Do lens affect camera metering ?


On a second thought, I realized that the lens shouldn't effect the metering. Since DSLR in camera meter is caliberated to read 18% grey, any change in luminance of light would only alter the values around 18% grey. The scene might look a bit dark or a bit light but since the camera meter is designed to read only the pre defined values around 18% gray, any lens should not effect the metering.

Henry, why should focal length change the metering, especially if you spot meter?
Focal length affects the field of view. Spot metering which is around 3-6 percent of the screen will cover bigger area of a scene/object in wide angle than a telephoto where everything is magnified.


Well-Known Member
The answer is yes and no.

The meter reads the voltage created by the amount of light falling on the sensor. More light means more voltage means higher readings.

A lens, by the nature of its construction, will allow a Certain amount of light to fall on the sensor (assuming a constant aperture). So the meter reading will be FOR THAT lens and that scene combination.

Professional cinematography lenses are thus calibrated for T Stops (transmission) instead of F Stops (Regular geometric aperture). This is because the T Stop is calibrated for light transmitted, and not just the geometric aperture.

(Two lenses of the same focal length at the same aperture may transmit slightly different amounts of light because of the nature of their construction. However, if the meter is reading off the sensor, you need not worry, since the meter is already reading the transmitted light).

Things become different if you are using an external light meter (as in professional Cinematography) and manually set the aperture. But those lenses already take into account the transmission loss.

In modern still cameras with TTL metering, these are non issues. Even in still photography, if you are reading off an external hand held light meter, the lenses are very well calibrated, and any slight transmission losses are too small to make aNy significant difference.

(@nishchaya : meters don't read the range just around 18% grey. They read the entire scene.They 18% number is just a geometric/mathematical mean of objects with the highest reflectance ((around 81 %, let's say white snow) to those with the lowest (around 4%, let's say charcoal). Multiply 81x4 and the square root is 18 (my numbers may be off a bit but it's around these figures). Meters are calibrated to be able to reproduce an object of average or mean reflectance 'faithfully'. It's not a gospel. It's subject to interpretation. I always underexpose to get more 'density' in the sky. When manually metering, you interpret the scene, the mood you want to create etc and choose an exposure setting based on your aesthetics and needs. When on auto, meters try and reproduce the scene in a way that average reflectance are reproduced 'faithfully'. If you point an auto exposure shot at a bright sky ONLY, it will probably underexpose. If you point it at a dark black object, it will over expose. In either case, it's trying to find a mid-grey.) in real life, green foliage is roughly 18% and some of us Indians have a skin tone of around 18%. I often find a reasonable exposure by reading a reflected meter off the back of my hand.
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