As a photo professor, this is awfully full of numbers and detail that would make my students' eys glaze over.  It's very informative and I like how you explain your methodology in detail.  
When we use bellowed-cameras after a while we intuitively know we get greater magnification with extension.  Then we need to do, and I find it simpler for my students to understand, is use a simple bellows extension factor in adding to the base exposure time.

Wonderfully explained on getting the right exposure with the Harman Direct Positive Paper

I first thought, ‘man, it’s just bellows-extension factor!’ But then, I realized that I now understand bellows-extension factor from the ground up, and it will not be a sorta irritating mystery any longer. This is the real deal. Thanks!

This is how a good instructor explains things. I wish I had professors that good in school.

This is one great VID.  Thank You, Joe.

Thanks, Joe, for these clear explanations ! The concept of aperture, focal length and so on was ... fuzzy from the time I used a film camera and shooting manual (well, almost 20 years ago, isn't it ?), and your video put me right back on track !

Great stuff.

Wow, so well explained, thank you!

Brilliant  Mr JVC.

Very good . You are a good teacher.

wonderful. clear, concise, illustrative. a masterclass.

you don’t need a table with actual diameter of you apertures
just divide actual extension (let’s say 270 mm for 1:1 macro) by focal length at infinity (135 in your case) and you’ll get a multiplier for your aperture
like f/8 becomes f/16 in my example (270/135)*8=16

joe, at 24:23 are you not also forgetting that as you set apature, you also vary depth of field, so as you go to the f 5.6/8/9 you are also making the dof shalower and perhaps de-focusing the subject; assuming of course as you are describing it in this video, you ARE seting focus/movements, Then doing exposure as a last step, as focus changes with the iris, so you have to re-focus afterwards, so more an A mode usage of the camera, with dof and focus point the priority, Not purely the exposure itself.

Isn't this the same as determining the bellows extension factor for the lens and focal distance? Factor = (Extension length/Focal length of lens) ^2

Mr. Van Cleave:  I learned a lot from watching your videos and want to try the process. Can you recommend a safelight or DIY set up for the Harmon paper? Many thanks!

Is this the same as calculating for bellows compensation? or does the bellows compensation need to be added onto the 'actual' aperture?

Is the aperture error accounted for when you calculate the bellows factor? Or should I add both? In general not just with this film

what if you just grab any digital camera, make a photo the way you like it (may be judge histogram?) and then just set up your LF camera on the same settings?

Joe, why don't large format lenses use the Cinema T system, then, since the f/stop is changing almost incalculably?

Hi Joe, I hope you are still monitoring the comments on these older videos.  In the video, when you measured the 135mm from the front of the ground glass, which part of the lens are you aligning to?  Front elements, back element etc... cheers!