To that issue, I contacted Jonathan Danforth whose website shows examples of his work. He specializes in Daguerreotype photography and works in this medium almost exclusively.
Some of the details that he pointed out deal with the work of an early Daguerreotype photographer named John Frederick Goddard (b. 1795, d. 1866). He was a chemist and lecturer in science at the Polytechnic of Central London. [...]
A second solution was to make the plate faster by double sensitizing, and Goddard, who published on the process in 1840, used bromide vapor in addition to iodine to increase the sensitivity of the daguerreotype. Goddard refumed the iodized surface of the plate with bromide, and his accelerator, which he called "quickstuff," could reduce a ten minute exposure to one minute. His work was of considerable significance for daguerreotype photography, as it reduced the required exposure from some fifteen or even twenty minutes to as little as ten seconds. Since this work was spoken of in print as early as 1840, it could well have been in use for the Altotting photograph. Furthermore, bromide was imported into America from Germany in the 1840's, so it may have been in use here in the states, though not nearly as early as in Germany. Danforth's comment to me, after I sent him a copy of the Constanze Daguerreotype were as follows:
"1. The man with the bad comb-over in the back row is standing at a very awkward angle. He would not be able to hold such a position for very long and certainly not for five minutes.
"2. I see no motion blurs of hands which is a dead giveaway in most long exposures. People can often keep their heads still but rarely their hands. Another such giveaway would be eyes but I can't see the eyes of the people in the scan you sent. The earliest close-up portraits (1839) featured subjects with pure-white eyes (from blurring) because they kept moving them around."
Danforth concluded his comments to me by saying, "I would estimate that this exposure was likely to be in the 20-40 second range at the most."
One final point deals with what happens to the Daguerreotype when the subject moves, or some body part moves during the photography. What appears in the final photo is not a blur of the kind present in contemporary photography, but is instead a white area having nothing present, as Danforth mentioned in the early 1838 close-up Daguerreotype photography of eyes.
The bottom line here is that there is no reason to believe that this photo was taken with a long exposure, and the technology existed to allow it to be taken with a very short exposure.
Second, we have Peter Alexander:
To follow up on my previous question about clothing and hair styles, I sent the photograph, with no indication of the subject or when it might have been taken, to our local stage & costume designer. Because she does a lot of 19th-century opera, she has studied fashion decade by decade, and engages in spirited arguments about images in posters and other advertising material if it is off for a given opera -- La Boheme, La Traviata, Elixir of Love, etc. -- by a few years.
With no knowledge of the disputed facts here, she wrote back: "I suggest that the clothing and hair, probably European, are 1840's"
In a follow-up phone conversation, she was more emphatic: they could not be 1880s. The women's costumes and hair are a dead give-away: the woman to the right of "pseudo-Constanze" might be 1830s and would be old-fashioned by 1840; the woman on the right is unmistakably 1840; and the men's costumes cannot be dated.