Overzealous

[Jackie Schuknecht]

When cleaning my sensor I inadvertently touched the rocket blower on the viewfinder which left some smudges. I tried to clean them off and it made it worse. I took it in for servicing and he cleaned the viewfinder but it still has some marks when I look through the eye-oiece. My question is will it effect IQ in the final image or is it just something I will see in the eye-piece?? Don't know how the image finally ends up on the sensor. Maybe someone can explain???

[John Chardine]

If it's just on the viewfinder glass Jackie, it will not affect the image. The lens projects the light back through the mirror chamber of the camera onto the sensor when you press the shutter release. To accomplish this, the mirror flips up and the shutter moves across the sensor to make the exposure. The mirror, focusing screen, pentaprism and viewfinder all function simply to bend the light up and back to your eye so that you can see what the sensor will see.

Sounds like you still may have a stubborn smudge on the viewfinder lens. I find a bit of lens cleaning fluid and a lint-free cloth usually does the trick in these cases.

[Jackie Schuknecht]

Thanks so much John, you have put my mind at rest.

[WIlliam Maroldo]

Jackie: indeed it will not effect the image at all. However, the fresnel lens(which is what you are referring to) could have been scratched, or gouged, and no amount of cleaning will fix it. However, it may one of the easiest parts to be replaced, as in most cameras it is interchangeable with different types. Bill

[Jackie Schuknecht]

Thanks Bill, I think it was the fresnel. Just some rubber marks on it:). What is the purpose of the fresnel. Does it magnify and brighten the light coming into the viewfinder? Can you replace it yourself or should it be done by a pro???

[WIlliam Maroldo]

Jackie: yes it does brighten the light somewhat. There are different types of fresnel lenses used in the camera viewfinders. Some have different grids and crosshairs imprinted on them or different type of focusing aid in the center. Professionals might need a specific type for the photography they are doing. Architectural photography would benefit from horizontal and vertical grid, for example.
A grid that was two equally placed vertical and two horizontal lines would help in applying the "rule of thirds", but I don't think they made them.
I used to repair cameras, years ago, and the top cameras had the capability of the user replacing the fresnel pretty easily, it just required a special holder to hold the lens and not get fingerprints on it. Fresnels were generally made of plastic, and I had a number of occasions where people tried to clean them with a solvent. Bad idea.
Even if the fresnel is not user replaceable, the viewfinder area is generally fairly accessible, since cleaning dust from the area is commonly needed, and replacing a fresnel shouldn't be a major undertaking.
regards~Bill

[John Chardine]

At least in the modern Canon xxD and xD series cameras the fresnel screen is interchangeable by the user.

[Jackie Schuknecht]

Thanks John and Bill.

[Ed Cordes]

If yo do remove the fresnel lens it is easy to do. However, I do believe you must be careful to place it back right side up or performance in th eviewfinder may suffer. I think one side is convex an one side flat. Check it out before you begin to play with it.

[Jackie Schuknecht]

Thanks Ed for the tips!