Nikon D850 Individual Focus Sensor Actual Coverage Area
When you look through your viewfinder, you could be forgiven for assuming that the little auto-focus sensor indicator squares on your screen are accurate. Think again.
Active focus sensor square
The little square(s) showing where your camera is focusing isn’t even close to showing you how big the actual focus zone is. I’m going to show you how you can discover for yourself the real size of the focus sensor. This technique should work for most (Nikon) camera models, even though I'm demonstrating the Nikon D850.
My focus-checking chart from the MTFMapper site
I use printed charts like the one shown above to check and calibrate my auto-focus. The chart is meant to be rotated by 45 degrees, and then my software can figure out where my lens actually focused versus where I pointed the focus sensor.
In this case, I used this test chart to find out how accurately my D850 indicates the area of sensitivity of its focus points are, compared to the little etched squares on the viewfinder’s focus screen. This chart makes it easy for the camera to focus on the right-hand edge of the big square in the chart middle, so you don’t have to guess where the camera focuses.
Before testing, I set the camera to be in AF-C mode and I select the “single-point AF” mode. I want the camera to concentrate on only one focus sensor, so I can find the boundary of sensitivity of that sensor. I use a sturdy tripod, and I pan the camera away from the rectangle edge I have focused on until I lose focus.
To measure the actual size of the focus sensor area, I make use of the little viewfinder focus-confirmation “dot”. When the camera sees proper focus, it displays a little dot while you’re activating auto-focus. When the camera loses focus, it instead displays a couple of little flashing triangles.
I make use of the transition from the in-focus “dot” to the flashing triangles to know when the camera has lost focus, while pressing the AF-ON button and moving the focus sensor away from the chart’s big rectangle edge. When the flashing signal starts, it means that your focus sensor has fallen off of the target edge. This indicator is located along the bottom left-hand side of the viewfinder.
Aim the single focus point at the edge of the rectangle edge
As shown above, I begin by centering the focus point over the middle of the target rectangle edge. I slowly pan to the right, watching to see when the camera focus indicator shows that it lost focus. This operation lets me find the sensor left-edge boundary. I repeat the test using the rectangle left edge to find the focus sensor’s right edge boundary while panning to the left. I mount the chart vertically and pan up/down to find the focus sensor boundaries in the vertical dimension.
I tried the test using other focus sensors, to see if they behaved the same; it appears that they behaved similarly. I only tested the “cross-type” focus sensors, since I was looking at both horizontal and vertical details. I'll mention that mirrorless camera focus sensors are usually NOT cross-type sensors, and they cannot detect pure horizontal details.
The real focus sensor boundaries
I drew a square in magenta to show the size and location of the focus sensor, as seen through the viewfinder. I have overlaid (in purple) the extent of actual focus sensitivity for the sensor that I measured in the vertical and horizontal directions. The boundary is almost exactly an entire ‘square’ away from the viewfinder sensor indicator area. The total area of sensitivity maps out to a perfect circle, as near as I can tell; I used a round focus target and panned diagonally to estimate the extent of sensitivity in various directions. I drew a red circle around the estimated area of sensitivity for the focus sensor.
There are times that it’s good to have some elbow room around a sensor and still hold focus. Other times, it’s the last thing you want. In any case, it’s good to know what your focus sensor is actually seeing. I’m not trying to pick on the D850 here; it’s certainly not alone in exhibiting this sort of “dishonest” focus response.
There are probably times that you noticed some unusual focus behavior. This analysis may shed some light on why that happened.