How to Align a Lens Resolution Target
Lens resolution measurement is a combination of software and special flat targets that get photographed and then measured by the software. It’s very important that you properly align a lens resolution target. Otherwise, it’s the old adage of “garbage in, garbage out”. Alignment falls into two categories: parallel to the camera sensor and no rotation error.
Rotational alignment is the easy part; you can use the edge of your viewfinder to align with the target markings. I always use “grid view” in my viewfinder for easy alignment.
So how do you ensure that your camera sensor is parallel to your target? Easy; you use a mirror. A couple pieces of rolled-up (removable) painter’s tape behind the mirror will hold a small mirror flat against the middle of your focus target. Make sure you press it flat, so there aren’t any gaps between the mirror and the target. Manually focus your lens at twice the distance from your target, and then move the camera around until you see your reflection through the lens. You probably want to test your tape on the edge of your target to verify it peels off without damaging your chart.
I have found that inkjet works much better than laserjet; the tape can strip off the toner in laserjet prints. Semi-gloss is probably the best surface for the charts, too. The heavier paper weights work better, as well. Around 600 dpi printer resolution is ideal.
If you have a flat surface like a wall behind the chart and don’t want to put any tape on your chart itself, you can try sticking the mirror to the surface until you align the camera, and then clamp the chart over the spot where you had temporarily stuck the mirror.
I had experimented with hanging a mirror like a necklace on a string, but it didn’t work very well. Another technique that does work well, however is to sacrifice getting measurements in the middle of the chart and cut a hole to mount a mirror with glue behind the chart, with the mirror showing through the hole. This works best if your chart is either glued (spray glue) or dry-mounted to mounting board, and you cut the hole through both the chart and its mounting board. If you use the “lensgrid” chart shown later in this article, the middle of the chart (an hourglass pattern) doesn’t get measured anyway.
I got a suggestion from Frans van den Bergh (the author of the MTFMapper program) to try using magnets to hold a mirror in place, so that the chart surface isn't harmed. I did some research, and found some outrageously strong neodymium magnets (32 mm diameter and 3mm thick) that are nickel/copper coated. These babies hold with a direct-contact force of 18 pounds! The thickness of my chart with its backing is 9.6 mm, so I knew I'd need really strong magnets to be able to hold the mirror with that big of a gap. Lo and behold, a magnet behind my chart and one stuck to the back of my mirror (with double-stick tape) keeps the mirror perfectly in place to align the chart. It is easy to remove the mirror when photographing the chart, and leaves no marks. I actually stuck a pair of magnets (side-by-side) behind the mirror to guarantee it's completely parallel to the chart; only one of the mirror magnets is stuck to the rear chart magnet. Thanks, Frans!
You can also glue little bubble levels to the edges of the chart, particularly if your chart is mounted in a frame. If your tripod also has bubble levels, then you can get the chart and camera into pretty close alignment.
Find your reflection
Notice in the shot above that you can barely see the chart itself. That’s because the lens is focused at twice the distance of the chart to see the mirror reflection. It should go without saying that you need a sturdy tripod. If there’s any vibration, you can easily see it in your reflection.
Focus on the chart
Now, focus on the chart. Your reflection basically disappears! You can be confident, however, that the camera sensor is parallel with the mirror, and therefore parallel with the chart, too. Once aligned, you can then peel the mirror off of the target. Note that the chart shown above is an older design, but is still supported for resolution measurements. A newer chart design is shown below.
I’d also recommend that you clamp the resolution target into position, so that it won’t move when you touch it (or when you peel off the mirror).
Align a Chart Using Fiducials
The MTFMapper program can use a chart design (“lensgrid”) that can measure any chart rotations in 3-D space. This doesn’t help you align beforehand, but at least you can tell how well you aligned the camera to the chart after the fact. Their website has resolution chart and focus chart downloads in PDF format. The latest as of this writing is called “mtfmapper_sample_test_charts_0.5_v4.zip”.
The LensGrid Resolution Chart with Alignment Fiducials
If you use the chart shown above, you can not only measure lens resolution, but you can also get feedback about how well you aligned the chart via rotation measurements in yaw, pitch, and roll axes.
Select the Chart Orientation option
Before you can see the chart orientation measurements, you need to select the correct option. This option is paired with the “lensgrid” chart type. When you get the resolution measurements of the chart, you’ll get an extra plot result, called “chart orientation”.
Chart Orientation results
The MTFMapper program uses standard math rules, with the “X” axis horizontal and positive to the right, “Y” is vertical and positive up, and “Z” is using the “right hand rule” being positive out of the screen. The “Pitch” is rotation about the horizontal X axis, “Yaw” is rotation about the vertical Y axis, and “Roll” is rotation about the Z axis.
In the shot above, the Roll value is +0.91 degrees. To reduce the roll error, I would need to rotate the chart about the Z axis (this axis is out of the screen) in a clockwise, or negative rotation direction. You might notice that the left side of the shot above shows a narrower gap at the top than the bottom, so chart rotation in the clockwise direction to get it vertical makes sense.
Many cameras have a feature in Live View called “Virtual Horizon” to help get the camera aligned on the “roll” axis, which is mainly intended to get horizons level. This can be used to decide if the camera or the chart (or both) isn’t level.
Chart rotated clockwise to reduce “Roll” error
The chart was rotated a little clockwise and then re-clamped. The Roll error was reduced from +0.91 to +0.12 degrees.
The next problem to fix in the chart shown above would be to fix the “pitch”. Pitch is sometimes referred to as “nose up” (as in aircraft).
When you get within about a degree on each axis, then you can be confident that the resolution measurements will be reliable.
So, what happens if you analyze chart shots that aren’t parallel to your camera sensor? You’ll get one or two sides of the chart with poor resolution readings. If you get poor readings, you won’t know if it’s your lens or the chart or both of them.
You still need to use a remote release and use “live view” with contrast-detect focus for maximum focus accuracy. Remember to turn off lens vibration reduction when your camera is locked down on the tripod, or it will actually add vibrations.
I’m using the MTFMapper program for my resolution testing software, but these same precautions are required for whatever analysis program you use. You can get the software from this site.