Why is my Canon 7D light sensor under-sensitive?

Question

It seems the light meter indicator in the viewfinder on my Canon 7D underestimates how bright the picture will be. This makes for overexposed photos whenever I use anything other than full manual mode, on the order of 2-3 stops. This while using no exposure compensation and evaluative metering mode.

Some possible culprits:

• I've used microfiber cloth to clean the camera, including the light meter IR sensor. I only ever cleaned the sensor and mirror by holding the camera front down and using a hand powered air blower. Is any other cleaning necessary?
• Is there any known problem with the hardware or software of these cameras which could account for this?
• Is there any setting, other than the ones mentioned, which could account for this?

Answer 1

Have you checked which metering mode you are using?

If you are for example are using average metering then the camera will use the whole frame to decide what setting to use. For example if the overall frame is too dark the camera will try compensate for that.

Normally the different metering mode are:

• Average metering - Uses the whole frame
• Center weighted metering - Use the surrounding points around the focus point
• Spot metering - Use the focus point

Another setting that you can check is the Exposure Compensation where you can say the image should be X exposure over or under what the camera think it should be. The normal symbol for this is +/-

Answer 2

In response to M. Clark's statement.

That is not the case on the Canon 6d and other Canon cameras I suspect,the mirror is just partially silvered and reflective glass and is not as fragile as some think, the reflective material is basically baked into the glass. If you have ever seen an old vacuum tube where it is silvered it is basically the same process where metal vapor fuses into the glass. And while it is true you want to avoid touching it the reason being is it is a real pain to clean unless you have the proper materials, you will end up leaving smears that are hard to get rid of. I have successfully cleaned it with high quality micro-fiber and good quality eyeglass cleaner, but I suggest that only as a last resort. When it is at rest in the mirror box it acts as a highly reflective mirror because of the angle it sets at. Also the light meter is not under the focusing screen but is reflected down below the the main sensor. On the 6D the focusing screen is easy to remove and they actually sell different ones that may suit your needs better. If you shine a small flash light at the mirror you can see there is actually a large opening under the mirror where light passes through and then hits another mirror and is directed down to the bottom of the mirror box to the light meter and auto-focus sensor. When you flip the mirror up as in cleaning mode the smaller mirror closes into the main mirror so it cannot be seen. For a time I thought the 6D was underexposing but it turns out it is actually doing a great job of metering across the scene. You do need to practice with it as the different meter modes are very sensitive and will give very different results scene to scene, be careful not to leave it in spot mode as that can really mess you up and give under exposures while center weight can leed to blown out scenes.

Answer 3

You should first check what Metering mode and the amount of Exposure Compensation you have selected. A good place to start for most situations is Evaluative metering or Center-weighted averaging. Using Partial or Spot metering modes can cause overexposure errors if the small area being metered is darker than the rest of the scene and the rest of the scene is what you are trying to correctly expose.

Most mirrors outside of cameras and telescopes have the reflective coating on the back of the glass, but cameras and telescopes usually have the coating on the front side. The coating is extremely fragile and should never be touched. If it needs cleaning use an air blower. If that is not enough, then you need to send the camera to a Canon authorized factory service provider. You may also have damaged the focus screen itself by cleaning with a cloth. The surface of the focusing screen is a very precise pattern of micro-prisms that are also easily damaged.

If you've cleaned the mirror in the light box it is likely you have removed some of the finish from the mirror's surface. This in turn has reduced the reflectivity of the mirror which feeds the light meter that is located above the focusing screen in the pentaprism housing. If you removed the focusing screen to clean the light meter and didn't reinstall the focusing screen correctly, such as putting it in with the wrong side facing the mirror box, it could also cause metering errors.

If you have replaced the original focusing screen with an aftermarket one you will also affect the accuracy of the camera's meter. Different screens allow different amounts of light to pass through them. Since Canon doesn't consider the 7D focus screen to be user replaceable, there are no custom settings available in the 7D menu to adjust for a different screen. Other camera's for which Canon offers several different focus screens do have custom menu options to tell the camera which one is installed so that the light meter is correctly calibrated for a particular focusing screen.