“100 lux”

1

At 12:45pm, I measured an interior light reading of 100 lux on the subject, facing camera.

Lux I treat as the intensity of light striking at object, not the intensity of the light reflected at the camera.

Depending on the conditions outside, the reading for the portrait can reach as high as >1200. If it is completely dark, the lux could be as low as 0.1 lux.

Outside, during a bright day the lux can be like 50,000 (wikipedia says the brightest daytime lux would be 120,000 lux).

A full moon is 0.25 lux.

In in my studio, I want 100-175 lux (ideally). I don’t want to set up a third light to control shadows, so I have to control the ambient lighting. Does that make sense?

I can’t adjust the sun or weather. But to adjust ambient light, I a several options:

• I can open or close the curtains.

• I can use different curtain thickness (a thin fabric dampens light).

• I can also place solid boards to eliminate light (light will seep in through the corners).

• I can choose a different time of day (10am versus 3pm versus 9:30pm).

• I can move (or angle) the subject away from the windows.

2

About six months into flash photography, I realized I wanted to exclude all ambient light.

I wanted the portrait to be visible only through lights added.

This means, if the picture was taken without flash, it should be completely dark.

My settings were:

{
f/8 (or smaller)
1/200
ISO100
} => 13.67 EV (=> 32,000 lux)

100 lux of light would barely expose the image (0.3% of total light), or 333 times less light (or a little over 8 f-stops, 2^8 = 256).

3

My fill light settings

{
f/1.4
1/60
ISO100
} => 7 EV (normal exposure) == 320 lux

100 lux is one third of 320 lux, so the ambient does 31.25% exposed (100/320).

By moving the ISO from 100 to 64, we effectively cut 66% more light, so the ambient becomes:

{
f/1.4
1/60
ISO64
} => 6.33 EV (normal exposure) = 508 lux

100/508 = 19.7% exposed.

A 4 stop ND filter also cuts exposure by 4 stops (!):

508 * 2^4 = 8128 lux

100/8128 → 1.23% exposure (or 2.0% at 100 ISO).

Not bad? 1.5-2% fill is enough to lift complete darks.

Plus, I am under-exposing my actual lights. The 1.5-2% will go up.

The fill is also tinted orange, because it reflects from the wood floor.