Test strip negatives to determine ISO, is this viable?

[Ed Bray]

I puchased some 4x5 Toyo Film Holders (7) which were pretty much like new, they were all sent dark side showing of the Dark Slide so was surprised to find when I opened one up that it had film in it. The film has a shallow semi-circular notch about 8mm wide and 2mm deep, the film has a green/grey emulsion with a very dark grey back. I then took all the holders up to my newly created darkroom (just absolutely light tight not set up as a proper darkroom) and checked them all, they all are loaded with the same film stock.

I have since done a bit of searching on the interweb and it would seem that a lot of suppliers have supplied sheet film with just a single semi-circular notch and the same manufacturers have even used the same notch for different film speeds. All I know at the moment is that it is not Shanghai 100 as even though that also has a semi-circular notch the film has a blue colour which is completely different to this film/

So, as I do not know what the film speed is I was thinking of doing an 4 strip test (ISOs 25, 50, 100 & 400) on one of the sheets of film with each strip based on part of an overall exposure to try to determine the ISO.

Assuming the exposure is based initially for an ISO400 film and with a constant aperture, I realise that I would have to have a number of varying exposures to build up the required exposure for the strip, assuming an aperture chosen to allow 1/200sec at ISO400 the film would be exposed, then the darkslide would be inserted about an inch (giving the strip an exposure for ISO400), for the following strip I would need another exposure of 1/200sec plus a further exposure of 1/100sec before pushing the darkslide in for the strip (giving enough exposure for a speed of ISO100), the following strip would have already had exposures giving ISO100 before a further exposure of 1/50sec which would give an exposure of the remaining uncovered film equalling a film speed of ISO50 before covering up the strip, the last strip will receive a further exposure of 1/25sec which would then give an exposure equal to that required of an ISO25 film.

So just to make sure I have got this correct:

ISO400..........ISO200..........ISO100..........ISO50..........ISO25

1/200............1/200............1/200...........1/200...........1/200
......................1/200...........1/200...........1/200...........1/200
.........................................1/100...........1/100...........1/100
.............................................................1/50............1/50
................................................................................1/25

I wouldn't bother with the ISO200 as not many films have been made at that speed. Speeds in Bold are the exposures required to complete the next strip.

So, provided I secure the film with some double sided tape (made less sticky first) does anyone see any problems with my suggestion? If so, any better alternatives other than shooting 4 seperate sheets at the various ISOs?

 

[Rich Ullsmith]

You are assuming that the films in the holders have not been exposed?

Can't see why your dark slide trick would not work. But, why spend so much time learning the speed of a film that you only have a few sheets of, which may or may not have been stored in a glove box for many years.

Perhaps if you develop a sheet, 10m @ 75 in whatever developer, then read what's on the border.

 

[Ed Bray]

You are assuming that the films in the holders have not been exposed?

Can't see why your dark slide trick would not work. But, why spend so much time learning the speed of a film that you only have a few sheets of, which may or may not have been stored in a glove box for many years.

Perhaps if you develop a sheet, 10m @ 75 in whatever developer, then read what's on the border.

Yes, I was, it was only a bit later that I realised that they could have been exposed and not processed.

Well, I have 13 sheets at present if they are not exposed, one for the ISO trial which if they are not exposed would leave me 12 to play with. I have been given a better way to do the trial by making a mask for the film-holder with a quarter cut out, then turn it for each of the 4 exposures.

Now that is a good idea. I use Caffenol CL semi-stand for all my processing so I should not have a problem with the film processing depending on it's age.

 

[Doremus Scudder]

... I have been given a better way to do the trial by making a mask for the film-holder with a quarter cut out, then turn it for each of the 4 exposures.

Ed,

FWIW, I think your original method of inserting the darkslide partway with each exposure is much easier. I have a holder with a darkslide marked for precisely this purpose.

Best,

Doremus

www.DoremusScudder.com

 

[Ed Bray]

Hi Doremus, thanks for your contribution, a fraction too rate as I have just finished making the mask

It is possible that the film is Velvia RVP as that has the same notch code, time will tell.

 

[Ed Bray]

Yes, after all the messing around and processing the film all I can say for sure is it is not print film, either colour or b&w.

From the notch code it would appear to be Fuji Velvia Pro RVP50.