UncleVanya, you are wrong, quantity of light that reaches the sensor is a matter of macro ratio, not a matter of register.
These two setups were used to perform some comparisons at 1:1. First, a Tamron 90mm macro F/2.8, then a Rodenstock Rodagon 135mm F/5.6 and some extension tubes and helicoid (
21.75 cm).
Both lenses were used at F/8 and 100ISO, and for a same subject in the same light conditions, the shutter speed is respectively 1/15" and 1/13" to get about the same light level on the subject. The difference between 1/15" and 1/13" is not significant because the camera vs target alignments are handmade and not very precise.
In fact, I measured the speed shutter, at 1:1, F/8, 100ISO, under the same lighting and on the same subject (a post-it block with a rectangle which dimensions are the same that an aps-c sensor) on 7 setups:
Reference---------------Focal length (mm)-----Speed
Oshiro (Venus)------------------------60-----------1/10"
Rodenstock APO Rodagon---------50-----------1/10"
Tamron SP Di------------------------- 90-----------1/15"
Panagor PMC-------------------------90------------1/13"
Rodenstock Rodagon--------------105------------1/13"
Rodenstock Rodagon--------------135----------- 1/13"
The Rodenstock APO Rodagon 50mm setup uses a 5.75 cm extension length.
The Rodenstock Rodagon 105mm setup uses a 16.25 cm extension length.
The Rodenstock Rodagon 135mm setup uses a 21.75 cm extension length.
You can verify that, at 1:1, with several extension distances, the shutter speed is about the same.
The slight differences may be due to alignment differences and different optical formulas.
By the way, I own several close-up lenses (Nikon 5T, 6T, Sigma life size converter, Raynox DCR-250, LOMO AL-4) and if you are still sceptical I can include some close-up lenses in my test setup to complete the table.
Your error is very common, when I began macro I did the same.