You want to measure how sharp your camera/lens combination is to make sure it lives up to its specs. Or perhaps you’d like to compare how well one lens captures spatial resolution compared to another you own. Or perhaps again you are in the market for new equipment and would like to know what could be expected from the shortlist. Or an old faithful is not looking right and you’d like to check it out. So you decide to do some testing. Where to start?
In the next four articles I will walk you through my methodology based on captures of slanted edge targets:
- The setup (this one)
- Why you need to take raw captures
- The Slanted Edge method explained
- The software to obtain MTF curves
In this first installment I will describe the most critical part of the process, how to capture images of targets with very good technique. There are many tutorials on achieving just that but let’s go over the basics: stable location, oversized tripod and head, delayed release, mirror up, electronic shutter or front curtain if available, Manual Exposure and ISO, Manual Focus, VR off, NR off, 14-bit Raw, proper, properly illuminated subject at least 50 times the focal length away, base ISO, stay away from the 1/10s to 1/300s shutter shock speed range, ETTR.
The ideal subject is a back-lit razor or utility knife blade which, if used properly, should provide absolute results, see Frans van den Bergh‘s or Jim Kasson’s setups for that. But for non-absolute, relative purposes such as comparing the performance of your own equipment a good resolution chart that can be printed at home by a decent printer on decent paper will do. A recent laser printer at its highest resolution works well, as do pro-sumer inkjets. The blacks should be uniform and 100% black, no striations. The paper should provide a uniformly white background, so gloss types are typically not a good choice because of the specular highlights they inevitably reflect. Soft matte papers are not good either, especially with inkjets, because of their visible rugosity and because they smear edges. A good compromise is quality multipurpose paper often used in office copiers/printers. It usually weighs 80 g/m^2 or more with labels such as ‘silky touch’ or ‘ultra bright’.
Choose the location for the test carefully. The tripod and head should be sturdy and positioned on a level, stable flooring, no old planks. Any vibration in the neighborhood, like people walking in the room or heavy trucks nearby, will most likely be picked up, so eliminate all possible sources of noise and vibration.
Set up the target ensuring that it is perfectly flat and square to the camera – the slanted edge method is so sensitive that it will pick up any tilt in the target whatsoever (and buckling in the case of a paper chart). The performance of your imaging system will vary with distance to subject so make sure that you are as far away as your intended application requires. If, like me, you do mainly landscapes the camera/lens should be at least 50 focal lengths away, 100 is better, aimed at the center of the chart and perfectly perpendicular to it. Make sure that the angle of incidence of the illuminant does not bounce close to the camera after hitting the target. In other words, watch specular reflections – ensuring that the camera is not inside the family of angles of the lighting.
Lighting is important. Illumination should ideally be balanced and uniform over the target. If you are going to look at individual color planes separately use a light source that provides close to ‘daylight’ color temperatures (D50, D55 or so), diffusely. By uniform I mean that it should not create a gradient of decreasing or increasing intensity locally across the chart. Both of these requirements can be tested by taking a raw capture of the chart with the illumination as set up and gathering statistics off four patches of pure white at the cardinal points and four more at the corners around the target with software like RawDigger.
Reread the checklist at the beginning of the post and remember to Expose the whites of the chart To The Right: raw data in the three color channels somewhere between 5000 and 10000 ADU is ideal in a 14-bit raw file. While checking for gradients above also ensure there is no clipping in the raw data of the region of interest that will be used to measure the edge.
Focus is critical and focus bracketing essential (LiveView/pinpoint focusing is a distant fall back). Ideally you will mount the camera on a rail, position it in its center, focus with LiveView, switch the camera to manual focus mode, pull it back a little and bracket several shots by moving the camera forward on the rail. Depending on focal length and target distance you will have to move the camera several millimeters after each shot and take many of them. If you don’t have a rail you can try to focus bracket manually by keeping the camera in one place on the tripod, focus in LiveView, switch it to manual focus mode and turn the focus ring one way then the other trying to cover peak focus. With modern fly by wire lenses manual focusing is very sensitive and even with absolutely minimal focus ring turns it will often miss perfect focus, so in this second case you will probably have to fit the resulting data to obtain the actual peak MTFs.
Once you have a picture of the target the best place to measure the full, unretouched spatial resolution information captured by the camera and lens that took it is straight from the raw file as it rolls off the sensor and electronics. No demosaicing, no processing, no sharpening (well, assuming no subliminal tricks by manufacturers under the hood) – just what the hardware saw according to the laws of physics, in order to judge its capabilities and IQ.
What about measuring it off the image of the chart as rendered by a raw converter, a TIFF or, heaven forbid, a JPEG? Read on.