Hi Bill, Many thanks for the explanation. I will follow the lines
With regards, Ayash. On Fri, 9 Aug 2002, William Robb wrote: > ----- Original Message ----- > From: Ayash Kanto Mukherjee > Subject: Twin flash for Macro: Some thoughts > > Hi Ayash: > You had best use both flash units in manual. As you know, in > auto, they will quench very quickly when used close to the > subject. Don't worry about point source or whatever, as that > isn't germaine to light measurement anyway. > Here is what I would do: > If you can keep the flash to subject distance constant from > setup to setup, then the calculations become simpler. > I would use the 285 as my main light, as it is the most powerful > of the two. > First, calculate the aperture needed, either via the guide > number or the calculator dial, based only on the flash to > subject distance. Next, calculate the number of stops you need > to open up the aperture. > > You can do it the hard way, based on the amount of lens > extension. > > The formula for that is : > > Extension factor = (extension from infinity focus)squared, > divided bt (focal length)squared. > > If you are using a macro lens, the extension factor is often > engraved on the lens barrel. > > Or, you can do it the easy way, using the TTL meter of your > camera. Meter a gray card at infinity, then meter it again at > the lens extension that will be used. The difference is the > number of stops you lose to lens extension. > > If you are using the second flash as a backlight, then you can > ignore it for the base exposure, but you will need to calculate > it's effect on the image, again using the guide number, or > exposure scale. Figure you want it a stop or so less bright than > the main light. > > If you are using the second flash as a fill, then you will want > to calculate the distance from the subject you want it to be, to > give an appropriate fill ratio. You will find that having the > fill somewhat less bright than the main will give the most > pleasing effect. > > Be sure to make careful notes, especially making careful > measurements of the flash to subject distance, as that is the > most important measurement for calculating the aperture. You > will find that you probably won't have to do a lot of testing > until you get it right, providing you follow a fairly scientific > approach. > > William Robb > > > > > Hallo all! > > > > I have two flash units. I want to use both the flashes > simultaneously for > > frontal lighting, sidelighting and back lighting macro shots > of flowers > > etc. I have a synchronization cord, eye and cord extensions (2 > m) to fire > > the flashes simultaneously. The problem is automodes of the > flash won't > > help me while doing macro macrophotography because the > f/stop-film > > speed-distance chart at the back of the flash is made on the > assumption > > that the source is a point source (Am I right?). When the > flash is near (1 > > m) to the subject, it is no more a point source but an > extended source. > > The only way that I > > can control the light is by adjusting the separation between > the subject > > (flower) and the flash or adjusting the output levels of the > flash. > > Has anybody got some experience in using twin non-TTL flash > for macro? I > > can not understand what aperture shall I set. Do I have to > calculate > > Bruhlich factor and prepare the nomograms? If yes, it becomes > tedious. > > > > The other thing that is not clear to me, is using the non-TTL > flash as a > > fill flash for macro shots. Is it possible to use the > automodes here? > > Or shall I adjust the output level? If the second is true, > what aperture > > shall I choose for a particular distance (1-1.5 m) for so and > so guide > > number. > > > > I use Vivitar 2800 (G.N. 22 m) and Vivitar 285HV (G.N. 36 m at > Normal > > setting of the zoom head). > > > > Thanks for reading. > > > > With regards, > > Ayash. > - > This message is from the Pentax-Discuss Mail List. To unsubscribe, > go to http://www.pdml.net and follow the directions. Don't forget to > visit the Pentax Users' Gallery at http://pug.komkon.org . - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
