Zoom question.


Oggy

Link Posted 23/04/2011 - 23:43
I should know the answer to this but I cannot for the life of me think of it.

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.

sorted78

Link Posted 24/04/2011 - 08:51
I don't know, but I think the key is the 'resistant' bit of weather resistant, not weather proof. I think that the WR sealing is good enough to prevent most water ingress, but not perfectly sealed, so it isn't air tight and probably wouldn't survive immersion in water for long.

cbrog

Link Posted 24/04/2011 - 21:47
I have an old Sigma 100-200 zoom you could use to pump up a tyre

Oggy

Link Posted 24/04/2011 - 23:04
140 reads! Someone must know.

prsjnb

Link Posted 24/04/2011 - 23:23
Oggy wrote:
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

Anvh

Link Posted 25/04/2011 - 12:55
I believe i've notice that the DA*16-50 does push some air out but it can be me... I've used the DA*16-50 inside a swimingpool but only the front element fogt up and not the inside so it seems to work somehow, assuming the DA*60-250 has the same build quality you've something to comapre it to at least...

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

Don

Link Posted 25/04/2011 - 13:01
air does compress... it isn't like you are trying to compress water....so it would seem like a possibility that a slightly porous gasket combines with sufficient room for the air to compress (or move from one chamber to another)would still equate to a water resistant seal that allows air to seep through when there is sufficient pressure/vacuum applied... and the slight seepage would allow the air pressure to stabilize fairly quickly...
Fired many shots. Didn't kill anything.
Last Edited by Don on 25/04/2011 - 13:02

SteveEveritt

Link Posted 25/04/2011 - 13:02
Am I right in reading that you have used the 16-50 under water!
My Flickr link

"Life is what happens to you while you're busy making other plans" (John Lennon)
Last Edited by SteveEveritt on 25/04/2011 - 13:03

Oggy

Link Posted 25/04/2011 - 13:44
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.

Don

Link Posted 25/04/2011 - 14:07
Oggy wrote:
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.
Last Edited by Don on 25/04/2011 - 14:20

Oggy

Link Posted 25/04/2011 - 22:29
I can partly see what you are saying Don, but if there were collapsible/expandable spaces they would still have to vent somewhere. If this were the case, I would expect some audible sign, not even on my 150-500 quickly extended or retracted.
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