Then...just right.
Some top-quality DSLR cameras have an ability in their circuits that allows you to play with the way they auto-focus. Specifically, it allows you to micro-tune the way the sensor sees the image so that it has the best possible focus with each lens you own. And this might be different for each lens.
This is not required for a number of mirror-less cameras because they use an on-screen focus trap that automatically adjusts each time it activates.
Is this necessary? Well, some autofocus lenses from the big makers are built very well, but still are operating at the limits of their manufactured tolerances. Combine this with a similar range of measurement in the camera body and you may go past what the AF mechanism can cope with. This there is a provision to measure this with separate lenses and to program a set of corrections that come into effect every time you clip a lens on.
That's what Daniel Ray was doing today in the workshop today as I used the mini-studio. He was taking a series of pictures of the
Spyder LensCal device to see whether the on-screen result agreed with what the AF was asked to do. He would place the AF indicator directly on the centre of the sloped scale and shoot several shots - then look at the results enlarged on the LCD screen. Any fore or back focus would become evident, and he could crank in a corrective factor for that lens.
Note - it can certainly change for each separate lens, and I have been told that it may change over time for an individual lens as it wears in.
The good news is it is an easy measurement to do ( I've even done it with a tape measure and a sloped board, but that was awkward and not as precise...) and the Datacolor people make the
Spyder LensCal available as an accessory that you can use at home - and use repeatedly.
This is not to do Daniel out of a job - remember that he can tune and clean lots more of your camera equipment in the workshop a lot safer than you can - but the
Spyder LensCal is pretty safe to use.
