Light Metering with a letus extreme
 

I am shooting with an ex-3 with letus extreme adaptor w/o the relay lens. So the question is how would I determine the EI with the camera using 2 lenses (the ex-3 lens and the lens mounted to the letus). I guess a more direct question would be how do you meter for (2) lenses?

Made a mistake. I have an ultimate not extreme. I guess the only difference would be the variation of speed of the ground glass.

A long time back there was an article written by a director of photography and published on the web. It described tests of the P+S Technik PRO35 and there was a table of light loss for given iris settings.

The table covered from f16 to widest. The light loss at f16 was mimimal, the amount of light loss increased with wider iris settings. The light loss was not constant through the entire iris range.

The f5.6 limit rule remains. Higher numbers or tighter iris settings will produce artifacts sooner or later with all adaptors. Some have a higher threshold than others.

For exposure, my personal preference is to use the camcorder zebras when using a groundglass adaptor. I assume a very general rule of 1.5 stops of light loss at the f1.2 iris setting on the lens on front of the adaptor. Various adaptors lose less light.

If stuck with the LCD screen alone, I attempt to set screen brightness so that the "pluge bar" in the bottom right corner of the in-camera colour bars is barely visible in the ambient light conditions I am viewing the LCD screen itself in.

To achieve best results, especially for focus, you probably should use a monitor or a separate large display when you can.

Selective focus can become very hit and miss otherwise. The separate monitor also enables you to finely trim the relay focus to a test chart.

I estimate you can get a final extra 50 lines or so of apparent resolution by setting relay focus to a chart with the groundglass running versus setting relay focus to the groundglass texture with the groundglass stopped. You still need to have the camcorder iris at its widest to make the camcorder lens depth-of-field shallow for the setting of relay focus to be the most accurate it can be. The lens iris on front is closed down as needed to compensate.

I could use a light meter and sometimes do when I want to assure myself how much darker chosen objects within my frame are than others but I prefer to use what the video camera instantly shows me, the what I see being what I get (MAYBE?).

It does not hurt to use the focus assist button on the camera to "zone-in" on areas within the image to check just what detail remains visible within them. Zooming in closer on the groundglass to select an area may give you a false indication as there is a slight increase in light loss as the camcorder lens reaches the tele end of its zoom range.

If you zoom in to examine a smaller area of the image to enable exposing for detail in a dark area, then zoom back to the normal view of the groundglass image, this will assure the detail within the area will remain visible. If however you want to bury something in black, then the slight increase in light transmitted by the lens when you zoom back wider may brighten something unwanted to visiblity. In practical terms, this brightness variation can be ignored as the camcorder lens is most often at the tele end of its zoom range already. If the rest of the image is lit, composed and exposed correctly then a slight crush in post of black would be all that is needed to disappear something.

Please do not regard me as a qualified commenter and take the advice of others more able than I who may comment here.

Adrian, I've done a lot testing re: light loss of different adapters, and metering with video.

My advice - use a waveform not a meter unless you are just using the meter to get you in the general ballpark for exposure. As Bob mentioned , though its counter intuitive, exposure with an adapter is not linear because the way light is scattered by the screen is complicated.
The exposure difference between f1.2 and f2 on the 35mm lens on some adapters is barely noticeable while between 5.6 and 8 may be well more than a stop. It will vary with lenses and the adapter.
Generally speaking in the area of f1.4 or f2 my experience is that an Ultimate or Extreme loses around 2/3 - 1 stop or more.