The new Nikon Z7 – investigating with Reikan FoCal

Unless you’ve been hiding under a rock, you’ll be aware of the release of Nikon’s first full-frame mirrorless cameras – the Nikon Z6 and Nikon Z7.

Our Nikon Z7 arrived at FoCal HQ last Thursday in the form of the “24-70 + FTZ Kit”. The documentation isn’t available from Nikon yet to control the camera from a computer, but with our experience in this area we had it talking to FoCal in a few hours, and have been putting it through its paces with the native S lens and a few adapted F-mount lenses.

Nikon ℤ cameras – a quick overview

There are far better places on the internet to get a general overview of the Nikon Z cameras (here at the Nikon site is a good start), but here’s a quick summary:

• New Z-mount with significantly larger diameter (55mm) than the F-mount (44mm). F-mount lenses are supported with the FTZ adapter.
• 24.5 MP (Z6) or 45.7 MP (Z7) backside illuminated CMOS sensors which include focal-plane phase detection AF pixels.
• 273 (Z6) or 493 (Z7) AF points.
• 12fps (Z6) or 9fps (Z7) shooting speed.
• Max native ISO is 51,200 (Z6) and 25,600 (Z7), with the Z7 going as low as 64 (native).
• Both cameras use the Expeed 6 processor, support 4K UHD, 10-bit N-Log profile, have silent mode shooting, a really excellent Quad-VGA EVF and sensor shift in-camera VR that compensates up to 5 stops.

and…

• Both cameras support AF Fine Tune

This last one is interesting as people generally assume that a mirrorless camera – where the AF sensor is part of the imaging sensor – does not need any AF calibration.

What this post is about

This post is both a general look at the Z7 and a more technical look at the camera, new Z-mount 24-70mm f/4S lens and operation with the FTZ adapter and the F-mount Nikkor 50mm f/1.4G lens.  It’s a chance to discuss the AF Fine-Tune capabilities of the Z7, but also to give a bit of a preview of functionality coming to FoCal soon.

The first part is chatty and less technical, and I’ll dive a bit deeper into the technical aspects later on.  I’ve dotted around a few photos I’ve taken with the camera and the 24-70/4S lens too.

To summarise what’s below: it’s an excellent camera!  The autofocus is very consistent with both native and adapted lenses, but we have shown that calibration will still give you improvements so it’s great that Nikon offers AF Fine Tune capability.

General thoughts on the Nikon Z

A bit of history… I first got my hands on a mirrorless camera back in 2010 – the Sony NEX-5.  Since then I’ve used the NEX-7, NEX-6, A6000, A6500, original A7, A7r mk 2, and A7 mk 3 as well as the somewhat quirky QX-1 “lens-style camera” and the original Canon EOS-M.  I’ve shot with all of these cameras fairly extensively and enjoyed the march of progress, adding higher resolution, better noise handling, apps and connectivity improvements, IBIS and an ever expanding lens choice.

My current camera of choice has been the Sony A7 mk 3, which I usually use with the fairly low-end Sony 28/2 or 50/1.8 lenses (they’re a great compromise between cost, performance and physical size), or with the Sigma MC-11 adapter and the EF mount (Canon) Sigma 35/1.4 or Canon 85/1.2L.  Prior to this I’d mainly used the Sony A7r mk 2 with it’s 42 MP sensor and in body IS – a fairly comparable camera to the Z7 (although one generation older as it’s been replaced with the A7r mk 3).

I was really excited to get my hands on the Nikon Z7.  Sony are great at pushing technology forward, but sometimes I feel they’re so intent on adding features they don’t give usability enough thought.  Take the ever evolving Sony camera menus.  On the NEX-5 they were pretty but disgusting to use – I could never find anything!  Each iteration has got a little better, but they’re still clunky and feel like an area that lacks care and attention.

The Z7 is a first generation camera, but I’m sure Nikon have paid close attention to Sony, Fuji and other popular mirrorless vendors to determine how to get that special “Nikon”-ness into a package that competes with the best alternatives.  And, on the whole, in my few days of using it I’d say it does this very well.

