But it's a delightful lens if you get the focus right, so I put up with the idiosyncrasies.

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Marianne Oelund Veteran Member • Posts: 7,788
Re: AF fine tune and focus shift for fast lenses
2

Ryan Mack wrote:

My understanding is that Nikon's AF system cannot take advantage of lenses faster than 2.8.

That's the Canon AF system, but only for its better bodies.  Their lower-level bodies, and all Nikon SLR's, will only use the f/5.6 circle for AF.

Does that mean for a 1.4 lens that it is focusing based on where the lens focuses at 2.8?

The AF system uses a parallel optical path, which must be aligned to achieve the same perception of focus as the image sensor.  The answer to your question depends on whether or not the AF system was aligned for the lens as it focuses at f/2.8.  The user can of course modify this with fine-tune.

Specifically, it seems many 58 1.4 lenses need a significant positive fine tune adjustment at 1.4 and that lens is known to have a significant frontwards focus shift at 2.8. Is that causation or are they unconnected?

Lens focus shift isn't seen by the AF system.

Starting with models released in 2014, Nikon added a compensation feature to AF, which basically works by biasing the fine-tune value according to which aperture the lens will be taking the image at.  This requires the camera to have tables of focus-shift values for various lenses.  Cameras made prior to that time, have no such compensation and must be fine-tuned using compromised settings.

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Ferguson
Ferguson Senior Member • Posts: 1,826
Re: AF fine tune and focus shift for fast lenses

Marianne Oelund wrote:

Lens focus shift isn't seen by the AF system.

Starting with models released in 2014, Nikon added a compensation feature to AF, which basically works by biasing the fine-tune value according to which aperture the lens will be taking the image at. This requires the camera to have tables of focus-shift values for various lenses. Cameras made prior to that time, have no such compensation and must be fine-tuned using compromised settings.

I had never heard that.  Is that also in the lens distortion database that's in the second firmware file?

And is that only in models since 2014?   Or firmware (e.g. D4 and D800 were 2013 but have later firmware updates)?

I need to go back now and experiment with the 85/1.4.  I used a D800 to do my initial testing, I'll give it a go with the D5.

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Marianne Oelund Veteran Member • Posts: 7,788
Re: AF fine tune and focus shift for fast lenses

Ferguson wrote:

Marianne Oelund wrote:

Lens focus shift isn't seen by the AF system.

Starting with models released in 2014, Nikon added a compensation feature to AF, which basically works by biasing the fine-tune value according to which aperture the lens will be taking the image at. This requires the camera to have tables of focus-shift values for various lenses. Cameras made prior to that time, have no such compensation and must be fine-tuned using compromised settings.

I had never heard that.

It's one of Nikon's "silent upgrades" that they've done over the years without mention in any product literature.

Is that also in the lens distortion database that's in the second firmware file?

Unknown at this time, but I'm hoping that it is, so new lens models will not require all camera firmware to be updated.

And is that only in models since 2014? Or firmware (e.g. D4 and D800 were 2013 but have later firmware updates)?

I haven't checked to be sure, but my suspicion is that firmware upgrades to older models do not include the feature.

I need to go back now and experiment with the 85/1.4. I used a D800 to do my initial testing, I'll give it a go with the D5.

That's a good idea.

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jopezu
jopezu Contributing Member • Posts: 611
Re: AF fine tune and focus shift for fast lenses

Starting with models released in 2014, Nikon added a compensation feature to AF, which basically works by biasing the fine-tune value according to which aperture the lens will be taking the image at. This requires the camera to have tables of focus-shift values for various lenses. Cameras made prior to that time, have no such compensation and must be fine-tuned using compromised settings.

something i will never forget after last years' struggle with my 400e lens where i was testing afft by stopping down through the range and not re-engaging af after each stop.

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primeshooter
primeshooter Veteran Member • Posts: 6,130
Re: AF fine tune and focus shift for fast lenses

Marianne Oelund wrote:

Ryan Mack wrote:

My understanding is that Nikon's AF system cannot take advantage of lenses faster than 2.8.

That's the Canon AF system, but only for its better bodies. Their lower-level bodies, and all Nikon SLR's, will only use the f/5.6 circle for AF.

Why do people go on constantly about lenses' that are f/1.4 are able to focus easier as they let in more light? I had heard you post before explaining that it wasn't the case. Is this just misinformation?

