thank you for your reply. I understand what you say. Uising the same film of course would get my investigation more accurate. But I consider the evidence of my problem quite above the differences that two films can have. I shot with the Biotar on many different films and the lack of contrast (and the previous underexposure) was well recognaizable despite the different character of the film.
The Kantmere I uploaded was shot in a very sunny day, light and shadow were very strong, aperture was always around 16: Kentemere or not, those negatives deserve to have a wider dynamic range.
I'm not saying that the Kentmere is the issue.
I'm saying that the development of the Kentmere is the issue - the Kentmere has been developed to a lower contrast, so the results are lower in contrast.
If you want the same contrast as the Tri-X, then just develop the Kentmere to obtain that contrast.
Although, as I mentioned, the Kentmere looks right as it is, and the Tri-X looks overly contrasty to me.
FURTHER EDIT:
And for clarity, if you use the Canon lens again, along with the Kentmere film, and develop the Kentmere film the same way and for the same time as you did with the film exposed using the Zeiss lens, those Canon exposed Kentmere negatives will also be less developed/have lower contrast than the Tri-X ones.
In order to have a reliable comparative test of the lenses, you need to use the same scenes and light, and you need to use the same film and development.
It is quite boring to do this, but the way to do the test is on the same roll of film in the same camera, taking picture after picture, first using one lens, then switching lenses and taking the same picture again with the other lens.
First the Canon lens (metering correctly), then the Zeiss (metering correctly). Subject after subject. Do this for 5 or 6 scenes (10 or 12 lens switches). Make sure you are doing this in light that isn't changing rapidly. Try to do it in a variety of different lighting conditions, and with a variety of different subjects. Try to use the same order each time (e.g. Canon first and Zeiss second). If you can, put something in each frame that identifies which lens was used - a marker or card or something else easily identifiable.
After you develop the film, you can compare the results - side by side. All film and development variables will be gone.