The EVF is a beauty.  It actually took me a day or two to realise it as it just felt very natural.  When the resolution and frame rate become high enough, you don’t really distinguish it much from an optical viewfinder and I think we’re pretty much there with this EVF.

Control layout is good.  It’s difficult to say it’s perfect or to criticise it much because it’s a pretty personal thing. I’ve spent a long time shooting with Sony cameras (which are all just different enough that I regularly end up in a muddle), so it’s always going to feel a bit odd switching to a different manufacturer.  I find pressing the function buttons on the front of the Z7 and turning a dial at the same time to be a bit awkward, but apart from that things feel very natural.  The controls work, and nice little usability options like EVF Priority from the viewfinder button on the top are very much appreciated.

XQD cards… technically I think they’re great.  A bit overkill for most requirements, and with a price tag to match.  I wonder if they’ll get more generally accepted?  Otherwise I fear the price tag is going to stay high, and there’s no other option for the Nikon Z cameras – no second slot of any kind.  Which is another contentious point, although I think somewhat over-hyped.  During all those film years, did people shoot with dual-film-rolls?  I think cards are reliable enough now for most cases, and the connectivity options of direct transfer to a phone/tablet after shooting should negate the need for a second slot.

On the whole I really like the camera, and I’m looking forward to using it in earnest to find out how it compares to the Sony offerings, and soon the Canon EOS-R.  Onwards now to the more technical section…

Analysis with FoCal

FoCal does more than just autofocus calibration – it can analyse aspects of your camera and lens as a system and compare your results to the norm so you can get a real-world idea of whether your system is behaving as it should.

The application has been going through a massive amount of development recently which is nearly ready for “public consumption”, and will bring a big batch of updates including a new look, simplicity and functionality to tuning and checking the performance of your kit.

For this post we used a new testing method (which we’ll talk about much more in a post coming shortly) to measure multiple aspects of the camera and lens.  In this single test we can look at autofocus performance (accuracy and consistency), aperture profile including focus shift information and general lens performance characteristics (including astigmatism and chromatic aberrations).

Why AF Fine Tune on the Nikon Z cameras?

As I said above, it’s generally assumed that cameras with the autofocus sensor built into the image sensor (i.e. mirrorless cameras) do not need any AF calibration.  There’s no chance of the AF sensor being misaligned or the pre-programmed internal calibration between the AF sensor and image sensor to be incorrect like there is in a DSLR (which has a separate AF sensor in it’s base).   However, Nikon has included AF Fine-Tune in their Z6 and Z7 cameras – so why is that, and is it really needed?

If the camera uses purely contrast-detect AF (CDAF), you wouldn’t need any sort of calibration.  CDAF works in the same way as a human focusing a lens – you look at how sharp the image is, and if you’re not satisfied with the sharpness you move the focus point a bit in a random direction and see if it gets sharper or not.  You repeat this process until you get a sharp image.  This is a closed-loop, iterative process – closed loop means you adjust and then check your result, and iterative means you repeat the process until you’re happy with the outcome.  The end focus position is probably about as good as you’re going to get (although you can never be quite sure as you have to adjust the lens to check, which might then mess up the focus!).  The down sides are that CDAF is quite slow and requires good light.

Phase-detect AF (PDAF) looks at two paths of light coming to the sensor and with a single measurement can determine both the magnitude (how much you need to adjust the focus) and the direction (towards the camera or towards infinity).  So one quick measurement can give you everything you need to know – then you drive the lens the right amount in the right direction and voila, perfect focus…

… not quite.  This is an open-loop system.  It’s fast, but it relies on your measurement being accurate and the lens doing exactly what you tell it to.  When the camera says move x amount in this direction, it assumes that x is the perfect amount, and the lens will correctly move to this point.

So what happens if your lens motor/gearing is a bit worn and doesn’t behave quite the same as it did when it was new?  What if the lens mount or lens elements are fractionally misaligned due to manufacturing tolerances or an accidental knock 6 months ago, and the focus isn’t measured quite correctly or the movement of the motor doesn’t quite move the lens element where is should be?  These are the sorts of things can result in a fractional offset of the focus position, and an out-of-focus image.

