In a panoramic photograph dated 1861 in St. Petersburg there are no people and not just no inhabitants, but no signs of the life of a huge city at all.
You can, of course, argue about shutter speed and exposure time, but the photographs prove the opposite.
Several moving objects are displayed correctly, for example ships, there are also stationary objects, these are only three coachmen sitting in horse-drawn carriages and one citizen standing alone in an empty city.
Therefore, the exposition is not the reason for the "disappearance" of people. And to think that railway stations, key administrative buildings and structures associated with religion were emptied "by the order of a photographer" sounds like a desire to "pull an owl on the globe" in an attempt to rationally explain the complete absence of population in a huge city.
The explanation of the absence of people (complete absence) on the streets by the fact that the photo was taken in the early morning is also not consistent. Shadows reject the possibility of white night and early morning.
The population of St. Petersburg in 1870 was supposed to be 682,300 people. We do not see any of them, or rather in the whole panorama we see: 2 carts with harnessed horses but without people, two more carts and two horses without people, two more carts in each, one person (coachman), and one person pensively standing near one of the buildings.
In total, out of a population of 682,300 people, we found 3 live people and 6 horses in a panoramic photograph.
The streets are clean, the vegetation is sparse, no stray animals, no random piece of paper on the ground, there are no signs of life - the city looks dead.
St. Petersburg in 1861 looks the same as Moscow in 1867 - ghost towns without population, or rather, cities fully equipped and ready to move in.