Friday, 27 June 2014

Lens Calibration (the proper way)

Readers of this blog might remember that I had focus problems in Iceland as a result of buying new equipment immediately before going on holiday.  Keen not to repeat the same mistake two years in a row I decided to splash out on a Spyder LensCal.  A simple piece of kit, which in principle should be simple to use.

That's the theory, anyway.  The device comes with a 1/4" mounting, so it's easy to fix it to a tripod.  That's the good news.  What's not quite so good is that the spirit level isn't accurate; and, more importantly, the focusing centre of the target is not directly above the mounting point.  This makes it very difficult to ensure that the camera and the LensCal target are parallel: this is crucial, as any rotation will invalidate the calibration results.

In the end I decided to think laterally (literally!) and put the LensCal on a focusing rail mounted sideways.  This allowed me to shift the target until the centre was directly above the tripod column, thereby making alignment much easier.  Even so there was no clear way to ensure that the target and the sensor planes were parallel other than by eye.  Another minor flaw in the target which I bought related to the alignment of the focus target and the diagonal "ruler".  The whole point of the system is that the "0" line is in exactly the same plane as the focus target.  On my LensCal this wasn't true, so I had to pad the ruler support slightly in order to make everything exact.  Being an anally-retentive engineer can be a bit of a pain sometimes...

I tried checking the focus calibration on the back screen of the camera, but this proved to be quite tricky - especially with the slower lenses.  As is shown in the picture above, the easy solution was to tether the camera to a laptop running Lightroom as this allowed me to view each captured image at 100% or 200% in order to check focus accuracy.

This is what the target should look like when set up properly.  The camera focuses on the central square, and the diagonal ruler indicates whether the auto-focus system is front- or back-focusing.

Zooming in, this is what the target looks like if the lens is front-focusing.  It's clear that the ruler in front of the target is in sharper focus compared to behind it.  A quick positive correction to the camera calibration fixes this immediately.

The same image with micro-focus adjustment applied.  It is easy to fall into the trap of checking the two "1" values and ensuring that they are equally soft.  Because of the laws of physics, the area of good focus extends further behind the focal plane than it does in front of it, meaning that the area behind the "0" line will tend to appear sharper than that in front.

#Spyder #LensCal