Zoom question.


140 reads! Someone must know.

One possibility is that there are vents in the body of the lens with hydrophobic (non-wettable) membranes with pores large enough to permit the free movement of gases, but small enough to prevent the ingress of particulates.
Jon

Never notice something with the DA*50-135 but everything is internal there so it doesn't change volume.
Stefan

K10D, K5
DA* 16-50, DA* 50-135, D-FA 100 Macro, DA 40 Ltd, DA 18-55
AF-540FGZ
Fired many shots. Didn't kill anything.
The relevant part is Boyle's law which states that in a closed system, the product of the pressure and the volume are constant. Thus if you half the volume, you double the pressure.
Some very rough (fairly conservative) figures - the area of the lens is 1 square inch and the volume of the lens changes by one third between min. and max. focal length.
Assume you put the lens on at max. focal length then moved it to min. focal length, the volume would decrease to two thirds and the pressure increase by a factor of 1.5 times it's original value. You now have 1 atmosphere acting on one side of the lens and 1.5 on the other. An atmosphere is about 14.5 psi so for your 1 square inch lens, you are going to have about 7 pounds force (about three bags of sugar) acting on the lens. I am sure we would notice this.
I am convinced they have to vent but the vent system has to be quite clever.
Don - have a look here.
The relevant part is Boyle's law which states that in a closed system, the product of the pressure and the volume are constant. Thus if you half the volume, you double the pressure.
Some very rough (fairly conservative) figures - the area of the lens is 1 square inch and the volume of the lens changes by one third between min. and max. focal length.
Assume you put the lens on at max. focal length then moved it to min. focal length, the volume would decrease to two thirds and the pressure increase by a factor of 1.5 times it's original value. You now have 1 atmosphere acting on one side of the lens and 1.5 on the other. An atmosphere is about 14.5 psi so for your 1 square inch lens, you are going to have about 7 pounds force (about three bags of sugar) acting on the lens. I am sure we would notice this.
I am convinced they have to vent but the vent system has to be quite clever.
your assumption is that they have to vent a lot the air inside them, externally is probably incorrect.
if the air is moving between chambers (or spaces within the lens) then the air pressure is probably greatly reduced..... depending on how you design it, you can take multiple tubes and baffles and create a collapsable/extendible tube where air moves freely between air chambers with minimal pressure and minimal air movement from outside the tubes....you just can't do it with TWO tubes.... unless those tubes have sufficient space (volume) to allow the air to compress...
i think the lens design incorporates both internal baffles/air chambers AND some air exchange from out side the system... but that overall, MINIMAL air exchange from outside the system...
Fired many shots. Didn't kill anything.
Oggy
Member
Worthing
Considering a lens like the DA" 60-250, what happens to the air inside it as you change focal length? It has to go somewhere or as you released the zoom ring, it would return to the focal length it was set to when you fitted it, but it would be difficult to vent it while retaining the weather resistance.
Confused,
Worthing.