The Mac version of FoCal is coming on very nicely, and we’re preparing for a public Beta release for FoCal Pro users within the next few days (keep an eye out!). Right now, we’re doing some final tests and creating the OSX version of the manual. In doing so, we have to run all the tests, and noticed something really interesting when we accidentally left IS (Image Stabilisation) on for the Aperture Sharpness test.
As a quick introduction: the Aperture Sharpness test measures the quality of the whole image capture setup (lens, and camera) through the range of apertures available from the lens. It plots the points for the different apertures against a scale of quality, with a higher value indicating a sharper image.
As you can see from the image above, the quality of the shots is all over the place! There is a general curve that you would expect from the Aperture Sharpness test – quality gradually improving from the smallest aperture (which will be most affected by diffraction), improving to a stop or two below widest aperture then the quality dropping again approaching wide open. For comparison – here’s a test that was run immediately afterwards, but with IS disabled:
So what’s going on? Well, we thought we’d probe around a bit… Initially we ran a set of aperture sharpness tests with various mirror lockup delays from 0.1s to 0.5s with IS active, and while these showed slightly odd results, on the whole they were better than expected.
Not deterred, we hacked around with the Focus Consistency test quickly to stop it changing the focus so we could control aperture and ISO (and therefore – importantly – shutter speed), as well as running multiple shots to get some confidence in the results and get a final numerical result (in the form of CoF which for the technically minded is the peak-to-mean ratio of the shot quality values). Looking back at the original “odd” aperture sharpness graph, we decided to change to f/14 to give a shutter speed or around 1/2s, and these are the results:
0.1s delay = 93.8%
0.2s delay = 93.9%
0.3s delay = 92.1%
0.5s delay = 91.2%
2.0s delay = 98.6%
The “delay” above is the mirror lockup delay (so the time between lifting the mirror and taking the shot). From this it looks like you need somewhere between about 0.5s and 2.0s for the IS to stabilise. For comparison, here are the results with no mirror lockup:
IS off, no mirror lockup = 99.0% (this serves as a control – anything from about 99.0% upwards can be considered perfect)
IS on, no mirror lockup = 96.1%
I’ll be honest… there’s not enough data here to draw any particular conclusions above what is already known – that IS takes a bit of time to settle and from the instant it’s enabled until it’s settled it will reduce the image quality, and if you’re going for longer exposures you probably want to make sure it’s disabled.
What is worth noting is that if you use FoCal, make sure you run tests with IS OFF!
For information, these tests were run with a Canon 1Dmk4 and Canon 100mm f/2.8L IS Macro lens.