Old Manual Lens - Exposure question

Hey all,

Merry Christmas and all that.

I have been playing around with my Petri 50mm 1.7 and have noticed and interesting phenomenon. I use Av mode and set the aperture manually. I have always had to set exposure compensation to under expose by around 2 stops with this lens. I was using Live View with this lens this morning and noticed that it was correctly exposing. Has anyone else experienced this and can someone explain what is happening. I can work with it but am just curious.

Cheers

Tim

Hi Tim.

I have been using manual lenses for many years and have never found a difference between LV and normal use. However, I did find I had to under expose by about 2 stops on my 450D, which I don't have to do on my 7D, or EOS-M.

Perhaps someone will be along and explain why you are experiencing this issue.

When you shoot in Live View, the sensor is seeing what it's seeing and exposure is measured from that, so it always has to be correct.

When you don't use Live View, the exposure reading is taken from a meter in the top of the camera looking through the focussing screen. That focussing screen lights up differently depending on the aperture of the lens. The firmware knows that, and with a Canon EF lens adusts the reading to adjust for the difference. But with an adapted lens, it doesn't know the lens's aperture (or what lens it is), so can't make the adjustment.

Some Canon cameras can be fitted with an -S type screen that isn't affected much at all by the lens's aperture, at least for an f/1.7 lens like yours. Since there's little to no adjustment required, the firmware doesn't need to know the lens's aperture and type, and the reading will be correct. At least, I found that to be so with the particular adapted (OM) lenses I used to use on my 5D.