Distance Scale & Hyperfocal Focusing

[rupal]

Hi all. I have noticed that the "distance scales" for most lenses i see only go up to 10m & then straight to infinity, lets say i have a 150mm lens (for 4x5) & i want everything from 7.1m to Infinity to be in focus, then 14.2m would be the Hyperfocal Distance to focus at @ f16 (According to my DOF calculator & COC of 0.066), but the Distance Scale for my 150mm lens only goes up to 10m & then Infinity. I could just stop down to f22 & then 10m would be the Hyperfocal Distance, but i am wandering if there is a way to know the distance the lens is focused at after 10 metres. ps, this is just a general photography question & not taking into account view camera movements etc.

Any advice much appreciated. cheers - Rupa

[rdenney]

Hi all. I have noticed that the "distance scales" for most lenses i see only go up to 10m & then straight to infinity, lets say i have a 150mm lens (for 4x5) & i want everything from 7.1m to Infinity to be in focus, then 14.2m would be the Hyperfocal Distance to focus at @ f16 (According to my DOF calculator & COC of 0.066), but the Distance Scale for my 150mm lens only goes up to 10m & then Infinity. I could just stop down to f22 & then 10m would be the Hyperfocal Distance, but i am wandering if there is a way to know the distance the lens is focused at after 10 metres. ps, this is just a general photography question & not taking into account view camera movements etc.

Take a tape measure and measure off 14.2 meters. Set up the camera at one end, and stand up a tripod (or something easy to focus on) at the other end to use as a target. Focus (accurately) on the target with the camera's ground glass, and then mark the scale with a piece of tape or a small dab of white paint.

Rick "whose large format cameras have no distance scales" Denney

[Emmanuel BIGLER]

...I am wondering if there is a way to know the distance the lens is focused at after 10 metres.

Hello !

The question is directly related to the capabity of the human eye aided with a suitable loupe to determine the proper focus of a 150 mm large format lens. It is also directly related with the precision and sensitivity of the mechanical system that allows you to focus.
An alternative problem would be : how precisely can I determine that my lens is focused on infinity and not closer.

Let's start with the second problem. Imagine that you bring your camera+lens to a technician equipped with a good quality collimator and suitable means to properly set your ground glass in the focal plane. You can imagine various means to retrieve this position with suitable pieces of metal precisely milled as spacers between your view camera standards or any other system, e.g. blocking your standards at the technician's bench and focus with an helical only for all subsequent shooting sessions ! ;-)

With a 150mm focal length, the theoretical offset with respect to the focal plane that brings sharp something located at 10m, 20m etc is given by the following chart :
- 50m : offset the ground glass by 0.45mm w/respect to the true focal plane;
- 20m offset = 1.2mm
- 10m : offset = 2.25mm
- 5 m : offset = 4.5mm

So in other words, assume that you know your focal plane position precisely, you can directly convert the displacements of your ground glass into metres to the subject according to Newton's formulae:
offset = f^2/(distance)
distance = f^2/(offset)

Now you need to be able to precisely set your offset within a fraction of a millimeter. This is easier to achieve with an helical focusing mount (you can mount your 150mm lens on an helical if you wish) but reaching a precision of one millimeter is, nevertheless, well within reach of a good translation system based on a rack-and-pinion lake many 4x5 cameras, either field cameras or monoarails view cameras.
If the hyperfocal distance is 20m, no need to focus more precisely than about one 1 mm.

-------

Now let's come back to the first problem, if I have no idea at all of where my focal plane is located and if I have no means to precisely measure the displacements of my standards, which precision can I reach by focusing an image on a ground glass ?

It is difficult to give a precise answer, it depends on the kind of ground glass you use and on the contrast of the image, related to the performance of the lens and the contrast of the subject.
The most precise focusing method is aerial focusing using the parallax method, something that was used for focusing large format repro benches in the past.

With a friend, we have tested our ability to manually focus and get the sharpest possible image of a USAF 1951 test target located at about 3 metres from the camera.
The camera was fitted with a digital back, so we could make a large number of test images very quickly.
The camera was a LF monorail with a rack-and-pinion focusing system, the focusing knob translates the standard by 2 cm for each full turn of the knob which is something very common in all 4x5 cameras.
With view camera lenses in the 90-210 mm range that were tested, we were able to reach something like 50 cycles/mm very consistently in the image. I remember that with a 100mm lens we reached this figure of 50 cy/mm at f/11 simply by a quick focusing with a good 6X loupe.
In those conditions, the maximum defocusing spot has a diametre of about 20 microns, and @f/11 this means that our focusing was the right one with a tolerance on the ground glass position of about plus or minus 220 microns.

Even if we consider that the focusing cannot be better that about .45mm of offset with respect to the right position, it means that we can appreciate if something is located at 50 metres instead of infinity with our 150mm lens, but this is the limit of our simple ground glass focusing system.

[going off-topic, medium format !]
I have tested the sensitivity of a split image rangefinder built in the ground glass of a vintage Rolleiflex TLR, the viewing lens opens @f/2.8
I have found that I can appreciate the proper focusing with a tolerance of plus or minus 0.15mm.
[end off-topic]

In principle, with a view 150mm camera lens offering a max aperture of 2.8, the same rolleiflex split image rangefinder could allow you to reach the same precision. Unfortunately most LF lenses open@4.5 or 5.6 and I doubt that we could reach the same figure of plus or minus .15mm.

So the practical conclusion is that in reasonable conditions with a 150mm lens stopped down to f/16 or f/22, everything located beyon 20 m is at infinity and therefore it would be meaningless to engrave an helical with : 50m, 100m, etc....

[rupal]

Thanks rdenney & Emmanuel for the helpful explanation. ps, i normally don't focus by hyperfocal distance, I usually follow the method described in "Procedure II" in the "How to focus the view camera" article & find i get good results.

thanks rupa