I am getting unexpected results using HDR and Bulb Ramping. What’s going on?

There are two quirks with the way cameras deal with Bulb modes; Minimum exposures, and exposure stepping.

The former means that many entry-level cameras have a 1-second minimum exposure, and the latter means that some camera models will ‘snap’ to the nearest exposure step, creating inaccuracies in the exposure duration.

Both of these issues are discussed in great depth in this article: Quirks with using Bulb mode

(Short link to this Knowledgebase article is http://tri.gg/kb-bulb)