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Author Topic: Variable length zooms and dust  (Read 1997 times)

Sporgon

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Variable length zooms and dust
« on: December 22, 2012, 01:21:50 PM »
I have a question regarding zoom lenses that change their physical length, such as 24-105L,  70-300L etc, and the potential for them to ingest dust, pollen etc into both the camera body and themselves by sucking and blowing air in and out.

Is this a fact ?

If this is the case I am puzzled as to why when fitting a weather sealed zoom of this type onto a weather sealed body you don't have problems in zooming the lens outwards against a vacuum that would be created in the body.
Now I know that weather sealed doesn't mean airtight, but even so I would expect resistance when trying to zoom in or out quickly as the air tries to pass the weather sealing gaskets.

Can anyone explain
« Last Edit: December 22, 2012, 01:30:44 PM by Sporgon »

extremeinstability

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Re: Variable length zooms and dust
« Reply #1 on: December 22, 2012, 01:29:12 PM »
I've only had problems with my push pull 100-400L, granted I've never owned a huge collection of lenses.  It for sure is a dust pump.  Looks like it was assembled in the Sahara desert. 

I've had it fog up inside the same area as well.  The only way I got it back out(without waiting forever) at the time was to blast the car heater on the lens then push pull pump it a lot, then cool it back down in the cold outside air, then repeat.  The expanding and contracting along with the pumping eventually dried it out.  Had to dry the car out from all the snow melt issues first. 

Seems the vacuuming stays out of the cam.

Sporgon

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Re: Variable length zooms and dust
« Reply #2 on: December 22, 2012, 04:40:22 PM »
Can't believe all the tech guys out there don't have an answer to this one !

neuroanatomist

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Re: Variable length zooms and dust
« Reply #3 on: December 22, 2012, 05:10:39 PM »
Dust-/weather-resistant lenses have o-rings under the switches, but the seal for the barrel extension is generally a brush-type seal that prevents entry of dust/water while allowing air flow so the barrel moves smoothly.  There is a difference in feel when extending/retracting a sealed vs. non-sealed but well-built zoom, e.g. 24-105L vs. 17-55.
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FTb-n

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Re: Variable length zooms and dust
« Reply #4 on: December 22, 2012, 06:59:17 PM »
Some lenses, such as the 17-55 f2.8, also have vents near the base of the filter threads.  Using a filter on these lenses can help reduce dust ingestion.  which begs the question, "why put vents where filters can defeat them?" 

For what it's worth, on my 17-55 I don't feel any additional resistance with the filter in place.  This lens does have a bit of resistance between 28 and 35 mm range when zooming, but I don't think its vacuum related.  This is one of those things that feels huge when playing with the lens, but you forget about it when using it and you're more focused (no pun intended) on getting the shot.
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Mt Spokane Photography

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Re: Variable length zooms and dust
« Reply #5 on: December 22, 2012, 07:14:20 PM »
What problem does a few specs of dust in a lens cause.  I've had a number of  zoom telephoto lenses, both sealed and unsealed, and some had a lot of visible dust in them.  In all cases it was a non issue, so I'd recommend not getting hung up on dust in a lens and worry about real issues.

Sporgon

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Re: Variable length zooms and dust
« Reply #6 on: December 22, 2012, 07:37:18 PM »
What problem does a few specs of dust in a lens cause.  I've had a number of  zoom telephoto lenses, both sealed and unsealed, and some had a lot of visible dust in them.  In all cases it was a non issue, so I'd recommend not getting hung up on dust in a lens and worry about real issues.

I never mentioned dust contamination in the lens. However bad dust contamination on the sensor is a real issue if you're shooting predominantly at f8-f16 as we are, and pro sensor cleaning is about £50 at shot + camera body down time. When we were using 24-105's a lot of the time sensor contamination became an issue really quick. Recently we've been more on fixed physical length lenses and the sensors have stayed remarkably clean.

BL

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Re: Variable length zooms and dust
« Reply #7 on: December 22, 2012, 08:02:06 PM »
i used to worry about dust and such until a small fruit fly crawled into my 55mm 2.8 mamiya sekor and died lying on the inside of the front element.  never noticed any additional flare, loss of sharpness, etc. 

i'd be more concerned about haze than dust, as that seems to be the culprit for issues affecting IQ (for me)

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Re: Variable length zooms and dust
« Reply #8 on: December 23, 2012, 07:01:30 PM »
My 24-105 gets constant use in extremely dusty environments I've had to have lens cleaned out when dust made it into the focus ring and it was making crunchy noises no dust has gotten further than this into the lens. But the amount of fine dust this lens is exposed to far exceeds that which most people would subject their gear to. I keep single use alcohol wipes with me and if I know its gotten very dusty I'll give the lens and particularly the extending barrel a wipe down in the field and clean the whole camera and lens in detail daily
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