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Flashing Stock Questions


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#1 ben g

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Posted 22 May 2009 - 07:03 AM

Hello,

I am looking at doing some tests with pre flashing stock. However, before i do this i want to get my head around a few things.

Firstly, how do i determine d-min for a given stock. When i look at characteristic charts for a stock it gives info for blue, green and red layers separately. How do I find the d-min of a stock? Am i just supposed to pick a colour (green is in the middle) and go off that?

I am going to flash the stock by shooting a grey card.

Now say a stock has a d-min of 0.50, i want to do a 0.10 flash. How do i find out how many stops under to shoot the grey card to get a 0.60 density?

Obviously this is going to vary depending on stock (and batch). I plan to shoot on kodak vision 3 500T 5219.

Any help would be appreciated.

Thank You.
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#2 Simon Wyss

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Posted 22 May 2009 - 10:28 AM

Now say a stock has a d-min of 0.50, i want to do a 0.10 flash. How do i find out how many stops under to shoot the grey card to get a 0.60 density?

Ben, what exactly do you mean with a stock has a d-min of 0.50? Is it not rather a definite value you are talking of? Do you know that these are logarithmic values, so D = 0.50 means 68.4 % opacity and 31.6 % transparency. D = 0.10 corresponds to 20.6 % opacity and 79.4 % transparency. We have the factor 3.3 between .1 and .5 log D, three and a third stops.
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#3 Simon Wyss

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Posted 22 May 2009 - 10:43 AM

We have the factor 3.3 between .1 and .5 log D, one and two third stops.

Editing is out of order. :angry:
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#4 ben g

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Posted 22 May 2009 - 08:31 PM

We have the factor 3.3 between .1 and .5 log D, one and two third stops.

Editing is out of order. :angry:


thank you for your help, unfortunatly i think it is over my head, do you know of any good material/articles i can read to understand this a bit better?
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#5 Brian Pritchard

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Posted 23 May 2009 - 04:37 AM

Hello,

I am looking at doing some tests with pre flashing stock. However, before i do this i want to get my head around a few things.

Firstly, how do i determine d-min for a given stock. When i look at characteristic charts for a stock it gives info for blue, green and red layers separately. How do I find the d-min of a stock? Am i just supposed to pick a colour (green is in the middle) and go off that?

I am going to flash the stock by shooting a grey card.

Now say a stock has a d-min of 0.50, i want to do a 0.10 flash. How do i find out how many stops under to shoot the grey card to get a 0.60 density?

Obviously this is going to vary depending on stock (and batch). I plan to shoot on kodak vision 3 500T 5219.

Any help would be appreciated.

Thank You.


Ben
The figures on the characteristic curves are the red, green and blue d-min. When you are dealing with flashing you need the visual density which measures all three layers. On the densitometer the visual filter is used for B/W.

If you want to be very precise (as it appears you do) you need to read the visual density of processed unexposed stock, then shoot your gray card at normal exposure and measure the visual density after processing. If the gray card has a density of 1.0, the Dmin is 0.5 and you want a flash of 0.1, then the difference is 0.4. The gamma of the neg is 0.6 so the change in exposure is 0.4 X 0.6 = 0.24 LogE. 0.3LogE is 1 camera stop so you would need to underexpose by 2/3 of a stop.

If you are able to take the measurements I would be happy to make the calculations if you are concerned about them.

The density you obtain from your gray card will depend on the reflectance of it, they are usually 16% reflectance and, of course the exposure you give it.


Brian
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#6 Brian Pritchard

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Posted 23 May 2009 - 04:50 AM

Ben
The figures on the characteristic curves are the red, green and blue d-min. When you are dealing with flashing you need the visual density which measures all three layers. On the densitometer the visual filter is used for B/W.

If you want to be very precise (as it appears you do) you need to read the visual density of processed unexposed stock, then shoot your gray card at normal exposure and measure the visual density after processing. If the gray card has a density of 1.0, the Dmin is 0.5 and you want a flash of 0.1, then the difference is 0.4. The gamma of the neg is 0.6 so the change in exposure is 0.4 X 0.6 = 0.24 LogE. 0.3LogE is 1 camera stop so you would need to underexpose by 2/3 of a stop.

If you are able to take the measurements I would be happy to make the calculations if you are concerned about them.

The density you obtain from your gray card will depend on the reflectance of it, they are usually 16% reflectance and, of course the exposure you give it.


Brian

Of course if you do not want to do these calculations then you just need to make a series of exposures of your gray card from normal reducing by 1/2 a stop each time. Ask your processing lab to measure the visual density of the dMin and each exposure. You only need to shoot enough frames to ensure that the camera is up to speed. You will then be able to get a close estimate of the required exposure. It didn't say on your post where you are from; if you are from the UK and you can't get the lab to do the measurements then do the exposure and get it processed and I will measure it for you if you send it to me.
Brian
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#7 Dominic Case

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Posted 24 May 2009 - 05:51 PM

Ben, what exactly do you mean with a stock has a d-min of 0.50? Is it not rather a definite value you are talking of? Do you know that these are logarithmic values, so D = 0.50 means 68.4 % opacity and 31.6 % transparency. D = 0.10 corresponds to 20.6 % opacity and 79.4 % transparency. We have the factor 3.3 between .1 and .5 log D, three and a third stops.

I'm sorry to be blunt and direct, but this information is (i) confusing; (ii) unnecessary to answer the question; and (iii) completely wrong. In fact it is SO wrong, I feel the need to correct it.

The concept of density was introduced (well over a centory) ago as it was useful to describe the behaviour of film emulsions simply. It is also a simple unit to work with: densities can simply be added, whereas tranparencies (or transmittances)have to be multiplied. Opacities are even harder. Simon, what is the combined effect of a 68.4% opacity filter laid over a 20.6% opacity filter? Quickly now! Pass? OK what is the combined effect of a 0.50 density filter and a 0.10 density filter? Answer, 0.60 density.

