Flash Bulb Guide Numbers for Shooting Tmax 100 Film
Hi everyone. In another thread we are discussing Guide Numbers for using flash bulbs, specifically Sylvania 25 and 25b.
If anyone has experience with this and has actually run film using these bulbs (25 and 25b class M) could you please shed some light on the most ideal guide numbers to use when shooting Tmax 100 (4x5 and 35mm)? If you've found better bulbs and/ or guide numbers please share your experiences.
I will be running some tests to compare a few sources of guide numbers but I have a limited supply of bulbs at the moment so a tested starting point or a blatant solution would be hugely helpful! I'd like to waste a few of these bulbs as I have to so that I can use them for more serious shooting projects.
Just an FYI, I'm running Tmax 100 4x5 through a modified Speed Graphic Pre-Anniversary with a Carl Zeiss Jena 135mm Tessar lens and I'm running Tmax 100 35mm through an Argus C3. Using both Sylvania 25 and 25b class M bulbs.
Ok, I just placed a call to a friend who has more experience going back decades then anyone else that I know and he provided me with the following solution that you can apply to any bulbs that you come across:
Guide numbers are figured by measuring light at a specific distance relative to the ISO (ASA previously) using a light meter and then multiplying the f-stop reading by ten.
Process (for ISO 100):
Have an assistant hold a tested and trusted light meter with hemispherical (domed) attachment (I use a Sekonic) 10 feet away from you. Use a tape measure to be as accurate as possible. Make sure that you are in a room that allows you to control your ambient light. Don't do this outside or in a room with bright open windows or anything that will cause any excess ambient light to throw off the reading. Have your assistant, at a 10 feet distance from the flash bulb, point the meter at the center of the bulb. Have the meter set to flash (not ambient or flash w/ cord). Have the assistant trigger the meter and immediately after you know they've triggered the meter ignite the flash bulb. Keep in mind that the longer you wait to ignite the flash the more skewed the reading will be because the meter will be recording ambient light from the moment it is triggered so ignite the flash bulb immediately. Now take the f-stop reading from the meter and multiply it by 10. This is your guide number for that particular flash bulb type. Use this guide number to figure the f-stops for any distance that you may be shooting just as you have before i.e. Guide Number divided by distance in feet (lamp-to-subject) equals f-stop.