But as long as the focus error is fairly consistent, then AF Fine Tune is your knight in shining armour to get that perfect focus back!

Other mirrorless cameras offering AF Calibration

There are a few other mirrorless cameras which offer AF tuning.  Cameras like the Olympus OM-D E-M1 even have the capability to adjust calibration for each individual focus point!

I’ve consistently seen reference to Sony mirrorless support AF Fine-Tune but this isn’t technically correct and I wanted to clarify here.  There are a few lens mount adapter available for Sony cameras, and at least 2 of them (LA-EA2 and LA-EA4) use Sony’s “Translucent Mirror” technology and incorporate a PDAF sensor in the base of the adapter.  When you use AF Fine Tune on a Sony mirrorless, this adapter PDAF sensor is what you are actually calibrating, so it’s only of use if you’re using one of these specific adapters.

Nikon JPEG Processing

A quick note about in-camera JPEG processing on the Z7.  This camera features great processing enhancements like vignette control and diffraction compensation.  These work well in post-processing the image to mitigate issues introduced by the lens, but they also mean that if you use JPEG mode for testing in FoCal, you’re measuring the performance of the camera, lens and post-processing engine.  And that’s not what we want to do – so all the following test results are based on 14-bit Z7 raw images processed with the FoCal Raw Engine.

AF Performance of the Nikkor Z 24-70mm f/4S lens

The test involves connecting the camera to the computer, aiming at the correct test distance at a well lit calibration target and then watching the results come in.

The first lens we ran through FoCal was the Nikkor Z 24-70mm f/4S lens – this is supplied as part of the Z7 kit.  You can see the results here:

Both show that the lens could benefit from a slight adjustment – an AF Fine Tune of -1.  They also show pretty similar peak sharpness (both around 2000).  There’s a little more AF variability at 70mm which is not unexpected, but both sets of results show very good consistency overall.

Here’s another view – this is the AF Error (including direction) at each measurement point.  The points in both cases are very close to the 0 line, showing little error from our expected focus position:

This lens looks to be working well on this camera.  Not only does it only require a tiny AF Fine-Tune, but it’s the same value at each end of the zoom range and the shot-to-shot focus error is very small.

General lens performance of the Nikkor Z 24-70mm f/4S lens

The new FoCal test measures a lot more than autofocus performance.  Let’s take a look at the image sharpness across a range of focus positions and apertures:

So, what are we looking at here?  I won’t explain in too much detail (that can come in a later post) but the vertical axis is the focus position, and horizontal axis is the aperture.  The colour of the background represents the sharpness relative to the absolute maximum sharpness of this lens – green is good (close to 100% of the best quality), and red is low quality (70% or less) going through yellow and orange on the way.

The other things of note are:

• Purple diamonds show the focus adjustment (in AF Fine Tune units) required for the sharpest image at each aperture (this is focus shift)
• The white diamond shows the sharpest overall point for this lens
• The dark horizontal line shows the calibration result (this is the same as the value of the best quality at the widest aperture)

Hopefully that’s not too confusing!

So what do the two charts show?  At 24mm, we can see that using autofocus is going to give a decent result across the aperture range: the dark horizontal line is the calibrated autofocus result, and it goes across the aperture range (from left to right) sticking nicely to the green area.  The hints of orange on the right are the results of diffraction and will affect every lens (side note: if you enable diffraction compensation on the camera and run in JPEG mode, the sharpness stays high towards the right of the chart.  The compensation is really quite good.)

At 70mm, AF will give you a decent result through to f/8, but if you want the absolute best image quality at a smaller aperture than f/8 you probably want to manually refocus a little towards infinity (tracking the line of purple diamonds through the green area).

