8x10 camera prototype, need help to focus at 30.4 feet

[Eric Constantineau]

Hi all,

I am making large format camera prototypes since a few months. My next one will be a handheld 8x10 wide angle camera. I will use a Schneider 121mm, a 8x10 back, and a lot of tape, black foamcore boards, and some home made aluminum parts.

This will be a focus free camera, I intend to focus the lens at 30.4 feet, which is the hyperfocal distance at the maximum aperture of f8, this will give me the maximum depth of field at all apertures. At f64, it will focus from 3.38' to infinity.

My question here is, how can I make sure that the lens will be focussed at 30.4 feet... Is there a way to calculate the exact distance between the ground glass and the lens flange when focussed at exactly 30.4 feet ?

thanks

Eric

[Robert Hughes]

Hello Eric. Take a look at Wikipedia's lens optics (http://en.wikipedia.org/wiki/Lens_%28optics%29) entry. This should get you going:

"For a lens in air, f is then given by:

http://upload.wikimedia.org/math/a/f/a/afad8f375f5846cea8f9e8091f145d60.png

f is the focal length of the lens,
n is the refractive index of the lens material,
R1 is the radius of curvature of the lens surface closest to the light source,
R2 is the radius of curvature of the lens surface farthest from the light source.

"The focal length f is positive for converging lenses, and negative for diverging lenses. The reciprocal of the focal length, 1/f, is the optical power of the lens. If the focal length is in metres, this gives the optical power in dioptres (inverse metres)."

In short, if you place your 121mm lens about 121mm out, it's going to be correct at infinity. for 30.4 feet, you'll want to lengthen it appropriately, maybe to 125 mm (just a guess).

[BradS]

the formula you need is:

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

so for the example you gave,

focal length is: 121mm
object distance is: 30.4 feet = 9266mm
and image distance is unknown.

solving for image distance we have,
image distance = 1 / (( 1 / focal length) - ( 1 / object distance ))

substituting in numbers, i get...

image distance = 1 / ( 1/121) - (1/9266))
and running those numbers through my pocket calculator...

image distance = 122.6 mm

[ic-racer]

If you then know your distance between the nodal point and the flange focal distance, you will be there. ( but you still have to measure this and get the lensboard right there etc.)

[Mike1234]

30.4? Can't you be more precise than that?? :D

[Skorzen]

Easiest way I can think of is to mount the lens in a camera and choose a subject that distance away, focus and measure. seems like the most fool-proof method to me. Of course this assumes you have another camera to use, does not have to be 8X10, just as long as you can mount the lens on it you should be good to go.

[Peter K]

In short, if you place your 121mm lens about 121mm out, it's going to be correct at infinity. for 30.4 feet, you'll want to lengthen it appropriately, maybe to 125 mm (just a guess).
This is only true for thin lenses. Also a single thick lens has a front and a rear main- or nodal-point. And the SA is a - real thick - compound lens. Also 121mm is only the nominal focal length, the real focal lenght - as Schneider mentioned it - is 121.6mm. And this can change from production batch to production batch.

So it's easier either to calculate from the rear plane of the shutter resp. front plane of the lensboard or from the middle of the front lens to the subject and from the middle of the rear lens to the image plane resp. ground-glass.

The distance from the front-plane of the lensboard to the image plane is 132mm at infinity for the SA 121. And the distance from front- to rear nodal-point is ca. 40mm.

And if the distance from the front-lens to the subject is 9.265m (3.4 ft) the distance from the rear lens to the image plane is 108.5 mm.

But I've mentioned before there is always some tolerance so your camera needs in any case an alignment possibility. This can also be distance rings etc.

Peter

[Ken Lee]

Dunno how demanding you are for image quality, but have you made any images with the lens set accordingly ? Are you happy with the results ? Do you intend to enlarge or scan portions of the negative ?

(My first lens was a 121mm Schneider Super Angulon, and it's a superb lens. I'm just suggesting a bit of conservatism, when it comes to depth of field. We wouldn't want you to go to all that trouble, only to be surprised).

[Dan Fromm]

Theory and calculations are nice, but as Skorzen suggested measurement beats theory. This because the focal length engraved on a lens is its "name plate" focal length, not its actual. The two can differ by as much as 5%.

A real example. I once bought 20 38/4.5 Biogons that had been fitted to aerial cameras. That lens name plate focal length is 38 mm. Per Zeiss' data sheets, its nominal focal length -- the focal length is the lens is made perfectly -- is in fact 38.5 mm. Now, my lenses all went into cameras with fixed flange-to-film distances. Absolutely not adjustable, and all the same because the camera bodies were very well made. Each lens was collimated to the camera by a shim that sat between the back of the shutter and the body. The lenses' actual focal lengths were measured by the camera maker and marked on them; the range was 38.3 mm to 38.8 mm. The collimating shims' thicknesses were engraved on them, measured to 0.01 mm, and each shim was marked with its lens' serial number.

Another. Anyone who uses a Graphic quickly learns that Graflex cataloged more than one focusing scale for the same nominal focal length. The various scales are all specific to actual focal lengths.

And another. I have a 260/10 Nikkor-Q (= Process Nikkor). The lens name plate focal length is 260 mm. Its nominal focal length, according to Nikon, is 266 mm. Its actual focal length is noted on the QC slip.

Eric, calculations based on a lens' nominal or name plate focal length -- as noted they're not always the same -- can be used to estimate what's feasible, can't be used to focus the lens. There's no substitute for measuring, and what you need to measure is flange to film distance at 30.4' +/- a few hairs. Go measure. Measure from the diaphragm, that's a good approximation to location of the lens' nodes. If you don't believe me, go to Schneider's Archiv and look the lens up

[Lachlan 717]

Any chance that you can bung the lens onto a helical mount? This would allow you to estimate the hyperfocal distance (as per many suggestions above) and fine tune it with the helical thread.

This would also give you a chance to focus closer if you so desired.

Just a thought...

[Frank Petronio]

Or, since you can get into the ballpark via math, fit a pc of glass wet sanded on one side to make an inexpensive ground glass, and get precise focus by using shims and washers. Or perhaps making a contraption for your lensboard out of wing nut bolts and t-nuts and washers?

[Jack Dahlgren]

Anyone ever cut apart a film holder and slide a piece of translucent film in it as a removable ground glass?

[Eric Constantineau]

Ok, first thing, ... ehhh wow !

this is my first post on this forum... I am just amazed by the quantity of excellent replies in such little time ! Thanks to all, for me this is a proof that there is a real community out there making the difference :-)

Ok, so I'll use the calculation as a base, and after that, I'll measure with a ground glass, with an helicoidal mount, thanks for all your replies, I'll post some news in a few weeks when I'll have some results.

Eric