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Q: How should I use a Promaster flash, dedicated for Nikon, together with my Nikon SP25 flash?
A: The dedication of the independent brand flash units is usually not quite complete. I would use the SP25, therefore, as the primary flash unit on the camera and connect the SP25 and the Promaster with a SC18 cord attached to the AS10 adapter. Because the Promaster flash does not have a socket for the SC18 cord, the AS10 adapter will provide a flash shoe for connecting the Promaster. Your SC17 cord does not allow a flash to be mounted on the camera and both flashes must be handheld or mounted on a bracket.
It is unlikely that a flash sold as a dedicated unit can harm the camera but the safest way is to use any off-brand electronic flash with a slave. The preflash of the flash unit on the camera is usually too weak to trigger a slave flash. The output of the slave flash is not controlled by the camera, except for the Nikon SU4 slave which allows a TTL control of the slave flash. The SU4 operates only with flash units dedicated for Nikon.
Q: Many of the pros use for nighttime shooting of car races either a Norman or Lumiquest studio head on a Stroboframe bracket. The big units are rated in watt/seconds but my Metz 70MZ-5 flash unit has a guide number. Is there an equation that I could use to convert a conventional guide number to watt/seconds and vice versa?
A:The guide numbers (GN) of an electronic flash unit are given by the formula GN (in feet) is equal to the square root of the product (factor x ISOx BCPS). The factor depends on the reflector, the flash tube, the zoom setting for zooming flash heads, and diffusers. The factor may vary between 0.03 and 0.07. Usually the factor is around 0.05-0.06. IOS is the rating of the film. BCPS, the beam candle power seconds, indicates light output and is measured by a standard procedure. The BCPS values of some older flash units were: 2500 for Vivitar 285, 3400 for Vivitar 292, and 2550 for the Sunpak 444D. Most manufacturers no longer state BCPS values and give only guide numbers for their electronic flash units.
For some flash units only the watt/seconds (Ws) are given. Watt/seconds indicate the electrical energy in the capacitor. A watt/second rating is not directly related to the light output of the flash and the guide numbers. Units with a lower Ws rating may have a higher guide number and vice versa. Usually the Ws ratings are given for studio units where guide numbers or BCPS values are meaningless. A studio flash may be aimed directly, bounced off an umbrella or a reflector, projected through an umbrella, and so on. The watt/second ratings are not used to derive guide numbers. The light output is measured with a flash meter to determine the exposure for the given set of conditions.
In conclusion, the answer to you question is quite simple. An equation for converting watt/seconds to guide numbers does not exist, because these two variables are not independently related. The guide number can be determined by test exposures or by using a flash meter to compare the light output to that of a flash unit with a known guide number.
Q:I am a beginner and would like to enter the monthly contests. Should I start with slides or prints?
A:Slides are a better medium for learning photography than prints. With slides you can improve your photographic techniques and develop an artistic vision. Prints require additional skills required for operating a scanner, the Photoshop, and the printer. Color management is an art to be learned.
Consider also the expenses involved. For making slides only a camera is needed. Prints require a scanner, a computer with large memory (at least 256 MB RAM, preferably much more), the Photoshop software, and a printer. In addition to the cost of processed film, the printer ink and paper add to the cost of making prints.
Q:I have a Vivitar 80-200mm zoom lens, F 4.5, Serial Number 22816491. Can you tell me which camera it is for?
A: The serial numbers of Vivitar lenses do not include a code to specify the lens mount. The lens has letters on its rear mount to indicate the suitable camera.
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