Let’s look at this in another way:

These two charts show the sharpness across the aperture range.  There are 3 lines on the charts:

• Red is the sharpness if you left AF Fine-Tune disabled or at 0 (so the “uncalibrated” value)
• Orange is the sharpness if you use the recommended calibration value (for this lens it’s -1 at both 24 and 70mm as we said above)
• Green is the sharpness if you manually refocused at each aperture to the absolute best quality

In both instances, we can see a slight sharpness improvement with the AF Fine Tune set correctly to -1 (the orange line is higher value than the red line).  We can also see at 24mm that sharpness peaks at around f/5, but at 70mm it’s sharpest wide open.  And again, at 70mm we can see an improvement if we manually refocused past f/8 (the green line starts to get significantly higher than the red and orange lines).

Note also that at 70mm, the orange (calibrated) line drops below the red (uncalibrated) line from f/7.1  In technical terms this means you need a slightly difference AF Fine Tune value across the aperture range of this lens*, but in practical terms the difference is very small.

(* no calibration method – be it in camera or lens-dock based – supports calibration at different apertures.  For a perfect calibration you’d want to adjust for focal length, focus distance and aperture…!)

Chromatic Aberrations

The new test also gives a couple of different ways of viewing minor distortions introduced through lens design, defects or physical damage.  First is astigmatism factor which gives an indication of the comparative sharpness when measured horizontally and vertically:

At both 24mm and 70mm for this lens, the absolute values are very small (around 1-2% at 24mm, and less than 4% at 70mm).  There’s nothing to be concerned about here.

The other measure is the ratio between red and blue sharpness.  A big difference can indicate that the presence of colour fringing in images as the different wavelengths will blur different amounts at the sensor plane.

Here, once again, there’s nothing to be particularly concerned about.  5-10% at 70mm is acceptable.  The rise above 10% past f/7.1 at 24mm may suggest the image will start to show hints of colour fringing, but diffraction will be blurring the image past this point anyway so it is likely to just be general image degradation.

Using the FTZ Adapter with the Nikkor 50mm f/1.4G lens

The FTZ mount adapter allows the use of most modern F-mount lenses on the Nikon Z cameras.  To test things out a bit, I used the Nikkor 50mm f/1.4G lens on the adapter and ran the new test.

Here’s the calibration chart:

Now we’re seeing something a bit more interesting.  It’s not hugely out of calibration, but there’s a definite (and repeatable – I checked several times) calibration requirement of 3-4 AF Fine Tune units of front-focus for this lens.

The AF Error chart shows great autofocus performance, very similar to the native Z mount lenses:

 

Looking at the general lens performance… this is a wide-aperture prime, and these types of lenses always have some compromises.  In order to get that wide aperture, the sharpness at the wide end drops away and we can see that with big areas of red and orange on the left side of the chart:

What’s maybe more interesting here too is the focus shift this lens exhibits.  The line of purple diamonds (showing the best sharpness available at each aperture) starts drifting away from our calibration result (the dark horizontal line) from about f/2.8.  By f/6.3 the lens is needing the equivalent of -17 AF Fine-Tune to get the absolute sharpest result!  It should be noted that the calibration line  does stay in a green area throughout the aperture range so you’re not going to be getting a bad sharpness by any means if you just use autofocus.

Looking again at the aperture sharpness profile:

 

The red line (uncalibrated) sits lowest for almost the full range, and the calibrated (orange) line is a reasonable improvement across the apertures.  The manually refocused (green) line shows a significant improvement from about f/2.8 through to f/13, so if you want to get the best from this lens and have the time to refocus manually, then that’s the way to go at these apertures.

The astigmatism factor is pretty decent across the range, sticking with a max of about  5%:

 

But the red/blue ratio is quite high around f/2.8 and a stop or so either side, meaning you’re likely to see some colour fringing from this lens (not unexpected for a fast prime).

Conclusions

Looking at the charts, the autofocus error of the Z7 with the native 24-70/4S and the 50/1.4G with FTZ adapter is excellent.  Nikon’s decision to leave AF Fine Tune as an option in this camera is a good one as both the lenses above could do with minor adjustment to get the very best from them.  It’s always worth tuning though as there can be quite significant sample-to-sample variation as we showed in this post.

The new test capabilities of FoCal will bring you the ability to test your system – be it the Z7 or any supported DSLRs – and compare your results against other users to find out how your system is behaving.  Stay tuned for more information!