Measuring exposure with new fangled cap

[swmcl]

Hi,

I have purchased a couple of those new white lens cap thingys. They are a screw-on cap which is a white diffuser for getting the white balance on a digital camera. So they sort of act like a giant lumisphere on an incident lightmeter.

OK so I want to use it to do pre-exposures on my LF setup. I hope to put an ND filter underneath it and then pre-expose the film.

BUT !

How on earth do I take measurements ?? I have fitted the 77mm one to my Nikon film camera a F100 with an 85mm lens and it says f8 - 1000th - 100 ASA. I then retract the lumisphere into the head of the lightmeter (Sekonic L358) and take another measurement. It says f8 - 1/250 - 100 ASA which is 2 stops more exposure. The reflective head on the lightmeter reads f8 - 1/6400th - 100 ASA which is nearly 3 stops less exposure than the Nikon ...

So...

Is it possible to take a reading from the ground glass or something ? Perhaps I should setup the LF with a known good lens and take an incident reading with a retracted lumisphere on the ground glass ??

I must admit to being a bit of a goose with exposures sometimes.

This is particularly tricky for me.

Cheers,

Steve

Edit: I think I do want to refer to a reading from the ground glass if possible as this would be a 'constant' if I use the same lens and shutter. I absolutely do not know what I'm doing here ...

[Doremus Scudder]

You're making a mistake thinking your white-balance diffuser cap should make your in-camera meter match your hand-held incident meter. Not the case.

Just use your in camera meter, read through the diffusion cap and underexpose appropriately; minus five stops to give a Zone 1/2 pre-exposure, minus four stops to give a Zone I pre-exposure, minus three stops for a Zone II pre-exposure.

That should be all there is to it.

Do make sure you have your diffuser cap pointed at the most even light source/area you can find and focus the lens at infinity.

Best,

Doremus

[swmcl]

Thanks for replying Doremus.

I'm trying to use the white balance lens cap on LF gear. There is no in-camera metering.

My hope is to find a method to use this setup so that I can use it with confidence in the field.

Rgds,

Steve

[Mark Barendt]

Steve,

The only bit of info you are missing to do the math is the filter factor.

As a starting point I'd do a normal dome out incident reading then retract the dome then with the filter in place of the dome take a second reading. The difference is your estimated filter factor.

You could shoot two test sheets to verify that factor.

Once you have your factor just add that into the calcs that Doremus provided.

[Jim Noel]

Thanks for replying Doremus.

I'm trying to use the white balance lens cap on LF gear. There is no in-camera metering.

My hope is to find a method to use this setup so that I can use it with confidence in the field.

Rgds,

Steve

PLace the cap over the sensor of the meter. Take a reading and compute the exposure to give you 4 stops less than the meter indicates. Set this lens/shutter combination on your camera. Place the cap on the lens and make the exposure. Remove the cap and make your normal exposure. You will have pre-exposed the film on ZOne I.
Jim

[E. von Hoegh]

Hi,

I have purchased a couple of those new white lens cap thingys. They are a screw-on cap which is a white diffuser for getting the white balance on a digital camera. So they sort of act like a giant lumisphere on an incident lightmeter.

OK so I want to use it to do pre-exposures on my LF setup. I hope to put an ND filter underneath it and then pre-expose the film.

BUT !

How on earth do I take measurements ?? I have fitted the 77mm one to my Nikon film camera a F100 with an 85mm lens and it says f8 - 1000th - 100 ASA. I then retract the lumisphere into the head of the lightmeter (Sekonic L358) and take another measurement. It says f8 - 1/250 - 100 ASA which is 2 stops more exposure. The reflective head on the lightmeter reads f8 - 1/6400th - 100 ASA which is nearly 3 stops less exposure than the Nikon ...

So...

Is it possible to take a reading from the ground glass or something ? Perhaps I should setup the LF with a known good lens and take an incident reading with a retracted lumisphere on the ground glass ??

I must admit to being a bit of a goose with exposures sometimes.

This is particularly tricky for me.

