## lens focal length and focusing questiongreenspun.com : LUSENET : Large format photography : One Thread |

I am a recent user of the Toyo VX125b camera with a 150mm/5.6 APO-Symmar lens. I have discovered that I would like to use a longer lens in many situations for close-up shots of details of objects for abstract compositions. Therefore, NOT focused at infinity. One of the drawbacks of my camera is that it has limited draw: 325mm absolute max.My question is: Is there a way (perhaps a mathematical formula)to figure how close an object can be to my lens and still be able to focus on it for a 240/5.6 APO-Symmar and/or a 300/5.6 APO-Symmar? Infinity of course will be no problem. It is the closeness of the object that I want to know about so I can pick a lens that I can use for detail work without having to get so close to my subject as I have to do now with my 150mm lens.

Thanks in advance for any help!

Scott Jones

-- Scott Jones (scottsdesk@home.com), May 15, 2001

I know this doesn't completely answer your question, but with a bellows draw of twice the focal length, you should get 1:1 reproduction.I'm not familiar with your Toyo, and I don't know if you can interchange bellows and/or rails, but a custom extended lensboard will help you to focus closer with a long lens.

-- Dave Brown (wolfgang@peakpeak.com), May 15, 2001.

Your bellows draw is 12.7 inches, for a 1:1 inlargement you would need a lens not longer than 160mm, so unless you plan on using an extended lens board your 150mm is about your limit. Pat

-- pat krentz (patwandakrentz@aol.com), May 15, 2001.

Perhaps I should clarify my question. I am not trying to do macro photography. I am trying to use a longer lens to get magnified images of objects without having to get phyically so close to the object as I do with my 150mm. Sometimes when working in the field I can't get in tight for the shot so I need a longer lens to get me in there visually. So I would still like to know if there is a way of figuring out how close I will be standing to my in focus object when I run out of bellows draw on my camera with the 240mm and the 300mm lenses listed above. Thanks again for any help!Scott

-- Scott Jones (scottsdesk@home.com), May 15, 2001.

With a focal length of 240mm and a bellows draw of 325mm you get a magnification of (325/240)-1 = 0.354. An object, that in reality is 10cm large, will be 3.54cm on the film. This is equal to a distance of 240/0.35mm = 0.677 meters. I think this distance is from the center of the lens and not from the filmplane, but I am not sure. For 300mm focal length the minimal distance you can get with 325mm bellows draw is 300/(325/300 - 1)mm = 3.6 meters.

-- Andrä Steiner (steiner.keg@liwest.at), May 16, 2001.

You want the simple conjugate focii formula of 1/F = 1/v + 1/u.

F is the lens focal length; v is the bellows draw (approximately); and u is the distance to the subject.

This formula should be mother's milk to any LF photographer. It takes about 20 seconds to work out on almost any pocket calculator.

-- Pete Andrews (p.l.andrews@bham.ac.uk), May 16, 2001.

Scott,Are you aware that by using the basic monorail you have 19 inches of extension with your camera? At least that's the way I read it on the B&H web site.

At any rate another method of achieving the "optical extraction" you desire is to use a telephoto design lens. I shoot with a Fuji 400 mm and recently tested the Nikkor 500 mm and the Osaka 400 mm and 500 mm telephoto designs.

The Osaka lenses have the very desireable attribute of requiring very little bellows extension in relation to their focal lengths and I found both to produce quite sharp images.

The Osakas also cost considerably less than either the Fuji or Nikkor lenses.

Joe Dickerson

-- Joseph A. Dickerson (jadphoto@aol.com), May 16, 2001.

The standard lens formula is 1/o = 1/f - 1/i, where "o" is the distance from lens to object, "f" is the focal length, and "i" is the distance from lens to image (or negative). This formula applies to so called "thin" lenses. I'm not sure how well it works multiple element lenses, but many authors reference it in their books. (e.g. A.A.) As for determining how close you need to be, the magnification is m=i/o.Applying this formula to your application, at 325mm maximum extension, you could get as close as about 900mm (1 yard) with your 240mm lens and achieve a magnification of 0.35. I think that your 300mm lens is practical only for infinity focus. The closest you can get with this lens is about 45 yards.

-- neil poulsen (neil.fg@att.net), May 19, 2001.

One comment about the suggestion to consider a telephoto design lens: While these designs typically require only about 60% of their focal length in bellows draw, they present two interesting challenges for the tabletop photographer. 1. One characteristic of these designs involves the moving of the primary nodal point (optical center of the lens) to a position somewhere in front of the the front element! This means that when you perform a tilt or swing with the front standard, the adjustment becomes very sensitive and makes finding the desired lens plane a bit tricky. 2. When the size of the image on your ground glass exceeds about 1/10th of the object's actual size, you need to calculate an exit pupil factor to achieve the correct exposure. This is why most tabletop photographers use monorails with lots of bellows and standard design lenses.You might consider equipping a lens board with what amounts to an extension tube, to which you can attach a shuttered lens. The tube would have to be large enough in diameter to avoid portholing and of course, it would need to have its inside surface be blackened and made non-reflective to avoid flare. Something like this might enable the use of a longer lens without getting a different camera. I hope this helps you.

-- Robert A. Zeichner (info@razeichner.com), May 20, 2001.

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