Having acquired some Sylvania Press 25`s, I hope to take some flash with my SG.
Unfortunately, The exposure guide printed on the box shows only Tungsten film speed.
Can anyone tell me how to convert Tungsten to ASA, as the rollfilms I am currently using are rated 400 ASA, assuming that these speeds are different?

Okay, I'm just too tired to figure out the speeds It may also depend on the age of your chart as they did double or so the speed rating somewhere back then...

But, my trusty old kodak flash chart shows a guide number of 500 for the 25's in a polished bowl for an ASA 400 film...

You can also read:
http://graflex.org/flash/technical.html

And then maybe someone else can offer more info

While all of this falls into the space of latitude, films have two speeds, one for daylight and one for tungsten.

Good old Tri-x was still blue sensitive so it's Tungsten ASA rating was lower than it's daylight ASA rating.

On the other hand old style ortho film was blue blind, and using blue bulbs and ortho film got you no where fast.

So check with whomever made your film as see if the Tungsten ASA is close. If you can't go with the Daylight ASA.

Thanks guys for your help. It`s got me started.

Hi, Brian! The old films were rated about a stop slower for tungsten light. A check of the film manufacturers web site should provide all the speed rating data for your film. I don't have the site address for Bill Cress but I believe he has current speed and guide numbers listed for current films. A web search for flashbulbs should bring it up. By the way, I believe clear bulbs are rated as tungsten light. Worth a check.

Quote:

On 2005-08-15 22:15, Les wrote: On the other hand old style ortho film was blue blind, and using blue bulbs and ortho film got you no where fast.

Les:

You have it backward. Ortho film is blue & green sensitive, and RED blind. You need blue bulbs to be able to use the daylight speed rating of ortho films, with clear bulbs you need to use the tungsten speed rating.

-Mike

...for example, "Tri-X" is rated at 400 daylight, 320 tungsten (or was, the last time I looked).

As I recall, clear flashbulbs had a color temperature of 3800° Kelvin, cooler than tungsten (usually 3200°K.) but warmer than daylight. Thus, one would expect the film to be a tiny bit more sensitive to a clear flashbulb than to a photoflood lamp.

As a practical matter, a couple of exposure tests ought to tell you whether this theoretical difference in density makes any practical difference at all, any difference in density that you really can see.

Some of us used to "overexpose" large-format "Tri-X" by continuing to use the older ASA indices (200 and 160), because this improves the tonal scale -- of course at the expense of some grain buildup.