Quote Originally Posted by sandermarijn View Post
Perhaps your negatives are not underexposed but underdeveloped. It can be hard to spot the difference. If there is sufficient detail in the shadows then your negatives are probably only underdeveloped.

In case there is underdevelopment this may have been caused by insufficient agitation or by too small a ratio of agitation time and total development time. (There are other possible causes of underdevelopment, but they shouldn't be relevant in this context.) Too little agitation in turn may give rise to uneven development. So yes, there could be a causal relationship between uneven development and underdevelopment mistaken for underexposure.

Two initial inversions is a bit little. Try at least 30 seconds of initial agitation. Proper initial agitation is crucial in obtaining evenly developed negatives.

Also, if you're using the smallest size Paterson tank, try to get one size bigger (the one that takes two 135 films). Agitate such that all developer is allowed to clear the film, in other words invert fully and not too short. This allows the agitation to 'look' the same to any random area of the film.

The film and the camera seem unlikely culprits.

Good luck.
sandermarijn, thank you very much for your input.
I have been using Spur HRX-III for two years now and developed many films (ca. 60) in it, sticking to the development instructions. I experienced this kind of uneven development like 3 or 4 times and was/am confused what might have caused this, when all other films turned out fine.