Using a spot meter TTL with WLF

I am going to do some experimenting with a reverse mounted RB lens on an RZ macro tube (an RB lens so I can control the shutter speed). I was originally thinking that I would need to use an RB metered finder to get TTL metering with this set up. Then I started wondering if I could get any useful exposure information by sticking my Pentax digital spot finder into the WLF.

The first thing I tried was with a 110mm f2.8 lens mounted normally and focused to infinity. The EV I got through the WLF was about 1 1/2 stops less than with the spot meter itself pointed in the same direction as the lens. Then I mounted the same lens on both macro tubes with the bellows extended and took a reading with the AE prism. Then I took a reading with the spot meter in the WLF, and it was 1 1/2 stops less than the reading I got with the AE finder. It appears that this method will work, as long as the image is bright enough so that the spot meter reads higher than zero. The RB lens will be a stop slower, but I will use a brighter screen, so hopefully I be able to keep the compensation to a stop or stop and a half.

Has anybody else ever tried this before?

That is pretty funny but I think to a degree it would work. block off all the extra light coming in the top and the amount of light hitting the screen should be something you could figure out how to use. In fact the RolleiFX meter takes readings off the viewing screen.

I figured out how to use my pentax spot meter on a light box as a densitometer by establishing a reference from a piece of exposed film with known zone 7. It works sort of.
Dennis

I haven't done it although I've thought about it, but then isn't it Sinar and Gossen that has GG metering probes for 4x5 cameras. The only thing I can think of is not being able to get a good spot reading in the right place on a small screen. It's not rocket science to figure exposure loss with a bellows and tubes on a RB/RZ. I say meter your dark tones in your scene and compensate.

Yes, I take spotmeter readings off the screen of my RB67.

Precautions include eliminating stray light, always reading the same part of the screen at the same angle, and reading evenly lit areas bigger than the metering spot.

The main application for me is in calibrating lens masks so that I can assign them a nominal T stop. A typical lens mask is a black card opaque except for a thin transparent cursive line cut into it that spells out the name of my portrait subject. The technique is to photograph the subject in front of a background with numerous out of focus pin point specular highlights. The out of focus highlights take the shape of the diaphragm in the optical system; the subject's own name!

Many guess how the "trick" is done but so far none have guessed right. My secret is safe on APUG, right?