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Abhiranjan, You didn't mention anyting about:
(a) film speed you're using
(b) type of subject material you're trying to photograph
(c) subject distances
(d) lens speed All of these things affect whether you can get a good exposure (avoid underexposure). The Vivitar 2800 has a respectable GN of 80 (in feet for ISO 100 film). It's not super powerful, but still more than the average consumer flash. Some flash tips: Film speed --
While this is not always the most desirable solution, increasing film speed increases your working range and decreasing it decreases the working range. This has a limited effect though. Doubling the working distance requires four times the film speed. A subtle effect of using a faster speed is picking up more ambient lighting and requiring less from the flash. When working in a large space or outdoors, with at least some ambient light, the background won't be quite as dark with a faster film as with a slower one. Working distance --
You should be able to get at least 14 feet from your flash in an average space inside a home or office. You can get an idea on your flash limit by using its guide number and lens aperture. How did I arrive at 15 feet? Assuming you're keeping the lens open at least to f/5.6 and using no slower than ISO 100 film, I divided the flash GN @ ISO 100 (80) by the 5.6 for 14.3 feet. The GN for your flash in feet for ISO 200 film is 113 and for ISO 400 film in feet, it's 160. At f/5.6, this is a working range of about 20 feet for ISO 200 film and about 28 feet for ISO 400 film. Lens Aperture --
I suggest working with this before trying faster film. With a GN of 80, you should be using a lens aperture of about f/4 or f/5.6 to keep a reasonable working distance. Indoor vs. Outdoor --
Indoors, light from the flash will bounce off the walls, ceiling and floors. This helps some by keeping light from the flash from dissipating. Outdoors without walls and ceilings close by is another problem. There is nothing to bounce the light from the flash back onto the subject material. The light from it dissipates completely. If the background is very distant, your flash won't even cover it. Also, if your subject is relatively small in the photograph with sizeable areas of very distant background, the photograph can look like someone is in a cave. In addition, the flash will attempt to put out more light to get the average of the entire image up to a proper exposure. The result can be a subject overexposed. Even if you get a negative properly exposed, the lab printing it can try to do the same when using automated print machines. Outdoors, it's important to pay attention to what the background is and how far away it is. Not quite as severe, but still a similar problem is large spaces indoors. This is the problem wedding photographers have working inside the large spaces of churches and reception halls compared to the average home. Other Bright Light Sources --
Pay attention to bright light sources that can "fool" the sensor on your flash. Your Vivitar controls its light output using a sensor on the front. Even though a bright lamp or window with bright daylight outside may not be in the image you see in the viewfinder, if it is close to an edge of the image, it can be within view of the flash's sensor. The flash then uses this as part of computing how much light is required. If the light from this bright source is not directly illuminating your subject, it can be underexposed. -- John
July 05, 2002
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