AF fine tune and focus shift for fast lenses

Started Sep 12, 2016 | Discussions
fPrime
fPrime Veteran Member • Posts: 3,514
Re: Viewscreen adjustment

snellius wrote:

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.

How's that done?  Is there a similar screw for the viewfinder?

fPrime

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snellius Senior Member • Posts: 1,060
Re: Viewscreen adjustment

For the parallax adjustment you have to remove the top cover.

fPrime
fPrime Veteran Member • Posts: 3,514
Re: Viewscreen adjustment

snellius wrote:

For the parallax adjustment you have to remove the top cover.

Thanks, that sounds like it would best be done at Nikon then I would take it.

fPrime

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Lance B
Lance B Forum Pro • Posts: 33,046
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 for the explanation.

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OP Ryan Mack Regular Member • Posts: 191
Re: AF fine tune and focus shift for fast lenses

Hi Marianne,

I was googling a bit more and found an older post of yours that confirms my suspicion that front focusing could be caused by rear focus shift and the focusing system seeing at an effective 5.6 aperture (my mistake originally thinking it was effective 2.8) : https://www.dpreview.com/forums/post/57656618

For the D810 you have mentioned software correction for focus shift. Any idea how that gets updated for newer lenses (firmware or distortion control data or something else)? And do you happen to know if there is correction data for the 58 1.4 yet?

Thanks for your time, Ryan

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

Ryan Mack wrote:

For the D810 you have mentioned software correction for focus shift. Any idea how that gets updated for newer lenses (firmware or distortion control data or something else)?

Not known yet, but with the release of new ver. 2.015 data, there may be an opportunity to find out; I'll check this with the 105/1.4E and see if it makes a difference.

And do you happen to know if there is correction data for the 58 1.4 yet?

I would imagine it was included in ver. 2.013, if not earlier.

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Leonard Shepherd
Leonard Shepherd Forum Pro • Posts: 21,212
Re: AF fine tune and focus shift for fast lenses

Marianne Oelund wrote:

They just think everything is brighter with a faster lens, but that's not how it works.

Commenting only on viewfinder brightness and depth of field if you have a 1.8 or 1.4 lens you can compare what happens to viewfinder brightness and dof by using dof preview at various apertures.

As a guide there is no increase viewfinder brightness or narrower dof in the viewfinder, wider than about f2.8 on high end DX, f2.5 on high end FX and f2.2 on the later film bodies.

You can see the narrower dof at f1.4 using liveview.

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