Originally posted by demp10 Shooting negatives and photographing bugs are totally different processes. Negatives are practically flat and require very thin DOF to guaranty correct focus and sharpness even when using a relative wide f-stop (e.g. F5.6 or F8). Bugs being 3-Dimensional require much greater DOF and super critical focus. The viewfinder is fine for focusing on a flat surface but for a 3-D object there is no comparison using Live View and enlarging 6x the subject.
You can use the green button to measure (after you close the lens, in which case Av mode will do the same) only if you have enough light. The camera's light meter is sensitive to around 0EV (some models are -1EV and others +1EV). If the light is not strong (e.g. 5EV) and close the lens down 5 stops, you are already at the camera's low limit. Using bellows you loose another 1 or 2 stops depending on the extension making things even worse. That's why all metering in camera's is done with the lens wide open and then calculate the exposure at the f-stop selected. The green button is for convenience for manual lenses and works great for average situations, but it has its limitations.
I agree with most of your statement. However, shooting negatives is just hard as bugs, I also shoot closeups of flowers, but not bugs. Negatives gives a particular problem because if your DOF is off and you focus on the center, than the edges will be out of focus. This is why many people reverse the lens to get a flat field effect. I use a flat field lens so I do not have to reverse it, but I have in the past before I got the lens.
I use window light to illuminate my shots because I do not have to do much color correction, so there is also plenty of light to operate the light meter. I assume that when the OP shoots bugs he is outside, thus would have the same amount of light. Having shot weddings for 40 years (10 as a full time professional) using a Mamiya TLR with bellows, I fully understand the light fall off of when extending them. I had to calculate the effect when taking close ups of the hands etc because the Mamiya is a totally manual camera. But, using the K5 in M will give correct readings when stopped down because the camera sees the light hitting it, not calculating what the light would be if it were wide open. This is especially true using window light.
When I shoot some of my flowers I use tungsten light and a black background. I also shoot those at f8 to f11 to get the good DOP. I have had good luck using my bellows for that. What I was saying to the OP was to try M and stop the lens down and let the camera do the work.
Attached is one of those shots using tungsten light and the bellows.