Simple method to check focus shift

[Thom Hogan]

What is not yet in the public domain is how Nikon will implement this new to Nikon feature - implying there might or might not be issues if someNikon ML lenses have significant aperture focus shift - depending on how the system is implemented.

Speaking very generally, it appears that Nikon designed almost all of their early Z-mount lenses to avoid focus shift. Moreover, the lenses where I've found focus shift to be present, that shift is usually post f/5.6.

Whilst the Z lens mount transmits focus information much more quickly and comprehensively than the F mount, the mechanical aspect of focus between one aperture and another if there is focus shift as "focus correction" takes time.

It's still unclear if Nikon ever employed focus shift correction. I believe they did; I first detected something that seemed to be that in the D850. But the real issue is really how fast you can move the diaphragm on the lens. Moving it from f/5.6 to f/11 isn't a big deal. Moving it from f/1.2 to f/5.6 requires more time.

 

[Thom Hogan]

If you're shooting flash photography at f5.6 in low light, the Z8 can now focus at f1.8, and switch to f5.6 for the exposure. If the lens has aperture induced focus shift, you could end up with a soft result. As he mentions, at some point stopping down increases the AF area enough to adjust for this.

For that particular use case, you could dial in AF-fine tuning. Focus shift is going to be consistent if it is present. Most of those using flash this way would be using fixed subject distances (or close to fixed) because to do otherwise would start to introduce flash exposure variability.

 

[Thom Hogan]

That is you are only testing this at a close focusing distance. Useful for macro but not General Photography where a distance of 8 to 15 feet is much more appropriate.

Nothing stops you from testing at the distance you use. For my testing of lenses for reviews, I tend to use what I feel is an "appropriate and likely" subject distance, which more approximates your "General Photography" theory. Trust me, there are lenses that have focus shift at those distances that you'd want to correct, and as I pointed out above, you can do this test today with any Z System camera by just putting it into video mode where apertures are respected.

The second error is your statement "All existing Nikon Z bodies focus with the lens stopped down (up to f/5.6), which avoids the potential effects of focus shift". This will only reduce the effect of focus shift at apertures close to f5.6 and how much reduction that produces will vary with each lens.

As I also noted elsewhere, I believe Nikon was very aware of what the focus system was going to do—remember, they first introduced it in 2011—and designed their lenses so that they didn't tend to have focus shift at max aperture up through f/5.6. There may be some, but it's so small on Z System Nikkors that I can't measure it. If you use adapted F-mount lenses, you need to pay attention to focus shift, though.

Your third mistake is sadly very common, that is using high magnification views to judge for image quality.

For the lenses I've tested, it's pretty clear when the focus shifts away. First, you have the focus indicator Horshack is pointing to. Second, I can and do see the difference at magnification (for what it's worth, don't do that on the Rear LCD, do it in the viewfinder, as you'll get better discrimination).

I'll also note I check for sharpness at only 50% peeps because why discard an image with a tiny focus miss if it will print perfectly good images.

You're missing a point here. You're not judging whether or not the image is "good" or not, you're judging whether or not you can see a focus shift on the fixed target. If you're doing it right and have 20/20 vision, you can.

As for my Z mount lenses they all are just fine however I have not turned the lens corrections off, so perhaps there are some hidden corrections that Nikon does to correct for focus shift. If so I really don't care. Just as I don't care about "defects seen in 200% or higher peeps. Because they are completely invisible at magnifications I do not print at.

Most of your argument is about "can't see it in my images." As I've long stated: "collect optimal data, process data optimally." Focus-shifted data is not optimal. When you apply some sharpening methods to slightly out of focus data, you get different results than you do from data correctly focused. Now, on many images used in "normal" ways (i.e. no large print or display), that probably doesn't show. However, it does when you're working at the image quality levels my clients ask for.

 

[Horshack]

Here is the procedure for those who prefer to read rather than watch:

1. Make sure the camera's "view mode" is set to "show effects of settings", so that aperture changes are reflected in the Live View preview. Set the mode to Aperture Priority.
2. Put your camera on a tripod and set up a well-lit AF subject close to your lens's minimum focus distance.
3. Set your lens to its wide-open aperture and use AF to focus on your AF subject
4. Switch the camera/lens from AF to MF mode. This enables the Z's phase focus indicator, where it will display a dot+arrows and red/green squares to indicate if your subject is in focus.
5. Without adjusting the focus, start stopping down the aperture and watch if the focus confirmation switches from a green box / solid dot to a red box / flashing arrows. If it does then this indicates the focal plane of the lens is shifting with aperture, ie it is exhibiting the effects of focus shift.
6. Repeat the procedure in step #5 but his time while viewing LV at maximum magnification, so you can visually see the effects of the focus shift on your target in real-time.

