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With the danger of dust on all SLR cameras, it only seems logical that adding a simple glass element that went between the lens and shutter (mounted on the camera) would both protect the camera's interior and sensor from any intrusive particulate matter.

Logically thinking, I don't think it should effect the picture quality at all, seeing how we have glass sky filters already.

There are a couple of questions that I have:

Are there any cameras that currently have this feature?

Would implementing this feature detract from the normal function of a camera?

Is this in any way impractical?

To clarify, the blue region is where the question refers to. Camera Diagram

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10  
I vote for a spring loaded cover that slides into place on the lens back and camera front when the lens is unmounted, instead. –  rfusca May 10 '12 at 23:28
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@Dylansq: Sure, but the volume of incoming particulate would be hundreds if not thousands of times lower than without. Would kind of be a game changer if such a thing would work. –  jrista May 11 '12 at 0:31
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Yeah, its an excellent idea, I'm just thinking one static part has less chance for failure then a whole bunch of moving parts. –  Dylansq May 11 '12 at 1:42
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There already is a protective glass element between the lens and the sensor. Only its very close to the sensor, and covers it. This keeps dust OFF the sensor. The dust you see is on the protective glass cover. –  cmason May 11 '12 at 12:19
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@MattGrum - well sure, I'm not advocating that it would be perfect - just better than nothing. –  rfusca May 11 '12 at 16:38

3 Answers 3

up vote 7 down vote accepted

Image quality and possibly expense

Introducing yet another element in front of the sensor will degrade quality of the pictures, and for various reasons.

a) being outside of protection of the shutter will mean that it is constantly in contact with the air and dust. This will mean it will require more constant cleaning than a sensor would. How often do you clean the mirror inside your SLR? Hardly ever because you know it won't show up on photos. How often are you going to clean this extra piece of glass?

b) The placement of being between the lens and the sensor will mean that any dust on the glass would be spread over a larger area of the sensor, degrading quality. We probably all know what an example of dust on the 'sensor' looks like. That dust is on the protective glass right in front of the sensor, so only really covers the few pixels directly under. Being on glass in front of this would cause a larger smudge to be affected by the dust.

c) CA, diffraction, and all of those other nasty things. You have just had the light of your subject travelling through an expensive lens filled with precision glass. That nice white L lens you bought has scored amazingly well on sharpness and contrast tests. Now, however all that precision engineered light is passing at a steep angle through a cheap piece of glass placed in front of your sensor. Unless they spend a lot of money developing some very good quality coated glass, it is unlikely that it will not effect the quality of your photo. And depending on what lens you have, and the angles of incidence that the light from it is hitting this glass, will likely change how the glass effects it. To make none of this happen, you would need multi-element glass, or effectively another lens inside the camera, which would increase cost and give you even more to need to maintain and clean.

Also, in the placement where you put the glass, it might not fit with all lenses. I know some rear lens elements protrude more into the body with others, and I don't have any measurements, but at a guess some have been designed to only just narrowly miss the mirror as it flips up. Putting this glass in the way would mean these lenses no longer fit those cameras.

To summarise, it's not practical, and would degrade image quality. The sensor already has filtered glass on it that can be cleaned, and that is protected by a shutter that opens when it needs to to minimise contact with the air. Introducing an extra glass element removes this simplicity and makes the whole process of keeping things clean more difficult.

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Re. point (b): wouldn't the smudge be spread out to the point of invisibility? And the assumption in point (c) that the glass would be cheap is unwarranted; any sensible manufacturer would use glass of quality consistent with the body. –  Reid May 11 '12 at 17:59
    
Actually, the smudge would be spread out only to the extent the used F stop would give. It's the same way it is now, F22 shows dust, f2.8 doesn't. Also, internal reflections in glass over the sensors can be seen and shown in high contrast images (astrophotos are an example). Based on the size of the reflection and f-stop used, you can see internal reflection from less than a millimeter off the sensor all the way to the back of the lens. –  smigol May 11 '12 at 19:29
    
To the comment on point (c) for the glass to match the quality of the best lens put on the camera it will need to be a good quality lens (or series of) itself. This will ramp up the cost of the body considerably, and limit the placement of the glass. For a professional or someone who buys good quality lenses, any glass less than perfect is going to stop them choosing that camera to buy. They choose if they want filters or not in front of the lens, but not being able to choose if every shot of their $manymany lens is affected by this glass would lose sales –  Dreamager May 12 '12 at 11:29
    
And if the professionals would be finding this extra glass a hindrance in some way, be it cost, maintenance, or image quality, then it seems unnecessary on lower end products as those users are less likely to notice the problems that dust ina body could cause –  Dreamager May 12 '12 at 11:31

The problem with having a glass screen over the lens mount is that it would be useless as the if the rest of the interior cavity is not sealed from dust. The protective screen would then prevent you from cleaning out dust that got in from other sources.

Camera bodies are not assembled in a clean room environment, so there is dust around the sensor from day one. I've had dust problems with fixed lens bridge cameras which are much worse because you can't get to the sensor to clean it.

A removable filter is one option, but it adds to the cost, and isn't a 100% solution in any case. Regular sensor cleaning is, I'm afraid the best option currently.

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Sigma DSLRs have this feature instead of dust-reduction which almost all others have. In the case of these Sigma DSLR, this also block IR light from reaching the sensor while other DSLRs have a filter right in front of it to do that.

The suggestion of @rfusca is one I thought of before. It would probably not be complicated to have a protective cover in place which gets opened up when a lens is mounted. It would be hard to make such a moving part completely sealed, so the sensor clean function would need to open it too (in addition to lift me mirror).

Remember though that if your camera and lens are not both sealed, particles can still enter the camera. Leaving an optical grade class there could help but it would probably add unwanted internal reflections when bright light are in the scene.

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Even if it wasn't perfectly weather sealed, it'd still be better than what is there now....nothing –  rfusca May 11 '12 at 1:25

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