View Full Version : Hyper-focal Focussing on a 4x5

15-May-2008, 10:45
Is there a way to set the front standard out, stopping down 3-4 stops and obtain an overall correct-focus across the image. Without neccessarily checking the ground glass, some interiors are so dark it's almost impossible to see the image even with the lens wide open!

I'm trying to do this on a Linhof TK 4x5 with 65 SA F8 & SA 90MM F8..WITH Bag Bellows on the 65 & standard bellows with the 90mm...It's hard enough now-Hate to think how this be when I step the format down & check the composition focus on that bitty 6x9 area..!

Ralph Barker
15-May-2008, 10:55
Although you can calculate and measure focus, doing so for interior shots isn't too practical. You may find it easier to supplement the lighting for focusing purposes, either with a high-intensity flashlight or laser pointer, and then turn that off for the exposure.

15-May-2008, 11:17
Think I'll have to upgrade my constant lighting rigs-Anyone got any old powerful light rigs, you're looking to pass on to another careful owner. I'd be willing to set the balance right with my own carefully looked after brit £s'..

Glenn Thoreson
15-May-2008, 15:38
An accurate focus scale and the hyperfocal specs for the lens in use will do the job. On interior work, you'll be working pretty close, though. Small apertures and lots of light will get you there. Big old flash bulbs are/were great for this.

Ralph Barker
15-May-2008, 16:10
You don't really need to upgrade your existing continuous-lighting kit for the exposure. Just add enough light so you see to focus, then turn that off for the actual exposure. I often use a 650W Arri fresnel for focusing, and then turn it off for the exposure. A normal flashlight, placed in the scene, can also work. Or, one of the high-intensity models might work instead of the high-wattage supplement focusing light. As noted, some folks use laser pointers, aimed at the desired focus point, and then focus on the red dot. With high-wattage lights, one just has to be careful about blowing the circuit breaker.

Emmanuel BIGLER
16-May-2008, 01:56
Is there a way to set the front standard out, stopping down 3-4 stops and obtain an overall correct-focus across the image.
It depends on what you actually consider as acceptably sharp.
One thing easy to compute, which is proper to each focal length (F.L) is the amount of total travel you have to apply to the front standard in order to bring the plane of sharpness from infinity to one metre (about 3 feet)
You simply compute the square of the F.L in millimetres and divide the result by 1000.
For the 90 you have to move the front standard by 8.1 mm
For the 65 you have to move by 4.2 mm
So in principle once you have precisely placed your infinity-focus position, setting the focus by moving the front standard half way to the position focusing to one metre, gives you the best possible position if you want to achieve an homogeneous image from 1 metre to infinity.
In other words, the hyperfocal distance will be equal to 2 metres in this setting.

But usually this cannot be achieved without sacrificing the image quality.

For a F.L. of 90 mm stopped down to f/22, the relevant circle of confusion to achieve an hyperfocal distance of 2 metres is about 180 microns, about twice as much as regarded as acceptable in 4x5".
So you'll have to stop down to f/45 which will bring some visible softness due to diffraction, but this kind of homogeneous softness is not unpleasant : just try !

For a F.L. of 65 mm, the same computation, hyperfocal distance at 2 metres, @f/22, yields a circle of 96 microns which is within the usual values for computed DOF tables in 4x5".
However a circle of confusion of 100 microns is far from what a good lens + film combination is capable to achieve in LF ! If we consider that 30 cycles/mm is something not very good, but acceptable, this corresponds to a circle of confusion of about 1000/30 i.e. 30 microns, not 100 microns !

So as a summary,
- with the 90, setting the front standard about 4mm from the infinity-focus position will bring everything "sharp" between 1 metre and infinity when stopped down to f/45.
"sharp" means in fact homogenously soft due to f/45 & diffraction but possibly acceptable, with a circle of confusion of about 100 microns;
- with the 65, you have to offset the front standard by about 2 mm with respect to the infinity-focus position in order to achieve everything sharp from one metre to infinity @f/22, the sharpness criterion being also a circle of 100 microns.

Relevant formulae are very simple, where "f" is the focal length
- required total travel "T" of the front standard to bring focus from infinity to one metre - 3 feet :
T(infinity->1metre) = (F.L in mm)^2 / 1000

- hyperfocal distance : H = f*f/(N*c)

- focus setting at a distance H brings acceptable focus from H/2 to infinity within the acceptable circle of confusion "c" for a given hyperfocal distance H.

- given a minimum distance for acceptable sharpness, this distance should be H/2 ; hence H is determined ; if the circle of confusion is taken as "c" (e.g. : 100 microns), the relevant f-stop is N = f*f/(H*c)

- amount of travel for the front standard from the infinity-focus position, required to bring sharp focus at a distance H : T = f*f/H

- circle of confusion for a given N and H : c = f*f/(N*H)