----- Original Message ----- From: Ayash Kanto Mukherjee Subject: Re: Twin flash for Macro: Some thoughts
Ayash, with the setup Warren describes, the bellows factor and guide number factor cancel each other out perfectly. Exposure must be increased based on an obverse square ratio of the lens to film plane distance; the exposure must be decreased based on an inverse square ratio of the flash to subject distance. All he had to do was calculate a primary exposure, based on the minimum bellows draw, and the film plane to subject distance that it gave when in focus. As he moves the camera closer to the subject, he is racking out the bellows. This has the dual effect of increasing lens to film plane distance (bellows draw), and decreasing at the same rate, the flash to subject distance. It is quite a clever design concept, actually. I have used TTL with success with macro, but the calculations needed to do it are based more on guesswork than science. William Robb > On Fri, 9 Aug 2002, W.Xato wrote: > > > Ihave an interesting set up you might be interested > > in. I used two cheapie Vivitar flashes on home made > > brackets which I attached to the front standard of a > > bellows. As you know, the inverse square law says > > that you must increase exposure as you rack out the > > bellows. > > That's right. By how much the exposure should be increased can be > calculated from the length of extension of the bellow and the focal length > of the lens used. > > > Also it says that as the flash gets closer > > to the subject, you must decrease the exposure. > > The problem lies here. By how much moving distance the flash be moved > towards or away from the subject amounts to how much decrease or increase > of exposure? Can you please give me some data along with the guide number > of the flash that you use. > > > But with the flash attached to the front standard, both of > > these cancel out. > > I have no idea for how much extension of bellow and separation between the > flash & subject such cancellation takes place. > > > With my 80mm macro lens and Velvia, > > I just set it at f8 and shoot away. > > If the guide number of the flash that you use are not matching with with > mine, the aperture value will be different in my case. However, it can be > adjusted to f/8 by adjusting the flash to subject distance. But hey, I > don't know how much separation there should be. > > > I have to increase > > the exposure 1/2 to 1 stop at max exxtension but it is > > easy to apply. > > I should say that you have calibrated your system well so you can easily > tell the in and outs of it. For me it is very difficult. > > > TTL is difficult for macro because > > often the subject is small in comparison to the whole > > macro scene. TTL reads the whole scene and your macro > > subject in the foreground will be overexposed. Macro > > flash is easier to do manually even if you have to > > calculate exposure and bellows extension factors. > > Yes, you are absolutely right. - 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 .
