Originally Posted by Dr Croubie
Here are my feeling about that bit of advise. It thinks it a crock of ... What kind of weather are they talking about?
First let me say, even with a filter they are not Water Proof. They are simply weather "Resistant".
I might not be an expert on the matter or their reasoning behind comments like that. But it doesn't make sense to me that Canon would make high quality optics just so you have to purchase something else that can probably reduce overall Image Quality by adding another air spaced glass element in front of it.
If you're going to be using your camera and lenses in an environment where water can leak into your lens you probably shouldn't be there first place. If there is that much water or it's raining that heavy, what is to stop it from seeping around the filter threads? The filter does not have a rubber gasket to make a water tight seal does it?
What kind of weather are they talking about anyways? Condensation from rapid changes in temperature on the lens elements or heavy rain, either way adding another air spaced piece of glass with just a few fine unsealed threads won't stop it.
If you're just shooting in light rain why wouldn't the lens hood help keep water drop off the lens enough. What's to stop water from getting into your camera? I've seen a few people shoot in the heavy rain who don't use filters. They prefer to cover their camera and lenses to protect their equipment from the rain. http://www.kata-bags.com/
Having said that, I know some people feel strongly about using filters. They have their opinions and reason and can keep them. I even use one occasionally myself.
Can a filter save you lens in the event you hit something with it? Of course it can, but then if the seal on the lens is so bad that you need a filter to seal out the weather you need to be careful that when that filter shatters you don't get any fine glass slivers down into your lens..
On Edit:
FYI, nowhere in the manual for the lens does it say it requires a filter to complete any weather seal. It does mention the rubber o-ring at the mount but says nothing about completing any weather seal. It does however say that the hood can be used to protect from rain.
Why wouldn't canon say anything about using filters to complete a weather seal in their manuals for these lenses.
http://gdlp01.c-wss.com/gds/3/0300004673/01/ef70-300f4-56lis-usm-en.pdf
The only thing the manual says about filters is that they are sold separately and that you can attach filters to the filter mounting threads on the front of the lens.
From the Canon Web site, the overview of the lens.
"Designed to deliver ultra-high quality images along with lightning fast
operation, the new Canon EF 70-300 f/4-5.6L IS USM telephoto zoom lens
brings a favorite zoom range to the celebrated Canon L-series of lenses.
It features two Ultra Low Dispersion (UD) elements for improved image
quality and reduced chromatic aberration, incorporates a floating
focusing mechanism for sharpness from close up to infinity, plus an IS
system that increases usability by approximately four stops. The Image
Stabilization system also features a function that allows it to continue
to operate, even when used on a rigid tripod. An optional Canon Tripod
Mount C (W II) is available as an accessory, for balanced mounting on a
tripod or monopod. Gleaming white on its exterior, the lens is dust-
and water-resistant, and now also features a newly-developed Fluorine
coating that resists smears and fingerprints, and significantly eases
lens cleaning. For any serious and critical EOS user, this new L-series
lens represents an excellent -- and portable -- solution to versatile
telephoto lens shooting."
"Additionally:
Weather sealing (resistant against dust and water) when used with a weather sealed camera body. Weather sealed Canon products are not completely sealed from the
environment, they are merely more resistant than non-sealed versions."
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