Cheers,

Steve

Edit: I think I do want to refer to a reading from the ground glass if possible as this would be a 'constant' if I use the same lens and shutter. I absolutely do not know what I'm doing here ...

A simpler way might be to preflash your film under the enlarger.

[swmcl]

How would this work ... (overnight musings...)

Take the camera outdoors in the clear sunlight and set it up facing a fairly uniformly coloured wall - like the shed wall which is a wheat colour.

Put a lens on that has a good amount of coverage and is in a reliable shutter. eg. 180mm Sironar. Focus at infinity.

If the ground glass has say at least 80% of the uniform wall on it then a measurement can be taken in the centre of the ground glass with some reliability.

Take a reflective spot reading from the wall for the film speed setting. Set the aperture to that on the meter. Say f16.

Take a retracted-dome measurement from the centre of the ground glass with same ASA and aperture settings.

The difference in measurements is the compensation figure for that lens assuming inifinity focus.

Place the white lens cap on the lens leaving the aperture the same. Put an ND filter on as well - probably at least 2 stops. Point the lens at a known good full-spectrum light source (the sun).

Take a ground glass reading as before. Apply known compensations.

Adjust the shutter to get a Zone 1 exposure or whatever is required.

Comments much appreciated. This is doing my brane in!

Steve

[Doremus Scudder]

Jim has it right. Just put the diffuser cap over your meter diffusing dome and take a normal incident reading. Try to point the meter and the camera at an even light source (the sun is pretty good...). Underexpose by the appropriate amount to give the right pre-exposure (or post-exposure if you like, there's really no difference.

Best,

Doremus

[Andrew O'Neill]

This is how I do it: First of all, I use a thin piece of translucent white plexi as a diffusor. I face the scene I am photographing. I place the diffusor over top of the metre's lens. I use a reflective metre.
I take a reading. I transfer that reading, minus 4 (or more, depending on how much/less pre-exposure I want) to the shutter/apeture. I hold the diffusor up against the lens. Then I make the pre-exposure.
With your incident metre, retract the dome and place your diffusor over top. Point at your camera and take a reading. Transfer that reading minus whatever how many stops required say 4. Place your diffusor over the lens and Bob's your uncle...
Pre-exposing film in the field is easier in my opinion, and better than exposing under an enlarger as you can pre-expose the film on the spot and taylor it to the scene. If you expose under an enlarger (providing you have one), you could end up with unused film sitting in a holder until you use it. Film that you can only use for a scene that requires pre-exposure. Not good if you are out of non pre-exposed film and a potential image is screaming at you.

[Doremus Scudder]

Andrew,

I don't believe that retracting the dome on the meter will give you a correct reading. With the dome, the incident reading should place the lighting at roughly middle gray (Zone V). Reading through the diffusion cap (or your diffuser plex) with the dome in place should give a middle gray reading of the light being diffused. This is then what needs to be underexposed to place it properly for a pre-(post)exposure.

I say, leave the dome in place.

Best,

Doremus

[Drew Bedo]

I haven't read all the responses, so I'm sorry if this is redundent.

What about using a focal plane probe? Sinar-six and sinar Booster-1 (for minolta meter) come to mind. The current model is a sinar probe with Broncolor meter.

This would be in-camera metering on your LF camera.

[swmcl]

Hi guys,

I have been testing... but I will also test using the technique of Jim and Andrew.

I will let you know what I find.

Cheers,

Steve

[swmcl]

OK ...

My observations are recorded in the attached pdf.

The conclusions ....

Almost every meter will read the same scene and give the same reading. So too the lenses. They will all pass a very similar amount of light for a given aperture.

So ... the real variable in this setup is the brightness of the ground glass. Mine is a stock piece that came with the Chinese camera. This means that it will not be especially bright.

What I have found is that if I shoot into the sun with a new-fangled white cap on the lens I can take a reading (with the lumisphere retracted) from the centre of the ground glass and extrapolate this reading to a final shutter and aperture combination such that it produces a Zone 1 or Zone 2 exposure on the film. In my case, I need to shoot at 15 stops higher shutter speed than the one suggested from the reading into the sun to get a Zone 1 exposure through a white cap.

Cheers,

Steve