Why 5.6! That seems ridiculous? Why doesn't the system in nikon slrs take advantage of at least 2.8, or wider?

Does that mean for a 1.4 lens that it is focusing based on where the lens focuses at 2.8?

The AF system uses a parallel optical path, which must be aligned to achieve the same perception of focus as the image sensor. The answer to your question depends on whether or not the AF system was aligned for the lens as it focuses at f/2.8. The user can of course modify this with fine-tune.

Specifically, it seems many 58 1.4 lenses need a significant positive fine tune adjustment at 1.4 and that lens is known to have a significant frontwards focus shift at 2.8. Is that causation or are they unconnected?

Lens focus shift isn't seen by the AF system.

Starting with models released in 2014, Nikon added a compensation feature to AF, which basically works by biasing the fine-tune value according to which aperture the lens will be taking the image at. This requires the camera to have tables of focus-shift values for various lenses. Cameras made prior to that time, have no such compensation and must be fine-tuned using compromised settings.

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- Marianne

Correct me if I'm wrong, but I constantly see people going on about focusing fast manual lenses with nikon DSLRs. They say something like 'well I just MF using the viewfinder at f/1.4'. Some admit to using the range finder dot but many argue that the screen shows 1.4 dof which I am sure it doesn't? It's correct isn't it, that the focus screen shows the light and dof for about 2.8 and nothing less, so with a 1.4 lens, MF must be with the rangefinder dots, right?

Lance B
Lance B Forum Pro • Posts: 33,045
Re: AF fine tune and focus shift for fast lenses

primeshooter wrote:

Marianne Oelund wrote:

Ryan Mack wrote:

My understanding is that Nikon's AF system cannot take advantage of lenses faster than 2.8.

That's the Canon AF system, but only for its better bodies. Their lower-level bodies, and all Nikon SLR's, will only use the f/5.6 circle for AF.

Why do people go on constantly about lenses' that are f/1.4 are able to focus easier as they let in more light? I had heard you post before explaining that it wasn't the case. Is this just misinformation?

I don't think the f1.4 lenses are thought of as focusing faster, at least none of mine focus fast. However, all my f2.8 lenses certainly focus faster than any of my other lenses.

I think that f5.6 is for the focusing system, ie the camera uses the f5.6 focusing circle of f5.6, BUT they can take advantage of the extra light of an f2.8 lens thus making it easier for the camera to focus. I do not think there is any benefit of extra light of lenses faster than f2.8, though. From what I remember, f2.8 is the brightest that is usable.

Marianne will/may have a better explanation or info on this.

Why 5.6! That seems ridiculous? Why doesn't the system in nikon slrs take advantage of at least 2.8, or wider?

Does that mean for a 1.4 lens that it is focusing based on where the lens focuses at 2.8?

The AF system uses a parallel optical path, which must be aligned to achieve the same perception of focus as the image sensor. The answer to your question depends on whether or not the AF system was aligned for the lens as it focuses at f/2.8. The user can of course modify this with fine-tune.

Specifically, it seems many 58 1.4 lenses need a significant positive fine tune adjustment at 1.4 and that lens is known to have a significant frontwards focus shift at 2.8. Is that causation or are they unconnected?

Lens focus shift isn't seen by the AF system.

Starting with models released in 2014, Nikon added a compensation feature to AF, which basically works by biasing the fine-tune value according to which aperture the lens will be taking the image at. This requires the camera to have tables of focus-shift values for various lenses. Cameras made prior to that time, have no such compensation and must be fine-tuned using compromised settings.

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- Marianne

Correct me if I'm wrong, but I constantly see people going on about focusing fast manual lenses with nikon DSLRs. They say something like 'well I just MF using the viewfinder at f/1.4'. Some admit to using the range finder dot but many argue that the screen shows 1.4 dof which I am sure it doesn't? It's correct isn't it, that the focus screen shows the light and dof for about 2.8 and nothing less, so with a 1.4 lens, MF must be with the rangefinder dots, right?

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JF69 Contributing Member • Posts: 863
Great idea
1

Ferguson wrote:

I need to go back now and experiment with the 85/1.4. I used a D800 to do my initial testing, I'll give it a go with the D5.

Great idea.....please do come back with your findings.

