 Christopher A. Walrath |
Back to School: Guide Numbers Hi all. It's your good buddy flash with yet another highly technickle question for ya. And I asked it here because folks like Alan, Mark and Greg frequent here so I'm hoping you guys can help me shag a few flies here, as it were. So, without further ado . . . Flash Guide Numbers. I have an old Promatic FTD4000. You don't need to be familiar with it to answer what I want to know. It has the adjustable flash zoom for use with different focal length lenses. It has the manual setting slide. This flash has a dedicated Pentax mount (came with it) but I am using it on a Mamiya M645j and a Minolta sr-T101. Inconsequential but possibly pretanant. It has a 'M' setting so it will more than do for my purposes. Now, I have never taken the time to learn and understand the use of guide numbers and how to apply them to my photography. But I want to start using more artificial light in my photography and I want to learn it all. How to determine a guide number. How ISO ratings come into play. How a guide number will affect my exposures in terms of exposure zones, et al. I want a crash course. Throw it all at me and make the posts as long and bookish as possible. Wanna take this opportunity to say hey to some long time cohorts. Mark, long time no yak. What's up? Alan, say hey there, O' King Of Dee Gobbledygook. Greg, um, hey. I'm counting on you guys. I have some literature, including the actual guide numbers for this flash. But I want more. Thank you
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Pete H |
Hey Chris, I'm not much into "bookish" so here ya go. http://www.shortcourses.com/guide/guide2-28.html
Pete
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Alan N. Marcus |
Hi Chris, A guide number is a numerical value used to estimate the aperture setting when using a flash unit. Generally the guide number will be supplied by the maker of the flash however, you can easily determine by test. Set up a suitable subject and place in a room with light colored walls with standard ceiling height (8 feet). Establish a camera-to-subject distance of 10 feet. Shoot a series of exposures each at a different aperture. Shutter speed is not a factor so long as it will synchronize with the flash unit. Dim the room lights so ambient light has little or no influence. Examine each image for best exposure. Note ISO setting used (assume 100). Note aperture used. Multiply aperture used in the best exposed frame by distance. Example: f/11 best at 10 feet distance - so 11 times 10 = 110. Thus 110 is the guide number for ISO used for similar surroundings as to ceiling height and wall proximity and wall reflectivity. For 200 ISO guide will be 220 for 400 ISO guide will be 440 etc. How to use guide number. Compose your shot – estimate camera-to-subject distance or note camera distance scale (assumes camera mounted flash). Divide guide number by subject distance. Example: subject distant is 20 feet so 110 divide 20 = 5.5 use f/5.6 (closest rounded f/number value). Likely this method will yield satisfactory results, however best to bracket. Likely you will be under exposed due to walls and ceiling height differences and subject lightness or darkness differences, all are major ingredients. Dance halls and sports arenas and open areas outdoors, require an additional stop or stop more. Flash units with adjustable reflectors change the angle of coverage of the flash. This has a profound effect on the guide number. Wide angles setting require opening up maybe one or two f/stops. As to the M setting: Likely this pertains to medium angle of coverage. However: Maybe it pertains to synchronization setting. Modern cameras require synchronization setting labeled X. This translates to - electrical contact to fire the flash; this is timed for the moment when the shutter reaches full open. This is the setting used for all electronic flash as they fire instantaneously i.e. no delay. Flash bulbs required a shutter delay, electrical contact was supplied 20 milliseconds ahead of shutter full open to allow time for the flash bulb to reach peak brightness (a chemical process). This synchronizes the flash with the shutter for the instant when the shutter attains full open. Thus the shutter was delayed a “medium” labeled M i.e. 20 millisecond delay. Other settings: SF for super fast flash bulbs, they required only 5 milliseconds delay. FP was for focal plane shutters, the FP flash lamp had an extra long peak brilliance to provide accommodation for focal plane curtain travel time. Best of luck, Alan Marcus (marginal technical gobbledygook)
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Christopher A. Walrath |
OK. I kinda get that. For the most part. Enough to move onto a another question. Do guide numbers translate numerically to a specific value of added exposure units (in keeping with AA's Zone System)? Where if you add 12 units of exposure to a subject, anything that was in exposure zone 0 gave one unit of exposure. Anything in Zone I gave two units. Zone II gave four units. Zone III gave eight units of exposure and so on. By adding 12 units of exposure, anything that fell on Exposure Zone 0 is raised to between Zones III and IV. Anything on Zone I is raised to almost Zone IV. Zone II raises to Zone IV. Zone III raises to Zone V. And so on. But is there a way to translate a flash guide number into units of exposure added to a subject? I think I worded that right. Try it.
