Ok....I have an XL1s and was thinking about bringing it out of retirement. For discussion purposes, I'm gonna use an XL2 as an example since that's probably the most popular camera to discuss now-a-days. And don't worry, I will apply the knowledge I receive here to my XL1s.)

OK, we all know that I can put a P&S Mini35 or RRM M1 on a Canon XL2 so I can use 35mm prime lenses.
ALTERNATIVELY
I can slap a straight adapter on it like this one: http://www.letus35.com/XL.html to use 35mm prime lenses.

NOW...

I understand the 7.2x factor when using the latter option, but this is what I don't understand:

On the XL2 the 20x lens is considered 5.4mm - 108mm in terms of Video. But in 35mm terminology, this lens is considered 39 - 778mm. (7.2x)
Ok, I got that much and from here on let's speak in 35mm terms.

Ok, Now here's the big question.....

If the lens that comes with an XL2 is 39mm - 778mm. Then wouldn't it be safe to assume that if I get the latter option (a straight adapter) that I can simply put on a SLR lens that is rated 20mm wide and the picture be much wider then the stock 20x lens that comes with an XL2?????

When dividing 20mm by the 7.2x factor that would give me a 2.7mm wide lens for an XL2 wouldn't it? So theorhetically, even though I have to multiply everything by 7.2, I should still get a wider lens going this route, wouldn't I?

So please tell me, why on earth do my DP's keep forcing me to rent Mini35's when I can just buy a straight adapter and be OK?? I should still get a shallower depth-of-field this way then using the stock 20x lens, won't I??

- ShannonRawls.com

Uh...no.

A 20mm lens is a 20mm lens is a 20mm lens, no matter what format it goes on. The depth of field remains the same, all that changes is the field of view. A 20mm lens setting on your XL2 (XL1, whatever) will give you a much more telephoto field of view than a 20mm on a 35mm camera; 7.2x more telephoto than a still camera and 4.4x as telephoto as a cine camera.

I think the confusion you are having is with your math, Shannon--don't divide the 20mm focal length by 7.2, instead multiply it to get the effective focal length on a 35mm camera that will give you the same framing as that 20mm lens mounted on the DV camera.

If this wasn't the case, I would feel pretty stupid for dropping $10K on a Mini35...!

My understanding of Mini35 is that you are not using the video camer lens at all to capture the primary immage. The primary image is thrown to the ground glass by the 35mm lens. It has all the depth of field characteristics of a 35 mm film camera, still or cinema. The video camera then records that image to your digital tape. Chip size or film size is what cause greater or lesser depth of field, as I understand it.

Let me try to address this,

Where does this 7.2 ratio come from? It comes from the different sizes of the two formats we are trying to compare. The still camera horizontal film size is 36mm while the horizontal size of the 1/3” CCD is almost 5mm, The ratio between the 2 is 36/5=7.2 (close enough)
What are you really talking about when you say:
” On the XL2 the 20x lens is considered 5.4mm - 108mm in terms of Video. But in 35mm terminology, this lens is considered 39 - 778mm. (7.2x)”
Let me detail it this way:
On the XL2 the 20x lens is a 5.4mm to 108mm in terms of focal length. But to cover a similar range of angle of view with a 35mm still camera, we would have to use 39 to 778mm zoom.
You see in this comparison, we are changing two parameters; the camera format and the lens focal length to keep the image looking the same (keeping the angle of view)

The fact is the focal length of a lens is a constant for that lens regardless of the format of the camera.
The format of the camera determines the angle of view of the image being shot.

Let’s take a 20mm still camera format lens as in your example:

I choose to place this lens on a still camera at 60x the focal length in front of a chart, that is 1.2meters (almost 4’ in front of the chart)
The width of the still frame as seen above is 36mm so at that distance the chart will have to be 36mmx60=2.16m wide to fill the horizontal size of the still frame. (A little more than 7’)

Now let’s mount that same 20mm lens on the XL1 camera using a mechanical adaptor, (no optics in this adaptor). Again I place this setup at the same 1.2meters in front of the chart. And as seen above the width of the CCD is 5mm so the portion of the chart seen by the XL1 will be 5x60=0.3m wide. (11.81”) This is in no way as wide as you expected and by the way it is exactly the coverage your original XL1 zoom would give where you able to set it a the 20mm position. (Including the same depth of field)
To get the 7’ angle of view at that distance on the XL1, you need to change the focal length and use a 2.8mm lens.

The Min35 works because it creates a physical film size image on a ground glass and that image, with its original angle of view and shallow depth of field, is photographed on the entire surface of the CCD via the optical relay.

I hope this helps.

to simplify it... the mini35 allows you to shoot 35mm and telecine it to the camera of your choice. Other adaptors attach lenses to shoot images into the camera like any video camera. The mini35 shoots 35mm to an intermediary surface and then the tape records that image from that surface (the spinning ground glass in the mini35's case).
Your DP likes it because he likes the look better. You can do "more" 35mm style shooting... rack focus, etc.

mm %'s not withstanding - I think it's the DP's aesthetic desire to use the product not as a solution for a lensing measurment.