Feroze I think you may be confusing coverage and giudenumber??? Coverage does usually not have anything to do with ISO or f-stops. The "coverage" you descibe here is not, as it is commom practice, the measure of the side of the rectangular square of light, that the flash will make on, let's say, a wall. Coverage usually describe an angle of view or a focal length (35 mm equiv.), meaning what you get in the viewfinder.
Guide number means aperture at the distance of 1 foot at ISO 100. For this flash the guidenumber is 18. Guidenumber 18 will at 100 ISO give f 18 at one foot and appr. f 8 at 2 feet, appr. f 4 at 4 feet - or f 32-45 at 0.5 foot (15 cm). I'm not shure this applies the same way to a macro flash, as it does to ordenary flashes, though. I guess in this case it (coverage) referes to the maximum distance to the subject. 98 cm is almost 3 feet. Guidenumber 18 should be just about sufficient/enough power to give you f 5.6 at a three feet distance (as it gives you a little more than f 4 at 4 feet= 124 cm (more because my table is derived from GN 16). What you do in practice is this: Use A-setting to automatically adjust light for the aperture set at the camera (if there are more than one to choose from). Don't go beyond 3 feet distance to subject, as you do not want aperture larger than f 5.6 for macros anyway. Otherwise use manual setting and a aperture/distance table, like the one described above. I wouldn't shoot macros at less than f 8. Jens Bladt mailto:[EMAIL PROTECTED] http://hjem.get2net.dk/bladt -----Oprindelig meddelelse----- Fra: Feroze Kistan [mailto:[EMAIL PROTECTED] Sendt: 20. maj 2004 21:27 Til: [EMAIL PROTECTED] Emne: Re: flash question Hi Jens, Thanks, unfortunately this one is not a pentax flash, its a vivtar macroflash 5000, a very base line model. I downloaded the manual which was all of 2 pages. The part I didn't understand its gives you the table for a 100/105mm lens, so for eg I'll get on ISO100 @5.6 coverage to 98cm, how do I calculate or convert this table if I was using a 50mm lens, is there a formula or something conversion table. Is there a standard?? Thanks Feroze ----- Original Message ----- From: "Jens Bladt" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Thursday, May 20, 2004 9:09 PM Subject: RE: flash question > Hello Feroze > The flash light spreads at an angle. Most flashes will cover an angle > equivalent to that of a 28mm lens for a 35mm camera system. The userguide > for diffent flashes will give you the vertical and horizontal coverage > (angles) for different lenses/focal lengths/angle of view. > > The espression coverage for this and that lens has to to with flashes that > will zoom (achange angle of view) when you zoom the lens/change the angle of > view. > > Please see user giuides for Pentax falshes at > Pentax USA: http://www.pentax.com/docstore/index.cfm?show=6 > Or Bojidar Dimitrov's Homepage (angle of view for Pentax lenses): > http://www.bdimitrov.de/ > > All the best > Jens Bladt > mailto:[EMAIL PROTECTED] > http://hjem.get2net.dk/bladt > > > -----Oprindelig meddelelse----- > Fra: Feroze Kistan [mailto:[EMAIL PROTECTED] > Sendt: 20. maj 2004 20:26 > Til: [EMAIL PROTECTED] > Emne: flash question > > > Hi All, > > Hate to disturb you guys with a photographic question, but could someone > direct me to a site or link that explains in simple terms what flash > coverage means. For eg. if it says that a particular flash covers 80 deg > with a 105mm lens then will it not cover then same amount of area with a > 50mm lens. > > Thank You > > Feroze > > > >