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Ferguson
Ferguson Senior Member • Posts: 1,826
Re: AF fine tune and focus shift for fast lenses

Lance B wrote:

primeshooter wrote:

Marianne Oelund wrote:

Ryan Mack wrote:

My understanding is that Nikon's AF system cannot take advantage of lenses faster than 2.8.

That's the Canon AF system, but only for its better bodies. Their lower-level bodies, and all Nikon SLR's, will only use the f/5.6 circle for AF.

Why do people go on constantly about lenses' that are f/1.4 are able to focus easier as they let in more light? I had heard you post before explaining that it wasn't the case. Is this just misinformation?

I don't think the f1.4 lenses are thought of as focusing faster, at least none of mine focus fast. However, all my f2.8 lenses certainly focus faster than any of my other lenses.

I think that f5.6 is for the focusing system, ie the camera uses the f5.6 focusing circle of f5.6, BUT they can take advantage of the extra light of an f2.8 lens thus making it easier for the camera to focus. I do not think there is any benefit of extra light of lenses faster than f2.8, though. From what I remember, f2.8 is the brightest that is usable.

Marianne will/may have a better explanation or info on this.

I thought that the vast majority of the focus issues with respect to aperture (e.g. better at 2.8 than 5.6) related to ANGLE of the light due to aperture, not the amount of light.

If you look at most decent DSLR's, they focus in darn near dark, so (for example) a restriction on focusing at F8 in bright sun is clearly not caused by inadequate light.  If it was about light, the "which focus spots do F8, F5.6, etc." discussion would relate to EV not aperture.

Now if F2.8 is better than F4, it does not necessarily follow that F2 is better than F2.8, or F1.4 is better than F2.    But if one could plot "speed/quality of focus" against f-stop, I wonder how it looks?   Is there a sweet spot, or is wider always better?   [And here focus speed means detection speed, not how fast it can move the motor, which is a lens mechanics feature.]

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Ferguson
Ferguson Senior Member • Posts: 1,826
Re: Great idea

JF69 wrote:

Ferguson wrote:

I need to go back now and experiment with the 85/1.4. I used a D800 to do my initial testing, I'll give it a go with the D5.

Great idea.....please do come back with your findings.

I'm completely confused now.

It has been some time since I tested the 85 with the D800, but it had a distinct focus shift. Now I cannot see it. It has been repaired, twice, since then for other things, but among the repairs they also "adjust focus". The 85 has never been in the shop.

I tried it as (1) focusing at F1.4, then shooting successive shots at F2, F2.8 and F4 without refocus, and (2) focusing at each of those f-stops for each shot.

Interestingly, there is almost no difference that I can see between the refocus and no-refocus, certainly within the range of AF variation. This would seem to imply there is no innate correction for AF Focus shift. Though interestingly, there does not seem to be any significant shift either.

So I tried the D5 and did the same thing. And got the same results. No difference in refocus/not, and no focus shift.

The test is quite subjective, interpreting exactly where the focus area is (my green z's). I put these into two collages, one for each camera, so people can interpret themselves. The actual focus target is vertical and a bit right of the edge of each segment in the collage, the crops are on the ruler which is at a shallow angle, maybe 30 degrees from horizontal.

I KNOW the 85 didn't do this a couple years ago, as you went through F2.8 the DOF front edge would be very near the target, now it is not.

I put the F4 versions adjacent so you can see how they compare, but the full res shots are there if someone wants to re-crop to put others adjacent.

Frankly I do not know what to conclude from this. Unless it's actually focusing when I do not adjust the focus button, there is no visible indication of a correction for aperture, but then again there's no evident focus shift either. And this lens is definitely known for one.

Very confusing. Perhaps just a failed test.

By the way, I set the AF Fine Tune to zero for each -- what I was looking for was not whether it was correctly tuned but changes between aperture.

Linwood

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Marianne Oelund Veteran Member • Posts: 7,788
Re: AF fine tune and focus shift for fast lenses

primeshooter wrote:

Marianne Oelund wrote:

Ryan Mack wrote:

My understanding is that Nikon's AF system cannot take advantage of lenses faster than 2.8.

That's the Canon AF system, but only for its better bodies. Their lower-level bodies, and all Nikon SLR's, will only use the f/5.6 circle for AF.

