Lens calibration

[arthurking83]

.... Just thought I'd ask about zooms as a friend recently did a few of her primes and they were quite a bit out. ....

And with this specific issue, you need to be careful about that 'out' actually is.
Not many people realise that some prime lenses(most of them will be 'faster types') have an issue known as focus shift.

If you don't know what it is, I suggest a google search, and more importantly look for lens review sites that test for it too.
I remember Photozone has a good pictorial description in their reviews.

That is, the lens will be focused at a specific distance, and without touching that focus distance, the lens is then stopped down through it's aperture range.
Focus shift shows that maybe at f/1.8 the image is low contrast/hazy looking, but that the focus zone is pretty much spot on, but as the aperture is stopped down, the focus distance is visibly changed.
From memory this will usually be behind the focus distance set.

Why this may come across as an issue is that the camera holds the lens fully wide open(for more light), but at that point the lens focuses at the perfect distance.
But if you set the aperture to say f/4 or f/5.6 or whatever smaller size, it still focuses 'wide open' but focus shift issue causes the lens to appear back focused.

Focus adjust for these lens is futile, as it then misfocuses wide open, but spot on only when stopped down.

[Mark L]

..........

While it doesn't actually do any harm, it is best done in private and is generally not spoken about in polite company. Be sure to wash your hands afterwards and try not to make a habit of it.

Thanks Tony. ......

Your last sentence made me -

I hope you don't follow Tony's advice of not making a habit of washing your hands.