infinity extension, macro extension, etc

Here's a puzzle. It may be answered when I decide how to mount this, but here goes...

A flange-mounted zoom lens with helicoid and focus distance only from 3.5-18 feet (studio lens)...no infinity. Flange registration distance is 100 mm (verbal info; I have to trust it for now).

I want to ANTICIPATE how much less flange distance I need to allow focus at infinity & include a removable spacer so that it can be used with the calibrated scale at the proper flange distance so it remains 'parfocal' (focus doesn't change with zoom change). When foreshortened to allow infinity focus, I expect it may become varifocal (refocussing required for zoom changes).

I would not expect there to be much difference in spacing.

Now, to go from 3.5 feet to a closer macro distance I do understand usually requires a significant repositioning/extension...like doubling for 1:1. Unless the math indicates a reasonably small spacer to improve the 3.5 foot minimum to something like 2 feet it might be worth doing, but not a tube.

It's going on a box for starters, then maybe a Speed Graphic to solve all the problems (shutter, extension, etc). For now, I don't want to the SG approach...the flange and mounting holes require a thick board or they will interfere with the front standard.

I can't imagine the difference in extension between 18 feet and 'infinity' being very much at all (50-150 mm zoom). It isn't on primes.

Does being a zoom throw wrenches into the works, or is physics physics and that's that?

Thanks

Having been involved in Optical Quality Control (they taught me a smattering about optical design) for a few moments in a previous life, I'm waiting, eagerly for a quick, concise (no more than three words) answer to this one.

All I can say is .. VOOF!!!

I think if they gave that one to me as a homework assignment -- I'd still be at it.

Um, Murray, if the rear of your monstrosity won't pass through your Speed's front standard, it almost certainly can't be focused to infinity at 50 mm. Yeah, the rules are the rules. There's little magic about zooms.

More seriously, buy yourself a good book. A. A. Blaker's Field Photography, to be exact. Out of print, look for a used copy with www.abebooks.com, www.addall.com, and www.amazon.com. It contains the magic formulas you need and even explains clearly how to use them.

Murry, Just one simple equation is required...

(1/focal length) = (1/object distance) + (1/image distance)

let's use the following short hand notation:

1/f = 1/ob + 1/im

In your question, the image distance is unknown and the object distance is infinity....

so, at the short end of the zoom range the focal length is 50mm so...

1/50 = 1/<infinity> + 1/im

I get an image distance of 50mm...

and I guess at the other end of the zoom range the focal length is 150mm - right?

so you'll need an image distance of 150mm to focus the 150mm focal length at infinity (this is the case, by definition for all 150mm focal legth lenses!)

bottom line, you need to get the rear nodal plane of the lens within 50mm of the film plane to focus at infinity.

further, to focus this lens at 18 feet (let's call it 5486mm)...

1/50 = 1/5486 + 1/im


---> im = 50.46 mm

so, when the 50mm lens is focused at 18 feet (==5486 mm), the rear nodal plane of the lens is about 50.46 mm from the film plane. So to focus this lens at infinity, you just need to be able to get it another half a millimeter closer to the film.

Hope this helps...

Brad.

OK all...

Re: fitting SG front standard...I found I can take the big flange off, resulting in a smaller barrel diameter...for future reference, like SG lens boards, this may make things easier.

Re: distance from film. It was on a rigid body camera. I was told 100 mm flange distance (registration?). The helicoid is marked 3.5-18 feet, so even at 50 mm or 150 mm, the helicoid does the movement. But obviously, the difference between 18 feet and infinity then amounts to a shim or tiny spacer...I suspect it'll be interesting tweaking that. So, internal or external movement aside, the math should still rule.

Hmmm, I was thinking (maybe I wasn't) it would be a bigger spacer than that, and envisioning short normal and long positions on a rigid mount...normal = 3.5-18 as scaled, shorter for ?-infinity scale to be determined and zoom vs focus behavior to be determined, and longer for however much closer than 3.5 feet I could get.

I was guessing 5 mm for experiment's sake, no calculation, and now applying 150 mm in place of 150 mm I get 154.2 for 18 feet...so this all helps give me some perspective.

Thank you for the 'grounding' info.

Time to start building...

Oh, Dan, I shot a roll of the 620 film from ancient Rome this weekend in the old Vollenda.

I decided to make a chart of the difference in barrel length as the helicoid was rotated and compared this to the distance marks. I compared the 50, 100 and 150 mm to focus distances & went around in circles trying to make things line up...I then normalized one column of data to make them 'match' calcualted & measured, and scaled the f.l. to correspond, thinking there might have been an e.f.l. vs nominal. There were also problems extrapolating...it didn't convince me I had a wrap on it all, but I can see that there isn't much difference to reach infinity. As I suspected, I can't pick one single spacer to allow conversion from infinity focus spacing to the parfocal scale distances for all focal lengths, but eventually (after a few other experiments) I'll put it on a Speed Graphic anyway & be able to do whatever.

Thanks for the exercise.