Long exposure

[JohnR48]

When taking a long exposure photo with a DSLR is it necessasry to cover the eyepiece to prevent light from entering the camera? I read that you are supposed to do this, but it makes me wonder as there was no explanation why.

Thanks

John

[Donald]

No.

Well ......... re-word that. No, not in the real world. But I'm sure there's someone, somewhere has spent a lifetime's study proving that there is a 0.000001% deterioration in the quality of the image because some light might have got in.

[herbert]

Hi John,

The exposure meter of the camera is actually housed up near the eye piece. It uses some of the light reflected from the SLR mirror to do the metering and calculate exposure. So if you do not have your eye up the the eye piece when composing the metering will suffer from stray light getting in. This is a problem when shooting from the hip (i.e. camera at waist level in a street photography style). In this case it is good to cover the eye piece.

However when the mirror is up it blocks nearly all the light that can come in from the eye piece entering the main camera body. So when the mirror is up there is negligible light leakage. This means that 'Live View' and any long exposure will not be affected.

In certain circumstances you may see light leakage though, for example if using a strong Neutral Density (ND) filter during daylight. Then the amount of light coming from the viewfinder and getting through the mirror will be higher in relation to the light coming through the lens.

The truth is that it is a rare scenario when the eye piece cover is useful: if you want the metering to be accurate when you are shooting without the eye ot the viewfinder (and not in Live View). Long exposures are probably done in manual or bulb mode and so using the eye piece cover is not relevant.

Hope this helps.

Alex