I am able to understand it now. Many thanks again. With regards, Ayash.
On Fri, 9 Aug 2002, William Robb wrote: > ----- 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 . - 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 .
