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  • Ed Dozier

Focus Peaking Analysis for Nikon Mirrorless Cameras

Nikon has a very capable feature set to enable manual focus assistance called ‘focus peaking’. Unfortunately, the camera menu system wording almost guarantees using it incorrectly.



Nikon Z9 rear screen focus peaking display


As seen above, the image details that are in focus get a color outline around them; in this case they’re red. If you look closer, you should notice at least a couple of problems with what is shown.


The first problem with focus peaking you should note is that many of the fine details are shown as ‘in focus’, even though they’re largely mush.


Another problem is that the nearly-horizontal edges aren’t ever shown as being in-focus; only the mostly-vertical edges are indicated as being sharp.



The Custom Settings Menu


Let’s take a look at the camera setup menus that activate and configure focus peaking.




Locate the ‘a Focus’ option


First, locate the Custom Settings Menu, and then open up the Focus option.




Nikon Z9 Option ‘a13 Focus peaking’


Next, scroll down to the Focus peaking option. Don’t bother looking for in-camera help to explain peaking; on my cameras, it offers no help.






Turn on the Focus peaking display


Be sure to activate focus peaking by turning it ON. After focus peaking is active, your camera display (viewfinder and rear screen) will show peaking whenever you twist the Z-lens focus ring. It will even work when the lens focus switch is set to “A” for automatic focus; you don’t have to switch to “M” for manual-only focus.


Some F-mount lenses may require a manual-focus switch setting to get the peaking display, and most non-Nikon lenses will also require the ‘manual’ focus switch setting to get a focus peaking display.




Focus peaking sensitivity



Please remember this: when Nikon says “high sensitivity”, they mean LOW focus resolution! With wide angle lenses, a setting of “3 (high sensitivity)” will often mean that the entire frame is shown as “in focus”. You will almost NEVER want a setting of “3”; maybe save it for use on a foggy day.


For most conditions, please set the peaking sensitivity to “1 (low sensitivity)”; this setting will be the best indicator of what is actually in focus. Even this setting using an un-magnified viewfinder can give you pretty sloppy focus results.


Nikon came up with the “high sensitivity” descriptor due to how they implement peaking. Very small changes in illumination levels between neighboring (horizontal) pixels will trigger peaking if the camera is configured to have a high sensitivity to these small light changes. Unfortunately, this small-change condition rarely means that the lens is properly focused.




Focus peaking color



Nikon gives you 4 color choices for peaking. With this, you can adapt to your subject’s color to get the best peaking contrast.




How to get better focus results


The key to getting critical focus is to use viewfinder/rear screen magnification. You magnify the viewfinder by using the little “magnifying glass +” button on the back of the camera. Multiple presses of this button will give greater viewfinder image magnification.





No screen/viewfinder magnification


The shot above shows the camera rear screen at NO magnification (it was too difficult to get a photo of the viewfinder view). The peaking sensitivity was set to “1 (low sensitivity)”. Even at the 1 setting, the “in focus” peaking is too sloppy and covers most of the frame.




Single-press of the “Magnifier +” button


Notice that the slightly-magnified screen view has significantly reduced the range of the red peaking display. This is a much better indicator of what is really in good focus.




Double-press of the “Magnifier +” button


At higher viewfinder magnification, the range of “in focus” is much narrower. By the way, notice how none of the nearly-horizontal edge detail is shown as “in focus” although it is clearly in focus.




Triple-press of the “Magnifier +” button


Bummer! At this very high screen magnification, there are NO focus peaking indicators. What to do? Again, the shots above are all using the “1 (low sensitivity)” setting.




Triple-press of the “Magnifier +” button, ‘standard’ sensitivity


I switched over to the “2 (standard)” peaking sensitivity. At the same high viewfinder magnification, peaking has returned! This setting is pretty sloppy at zero viewfinder magnification, but at high viewfinder magnification it is really good.



Horizontal edge detail



No peaking to be seen for horizontal edges


Note the total absence of horizontal-edge peaking indicators. The reason for this is how Nikon implemented focus-peaking in their firmware. The camera is only looking at left/right pixel neighbors for changes in illumination level.


If you’re looking at a subject made up of only horizontal detail, then you’ll need to briefly roll your camera about the lens axis to confirm focus.



Focus-peaking math


If you’re interested, here is a little discussion of how the low-level camera firmware operates to figure out focus-peaking. Don't freak out about the math.


First, the camera makes up little lists of a pixel with its left-right nearest pixel neighbors. These lists consist of a pixel number and its illumination level.

Next, these lists are run through a ‘linear regression’ to calculate an equation that best fits a line through the plot of pixel-number versus pixel-illumination.


The equation of a line in most math books looks like this:


Y = m*X + b


The Y shown above represents pixel illumination.


The m above is the ‘slope’ of the line, where a bigger (absolute) slope value is steeper.


The X is the pixel number.


The b is where the line crosses the vertical (Y) axis.


When the line slope of pixel/illumination reaches a critical value, it means that the camera found a large-enough brightness change to indicate an in-focus edge and activates focus-peaking at this pixel. This process is repeated all around the image sensor, using a bunch of little lines.


The camera menu peaking sensitivity is then related to each of these tiny rows of pixels. For high sensitivity, it activates peaking with fairly low slope values. For low sensitivity, it demands a high slope value.




Sample of a line with low illumination changes


Above, I show some made-up data of a short horizontal row of pixels somewhere on the sensor. I used pixel 900 through 910 (out of over 45 million pixels!). The brightness at these pixels ranges from ‘50’ to ‘94’ units.


Using Microsoft Excel, I performed a linear regression on the data to calculate the equation of a line that best fits this data. The plot shows the original data (blue) and the best-fit line (red). The calculated slope of this line is “4.6”, which is fairly low. It would take a “high-sensitivity” focus peaking setting to decide that this represents an in-focus edge, and displays it in the viewfinder.


If I had set the focus peaking to “low-sensitivity”, then this low slope value wouldn’t trigger any peaking to be displayed at this location in the viewfinder. A ‘high-sensitivity’ setting, however, would trigger peaking in the display, because it is sensitive to even small slopes.




Sample of a line with large illumination changes


Using my fake data above, the line was calculated to have a slope of “17.5” or so. This is a much steeper line (and higher slope value). Focus peaking would get triggered at ‘low sensitivity’ for a steep slope like this line has, as well as ‘high sensitivity’. This portion of the viewfinder would probably show peaking at any sensitivity setting.


Nikon could have chosen to do the same peaking scheme using vertical columns of pixels, but decided not to do so. Most of the time, that decision has proven to be just fine.



Summary


I hope that gives you a better insight into what focus-peaking is, and how to use it to the best advantage. Stick with the lowest sensitivity that you can, and use the highest viewfinder magnification that you can to get the sharpest focus.


Focus peaking makes using manual-focus lenses better than in any other time in history, especially when combined with the mirrorless camera viewfinders that allow magnification.

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