Depth of Field on the Intrepid 4x5 Mk4

Another technical topic and another camera hack. 

This dive into landscape photography is a journey with a familiar pattern, learn, apply, repeat, master(?). I am by nature and background technical and photography (especially large format) is technical in the sense that one must understand and master a series of techniques to extract the most from the camera and film to perfect an image. This of course helps not the least in the artistic and aesthetic aspects of photography in general. I am of the conviction that a good creative outcome is not helped by a lack of good technique(s). Out of focus or poorly exposed images in my view do not a good image make. This is my own style. I don't really like expired film for this reason though there is a whole legitimate set of expressions involving expired film, cross processing, Lomography, etc. This goes way back to Man Ray and his solarization of black and white prints and perhaps even further. Nothing wrong with it; it just isn't my bag.

My recent outings with my 4x5 camera have left me feeling like I don't like my understanding depth of field (DOF) on the 4x5 well enough. My current technique is to focus on the nearest point I want in focus and then guessing how much I need to stop down. Often times I close the aperture down to see where the image looks sharp by examining on the ground glass. This is difficult as the image is dim and difficult to see. 

I had begun compiling tables of DOF for each of my lenses across my 3 different lenses. The idea was to keep a cheat sheet in my backpack. I was researching more details for this and I came across an article on DarkSlides by Frank Sirona (https://www.thedarkslides.com/using-calculators-in-large-format-photography/) about Rodenstock DOF calculators. The article also has lots of practical information about selecting the correct aperture between good depth of field and diffraction so is worth a read. 

210mm lens DOF table for 4x5. You have to know the near and far distances to determine the aperture. I had planned to use a small range finder for this. Now it all sounds like a real pain!!

The calculator relies only on the lens movement distance required to focus on the near and far focus points for an image. The beauty of this method is that it is independent of lens focal length. (It does need to consider the subject magnification especially for very close up subjects.) No single table applies to all lenses. The article by Frank Sirona gives a table version of the Rodenstock calculator. Now all one needs is the lens extension between focus points in millimeters. Many monorail cameras have the lens extension scale on the camera. My Intrepid does not. This got me thinking about how to make this easier.

Lens extension vs aperture.

I setup my Intrepid 4x5 Mk3 that I have updated with the Mk4 base a little while back. I used calipers to measure the lens extension for one turn of the focus knob which turns out to be 8mm/turn. Next I made a simple scale to stick on the focus knob that shows angles that correspond to millimeter and half millimeter increments. (Something similar should be possible on the Mk3 but I have not tried it.)

1mm and half mm indications for front of Intrepid focus knob.

In practice one focuses on the far focus point, note the dial setting, focus on the near focus point, note the new dial setting. Calculate the difference in millimeters. Look up the required aperture for this difference. Set the aperture on the lens. Set the focus to halfway between the near and far focus points. 

Below is a series of photos I took showing the method in action. I aimed my Intrepid with a 210mm lens down the fence in the back garden. This gives me a range of different focus points to try out. In this example the near focus point is the corner on a barbecue while the far focus point is a gate hinge.  These are photos of the ground glass so they show significant vignetting and grain from the ground glass. 

I start by focusing on the far point which I chose to be the gate hinge. This is done at f5.6 the widest aperture of the lens. I note the setting on the knob scale. 

Far point in focus.

Near focus point out of focus.

Here is the focus knob position for the far focus point.

I then focus on the near focus point on the barbecue and note that setting on the knob scale. 

Near focus point in focus

Far focus point out of focus.

Near and far focus points on focus knob. 3mm extension shown. Mid-way point on knob (1) should be turned to be on top when photo is taken.

The difference is 3mm. I look up the 3 mm extension from the table above and find the aperture of 16 2/3. I set the aperture to f16 2/3 and then set the focus to the mid-point between the two focus points. The photos below show the ground glass image with both focus points in focus. Mr Sirona emphasizes that the correct focus point is midway between the two extremes. Conventional DOF practice is to use the 1/3 and 2/3 rule where the close focus point is 1/3 of the way from the final focal point and the far focus point is 2/3 behind the final focal point.

Far focus point in focus.

Near focus point in focus as well.

Conclusions

There are few other insights garnered from this article. First there are different tables for tilt or when magnifications become significant. This example should suffice for a lot of landscape work however. 

Mr. Sirona also recommends stopping down one more stop to reduce the circles of confusion when very large prints must be made. This limits the DOF range in the subject and forces more reliance on movements and composition where possible. 

Another insight is that if diffraction limited DOF is required then the focus extension cannot extend beyond about 8mm which is one turn of the Intrepid Mk4 focus knob. This gets into Mr. Sirona's 'yellow zone' where diffraction encroaches on sharpness. This means a quick rule of thumb is that if I can get the near and far focus points in one turn of the focus knob then I just need to set the aperture to f45 1/2 to know these two points and ones in between will be in focus. The limitation to this method is that I am incurring some softness due to diffraction. 

The minimum recommended aperture for 4x5 if 22 2/3 where diffraction is not the limit is a lens extension of 4mm. This corresponds to half a turn of the knob. So lacking a table these are useful numbers, They result in exposures that may be longer than necessary but the image should be sharp at least. 

I have printed and glued the dial to indicate distance by angle. This can work as rough guide and should improve my results. Additionally I plan to make a card that fits around the focus knob with guide points for each f-stop. Then make a new knob front consisting of two discs held together in the center by a screw and nut. The back one I would glue to the front of the knob. The front disc would have single arrow and could turn independently of focus knob. 

This system would work by first selecting the far focus position. Then align the arrow on the disc straight up to the 'f45 1/2' position (this is 0mm or 8mm after one rotation).  Next adjust the focus to the near focus point. This requires the knob be turned counterclockwise until the near focus point is in focus. The arrow will point to the required aperture to keep both in focus.  

Plate and Dial Mockup