As far as Simon's example calculation is concerned, yes it's true that the opacity of a 0.50 filter is 3.3 times that of a 0.10 filter (assuming that the figures of 68.4% and 20.6% opacity are correct. )That doesn't tell us much about their relative densities.

In fact, if you insist on using the linear terms rather than the simpler log terms, you need to compare the transmittancies: 31.6% and 79.4%, whch have a ratio of about 2.5:1.

A ratio of 2:1 would be a difference of one stop. 2.5:1 is about a stop and a third, which you would only know if you know your log tables. Much easier to subtract 0.10 from 0.50 and know that the difference, 0.40 is about a stop and a third (0.30 is one stop).

But this is still only the difference in densities. Not only are you comparing the wrong numbers, you are overlooking the factor of the gamma of the negative stock - which Brian brings in to the action, straightforwardly and simply.

Good information on this site may be useful to one or to many people. Bad information is worse than no information. If you don't know the difference, please don't post.
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#8 Paul Bruening

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Posted 25 May 2009 - 07:53 AM

You know, Dominic, a comprehensive set of stop-based tables for every stock type would be enormously helpful to so many shooters who just want to know what stop to set for their film to get a dependable and predictable amount of flashing. Our humble bowing, worship and offerings of agricultural products, even the throwing of virgins into your volcano would go up if you could chart this out and provide it in another book.
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#9 Dirk DeJonghe

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Posted 25 May 2009 - 10:37 AM

It has been a long time since I did this (flashing Ektachrome 7242 to match Ektachrome 7252) but this is how I did it without too much calculations.

Set up a grey chart with light at 45% angle so as to not reflect light. Defocus so the grain of the cardboard is blurred.
Do a series of test exposures starting from minus 4 stops going up 1/3 of a stop at a time. Note everything carefully.
Mesure the negative D-Min and measure each test;
Subtract the density of the D-Min from the density of the test. If you get a Delta of 0.10D you could call it a 10% flash, sounds like you know what you're doing, etc.

Shoot a few scenes with various flashing levels and have them printed or transfered. Take your pick.
If you preflash, don't do it too long beforehand. Preflashing helps to get more shadow detail.
Don't forget to rewind the preflashed negative in a darkroom. Flashing original camera negative is risky business.
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#10 Simon Wyss

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Posted 25 May 2009 - 10:59 AM

Dominic, I am getting out of the way. You are so right.
Yet the initial question is not answered, and it is not an easy one to satisfy, IMO.

“Firstly, how do i determine d-min for a given stock.”

What does Benjamin actually want?
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#11 Dominic Case

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Posted 25 May 2009 - 05:13 PM

Paul wrote:

You know, Dominic, a comprehensive set of stop-based tables for every stock type would be enormously helpful to so many shooters who just want to know what stop to set for their film to get a dependable and predictable amount of flashing. Our humble bowing, worship and offerings of agricultural products, even the throwing of virgins into your volcano would go up if you could chart this out and provide it in another book.

Along with others in this strand (Dirk & Brian) I am convinced that any special technique such as flashing, forcing, bleach-bypassing etc, should be done on the basis of testing. While it may be possible to shoot a grey card at two stops under and get a precise and predictable grey density, it's more important to see what effect that density has on your image. The visual effect will depend very strongly on what the image is like.

Doing it purely by numbers is like advising someone that the negative density of an image will be about 0.18 less if they underexpose by one stop. Yes, but that tells you nothing about what the image will look like - in the shadows, in the saturated colours and so on.

So please ignore the mass of numbers and calculations in my previous post, unless you really want to work with sensitometry. I just wanted to correct an earlier post.

So Paul, please save the virgins for a more deserving cause - they are a precious commodity :)

Dirk wrote:

Set up a grey chart with light at 45% angle so as to not reflect light. Defocus so the grain of the cardboard is blurred.

Good advice throughout. By the way, if you don't have a 18% grey card, you could use a white one - so long as it is perfectly matt. You'd need about 2 or 2 1/2 stops less than the grey card, assuming a reflectance of 90% to 95%.

Flashing original camera negative is risky business.

It certainly is, and that is why a test is some important. A mistake in the relatively unfamiliar flashing operation will stuff up your shoot.

Finally, Simon correctly points out:

Yet the initial question is not answered, . . .
"Firstly, how do i determine d-min for a given stock.”
What does Benjamin actually want?

Quite simply, look at the curves on the manufacturer's website - the D-min is the lowest value of density - at the left hand end of the characteristic curve - or ask the lab to measure it on the stock that you are using. Dmin values do vary from stock type to stock type but typically you will get 0.17 to 0.22 Red, 0.55 to 0.65 Green, 0.90 to 1.10 Blue.
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#12 ben g

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Posted 26 May 2009 - 01:37 AM

Thanks everyone for their replies.

I think my initial problem was not understanding how to read the characteristic curve of a stock properly, and a general non-understanding of flashing, negative density, etc.

However over the last couple of days thanks to a lot of reading, and everyones posts, im pretty sure i understand how to flash a negative. I understand that testing is still important, but i just wanted to get my head around the fundsamentals, and understand what is ment when someone says they want to flash 20% (or 0.20) [i understand that % is not correct, but it seems like a lot of people refer to it that way].

I actually did some tests yesterday using 35mm stills photography (in preparation for the real test) and i had some really great results.

So once again, thank you all.

Benjamin Gilovitz.
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#13 Dominic Case

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Posted 26 May 2009 - 06:11 AM

So once again, thank you all.

Happy to be of some assistance.
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