Photographic efficiency
 

Perhaps the chief advantage of the use of the photographic plate in astronomy is its ability to record many picture points simultaneously. It also has the ability to integrate the energy which is incident on it and to build up images of objects which would otherwise remain unseen. In these senses, it can be said to be efficient.
The photometric accuracy which can be obtained depends on many factors but it is rarely better than 5%. Most photographic measurements provide photometry with an accuracy in the range 5–10%. A major drawback of photography is that its quantum efficiency is very low. The interaction of photons with the silver halide crystals in the emulsion provides blackened grains on development of the plate and their distribution gives rise to the image. Each blackened grain corresponds to the response of emulsion to energy which is arriving at a particular point. The response, therefore, behaves in a quantized way: a certain number of recorded events (blackened grains) are obtained according to the amount of energy or number of photons which are incident on the plate. The quantum efficiency, defined as the ratio of the average number of blackened grains to the number of incident photons, is typically about 0·001 or 0·1%.
In order to have some appreciation of the absolute sensitivity of photographic emulsion, it is useful to remember the adage that when using some particular equipment,
 

what the eye can see can be photographed
with an exposure of a few minutes.
 

Now a star image can be recorded with certainty if its image is the result of an accumulation of a few tens of blackened grains, say fifty. By using the value of the quantum efficiency given earlier, the number of photons required to produce this faintest detectable star image is equal to 5 × 10⁴. We have already seen that the eye can detect a star image if it is receiving of the order of 200 photons s⁻¹; therefore, the exposure time required to match the detectivity of the eye is given by
5 × 10⁴/200 s ≈ 4 min
so confirming the adage. Exposure times longer than a few minutes will, in general, allow stars to be recorded by a telescope which are too faint to be seen by eye using the same telescope.