Why do people go on constantly about lenses' that are f/1.4 are able to focus easier as they let in more light? I had heard you post before explaining that it wasn't the case. Is this just misinformation?

Many people do not understand that the additional light brought in by faster lenses is entirely peripheral to the light brought by slower lenses.  They just think everything is brighter with a faster lens, but that's not how it works.

The AF system must be selective about the pathways that it accepts light from, that is, one from each side of the exit pupil (and/or one each from the top and bottom).  In order for the system to work with slower (f/5.6) lenses, it must be designed to use light paths that are included within the f/5.6 circle.  This means that all of the extra light provided by lenses faster than f/5.6 will be blocked from entering the (Nikon) AF module.

Why 5.6! That seems ridiculous? Why doesn't the system in nikon slrs take advantage of at least 2.8, or wider?

Even in the Canon cameras which do provide some f/4 and f/2.8 capability, the f/5.6 points are primary.  The camera uses those first, then when focus is nearly achieved, it will switch over to a more sensitive point for the final precise positioning.

Correct me if I'm wrong, but I constantly see people going on about focusing fast manual lenses with nikon DSLRs. They say something like 'well I just MF using the viewfinder at f/1.4'. Some admit to using the range finder dot but many argue that the screen shows 1.4 dof which I am sure it doesn't? It's correct isn't it, that the focus screen shows the light and dof for about 2.8 and nothing less, so with a 1.4 lens, MF must be with the rangefinder dots, right?

You are correct and the original-equipment viewscreens in today's cameras are not adequate for accurate manual focusing at apertures wider than f/2.8.  In fact they are often not adequate for f/2.8 or even f/5.6 in some cases, as the manufacturers no longer care about how accurately the screen is aligned for showing true focus.

I was taking some photos in the garden last weekend, with an AI 105/2.5 lens on a D4 body.  Even at f/5.6, if I focused the lens for best sharpness on the viewscreen, the image turned out severely back-focused.  It was very frustrating, I can assure you!  I had to resort to using the AF dot indicators to obtain correct manual focus.

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primeshooter
primeshooter Veteran Member • Posts: 6,130
Re: AF fine tune and focus shift for fast lenses

Marianne Oelund wrote:

primeshooter wrote:

Marianne Oelund wrote:

Ryan Mack wrote:

My understanding is that Nikon's AF system cannot take advantage of lenses faster than 2.8.

That's the Canon AF system, but only for its better bodies. Their lower-level bodies, and all Nikon SLR's, will only use the f/5.6 circle for AF.

Why do people go on constantly about lenses' that are f/1.4 are able to focus easier as they let in more light? I had heard you post before explaining that it wasn't the case. Is this just misinformation?

Many people do not understand that the additional light brought in by faster lenses is entirely peripheral to the light brought by slower lenses. They just think everything is brighter with a faster lens, but that's not how it works.

The AF system must be selective about the pathways that it accepts light from, that is, one from each side of the exit pupil (and/or one each from the top and bottom). In order for the system to work with slower (f/5.6) lenses, it must be designed to use light paths that are included within the f/5.6 circle. This means that all of the extra light provided by lenses faster than f/5.6 will be blocked from entering the (Nikon) AF module.

Why 5.6! That seems ridiculous? Why doesn't the system in nikon slrs take advantage of at least 2.8, or wider?

Even in the Canon cameras which do provide some f/4 and f/2.8 capability, the f/5.6 points are primary. The camera uses those first, then when focus is nearly achieved, it will switch over to a more sensitive point for the final precise positioning.

Correct me if I'm wrong, but I constantly see people going on about focusing fast manual lenses with nikon DSLRs. They say something like 'well I just MF using the viewfinder at f/1.4'. Some admit to using the range finder dot but many argue that the screen shows 1.4 dof which I am sure it doesn't? It's correct isn't it, that the focus screen shows the light and dof for about 2.8 and nothing less, so with a 1.4 lens, MF must be with the rangefinder dots, right?

You are correct and the original-equipment viewscreens in today's cameras are not adequate for accurate manual focusing at apertures wider than f/2.8. In fact they are often not adequate for f/2.8 or even f/5.6 in some cases, as the manufacturers no longer care about how accurately the screen is aligned for showing true focus.

I was taking some photos in the garden last weekend, with an AI 105/2.5 lens on a D4 body. Even at f/5.6, if I focused the lens for best sharpness on the viewscreen, the image turned out severely back-focused. It was very frustrating, I can assure you! I had to resort to using the AF dot indicators to obtain correct manual focus.