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Christopher A. Walrath |
Sorry, Alan. Our posts crossed. ANd thank you for the answer. It raises another question. Why is the guide number figured assuming ISO 100? Is there another reason other than someone once said so?
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Alan N. Marcus |
Hi again Chris, I only used 100 ISO as an example. You calculate a guide number based on the ISO setting operational on the camera at the time of the test. Example the camera is set to ISO 400. You position the test subject-to- camera distance at 10 feet. You shoot an exposure sequence and the frame at f/16 is best. Thus guide number = 16 time 10 = 160 and this is valid for 400 ISO. The guide number system solves based on the law of the inverse square. If the subject-to-flash distance is doubled the light arriving at the subject is reduced to 25% of it’s original value i.e. 2 f/stops. Example guide number is 200. At 10 feet the aperture is 200 divided by 10 = f/20 round to f/22. If lamp-to subject distance changes to 20 feet than 200 divided by 20 = 10 rounded to f/11 i.e. open up two f/stops. In a zone system, each unit = 1 f/stop. Thus the difference between zone 5 and zone 6 is one f/stop. That translates to a guide number change based on the square root of 2. Thus to open up one f/stop by guide number alteration one would either divide of multiply by 1.4. This being the case, if 200 is the guide number then to open up one f/stop the revised guide number is 200 divided by 1.4 = 142. To close down one f/stop the revised guide number is 200 times 1.4 = 280. More technical gobbledygook from Alan Marcus
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Alan N. Marcus |
Hi all, I re-read my first post and it contains an error. For self punishment I will put on the dunce cap and sit in the corner for 1 hour. WRONG: CORRECTION: Explanation: Light falls off with distance according to the law of the inverse square. Light starts its journey as a fixed amount. As it travels, it spreads out with distance and it is forced to cover a larger and larger area. If the distance to the light source is doubled, the light must cover 4 times more surface area. Consider a slide projector focused on a wall projecting a square image. Say it’s 10 foot projector-to-wall distance, and the image measures 4 feet by 4 feet. Thus the image area highlighted by the light is 4 times 4 or 16 sq. ft. Should we move the projector back away from the wall, say to 20 feet, we refocus and the image will be enlarged 2x in magnification. Now the light must cover an area 8 feet by 8 feet; thus the area of the revised image becomes 8 times 8 or 64 sq. feet. Thus if the image size doubles the light is now stretched out and must cover 4 times more surface area. Thus the brilliance at the wall is reduced to 25% of its former brightness. (law of the inverse square). The magic factor to calculate a 50% change or 1 f/stop change which is a 2x change in brilliance is 1.4. We can multiply or divide distance by this factor to calculate a doubling or halving. Personally I prefer to half by multiplying by the inverse of 1.4 which is 0.7. Like many old guys who used a slide rule in their youth I prefer to multiply rather than divide. Multiplying by the reciprocal is the same as dividing by the original number You will find the 1.4 factor and its reciprocal 0.7 are used to calculate the progression (number set) for f/numbers and ISO and filter factors and the like. More gobbledygook from Alan Marcus
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Christopher A. Walrath |
Thank you Alan. I will now go and formulate more questions as necessary. If you see smoke coming from central Delaware, it's just me. Thinkin'. Piece and Hippyness
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Alan N. Marcus |
Hi Chris, The smoke you see off to your west is me. I live just across the street from Disney Land. Also you should know; just because there is snow on the roof, doesn’t mean there’s no fire in the furnace. Photo science makes heavy use of a number sets comprised of the factor 1.414 (rounded to 1.4). Check this one out: 1 – 1.4 – 2 – 2.8 – 4 – 5.6 – 8 -11 -16 -22 -32 -45. If the lamp to subject distance is 8 feet, moving the lamp to the 11 foot mark, decreases the light energy on the subject by 1 f/stop or 50%. Moving it back to the 16 foot position cuts the light in half again or 2 f/stops reduction. Moving it back still further to 22 feet cuts it in half again, a three f/stop reduction. Moving a lamp from the 5.6 foot mark to the 4 foot mark doubles the light on the subject an increase equal to 1 f/stop or 100% more light energy Moving it closer to the 4 foot mark doubles the light energy again i.e. one more 1/stop.
Best regards,
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Christopher A. Walrath |
You know, my numerically oriented thought processes probably would have picked up on the eventually. In fact I think my mind grabbed it for a moment a couple of days ago but I was at work and it left just that quick and by the time I clocked out the mental scent was cold. I wanted to do something with the 1.414 factor series and that was it. Thanks, Alan. Piece and Hippyness
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