### Trusting the formula?

greenspun.com : LUSENET : Large format photography : One Thread

Bear with me folks...

I know that one can work out focusing distance from the old formula;

1/p + 1/q = 1/f

where p is bellows extension, q is lens-subject distance and f is focal length. BUT, whenever I've seen that formula quoted it's always been followed by a caveat along the lines of "for a theoretical lens of zero thickness". My question is, how important is that caveat in practical terms?

The reason I ask is that I only get 300mm of draw on my Wista bellow but I'd like to use a 300mm lens (the Nikkor M 300/9 is the one I have in mind). Don't want to muck about with extension tubes (needs to be a small, light and reasonably cheap solution) but I figured that if I could find a recessed board, maybe 30-35mm deep, and mount it in reverse then I'd be able to focus down to 3m or so - which is ample for me. However, if that formula is way out for modern lens designs or one should be measuring from the rear-element rather than the centre, then it could be an expensive mistake!

Any words of wisdom would as always be much appreciated.

-- Stuart Whatling (sw@dial.pipex.com), July 26, 2001

The formula is exact even for thick lenses IF you measure the distances from the correct places. The correct places are known as the "Principal Planes". The lens-subject distance should be measured from the front principal plane and the lens-film distance from the rear principal plane. For most lenses, the principal planes fall somewhere inside the lens, typically but not always very close to the aperture diaphragm. The important exception is true telephoto design lenses, in which the rear principal plane is positioned in front of the lens in order to reduce the bellows draw.

A Nikon brochure that I have gives the positions of the principal planes of their LF lenses. The rear principal plane of the 300 mm Nikkor-M is located 18.8 mm from the rear-most surface of the rear glass element. This is a mm or two in front of the aperture diaphragm and a mm or two behind the rear-most surface of the rear lens of the front half of the lens. Measuring from the aperture diaphragm will be accurate enough for most purposes.

I think Wista or someone makes an extension lensboard for the purpose that you describe.

-- Michael S. Briggs (michaelbriggs@earthlink.net), July 26, 2001.

Stuart: An extension lens board will work, but there is another solution to gaining a bit of bellows draw on the Wista/Tachihara. Tilt the front standard forward, raise the lensboard up to disconnect the little tab at the bottom on the lens board that rides in the standard, and then tilt the lens back to verticle. The front standard then becomes center tilt and you will need to tighten the rise and fall knobs to make sure everything stays put. You will need to use a bit of rise to recenter the lens on the front standard. Takes a lot longer to describe it than to do it.

Regards,

-- Doug Paramore (dougmary@alaweb.com), July 26, 2001.

The above feature *does not* exist on the current Wista DX models. That is, the front standard can not be released from the rails. The original Wista woodfield is the current Tachihara design, I believe, and should have this feature.

-- Richard Ross (ross@hrl.com), July 26, 2001.

Thanks for the thoughts guys,

Perhaps I should have clarified by pointing out that it's a Wista VX - not one of the wooden jobbies.

I've just spoken to my local LF shop ("Teamwork" in central London) and I understand from them that Wista do indeed make an extension lens-board - presumably just because people want to be able to use 300mm lenses on their 300mm bellows! The long bellows and extension rails would be totally impractical in the field and a bit OTT given that I only need a couple of centimeters extra. I'm also curious to see whether I can reverse it for use as a w/a sunken panel.

Will report back next week once I've had a look at these extension boards.

Cheers everybody, Stuart

-- Stuart Whatling (sw@dial.pipex.com), July 27, 2001.

Just as a follow up, I now have a set of Wista extension tubes sitting on the desk in front of me. Not cheap (about 170 pounds sterling) but well made and perfect for what I wanted. They're lightweight, easy to fit and will allow me to use lenses up to 360mm on my Wista VX (300mm bellows). A hell of a lot more practical in the field than an extension bed.

FYI, the set consists of a normal wista/linhof style board with a 33mm long tube sticking out of the front (the diameter of the tube is 84mm so vignetting shouldn't be too much of a problem). Two other tubes (each 28mm long) can be screwed onto. A special mounting plate then screws onto the front - with all three tubes, plus the front board, total extension is 104mm and the weight is 350g. With just the single tube (extension = 46mm, weight = 235g) and with a 300mm lens fitted it doesn't take up much more space in the bag than a normally moounted lens but will let me focus to <3m.

Problem solved.

-- Stuart Whatling (sw@dial.pipex.com), July 31, 2001.