thank you for your help
I did consider focus stacking, but there is a small problem
one should take several images and in each of them focus should be at the different point in the shot...
there are two ways to change the focus - on the lens and by minimal movement of the camera...
several people said second option is the right one, and it's done with use of macro-focusing rail - which i don't have...[/Qu
in the other words - since it is impossible to move a tripod precisely just for 2 mm I'd have to refocus on the lens for each shot...
any experiences with that ?
Yes, I use both methods, depending on the specific scenario.
Ideally, one would keep the entrance pupil of the lens stationary, and only change the sensor position to focus at a different plane. That indeed requires a very special setup, a rail, a lens holder and a camera body holder with a bellows in between. The benefit is that perspective doesn't change between focus brackets because the entrance pupil remains stationary with respect to the subject. This is a very specialized type of focus stacking, and more common with extreme macro magnifications and dedicated reversed lenses.
The next best solution is either a focus rail with an accurate positioning capability, or refocusing the lens (either manually, or automatic with e.g. Helicon Remote). The (arguably) better of these two solutions is probably the one where the entrance pupil moves the least. However, it is very difficult to find out where the entrance pupil is when using internal focusing lens designs. Nevertheless there is a reasonable chance that refocusing the lens changes the perspective the least. Helicon Remote is a great help if you can shoot tethered.
If you (plan to) do this type of work a lot, then I can whole-heartedly recommend a device like the Stackshot by Cognysis. While a relatively steep investment, it will save you huge
amounts of time. I just set it up and go do something else while the stackshot goes through the series of shots with extremely high mechanical accuracy. Especially with magnificatons larger than 1:1, where DOF is practically non-existent and an image may require 100 shots or more, it's an indispensable workhorse.
So, try refocusing first to see if focus-stacking suits your needs. You can always invest in a T/S lens (if you subjects are flat but only tilted) or other hardware later, if your requirements justify it.