Pre Exposure Technique?

[dancqu]

Perhaps a mental block on my part but I am
having difficulty visualizing a proper method
for determining correct exposure.

I know some utilize pre-exposure and A. Adams
did make use of it. Any clues would help. Dan

 

[CBG]

Try it and see.

For a simplest case - Get a white or neutral card Meter it by reflection under the predominant light on your subject. That would if not adjusted make it come out in the middle of the scale - zone 5 - but what you want is to add a slight overall exposure that will boost the film jsut above the threshold so that faint shadow detail will be visible, even if it is below zone 1.

So, a good start would be to reduce your exposure for the card four stops so it should just barely get above the threshold - ie zone 1. Put that card in front of the camera and shoot it - out of focus - then shoot thew subject.

Test shoot at zone 2, zone 0 - reduce the card's exposure three stops or five from metered... Make notes and put marked cards in the scene so you will be able to tell which shot was produced how.

Best,

C

 

[DrPablo]

I do it with a gray card, much as described above. I usually pre-expose on zone 3, sometimes on zone 2. For a high DR scene, and especially with slide film, a zone 3 pre-exposure adds a lot of luminosity to shadows. But you may want to avoid pre-exposure if you have big, broad shadow areas, because it may make them look fogged -- it's better for things like the underside of foliage.

 

[Bruce Osgood]

My understanding of pre exposure is to use a translucent material, such as a Pringle potato chip tube top, and cover the lense. Point at the subject to insure correct light and add exposure by the amount you wish. ie: If your metered exposure is 1/125 @ f-8 you would expose 1/125 at f-4 thru the transluent sheet and then make your exposure of 1/125 @ f-8 on top of that -- a double exposure.

The pre exposure is to add more density to the negative throughout its' spectrum without changing the ratio of hi to low.

 

[DrPablo]

I've always used a gray card, and Ansel's description of pre-exposure was with something opaque. It doesn't matter, though, as long as you're taking a reflective metering off of it, it's completely uniform, and (if you're shooting color) it's neutral.

A caveat with this last one is that you can use preexposure with something yellow to warm up the shadows on color film.

 

[Mick Fagan]

I have done very much what Bruce (Camclicker) has mentioned. However I used a Wallace Expo/Disc.

The Wallace Expo/Disc (I have two sizes 52mm & 72mm) came in quite a range of sizes. The original intention of this was for turning your camera meter into an incident meter.

The basics are that you put the disc on the front of the lens, point the camera to the light source then adjust your settings and you will have a perfect highlight setting for slide film. Of course you can extrapolate shadow settings and many other things as well.

Where the Expo/Disc really shines though, is it's colour neutral light transmission. The discs have an outer surface designed to spread light evenly, an inner with different filtering material, finally there is the rear which is semi opaque and also colour neutral.

My two units came with a calibration chart giving the actual deviations of transmission and colour.

To give you an idea this is the standards sheet I received with one of the discs:-

XMSSN: 18% +- 1/6 stop. Followed by a hand written D/w = 0.78 Which I don't understand.

COLOR: 04 R / 00 G / 00 B (This is the deviation from neutral of the three colour filters I assume)

The above data was carefully taken with a BMI color densitometer frequently checked against a National Bureau of Standards step wedge.

Now to what the instructions that came with both of my Wallace Expo/Disc units say about Flashing.

Pre-Exposing or "Flashing" Film.

Photographers sometimes flash film before using it in order to reduce overall contrast or to modify colour in the darkest areas - without altering the mid-range and lighter image tones. This is particularly useful when working with colour slide film.

1. Mount your Expo/Disc in front of your camera lens and point it in any direction, depending upon the colour of flashing required.

2. Adjust the exposure settings to provide for "normal" exposure, and then increase the aperture number or shutter speed by four full stops.

3. With the Expo/Disc still in front of the lens, trip and re-cock the shutter without advancing the film.

Note: When they can anticipate the colour of flashing they will be wanting, some photographers pre-expose entire rolls of film in advance.

End of the instructions!!!!!!!

