Thanks everyone!!!
This is why depth of field preview button/lever is a critical feature in macro photography so you can check what is in focus.
That's virtually impossible with the actual lighting vs the strobe lighting, IE can't see s**t through the viewer on a f/22 DOF Preview. But a good thought none-the-less.
Depth of field is purely a function of f/stop and magnification.
So no matter how you achieve the magnification you seek - macro lens, extension tubes, change in focal length, you will end up with the same depth of field if you use the same f/stop.
You may end up with different working distances with different combinations, and that can change both the perspective and the ease of lighting, but that won't change the depth of field.
I should have said the apparent depth within the frame. This changes when you get closer and extend the lens to 1:1. You get a "cropping" effect, which lowers the appearance of the depth. I know what you mean, I wasn't clear enough.
What Matt said.
Stone, there are no magic bullets. It would be nice if they existed, but they don't, except in fairy tales and at least one opera.
Dan, I wasn't looking for magic bullets, not sure that particular phrase applies here, I was looking or info, that's all, not the "perfect" lens, simply info on how they compare. I have used the 50 for YEARS it's probably the best $200 I spent on a used lens. I swear the images from this lens are still sharper than my newer 50 1.4 II lens and that's pretty good sine the 1.4 is purported to be sharper than the 1.2 L version. But I digress. I got the 100mm recently because I want to shoot in not-so-friendly environments and need the added protection the L offers, and the price right now is phenomenally cheap.
The 'effective' stop at close focusing distances should be though of more like a T-stop rather than an F-stop - it does not alter the depth of field but only the light transmission. Same as on LF when focusing closer.
Otherwise, what Matt said about DoF & image magnification in the macro range.
The most critical thing to bear in mind is what size your final print will be as that define your circle-of-confusion for DoF calculations.
VERY true, these will be HUGE prints, BIG, like a WALL, "UUUUGGE" *laughing to myself*
Honestly most of my work has been on the RZ67 with a 180mm and 2 extenders, I recently switched to the 645 format because I realized I didn't need SO much detail and the prints still translate well in 20x24 as long as the film is processed well, and it's a lot easier as a camera to manage in the positions I'm shooting in. From a physical standpoint. Although I'm still debating, I don't like switching formats mid-project, it's dangerous, but also, there's my back and my health to consider. hah!
For school however I needed to go 35mm for D***L purposes, since some of the courses are non-film based.
First, a lens's "marked" focal length is determined by when it is focused at infinity. A 50mm lens focuses at infinity at 50mm, 100 at 100, 500 at 500... The real focal length depends on where you focus. Adjusting focus with most 50mm lenses internally changes the "actual" focal length in use.
The simplest way to focus on things closer than the infinity focus point is by moving the lens away from the film, so, a 50mm lens moves out to a minimum focal local of 100mm when the 50mm extender is added. That combo can not be made to focus at infinity, the focus range becomes very short and very close to the camera.
The 100mm lens would need to move to 200mm to focus in a similar manner, and so on... (the exact point of focus will not be the same but that's a subject for a different discussion)
So no, the 100mm lens will not do the same work as a 50mm lens with a 50mm extender. The 50+50 setup will focus much closer to the camera.
The maximum diameter of the lens aperture of a 50mm lens at f/2.5 is 20mm. (50/2.5=20)
When the same lens is used at 100mm (50+50), the f# is 5 (100/20=5)
THIS!!!! Makes all the sense in the world!!!! THANK YOU!!!! ah this is what I was looking for. You're a god among men sir...
So, if the 100 is a pure without extension tubes, then it's 100/2.8=38
I get the whole "double the focal length = 1:1" aspect, same as large format with bellows. For some reason I never noticed that any adjustment needed to be made when using non-bellows cameras and my one macro lens. I'm assuming that it has something to do with the fact that I wasn't using strobes and I just never noticed the drop. But I swear when I had a MF D back and used a macro lens I didn't have any falloff either, but that could have been just a lapse in observation.
Thanks for all the info guys!