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03-31-2011, 07:27 PM - 1 Like   #16
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QuoteOriginally posted by secateurs Quote
Instead, (if the camera is completely airtight) the air inside the camera gets compressed and uncompressed. There will be a small amount of air circulation around the inside of the camera, but not at nearly the velocity as with your giant hole.

Your assumption is that the camera NEEDS to breathe. Maybe the camera is designed to be able to handle changes in internal pressure. I don't see why not! I can see how high velocity air movement might be a problem, but changes in internal pressure should be quite ok.
Changes in internal pressure are not ok. A DSLR or lens can never be airtight if it changes size. To demonstrate why, let's run some calculations:

Let's take the DA 18-250 for example. 62mm filter diameter, zooming approximately doubles its volume, thus halving its internal pressure (if airtight).

Area of front element: pi*31mm^2 ~= 0.00302 m^2
Pressure differential: 101325/2 pascals = 50662.5 pascals
Total force on front element: 50662.5*0.00302 = 153 newtons ~= 15.61kgf

A whopping 15.6kg (34lbs) of force pressing down on the front element, more than 30 times the weight of the lens itself. And that's not even considering the force on the barrel, and the camera itself, which would be much greater (due to the greater surface area). Even if you could summon the Herculean strength to zoom the lens, the thing would spontaneously implode.

QuoteOriginally posted by Schraubstock Quote
Is it possible that the few gaps through which negative/positive air pressure can now vent is making sucking in dust worse? Because air pressure will increase around those few small gaps it can find. (And thereby increasing the power to suck in more dust).
Incorrect. The pressure differential is the difference between external pressure and internal pressure. The size of the hole is completely irrelevant.

Pressure is a measure of force per unit area; that is, force for any given unit of area is the same regardless of total size of the hole. A given size piece of dust in front of the hole would experience the same force, regardless of whether it is a pinprick or a gaping hole. The difference the size of the hole makes is the rate of pressure equalization. A smaller hole will take much longer to equalize than a big hole. Of course, a camera, even a weather sealed one, is nowhere near airtight, so the time difference is negligible compared to the time it takes to zoom a lens.


Last edited by Cannikin; 04-01-2011 at 07:34 AM.
03-31-2011, 07:44 PM   #17
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QuoteOriginally posted by calluses Quote
I've seen at least 2 reports of dust developing UNDER the LCD screen on the K-5, possibly due to the use of zooms. Yes, under the screen so I'm not sure how one would clean that. But if it's related to what the OP is talking about, then I hope those of you who are ridiculing the OP will not experience the same problem. :ugh:
Pure speculation based upon NO evidence.

It's a bit like the 3rd party Battery scaremongering.

I stand by my analysis. The OP is paranoid.
03-31-2011, 08:32 PM   #18
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QuoteOriginally posted by Smeggypants Quote
I stand by my analysis. The OP is paranoid.
I would put it more mildly:

The OP needs to take more photos, then he wouldn't have time to worry about such things.
03-31-2011, 09:40 PM   #19
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QuoteOriginally posted by selar Quote
I would put it more mildly:

The OP needs to take more photos, then he wouldn't have time to worry about such things.
Indeed.

My shooting buddy used to be paranoid about taking too many pics because of the finite shutter life. I told him to 'get a life' and just take the shots. You can always buy a new shutter. You can't recreate a 'moment'.

Peeps should just go and take pics.

This is the most OCD/Paranoid thread I've ever seen.

03-31-2011, 11:29 PM   #20
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Eiköhän se pöly mitä kameran sisällä on ole sitä pölyä, kun linssiä vaihdettaessa menee. Ja se linssien pölysuojaus ole sitä suojausta millä suojataan rahavirtoja valmistajan taskuun.
03-31-2011, 11:33 PM   #21
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Wow, guys I had no idea that this little patch (which is removable in one second) I stuck on to my camera to try out an idea and theory I had would invoke such violent passion of disgust in a lot of you.

The responses this post generated spans the spectrum from understanding to misinformed to hysteria.

I have not sentenced my camera to death by just opening the microphone hole and sticking a little sticky disk over it. If any of you think I have compromised the waterproofing of the camera may I point out that when I feel the urge to take photos under a sprinkler I have no trouble taking the sticky thing off and put the the plug back in. You can also grant me enough intelligence to recognize when such a situation presents itself.

QuoteOriginally posted by selar Quote
Wouldn't they have to be a sealed system if they are to be weather and dust resistant. If you relieve pressure by venting air, you'll need to suck it back in when extending, wouldn't you and that would be when dust gets sucked in.
The camera is not totally sealed because if it was you would not be able to move in and out of zoom (especially with a long extending zooms) without feeling great pressure. (take a bicycle pump, hold your thumb over the vent hole and pump away, you get the idea). So whether you like it or not air gets sucked in even with a weather sealed body and with it dust. Where it gets in ? who knows ? All I have done is to take out the guesswork where it gets in and re-directed it through the mice hole where I can control the dust ingestion by providing a little filter.

