How do dedicated light meters account for light transmittance?

Question

Dedicated light meters seem to be well-regarded for their versatility and accuracy of metering.

What puzzles me about them is how can they account for the different light transmittance of different lenses? It's easy for an in-camera metering system that meters through the lens so any peculiarity in transmittance is already applied to light when it reaches the metering sensor. But a standalone meter does not have the privilege of looking at the world through the lens it meters for. I tried looking it up in some light meter manuals and Sekonic FAQ, but this issue was not touched upon at all.

This leaves several variants how it could be handled -

• light meter uses T-stops for aperture display/input, and photographer is expected to perform the adjustments to and from F-stops (which are used by cameras);
• light meter assumes some median transmittance of lenses of its era, and automatically accounts for that;
• photographers simply ignore the matter - even a 75% transmitting lens will be off only by about half a stop, no big deal;
• each light meter used is expected to know that you have to dial in some exposure compensation to account for the light loss;
• ...

So how does it work in real world?

Answer 1

If not knowing the transmittance of your lenses is really bothering you, you could test them and see as this DPReview user did. Most of the lenses tested, about 25 Nikon lenses, averaged in the 80-90% light transmittance (telephoto lenses were less). Compensate accordingly. In the real world of photography, though, a safe assumption is that most don't worry about it.

Answer 2

Like you this bothered me as well. Depending on the lens, the exposure was always underexposed to some degree. I use a Sekonic L-758DR, but in the end I don't calibrate using the provided software and tests required. My gear is Nikon.

Effectively their camera calibration procedure takes lens transmission loss into account, along with many other variables I didn't want to get into. There has to be something simpler.

Eureka! I discovered that DXO Mark tests lenses in addition to their reference sensor evaluations and a myriad of others I'm sure. To my total surprise lens transmission loss is identified for literally hundreds of lens. So off I go with numbers in hand. For each lens I dial in the loss as compensation and the exposure is bang on.

For every lens there is a mechanical term called f-stop that is universal for all lens. Due to the light loss within a lens there is also t-stop. For example my 105mm f/2.8 t-stop is t/3.4 and my 200-400mm f/4 is t/4.9. Perhaps not significant for some lens but I now dial in the appropriate loss as required.

Trust this helps