I have myself, done all of the above with great success. My country has extremely contrasty lighting, using a whole roll of flashed film does work in lowering the contrast, making colour printing much easier. I have mainly used it with colour negativefilms. But I do know it works just as well, with slide film.

I believe that the current Wallace Expo/Disc units are shipped without these instructions. Mainly they are sold to get correct white balance for electronic cameras which have great difficulty in getting, an absolutely correct, white balance. The Expo/Disc is perfect in fixing that.

Mick.

 

[richard ide]

I set up a very dark safelight about 12 feet from the bench where I load film holders. I made test exposures to give me between .05 and .10 density above base fog. I can flash film before or after exposure with no calculations and repeatable results every time.

 

[dancqu]

...a zone 3 pre-exposure adds a lot of luminosity to shadows.

Effectively the film speed is boosted one two stops? Dan

 

[DrPablo]

Effectively the film speed is boosted one two stops? Dan

Only the shadows get a boost, so you can't really talk about a change in film speed. Pre-exposure lowers contrast by virtue of elevating shadow values. The higher your preexposure the more of a boost in the shadows.

I understand the rationale behind doing it with test exposures, but honestly this is very easy. I just take a gray card, meter it in the light just in front of the camera, drop exposure by 2 stops (for zone 3) or 3 stops (for zone 2), then take a picture with it out of focus in that same lighting. Pretty simple.

 

[Lee L]

I have used the Wallace Expo-Disc in the same way and to the same effect as Mick describes. It's now being marketed somewhat differently, for digital color balance purposes, and there is a "digital" version, which is somewhat different physically, but as far as I can tell, would perform the same with film as the original that I have. The price has also gone up significantly. I have one that fits the majority of my SLR lenses and a larger one for my 6x9 rangefinder. I also use them pressed against the front of lenses with other filter sizes. As Mick noted, the Expo-Disc was initially designed to provide an incident reading with in-camera meters and an exposure at the camera reading gives you the equivalent of a gray card exposure for color balancing purposes in the darkroom.

In my Adams books, the preflashing device most prominently featured is not opaque, but translucent, using two pieces of square milky white plex (perspex) with a cardboard spacer between them around the perimeter. This is used pressed right up against the front of the lens. The spacing prevents hot spots and allows one to insert gel filters to color correct the shadows to some extent with color film. I've used one of those to good effect as well. You could do the same with an Expo-Disc and a glass or gel filter. Since optical Wratten gel filters have gotten so expensive now, I'd suggest Lee or Rosco theater gel filters for these pre-exposure color corrections.

You can also use these same tools, held against the enlarger lens, for pre-flashing paper in the darkroom with the negative left in place in the enlarger.

Lee

 

[Jim Noel]

A styrofoam coffee cup, or the lower part of one, makes an excellent diffuser through which to meter and pre-expose. Just don't leave it at the scene as trash.

 

[dancqu]

Only the shadows get a boost, so you can't really talk
about a change in film speed. Pre-exposure lowers contrast
by virtue of elevating shadow values. The higher your
preexposure the more of a boost in the shadows.

I understand the rationale behind doing it with test exposures,
but honestly this is very easy. I just take a gray card, meter it
in the light just in front of the camera, drop exposure by 2 stops
(for zone 3) or 3 stops (for zone 2), then take a picture with it
out of focus in that same lighting. Pretty simple.

I'll doodle some curves for a better understanding. In the
past I've thought of it as placing low level subject values
above or higher on the toe. Dan

 

[dancqu]

Perhaps a mental block on my part but I am
having difficulty visualizing a proper method
for determining correct exposure. Dan

My mental block concerned a tie twixt the metered
correct scene exposure and the correct pre-exposure.
DrPablo's method of determining correct pre-exposure
by metering and using a gray card in what ever level
of lighting reminded me that two exposures are
independent of one another.

His method though requires another metering and a card.
An alternative might be a fabric weighted at bottom and
at top a grip. The whole would roll upon itself and be an
easy carry when off trail through rough terrain.

Another alternative would use calibrated diffusers;
the styrofoam cup method. A second metering would
not be needed. Simply place the correct diffuser over
or in front of the lens for a pre or post exposure. Dan