[Paul the Sunman]

I notice that the distance scale on the D FA* 70-200 has a very wide range labelled as "∞". Why a range rather than a set point? It can't be a hyperfocal range, as that would depend on aperture and focal length.

[Adam]

I notice that the distance scale on the D FA* 70-200 has a very wide range labelled as "∞". Why a range rather than a set point? It can't be a hyperfocal range, as that would depend on aperture and focal length.

My guess it would be for the sake of AF accuracy, so that true infinity doesn't end up being outside of the AF range.

[johnc]

maybe it reflects that the true infinity position depends on temperature, sample variation, focal length?

In all honesty, I find some of the focus ring markings on some lenses mystifying nowadays!

John

[tomO2013]

Humidity can have a huge impact on the infinity point on all lenses. Telephoto are particularly affected by this.

[Paul the Sunman]

Nobody expects the lens markings to accommodate variations in atmospheric conditions. "Infinity" should simply mean "light rays arriving at the lens parallel". The atmosphere can act as an additional lens that changes the true distance. But with the D FA*70-200 there's about a 30° throw of the focus ring that is all denoted as "∞", which seems an excessive allowance for "sample variation".

[Na Horuk]

Good answers already. I guess its all of the above.
a) to allow for changes in material size due to temperature, pressure variations (these could potentially even push infinity out of the accessible range)
b) to allow AF to run to actual infinity without violently hitting the hard stop (and to let AF overshoot without problems)
c) to save costs from having to spend a lot of time on each lens precisely calibrating the focus ring

What you have to do is figure out where true infinity is on your lens mounted on your camera. Then you can remember where to focus, just before the infinity sign or just after or right in the middle..

But with the D FA*70-200 there's about a 30° throw of the focus ring that is all denoted as "∞", which seems an excessive allowance for "sample variation".

Might be because Pentax expects people to use that lens mostly near infinity, so having the option of precise control in that range might be more important. Just like many film lenses used to have a very long focus throw in a certain range, so you could focus accurately to the millimeter. Or macro lenses, which have a lot of focus throw devoted to the macro range

But I do agree with you that 30 degrees seems like a lot. I don't have that actual lens, the answers I listed are generic, as we have had plenty of threads where people asked about true infinity on various DA lenses. Is it really 30 degrees? It seems sensible enough in the sample photo here: https://www.pentaxforums.com/lensreviews/hd-pentax-d-fa-70-200mm-f28-ed-dc-aw.html
Do note that the lens has Quickshift technology. On some lenses that means the focus ring can be manually turned on and on

[pentaxus]

ED glass expands and contracts with temperature more than older types of glass. It seems like the more "fancy" glass in a lens the more infinity can vary. My Pentax 85mm f/2 focused right on infinity. My 85mm 1.4 changed with temperature.

[mee]

I had no idea humidity / temperature can alter the physical dimensions of the optics in our camera lenses! Are weather sealed lenses less affected or does that aspect not matter?

[virusn3t]

Mmmmmmhhhh...... Honestly? I didnt even noticed that, so far i only have tested the lens in portrait distances and it nail the focus, so, why worry about.... i just hope doesnt mean any possible failure in the future.

For those who dont know wtf we are talking:

The "line" for infinity is almost the same lenght for the distance of 10 to 4 mts (30-13ft)

This is the place for the "hard stop" of my lens, idk about the others:

[noelpolar]

ED glass expands and contracts with temperature more than older types of glass. It seems like the more "fancy" glass in a lens the more infinity can vary. My Pentax 85mm f/2 focused right on infinity. My 85mm 1.4 changed with temperature.

So in simple terms..... old lenses are cooler and can provide great images when feeling the heat.

[Paul the Sunman]

Temperature and humidity fluctuations in the atmosphere strongly affect the refractive index, thus deflecting light rays from their otherwise straight paths. This is a property of the atmosphere, not the lens. However, temperature also affects the lens (and camera; see this paper), which is what is being referred to above. I doubt humidity has a strong effect on the lens.

[zapp]

Why does it matter to you. Lens turns, camera determines focus, ...
Did anyone try whether infinity for 70 mm is at a different point than for 200 or 135...
Infinity is also not well defined and the values vary between manufacturers. 30° have no meaning unless you know the "gearing" of the lens.

Maybe we have this mark to figure out the focus shift in case we use IR film :-)