Cannikin
"Incorrect. The pressure differential is the difference between external pressure and internal pressure. The size of the hole is completely irrelevant."

Really ? When I use my air compressor in the workshop the strength of air escaping the trigger gun at the end of the hose is directly proportional to the size of the hole in the gun. (Not the volume) .

secateurs

"If I were you I would not be messing around with it. Your camera didn't evolve by random chance - there was a lot of though and information put into it. Don't think you can necessarily make improvements on it by your uninformed fiddling. You're really disrespecting the Pentax engineers. If you don't believe they can handle such a simply engineering problem, why did you trust them enough to buy the K-5 in the first place?

Not to mention you have probably voided the warranty. If dust/water DOES get into your camera now, you have no one to blame but yourself.
"

You are way off the mark with these insulting comments! Trust me, I have a brain and can think things through !

Best regards to all of you
04-01-2011, 12:15 AM   #22
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QuoteOriginally posted by Schraubstock Quote
Cannikin
"Incorrect. The pressure differential is the difference between external pressure and internal pressure. The size of the hole is completely irrelevant."

Really ? When I use my air compressor in the workshop the strength of air escaping the trigger gun at the end of the hose is directly proportional to the size of the hole in the gun. (Not the volume).
You're confusing pressure with force. Pressure is force per area (P = force/area). Change the size of the hole, and the total force exerted by pressure will indeed change, but pressure remains constant no matter how big the hole is. Pressure in an enclosed space is by definition the same on all surfaces at the same depth (for an object as small as a camera and a fluid as low density as air, depth can be regarded as constant).

The maximum force experienced is limited by the size of whichever is smaller, the hole or the object (i.e. dust). If the object (like the tool you're using to measure force) is larger than the hole, then yes, the force experienced will be proportional to the area of the hole: force = pressure*area, the bigger the hole, the more force.

However, the piece of dust is obviously smaller than the hole (otherwise it would never get in in the first place). Pressure is still constant, as before. The maximum force is limited by the size (surface area) of the dust particle, which is constant. Therefore the maximum force experienced by a given dust particle is constant, no matter how big the hole is (as long as it's bigger than the dust particle).

Last edited by Cannikin; 04-01-2011 at 12:39 AM.
04-01-2011, 12:34 AM   #23
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QuoteOriginally posted by Smeggypants Quote
So you never change lenses then? or do you do this in a 'Clean room'

What has that got to do with it, we are talking about dust that may or may not enter the body by virtue of air movement through zoom actuation, its air flow within the body, and thereby (possibly) distributing dust into areas, body and lens, where otherwise it would not go.

Greetings

04-01-2011, 01:04 AM   #24
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QuoteOriginally posted by Schraubstock Quote
The responses this post generated spans the spectrum from understanding to misinformed to hysteria.
I agree, Labelling the OP as paranoiac and stating that he has voided his warranty is going a bit far, and frankly i'm surprised people are reacting in this manner. This is a relatively open forum, so be warned that pretty much everything you post is open to either criticism or praise...and sometimes both.

Of course there is air moving inside the camera every camera short of a nikonos is far from airtight. However I suspect the Pentax engineers saw that as the lesser of the two evils and chose to control where the air would be coming from, and make sure the paths the air takes doesn't adversely affect the performance of the camera.

On my canon DSLR I have used a Canon EF 100-400mm f/4.5~5.6L IS several years ago and this lens has a well known reputation for being a dust pump. Which is why I personally have an aversion to push-pull type zoom lenses. My sigma 100-300mm f/4 APO EX DG is internally focusing and internally zooming lens - with an 82mm front filter thread I have no doubt it can move the air quite a bit inside it. But most of that air will have to get past the rear element of the lens, the primary mirror, the secondary AF mirror,the shutter and finally land on the sensor cover glass. The sigma 100-300mm f/4 doesn't extend like the Pentax 60-250mm f/4 SDM does which is actually one of the primary reasons why I didn't buy it.

Last edited by Digitalis; 04-01-2011 at 01:12 AM.
04-01-2011, 01:44 AM   #25
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Smeggypants

Excuse me, who is this aimed at, yourself, me or all the other posters here.
04-01-2011, 03:15 AM   #26
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QuoteOriginally posted by Cannikin Quote
You're confusing pressure with force. Pressure is force per area (P = force/area). Change the size of the hole, and the total force exerted by pressure will indeed change, but pressure remains constant no matter how big the hole is. Pressure in an enclosed space is by definition the same on all surfaces at the same depth (for an object as small as a camera and a fluid as low density as air, depth can be regarded as constant).

The maximum force experienced is limited by the size of whichever is smaller, the hole or the object (i.e. dust). If the object (like the tool you're using to measure force) is larger than the hole, then yes, the force experienced will be proportional to the area of the hole: force = pressure*area, the bigger the hole, the more force.

