In reference to: > >If you do night bulb shots and you know that a > >clear number 25 flashbulb is about F16 at ten feet (100 ISO with an old > >Kodak Brownie 5 inch reflector), and you know about the inverse square law, > >then you will also know that when you back up to 30 feet (three times as > >far) to blow your bulb, that your exposure will then be about F5.6 (three > >times as much light coming into your lens as F16).
Warren Sunkel thankfully pointed out: > Sorry to be nit-picking, but f/5.6 is *eight* times more exposure than > f/16. You're not nit-picking, you're very observant (and you made it to the end of my post!). That's what I get for not reading what I type before I send it. Thanks for pointing out my typo! 'F5.6 (eight times as much light coming into your lens as F16)' is correct and what I thought that I had typed. Pointing out this correction is definitely important here! When you are thinking one thing (three more F stops) and typing another (eight times as much light than F16), this is no good when you are explaining it to others. To clear up any confusion, F5.6 is three stops more exposure than F16, which is eight times more LIGHT than F16. I often do this in the studio with the shutter held open on B in the dark as I 'pop' off as many flash strobe bursts as I need to get the equivalent exposure with a wider aperture. And by doing so, I can use smaller apertures and get greater depth of field (very important with large format as you know) than I would otherwise be able to; (I only have 2400 watt seconds max.). To make (I hope) a clearer example of the example above, once I found my correct exposure to be at 5.6, I would then close my lens down three stops more to F16 and need eight times as much light (three stops which is eight pops) to get the correct exposure (with either more depth of field in the studio, or more bulb pops in the same exposure for more even lighting on the depot). Assuming I do not change the flash setting (or bulb type) or the distance: At 5.6 = one strobe pop At 8.0 = double the light = two pops At 11 = double the light again = two additional pops = total of four pops At 16 = double the light again = four additional pops = total of eight pops I hope I explained this much better this time. Sorry for any confusion. And Warren adds: > The correct exposure at 30 feet would be f/5.3 (f/stop = guide number / > distance). Yes, on paper and by the math, but I said ABOUT F5.6, because with all of the other variables here such as the ambient light (and white depot) that also contributed to my exposures that night, a third of a stop accuracy is only possible to achieve with a night shot by accident (or perhaps a box of Polaroid!). Bracketing always gets me the shot at night, but an initial calculated exposure to base your bracketed exposures on gets me in the ballpark, and avoids drastic exposure errors and lots of wasted film. I also recommend bracketing on night shots of anything. Too many variables to ruin your shot if you don't do so, and even with correct meter readings, the color temperature of any ambient light in your shot may not always register quite like you suspect on film sometimes, especially if you don't know what kind of lights they are. Even the same type of lights at night often show up as different color light sources from light to light in the same shot. I know you are thinking now Warren... black & white film! :) And Warren says: > You came up with the right answer, but your logic was a little shaky. Please feel free to give your best shot at explaining the example above. I did. Thanks! I'll read this one before I send it. :) > Later, > Warren > > Post this to the list if you desire. Thanks, Warren. Regards, Dave Cohen Photographer [EMAIL PROTECTED] --> SPORRS: Serious Photographers of Railroad Related Subjects
