Roger, roger on that. Thanks.
Yes, I did address the matter of linear and non-linear in my
Originally Posted by Donald Miller
post just previous your last.
Howard Bond agrees with you; compression in the lower zones.
But Howard in graphing of the curves, pre vs no pre-exposure
has moved the pre-exp. curve speed point one zone to the
left, ie he has doubled the speed of the film or paper.
To put it another way his pre-exp. start of toe is at half the
EI as his start of toe for the post pre.; the for real exposure.
In a previous post this thread I did mention the increase in
EI when using pre-exp.
In effect a pre-exp. of zone 1 places the post zone 1 exp. at
a zone 2 density. That density is towards the more linear
portion of the curve. Mr. Bond sees a compression which
is not really there. Dan
I recognize what you are saying about the effect of preexposure reducing the EI. While that is true for the lower zones it does not translate linearly to the upper zone exposures.
Originally Posted by dancqu
It's interesting since what I am about to state is what Howard told me at a workshop almost twenty years ago.
If we recognize that each stop of increased exposure amounts to a doubling of light then we can readily assign a numerical value to those stops for the purpose of illustrating the effects of pre exposure to non image bearing light This is what I have done in the following.
The pre- exposure in this example is at a zone I luminance to non image bearing light. I doubt that the post will have the columns neatly aligned as I have composed them. I would ask that you sort them out in a understandable format.
Zone Numerical value Pre-exposure New Value
I 1 1 2
II 2 1 3
III 4 1 5
IV 8 1 9
V 16 1 17
VI 32 1 33
VII 64 1 65
VIII 128 1 129
As one can see the relative effects of pre-exposure are greatest in the lower zones and virtually non existent in the upper zones. My negatives bear this out.
You've got that bassackwards. Pre-exposure increases the EI
Originally Posted by Donald Miller
as one might expect and as H. Bond graphicly depicts. Dan
That last post of mine may need some explanation.
The film or paper does not magicaly become more fast.
The effective increase in speed is due to the pre-exposure.
I'd think pre-exposure most usefull with very high contrast
scenes or when for some reason an extra stop in film speed
would be nice to have. Otherwise give the film an extra
stop or two. That alone will lift the shadows into a
more linear portion of the curve. Dan
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Notice in the table that Dan made three posts up, the data he gives for a fogging pre-exposure of the film is exactly the same as that which would be seen from the effects of lens flare.
Look in a copy of "Basic Techniques of Photography" by Ansel Adams, John Paul Schaeffer, in the first edition (I'm don't have the revised edition so it may be different), Schaeffer has two examples that demonstrate the use of preexposure.
One example is to reduce the contrast of a high contrast scene by using the fog to compress the density range in the low values - see Dan's chart above. Same effects as seen from lens flare.
The other example he shows for using a fogging preexposure is for use with low contrast scenes. I believe this example used a greater exposure than the previous one and was used to give a slight increase in tonal separation by pushing the lower zones much higher up onto the film curve so that they were no longer on the toe area. I suspect this was more benificial on films that have long toes, like Tri-X.
So I believe, that based on the amount of pre-exposure that is given, the type of film, and the original scene contrast, this technique could be used for two different effects.
I have actually preexposed as high as a Zone IV and then exposed for highlights with normal development.
For low contrast scenes why not just give more exposure to get off the film toe? This gets you up on the straight line portion. And without the contrast reduction in the shadows that results from pre-exposure.
Ansel has a chapter dedicated to this subject in "the Negative" (Filters and PreExposure). He explains the lighting/contrast conditions where this is most appropriate and compares the process results to overexposure.