Doug Kerr
Well-known member
By the transmission of a lens we mean the fraction of the light captured by its entrance pupil that is actually delivered to the focal plane. This primarily recognizes that fact that light is lost by reflection at each air-to-glass or glass-to-glass interface, and a bit by actual absorption in the elements.
Based on some of the (made long ago) assumptions in the equations for exposure metering standards, I had come to think that perhaps a value of 90% was to be expected in a typical case - maybe even less, given modern advances in lens coatings.
This morning, in connection with some measurements I was taking, I needed to get some idea of the transmission of the lens I was using (a Canon EF 24-105 mm f/4L IS).
I found that DxO Labs nicely determines this in their modern DxOMark lens testing suites, so I checked their finding for that lens.
I was shocked to learn that at the maximum focal length (where the transmission is highest), the value was about 62%. At the minimum focal length, it was about 58%.
Note that, with through-the-lens exposure metering systems, this does not make the automatically-determined exposure incorrect. But it does mean, that for any given scene luminance and exposure index, the exposure recommendation is over 1/2 stop greater than for a lens with transmission of, for example, 90%.
Looking at it another way, this f/4.0 lens has a T-stop value of about T5.1. That is, its effect on exposure is as if its maximum aperture were f/5.1.
Best regards,
Doug
Based on some of the (made long ago) assumptions in the equations for exposure metering standards, I had come to think that perhaps a value of 90% was to be expected in a typical case - maybe even less, given modern advances in lens coatings.
This morning, in connection with some measurements I was taking, I needed to get some idea of the transmission of the lens I was using (a Canon EF 24-105 mm f/4L IS).
I found that DxO Labs nicely determines this in their modern DxOMark lens testing suites, so I checked their finding for that lens.
I was shocked to learn that at the maximum focal length (where the transmission is highest), the value was about 62%. At the minimum focal length, it was about 58%.
Note that, with through-the-lens exposure metering systems, this does not make the automatically-determined exposure incorrect. But it does mean, that for any given scene luminance and exposure index, the exposure recommendation is over 1/2 stop greater than for a lens with transmission of, for example, 90%.
Looking at it another way, this f/4.0 lens has a T-stop value of about T5.1. That is, its effect on exposure is as if its maximum aperture were f/5.1.
Best regards,
Doug