Lens focus ring markings not matching actual distance

Started 3 months ago | Discussions thread
revio Senior Member • Posts: 1,811
Re: On being over precise

cba_melbourne wrote:

Tom Caldwell wrote:

revio wrote:

Tom Caldwell wrote:

But in general terms no lens should be capable of having the focusing ring move past the hard infinity stop.

......

Some lenses even have an additional mark for infra red photography and some tolerance might be needed for changes in heat from extreme cold to quite hot - especially longer lenses....

Autofocus lenses actually must be able to focus past infinity:

- the AF control loop needs to go slightly past infinity and then return to the optimal position, it is the way how it detects the optimum focus.

- AF motors move fast and cannot stop instantly. Some leeroom past infinity avoids the mechanism to bump into the end stop - and it makes AF faster as the motion does not need to be slowed as much when approaching infinity.

- zoom lenses are not always parfocal. The exact location of the infinity focus point shifts at different zoom settings.

- lenses that employ low-dispersion glass elements are particularly susceptible to focus shift due to thermal expansion

***

- in some manual lenses, the focus point shifts a little with different aperture settings - it's called focus breathing

- even the most precise cameras do have a mechanical tolerance for the flange-sensor distance

- It is actually much easier to find the best focus point, if a manual lens focuses very slightly past infinity. It gives you a visual confirmation, that you indeed are at the optiptimal focus.

***

- some lenses use rear mounted filters. Such a filter between lens and sensor (or sometimes inside the lens via a side slot) does change the focus position. It shifts the focus position by about one third the thickness of the filter, which is a lot... and such lens must still be able to be focused to infinity with or without a filter inserted.

All that is so true, including Tom Caldwell´s own clarification (or what I shall call it).

BUT, my two Oly PRO lenses don´t focus "a very slight bit" past infinity, their markings show clearly they go about a *centimeter* each, past the infinity mark (measured from the middle point of the infinity marking).

The 40-150 goes 11mm past, the 12-40 goes 9mm past.

Defined as how much the infinity distance marking on the focus ring pass by and beyond the reference marking.

I do not complain about that, to be clear, I simply note that this is how those lenses are made, not a problem in any way

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