You can already do this test without the Z8 firmware: simply put the camera into video mode, which respects the aperture.

The method I described works on all Z bodies and firmware versions in photo mode. Video mode is a good idea though since it supports apertures smaller than f/5.6, although I wouldn't expect most fast lenses to exhibit noticeable shift beyond f/5.6, esp when DOF is considered.

 

[Horshack]

It seems very probable the "slightly delayed" shutter release time is to allow for any necessary re-focus operation after the aperture is stopped down from wide open to take the image.

The "slightly delayed" shutter release warning Nikon documents is in the Z9's MF version of this feature, where AF isn't a consideration, which means they're likely referring to aperture movement time instead.

 

[anotherMike]

The second error is your statement "All existing Nikon Z bodies focus with the lens stopped down (up to f/5.6), which avoids the potential effects of focus shift". This will only reduce the effect of focus shift at apertures close to f5.6 and how much reduction that produces will vary with each lens.

His statement is largely correct. Here's why:

1. Focus shift is largely caused by spherical abberation, likely common third order, perhaps some fifth order. That particular aberration is not field dependent (meaning, you see it everywhere in the field), but is aperture dependent so that if you stop down a couple of stops from wide open, it's pretty much gone.
2. The amount of focus shift, as you say, will vary per lens.
3. So if you think about Nikons initial implementation, given quite a lot of their catalog is at least 2 stops wider than F/5.6 (obviously not the F/4 lenses), if there were any focus shift, by focusing at the shooting aperture (if below F/5.6), you eliminate any issues caused by it, and if the shooting aperture is F/5.6 or above, meaning you're now focusing at F/5.6, you're likely past the point where spherical is the predominant aberration so you don't have to worry about it. Obviously, again, if you're shooting one of the F/4 lenses, or the slower tele zooms, it's a different story.

Where it gets tricky is that some lenses have differing correction for spherical aberration at distance (usually not much spherical) versus in portrait range (where the designer might be willing to trade some things off for better bokeh and OOF transition), and as such, it's possible we're going to see some focus shift at portrait distance range with some lenses, but not with others, and thus not with all lenses. That means some evaluation/testing (as the OP suggests) is in order, so one can understand their lenses behavior at the distances they normally shoot those lenses at, if they wish to use this new feature.

My intended approach, once this firmware comes out and I test it is likely to be to leave the setting "off" in all situations except where I really need it: Studio/strobe situations. I'll certainly test all lenses I commonly use at the portrait distances to see which ones have some focus shift and which ones don't.

 

[Hooligans Imagery]

So with all the debate if this works or not, we still don't know how Nikon will implement this.

Horshack, I respect all you have done scientifically for this community.

Do you believe that due to your findings this feature will have limited functionality?

 

[Horshack]

The Z9 FW 5.0 implements the upcoming Z8 FW 3.0 Maximum Aperture in LV feature but only for MF operation. The manual page for that feature lists several warnings:

• Regardless of the aperture setting, the lens will always be at maximum aperture when in MF mode. Be sure to avoid pointing the camera at the sun or other powerful light sources. Failure to observe this precaution could result in damage to the camera’s internal circuitry.
• The timing of the shutter release may be slightly delayed.
• The frame advance rate may drop.
• Flicker may appear on the shooting display in following conditions...

Nikon takes that first warning seriously because it's repeated in the menu inside the camera when you attempt to turn Max Aperture on:



Here is an online thread where the risk of sensor sun damage is discussed, including some formulas:

https://www.cloudynights.com/topic/910808-camera-sensors-damaged-by-unfiltered-lens/

 

[Horshack]

So with all the debate if this works or not, we still don't know how Nikon will implement this.

Horshack, I respect all you have done scientifically for this community.

Do you believe that due to your findings this feature will have limited functionality?

I think the feature will be fine. DLSRs and other MILC systems focus with the lens at maximum aperture, perhaps or presumably without focus shift correction.

 

[Leonard Shepherd]

It's an issue if you use the new low-light focus feature.

If the mentioned by Nikon possible slight delay in taking the first shot and slightly slower fps available is sufficient to allow for any necessary re-focus for any focus shift - there should not be a focus accuracy issue.

Testing when firmware 3 becomes available will help clarify if there is an issue and, if yes, with which lenses.

Again if yes then the choice will be between not using the feature and focus accuracy/viewfinder brightness as now, or using the feature for potential AF and viewfinder plusses with the possibility of some focus accuracy issues and slightly reduced fps in some shooting scenarios.

Yet again any issue might be slight using an f4.5 lens aperture for viewing when shooting at f5.6 and greater using an f1.8 lens aperture for viewing when shooting at f5.6.