Don't get crazy here!
Originally Posted by Michael Roberts
Don't get crazy here!
Tin Can
Hah! I should have mentioned my point of reference is the "Four Mile" hike (10.5 mi round-trip) my wife and I did last month in Yosemite from the valley floor--3,200 ft elevation gain--up to Glacier Point and back down.
I taught at RIT that depth of field exists with 2/3 of its range located behind the POF and 1/3 of its range in front of the POF especially for wide angle lenses, but then goes to around 50:50 for longer optics. The reasoning behind the change from 1/3 in front of POF and 2/3 behind POF to 1/2 in front of POF and 1/2 behind POF totally escapes me...
I have spent hours fooling with DOF on this site.
https://www.dofmaster.com/dofjs.html
As focus can elude me I have tried various schemes suggested, which is usually a choice of 2.
Tin Can
You are right. ⅓ + ⅓ doesn’t equal 1!
50/50 is also for macro.
Those 1/3-2/3 and 1/2-1/2 are just rules of thumb, not exact. Consider for example a lens focused at its hyperfocal distance - the acceptable focus will extend from half the hyperfocal distance, to infinity, so the 1/3-2/3 rule doesn't apply.
It's easiest (maybe) to think about this in the image space (ahead of/behind the film plane), rather than the subject space. The cone of light forming an image at the film is symmetrical in front of and behind the image. So the depth of focus is equal in front of and behind. However, equal displacements in the image space only translate to equal offsets in focus distance when you are near 1:1. For non-macro distances let's work out an example. Say my circle of confusion is 0.12 mm, I'm using a 150mm lens at f/16, and I'm focused 5 meters away. That requires an extension of 154.6 mm. My depth of focus is 16*0.12mm = 1.92mm in front of and behind the film. So subjects that require an extension between (154.6-1.92) and (154.6+1.92) mm will be in acceptable focus. The corresponding subject distances are 8.55 meters and 3.6 meters. So I have about 1.4 meters depth of field in front of the subject, and 3.55 meters behind it, which is not quite the 1/3-2/3 rule.
I calculated all of these numbers using the usual lens equation 1/f = 1/image_dist + 1/subject_dist. Rather than dork around with a calculator, if you go to the DOF calculator that Randy linked, https://www.dofmaster.com/dofjs.html, try plugging these numbers in. Then try changing the focus distance from 5 meters to 2 meters, or to 10 meters. You'll see that the DOF is close to half-and-half at close focus distances; as you focus farther away, the DOF behind the subject increases. Of course, objects far behind the subject are small on the film; it's possible to write this all in terms of magnifications.
Got the main camera body all glued up. 14.8oz so far. Also chopped and ripped most of the pieces for the spring back, so the next job will be to glue that up.
Proceeding wonderfully!
And now I see I could use one of my existing 8X10 backs on a copycat of your camera.
Pictures are worth 1000 words!
Tin Can
Absolutely you could make one of these and use a standard 8x10 back from another field camera. I am making this one just for 4x10 (not 5x8), so I'm using a horizontal design for the spring back, i.e., the outside dimensions are 10 1/2 x 11 1/2 instead of 11 1/2 x 11 1/2. This back will be rectangular, but not square, since I won't be reversing the back from landscape to portrait mode.
If I want to make a 4x10 portrait I will need to rotate the whole camera.
Here's the one I built to use a 121 f/8 Super Angulon at a fixed hyperfocal distance. The lens board is positionable to either be centered for a full 8x10 or aligned over each half of the 8x10 holder with a split frame dark slide for 4x10s.
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