Expodisc question

Just got an expodisc and am having trouble with the exposure part .. the user guide says that the correct exposure reading will be obtained with the expodisc in place, but the expodisc absorbs two stops of light .. so you just add two stops to get a correct exposure, no problem.

But why does the user guide say a correct exposure is obtained with the disc in place and no exposure compensation is mentioned?

I get a +1 stop difference using my balens WB cap, but it doesn't seem to affect the WB results

I think the user guide that comes with the expodisc was pretty limited. Their full manual is here: http://www.expoimaging​.com …es/instructions​/EN_17.pdf

Scroll to: 2.d. Metering for Exposure with the ExpoDisc

And: 1.g. Correctly Exposing the Custom White Balance Capture

Brandon72 wrote in post #13919087
I think the user guide that comes with the expodisc was pretty limited. Their full manual is here: http://www.expoimaging​.com …es/instructions​/EN_17.pdf

Scroll to: 2.d. Metering for Exposure with the ExpoDisc

And: 1.g. Correctly Exposing the Custom White Balance Capture

Thanks .. that's a lot more informative than what came in the package. In that guide it says:

1. Stand in the same lighting as your subject (although not necessarily beside it) and put
your camera in its manual exposure mode and mount your ExpoDisc in front of the
camera.
2. Then point the camera and ExpoDisc in the direction directly opposite that in which
you will be shooting to make the exposure.
3. Turn on the camera meter, adjust the aperture and/or shutter speed settings until the
camera meter indicates good exposure conditions.
4. Return to your shooting position (if you left it to do your metering) and remove the
ExpoDisc from in front of the camera lens.
5. Aim the camera, check the framing, and shoot.

Doing the above results in two stops underexposure, and my Sekonic meter shows two stops of light loss when the Expodisc is placed in front of it as well.

So the Expodisc loses two stops of light, which is not surprising since it's about a quarter inch of plastic. What is surprising is nowhere in their documentation is there mention of this light loss .. in fact my understanding of their instructions is to just place the Expodisc on the lens, and your camera's meter reading will be correct. Which is not the case.

Yes but, that's how it's able to set a proper exposure with it. "Calibrated 18% light transmission for exposure reference" - it acts as an 18% grey card.

The steps you quoted are for a front-lit subject. Stand exactly where your subject will be standing. Put the expo-disc on. Point your camera towards the light source and dial your in-camera meter to zero. Stand between the subject and the light source and take a picture of your subject. Your exposure should be within 1/3 of a stop or so of the proper exposure.

The methods for side-lighting and back-lighting are similar but slightly different.

This works for me. Is that what you did, and it still came out 2 stops underexposed? I'm just wondering if you maybe read "Then point the camera and ExpoDisc in the direction directly opposite that in which you will be shooting." in a different way and pointed away from the light source or something?

Thanks Brandon for your input on this, here is what I do:

Standing at the exact same position as my subject, placing the expodisc on the lens, aiming the lens back toward the light source (and my eventual shooting position) and setting the cameras exposure to the exact correct exposure .. then going back to the light source (ie. directly opposite to where I took the reading) and shooting the subject in the exact same place (and exposure) I took the original reading from results in two stops underexposure. Everytime.

And as far as I know this is the correct procedure, and the procedure which should result in a correct exposure +/- one third of a stop, according to Expodisc's user guide.

But I don't need to go through all this to know the Expodisc incident exposure reading will be wrong .. taking my Sekonic L358 light meter with the lumisphere retracted so it just measures just very direct incident light I get a reading two stops lower with the Expodisc placed in front of the meter that I do without it.

So incident light reaching the camera with an Expodisc in front of the lens is reduced by two stops .. why do they not tell you to adjust your exposure plus two stops to get an accurate exposure when you take off the disc and take a photo?

mmahoney wrote in post #13919728
taking my Sekonic L358 light meter with the lumisphere retracted so it just measures just very direct incident light I get a reading two stops lower with the Expodisc placed in front of the meter that I do without it.

The Sekonic Lumisphere is already doing what the Expodisc also does. It's cutting light to give you your 18% grey reading. Placing your expodisc over it means you're cutting out twice as much light than you should be to correctly set your exposure according to 18% grey. So of course it will come out as two stops lower.

Using your Sekonic meter with the Lumisphere extended, take your incident light reading (without the Expodisc over it).

Then, from the exact same position, place the expodisc on your camera and dial in the settings your Sekonic meter gave you. Point your camera towards the light source just as you did with your Sekonic meter. The in-camera metering should be at 0 +/- 1/3rd of a stop. What does your in-camera meter say when you do this?

Because you shouldn't have to. It should be giving you a proper exposure reading.

Brandon72 wrote in post #13920016
The Sekonic Lumisphere is already doing what the Expodisc also does. It's cutting light to give you your 18% grey reading. Placing your expodisc over it means you're cutting out twice as much light than you should be to correctly set your exposure according to 18% grey. So of course it will come out as two stops lower.

Using your Sekonic meter with the Lumisphere extended, take your incident light reading (without the Expodisc over it).

Then, from the exact same position, place the expodisc on your camera and dial in the settings your Sekonic meter gave you. Point your camera towards the light source just as you did with your Sekonic meter. The in-camera metering should be at 0 +/- 1/3rd of a stop. What does your in-camera meter say when you do this?

Because you shouldn't have to. It should be giving you a proper exposure reading.

You are right sir .. maybe I should stay away from math & meters until I've had my coffee. Anyways Brandon, thanks for explaining this .. I was missing the 18% grey part