RE: Extension tube K
Fantastic!!! Thanks a lot!!! I think I'll give it a try with TTL cameras first. Cheers Guys!!! Andy -Original Message- From: John Coyle [mailto:[EMAIL PROTECTED] Sent: Sunday, January 04, 2004 10:00 AM To: [EMAIL PROTECTED] Subject: Re: Extension tube K Andy, the basic formula for calculating the additional exposure required is given in this table (taken from The Asahi Pentax Guide, Focal Press 1967): Tube No Magnification Exposure factor 1.0.35x1.8 2.0.52x2.3 3.0.69x2.9 1+3.0.86x3.5 2+3.1.04x4.1 the assumption is you are using a 55mm lens focussed at 18in. The factors may vary slightly for the K tubes, but this may get you going in the right direction! . John Coyle Brisbane, Australia - Original Message - From: Andy Chang [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Saturday, January 03, 2004 11:12 PM Subject: Extension tube K Hi guys, A belated Happy holiday to you all!!! I have just acquired a Pentax Extension Tube set K. I had no idea what it does first but after looking at Boz's web site, I have some idea how it works. It was a bargain and I couldn't resist bidding for it. According to Boz's site, it is a manual extension tube set and I guess the differences between the Auto set and the manual set is about the A position on the A and later lenses. If you remember, I have asked about losing F stops using teleconverter. I'm just wondering if it also applies on the extension tubes. By adding extension tubes behind the lens, do I have to adjust the exposure accordingly? Thanks Andy
Re: Extension tube K
On 3 Jan 2004 at 21:12, Andy Chang wrote: If you remember, I have asked about losing F stops using teleconverter. I'm just wondering if it also applies on the extension tubes. By adding extension tubes behind the lens, do I have to adjust the exposure accordingly? Yes, consider the way the tubes work. When using tube the image projected on the film plane is only a sub section of the image that would appear if the lens were closer (hence the effective magnification). You can then assume that the available illumination in that projected image is less over the area of the film as it is spread over a wider area. You can calculate the losses but it's difficult and lens dependent to an extent, this is one of the areas where TTL meters reign supreme. Cheers, Rob Studdert HURSTVILLE AUSTRALIA Tel +61-2-9554-4110 UTC(GMT) +10 Hours [EMAIL PROTECTED] http://members.ozemail.com.au/~distudio/publications/ Pentax user since 1986, PDMLer since 1998
Re: Extension tube K
Rob Studdert said, about extension tubes: You can calculate the losses but it's difficult and lens dependent to an extent, this is one of the areas where TTL meters reign supreme. Not difficult really, so much as _annoying_. But yeah, this is one place where having TTL metering is soo nice. That's one of the things I liked about having a Spotmatic -- since my bellows and my extension tubes are screwmount, I need to replace either the Spotmatic or the screw-to-K converter (yes, that got stolen too) so I can shoot macro with less calculation again. But you still have to do the math when you use a flash, unless you also have TTL-flash mode available. (I guess the PZ-10 will do that for me, if I get the converter.) My bellows has a scale engraved on it so that you can just look up the exposure factor for a given extension ... as long as you've got a 50mm lens attached (with some fudging for the in-between amounts, of course). -- Glenn
