It’s Not a Mirror Bumper

by Johnny Martyr

Do people still reiterate that once-famous line from the 90’s movie Kindergarten Cop where Arnold Schwartenegger says “It’s not a tumor” in his characteristically out-of-place Austrian accent?

Well… (in my best Arnold) It’s not a mirror bumper!

Man, everything about this post is showing my age.

It’s been about 24 years since I picked up my first vintage SLR and over that span of time I still see people refer to the foam rubber strip above the flipping mirror of SLR’s as a “mirror bumper.”

To be honest, I don’t think this part has an official name so I’ll concede to call it a “mirror bumper” for the sake of discussing it.

But it’s not a mirror bumper.

It’s important to understand what this part’s actual purpose is. And it’s not to cushion the mirror, no matter what people who haven’t actually read any camera repair manuals might imagine.

Contrary to popularly repeated misconception, the “mirror bumper” is not there to stop the reflex mirror from damaging itself. If you fail to replace an aging “mirror bumper”, you will not damage your mirror (not the way that you think anyway). That is not why you should replace your “mirror bumper.”

The “mirror bumper” is actually a light seal.

It stops stray light from the viewfinder from entering the optical path of the taking lens and striking the film.

When the reflex mirror flips up just before the moment of exposure, the “mirror bumper” creates a light tight seal from the viewfinder. That is it’s purpose. Not to cushion the action of the mirror.

Sure, when the mirror slaps the ceiling of the mirrorbox, the “mirror bumper” cushions the hit. But this is merely a side effect. If you’ll notice, the mirror doesn’t actually come anywhere close to hitting the interior of the camera. It doesn’t require a cushion. If it did, why isn’t there a “mirror bumper” below the reflex mirror to cushion it when it returns?

Furthermore, a sidecar to this “mirror bumper” myth is that if your exposures are bad, it could be because the “mirror bumper” is deteriorated and causing the mirror to stick to it and interfere with exposure time.

No. If your camera is not exposing images correctly, just replacing the “mirror bumper” is not going to change anything.

Look, it’s great that you want to learn about your camera and participate in maintaining it yourself. But please, if this is your intent, get the facts straight so you understand why you are doing what you’re doing. And perhaps more importantly, learn when it’s time to reach out to a professional. If you want to learn camera repair, don’t learn it from a hobby photographer. Learn it from a camera repair technician.

Replacing the “mirror bumper” alone will not resolve any problems you’re having with your camera. All that it will do is make the mirrorbox cleaner and light tight. Admittedly, that is important. But if your “mirror bumper” is deteriorated, you have bigger problems that you are overlooking if you have been lead to believe that replacing one part was all you needed to do.

I like to view the “mirror bumper” as a barometer for the condition of any SLR. It’s an easy light seal to use to asses the condition of all the other light seals and, by extrapolation, how long it’s been since the camera was professionally serviced.

You see, if the “mirror bumper” is crumbling, falling apart or feels sticky and smooshy, you have alot of problems. Ones that you probably aren’t qualified to address with a $10 pre-cut light seal kit.

The “mirror bumper” is made of the exact same type of light seal foam as is used throughout the camera. In a typical SLR, there are more light seals than just those in the channels in the perimeter of the film door and hinge. There are usually foam light seals underneath the top plate also. Sometimes they act as light seals, sometimes they are just used for insulating electronics from their surroundings and sometimes they act as both. But all that foam rubber is made out of the same stuff. And it will all age and deteriorate at pretty much the same pace.

So if your “mirror bumper” is deteriorated, all the foam in the entire camera is also deteriorated – providing that someone didn’t previously replace all the more difficult-to-access foam but left the original, easily replaced mirror foam alone. That wouldn’t make sense, would it?

Most light seal replacement kits and tutorials only cover replacement of seals that are in the mirrorbox and film compartment. So if you follow these tutorials, you might look down at your work and feel a sense of pride. But unfortunately, this leaves the inside of your camera with bad foam.

And the problem with decaying light seals, besides that they fail to block light, is that they turn into crumbly goo and make a mess. The particles that are shed from a bad strip of foam can even cause mechanical and electronic problems.

There are plenty of people reading this and saying to themselves that they’ve re-foamed hundreds of cameras and never had a problem. Of course not. The issues that decaying light seal foam causes are often going to go unseen unless you’re shooting outdoors with high speed film or suddenly, your meter stops working or your frame registration is slightly off. Would you associate these issues with lack of light seal replacement?

But then there’s the other problem. If your light seals have fallen apart, this tells me that your camera hasn’t been serviced in decades. Because it takes decades for light seal foam to fall apart. And if your camera hasn’t been professionally serviced in decades but you just replaced only the “mirror bumper” or only the exterior light seals, what you just did was not entirely different from putting brand new tires on a car that hasn’t had an oil change in many years. You may be able to drive it around a bit further and feel comfortable but at some point, your engine is goin to blow and you’ll be left sitting on the side of the road admiring those nice new tires.

When a camera fails due to neglect, it’s often less dramatic of course. But you get the idea.

Take care of your gear, folks. Support your camera repair technicians. And no, it’s not a “mirror bumper.” It’s the tip of the proverbial iceberg.

Thanks for reading, happy shooting!

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