Thank you, confirmed my suspicions!

jopezu
jopezu Contributing Member • Posts: 611
Re: AF fine tune and focus shift for fast lenses

I was taking some photos in the garden last weekend, with an AI 105/2.5 lens on a D4 body. Even at f/5.6, if I focused the lens for best sharpness on the viewscreen, the image turned out severely back-focused. It was very frustrating, I can assure you! I had to resort to using the AF dot indicators to obtain correct manual focus.

i'd had the same exact experience. toiling with the diopter didn't seem to help either. it seems like there's a substantial range of apparent sharpness in the viewfinder that the actual focus point will fall into (often on the outer edges as in your case).

it must have been a real joy to use older split-prism viewfinders with manual glass and gunning your shots down by hand. pdaf is fantastic, but when it does fail (like i've experienced in extremely warm incandescent lighting) it does so to a magnitude that i can't help but feel that i'd have been able to overcome with a decent viewfinder that allowed accurate manual focus.

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fPrime
fPrime Veteran Member • Posts: 3,514
Re: AF fine tune and focus shift for fast lenses
1

Marianne Oelund wrote:

primeshooter wrote:

Marianne Oelund wrote:

Ryan Mack wrote:

My understanding is that Nikon's AF system cannot take advantage of lenses faster than 2.8.

That's the Canon AF system, but only for its better bodies. Their lower-level bodies, and all Nikon SLR's, will only use the f/5.6 circle for AF.

Why do people go on constantly about lenses' that are f/1.4 are able to focus easier as they let in more light? I had heard you post before explaining that it wasn't the case. Is this just misinformation?

Many people do not understand that the additional light brought in by faster lenses is entirely peripheral to the light brought by slower lenses. They just think everything is brighter with a faster lens, but that's not how it works.

The AF system must be selective about the pathways that it accepts light from, that is, one from each side of the exit pupil (and/or one each from the top and bottom). In order for the system to work with slower (f/5.6) lenses, it must be designed to use light paths that are included within the f/5.6 circle. This means that all of the extra light provided by lenses faster than f/5.6 will be blocked from entering the (Nikon) AF module.

Why 5.6! That seems ridiculous? Why doesn't the system in nikon slrs take advantage of at least 2.8, or wider?

Even in the Canon cameras which do provide some f/4 and f/2.8 capability, the f/5.6 points are primary. The camera uses those first, then when focus is nearly achieved, it will switch over to a more sensitive point for the final precise positioning.

Correct me if I'm wrong, but I constantly see people going on about focusing fast manual lenses with nikon DSLRs. They say something like 'well I just MF using the viewfinder at f/1.4'. Some admit to using the range finder dot but many argue that the screen shows 1.4 dof which I am sure it doesn't? It's correct isn't it, that the focus screen shows the light and dof for about 2.8 and nothing less, so with a 1.4 lens, MF must be with the rangefinder dots, right?

You are correct and the original-equipment viewscreens in today's cameras are not adequate for accurate manual focusing at apertures wider than f/2.8. In fact they are often not adequate for f/2.8 or even f/5.6 in some cases, as the manufacturers no longer care about how accurately the screen is aligned for showing true focus.

I was taking some photos in the garden last weekend, with an AI 105/2.5 lens on a D4 body. Even at f/5.6, if I focused the lens for best sharpness on the viewscreen, the image turned out severely back-focused. It was very frustrating, I can assure you! I had to resort to using the AF dot indicators to obtain correct manual focus.

When I installed a Katzeye split prism focusing screen in my D700 I could immediately tell that it was out of alignment with PDAF focus. There appeared to be a slight back focus on MF when using the split screen. Fortunately the resolution was to adjust the manual focus adjustment screw with a 1.5mm Allen wrench. A simple 10 degree counter clockwise turn brought the view screen into perfect alignment.

fPrime

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snellius Senior Member • Posts: 1,060
Re: Great idea

If you really want to discover the phenomenon focus shift, then you can experiment best with a 50/1.4. Also very good for this is the AF 80-200/2.8D at 200mm. Test at different apertures and compare the results from far to near. Far everything is fine (constant), and nearby there is a huge shift when changing aperture. Since the D70 is the 80-200 (together with two other lenses) listed in the user manual as a potential source of a problem. With the increase of the number of pixels, these problems are now visible to much more lenses. So it's good to be informed and to act accordingly.

