Multiple exposure and prefoging

[Silver key]

Hello ! This one is a weird one but you'll see why in a few weeks. So basically, it's a "no questions asked" !

If i prefog on Zone V how many stop should i underexpose the next frame to have a normal exposure in total ?

In other words :
1st exopusure : normally exposed grey card
2nd exposure : normally exposed subject - X stops
Total exposure : normally exposed subject
X : ?

I'm guessing -2 stops but i just want to make sure.

Thanks for the help in advance !

 

[cowanw]

This sounds like fun as a mental exercise.
If your first shot is a normal exposure of a grey card then in total your highlights can be no whiter than middle grey.
If you then meter a subject of a 3 stop range and give it a normal exposure the middle grey zone will be will be zone 4, your highlights should be zone 4+a bit and your shadows zone 4 minus a bit.

Or not.

 

[PanaDP]

I believe you've done it backwards, cowanw. With a zone V exposure, shadows can be no deeper than middle grey but highlights can go denser. Think of those zones like empty glasses of water and exposure like adding water that you can never remove. An even zone V fills every glass halfway up.

 

[Silver key]

So there is no way i can go back to a normal exposure ?

 

[cowanw]

PanaDP
Well I have been thinking about it all night and got off on a tangent of combining negatives, but that is not what we are doing. The first exposure will make the negative middle grey. The second exposure will only add density, but the grey and white will add more density than the black. When the resulting negative is printed, so that the middle grey of the negative is middle grey on the print, the black area of the negative will have less density than the grey or the white and print darker.
In the glass analogy, the process of printing will move the zone V mark and will be measured from the top of the glass as a reversal (sort off).
Silver key
This is not prefogging in the usual sense. and not you cannot.

 

[RobC]

when you flash a negative it affects the shadows and not the highlights(not significantly enough to see it). When you flash a print it affects the highlights and not the shadows(not significantly enough to see it).

Its like fill flash, it lifts the shadows but not the highlights.

what your second exposure will result in will depend on its contrast range and where you place it on the curve but nothing in the result will be less than middle grey due to your first exposure which is added to by the second exposure.

 

[RobC]

So there is no way i can go back to a normal exposure ?

Correct. The result will be middle grey and lighter but nothing darker than first exposure.

 

[Silver key]

Thanks a lot guys, i'm glad i asked !
@Cowanw : i know, doing something different is the aim !
@RobC : thanks for the great piece of added information.
I need to rethink the technique but i think that what i want to achieve can be done while i'm Printing, not when i'm Shooting !

 

[RobC]

Just to be clear, flashing a negative pushes the shadows up (lightens them). Flashing paper pushes the highlights down (darkens them).

 

[DREW WILEY]

If you prefog for Zone V itself, your image is likely to have poor definition and relatively horrible tonal separation. But if you add .60 (2 stops) of neutral density to your flashing diffuser, that equates to Zone III anyway, which is a more realistic target. In general, I happen to find flashing to be a very poor choice for handling excess scene contrast. With black and white film, it's way better to simply have the correct film and develop it properly for the contrast. Once in awhile I will employ the equivalent to a Zone III flash with color neg film, in combination with a warming filter to correct the blue bias of shadows in open sun without affecting the color balance of the midtones and highs.

 

[RobC]

I think that if you want to lift the shadows on film then using fill flash is the way to go. But it sounds as though he got it round the wrong way and really wants to keep his highlights under control.