However, the piece of dust is obviously smaller than the hole (otherwise it would never get in in the first place). Pressure is still constant, as before. The maximum force is limited by the size (surface area) of the dust particle, which is constant. Therefore the maximum force experienced by a given dust particle is constant, no matter how big the hole is (as long as it's bigger than the dust particle).

"When I use my air compressor in the workshop the strength of air escaping the trigger gun at the end of the hose is directly proportional to the size of the hole in the gun."

This observation still stands, instead of "strength" a better word could have been velocity. Every protester who has had the unfortunate experience of having been the recipient of a jet of water from a water canon will understand this principal. Have a look at the pipe just behind the water canon nozzle and you will see some sort of an S-bend to introduce resistance and thereby velocity of the water jet.

Greetings
04-01-2011, 04:03 AM   #27
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QuoteOriginally posted by Schraubstock Quote
So then I got thinking, wouldn't it be good if I kept this microphone hole open all the time, this way at least I have control over where the air flow takes place.
Maybe Pentax already have control of where the air flow take place and how to deal with it, that is if you don't open up other holes in the camera.

So the question is, where does the air ventilate if you keep the microphone plugged?
04-01-2011, 04:15 AM   #28
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QuoteOriginally posted by Schraubstock Quote
"When I use my air compressor in the workshop the strength of air escaping the trigger gun at the end of the hose is directly proportional to the size of the hole in the gun."

This observation still stands, instead of "strength" a better word could have been velocity. Every protester who has had the unfortunate experience of having been the recipient of a jet of water from a water canon will understand this principal. Have a look at the pipe just behind the water canon nozzle and you will see some sort of an S-bend to introduce resistance and thereby velocity of the water jet.
Sorry, but this is still not valid. You're talking about hydraulics, which is a completely different principle to atmospheric (static) pressure.

A hose or water cannon is powered by a pump. A pump is designed to move a certain volume of water in a given amount of time (e.g. liters per minute) by applying a constant total force. Because the nature of the pump means that you must move a given volume of water in a given amount of time, the pressure increases when the area (and thus volume of water that can pass through at a given time) decreases.

This is not a valid extension to atmospheric pressure. The atmosphere is not powered by a pump. The atmosphere does not have to move a certain amount of air in a certain amount of time. Atmospheric pressure (not atmospheric "force") is the result of the static weight of a column of air of any given cross-sectional area. Decrease the size of the area and you decrease the size of the column of air, thus the force decreases and the pressure remains the same. You cannot "channel" atmospheric pressure. Atmospheric pressure at sea level is defined as 101,325 pascals. This is constant at any point on Earth at sea level, no matter if you are looking at a 1mm^2 or a 1km^2 area. Pressure differential is the net result of atmospheric pressure outside (constant) pushing against the pressure inside. Any hole is completely irrelevant (heck, there doesn't even have to be a hole at all). This is basic physics and fluid mechanics.

Try this. Take a bottle of water and poke a tiny hole in the bottom. Take another bottle and punch a huge hole in it. Which produces a bigger flow? Obviously the big hole. The tiny hole produces a very tiny trickle. The static pressure exerted by the water is exactly the same in both circumstances. The bigger the hole, the more water molecules get moved at a time, but for any given molecule of water (analogous to a dust particle), the force exerted on it is exactly the same. The end result is the same (empty bottle), just that the smaller hole takes a much longer time to do it.

This is the principle behind static pressure, and completely different from a pump/hose which moves a constant amount of water in a constant amount of time.

Last edited by Cannikin; 04-01-2011 at 05:36 AM.
04-01-2011, 04:38 AM - 1 Like   #29
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QuoteOriginally posted by Digitalis Quote
I agree, Labelling the OP as paranoiac and stating that he has voided his warranty is going a bit far...
Thanks Digitalis

It has been my lifetime experience when people don't want to or cannot understand something or feel superior they resort to hurling insults. Normal human behavior.

Maybe the moderator should close this thread or perhaps even remove it from the forum, I am not thin skinned and if I am wrong, o.k. tell me, no problems, back it up with a valid argument and all will be well. I came here to contribute and not have to listen (read) to all this tripe.

Greetings
04-01-2011, 06:08 AM   #30
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Nah, don't close the thread yet; it's still in its "informative and entertaining" phase. Hasn't quite reached "tiresome and draggy".

What can you do? Perhaps the people who're criticizing the proposed ideas here haven't owned cameras and/or lenses that're susceptible to this kind of dust problem and haven't had to consider the issue to any great degree.

i've had a couple of dust pump lenses, one of them when i got it had hidden recesses of dust inside it which when mounted onto my camera, and when zoomed, the inflowing air would free up and spew dust all over the inside of the mirror chamber.

It was a SMC-F 35-135mm. Fantasic sample of a fantastic lens, but it was hell cleaning up the mirror box from it. Finally had to take a vacuum cleaner to both the inside of the lens and camera.

My trusty compact cam is a Canon A650IS, love the thing, and its lens is another dust pump, and it can't be cleaned without disassembling the camera and lens. SOL when a speck of dust decides to settle down onto its sensor.
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