Marianne Oelund Veteran Member • Posts: 7,788
Viewscreen adjustment

fPrime wrote:

When I installed a Katzeye split prism focusing screen in my D700 I could immediately tell that it was out of alignment with PDAF focus. There appeared to be a slight back focus on MF when using the split screen. Fortunately the resolution was to adjust the manual focus adjustment screw with a 1.5mm Allen wrench. A simple 10 degree counter clockwise turn brought the view screen into perfect alignment.

There isn't a "manual focus adjustment screw."  The one you probably turned, is the adjustment for the reflex mirror rest position.  That is there to adjust vertical framing for the viewfinder, not focus.  In some cameras, it may also impact the AF-point vertical registration and the AF tuning.

In all cameras, it also affects viewfinder image vertical skew, which means that although you may have improved focus accuracy at the center of the screen, it's been altered differently at the top and bottom.

Viewscreen focus corrections need to be done by changing the shims - unless you don't care whether or not the viewfinder gives you the correct image framing, or top and bottom focus.

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primeshooter
primeshooter Veteran Member • Posts: 6,130
Re: AF fine tune and focus shift for fast lenses

fPrime wrote:

Marianne Oelund wrote:

primeshooter wrote:

Marianne Oelund wrote:

Ryan Mack wrote:

My understanding is that Nikon's AF system cannot take advantage of lenses faster than 2.8.

That's the Canon AF system, but only for its better bodies. Their lower-level bodies, and all Nikon SLR's, will only use the f/5.6 circle for AF.

Why do people go on constantly about lenses' that are f/1.4 are able to focus easier as they let in more light? I had heard you post before explaining that it wasn't the case. Is this just misinformation?

Many people do not understand that the additional light brought in by faster lenses is entirely peripheral to the light brought by slower lenses. They just think everything is brighter with a faster lens, but that's not how it works.

The AF system must be selective about the pathways that it accepts light from, that is, one from each side of the exit pupil (and/or one each from the top and bottom). In order for the system to work with slower (f/5.6) lenses, it must be designed to use light paths that are included within the f/5.6 circle. This means that all of the extra light provided by lenses faster than f/5.6 will be blocked from entering the (Nikon) AF module.

Why 5.6! That seems ridiculous? Why doesn't the system in nikon slrs take advantage of at least 2.8, or wider?

Even in the Canon cameras which do provide some f/4 and f/2.8 capability, the f/5.6 points are primary. The camera uses those first, then when focus is nearly achieved, it will switch over to a more sensitive point for the final precise positioning.

Correct me if I'm wrong, but I constantly see people going on about focusing fast manual lenses with nikon DSLRs. They say something like 'well I just MF using the viewfinder at f/1.4'. Some admit to using the range finder dot but many argue that the screen shows 1.4 dof which I am sure it doesn't? It's correct isn't it, that the focus screen shows the light and dof for about 2.8 and nothing less, so with a 1.4 lens, MF must be with the rangefinder dots, right?

You are correct and the original-equipment viewscreens in today's cameras are not adequate for accurate manual focusing at apertures wider than f/2.8. In fact they are often not adequate for f/2.8 or even f/5.6 in some cases, as the manufacturers no longer care about how accurately the screen is aligned for showing true focus.

I was taking some photos in the garden last weekend, with an AI 105/2.5 lens on a D4 body. Even at f/5.6, if I focused the lens for best sharpness on the viewscreen, the image turned out severely back-focused. It was very frustrating, I can assure you! I had to resort to using the AF dot indicators to obtain correct manual focus.

When I installed a Katzeye split prism focusing screen in my D700 I could immediately tell that it was out of alignment with PDAF focus. There appeared to be a slight back focus on MF when using the split screen. Fortunately the resolution was to adjust the manual focus adjustment screw with a 1.5mm Allen wrench. A simple 10 degree counter clockwise turn brought the view screen into perfect alignment.

fPrime

Do you still have it in situ? I notice they dont do them now? Luckily im af lenses only.

fPrime
fPrime Veteran Member • Posts: 3,514
Re: AF fine tune and focus shift for fast lenses

primeshooter wrote:

fPrime wrote:

Marianne Oelund wrote:

primeshooter wrote:

Marianne Oelund wrote:

Ryan Mack wrote:

My understanding is that Nikon's AF system cannot take advantage of lenses faster than 2.8.

