Originally posted by secateurs 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.
Originally posted by Schraubstock 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.