That's the Canon AF system, but only for its better bodies. Their lower-level bodies, and all Nikon SLR's, will only use the f/5.6 circle for AF.

Why do people go on constantly about lenses' that are f/1.4 are able to focus easier as they let in more light? I had heard you post before explaining that it wasn't the case. Is this just misinformation?

Many people do not understand that the additional light brought in by faster lenses is entirely peripheral to the light brought by slower lenses. They just think everything is brighter with a faster lens, but that's not how it works.

The AF system must be selective about the pathways that it accepts light from, that is, one from each side of the exit pupil (and/or one each from the top and bottom). In order for the system to work with slower (f/5.6) lenses, it must be designed to use light paths that are included within the f/5.6 circle. This means that all of the extra light provided by lenses faster than f/5.6 will be blocked from entering the (Nikon) AF module.

Why 5.6! That seems ridiculous? Why doesn't the system in nikon slrs take advantage of at least 2.8, or wider?

Even in the Canon cameras which do provide some f/4 and f/2.8 capability, the f/5.6 points are primary. The camera uses those first, then when focus is nearly achieved, it will switch over to a more sensitive point for the final precise positioning.

Correct me if I'm wrong, but I constantly see people going on about focusing fast manual lenses with nikon DSLRs. They say something like 'well I just MF using the viewfinder at f/1.4'. Some admit to using the range finder dot but many argue that the screen shows 1.4 dof which I am sure it doesn't? It's correct isn't it, that the focus screen shows the light and dof for about 2.8 and nothing less, so with a 1.4 lens, MF must be with the rangefinder dots, right?

You are correct and the original-equipment viewscreens in today's cameras are not adequate for accurate manual focusing at apertures wider than f/2.8. In fact they are often not adequate for f/2.8 or even f/5.6 in some cases, as the manufacturers no longer care about how accurately the screen is aligned for showing true focus.

I was taking some photos in the garden last weekend, with an AI 105/2.5 lens on a D4 body. Even at f/5.6, if I focused the lens for best sharpness on the viewscreen, the image turned out severely back-focused. It was very frustrating, I can assure you! I had to resort to using the AF dot indicators to obtain correct manual focus.

When I installed a Katzeye split prism focusing screen in my D700 I could immediately tell that it was out of alignment with PDAF focus. There appeared to be a slight back focus on MF when using the split screen. Fortunately the resolution was to adjust the manual focus adjustment screw with a 1.5mm Allen wrench. A simple 10 degree counter clockwise turn brought the view screen into perfect alignment.

fPrime

Do you still have it in situ? I notice they dont do them now? Luckily im af lenses only.

Yes, and notwithstanding Marianne's cautionary note, no noticeable change in PDAF focus accuracy either.

fPrime

 fPrime's gear list:fPrime's gear list
Nikon D700 Nikon D200 Canon EOS 5D Nikon D1X Fujifilm FinePix S5 Pro
snellius Senior Member • Posts: 1,060
Re: Viewscreen adjustment

Marianne Oelund wrote:

fPrime wrote:

When I installed a Katzeye split prism focusing screen in my D700 I could immediately tell that it was out of alignment with PDAF focus. There appeared to be a slight back focus on MF when using the split screen. Fortunately the resolution was to adjust the manual focus adjustment screw with a 1.5mm Allen wrench. A simple 10 degree counter clockwise turn brought the view screen into perfect alignment.

There isn't a "manual focus adjustment screw." The one you probably turned, is the adjustment for the reflex mirror rest position. That is there to adjust vertical framing for the viewfinder, not focus. In some cameras, it may also impact the AF-point vertical registration and the AF tuning.

In all cameras, it also affects viewfinder image vertical skew, which means that although you may have improved focus accuracy at the center of the screen, it's been altered differently at the top and bottom.

Viewscreen focus corrections need to be done by changing the shims - unless you don't care whether or not the viewfinder gives you the correct image framing, or top and bottom focus.

These washers differences 0.05mm, if you want more precise adjustment this is done with the mirror position. But after the mirror is adjusted it's necessary to realign the viewfinder for paralax.

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