Practical consideration of the variables dynamic range, sensitivity, full well capacity and pixel area

You are here: Nature Science Photography – Contrast – Contrast in photography

Now we have the necessary knowledge to comprehensively interpret the data from the Canon 1D Mark II and S70 dynamic range tables.

An important factor in camera performance at high sensitivities is the ability to capture enough light. Small pixels have a problem with exactly that. The conversion factor transforms the number of captured photons into data values directly. We refer to the mark of 1 electron per data value as the unity gain. Once the conversion factor falls below this threshold, further sensitivity increases are futile as they only further reduce the dynamic range without recording a weaker signal. With the compatible Canon S70, unity gain occurs at around ISO 100. On the other hand, the full-frame DSLR Canon 1D Mark II only achieves unity gain at a respectable ISO 1300, indicating a 13-fold increase in sensitivity. This factor is also reflected in the pixel sizes. For the S70, this is 2.3 µm, for the 1D MK II 8.2 µm. The ratio of the resulting areas is (8.2*8.2)/(2.3/2.3) = 13.

For these reasons, dynamic range and pixel size represent a closely related pair of values. Manufacturers derive their key values from a compromise between signal-to-noise ratio on the one hand and general sensor size and cost on the other. However, as is often the case with compromises, not everyone finds this trade-off acceptable.

The size of the pixels or their area is therefore the key to a large dynamic range and a good signal-to-noise ratio. The larger the sensor, the further apart the individual pixels can be, and as the distance increases – up to a certain optimum distance – the proportion of leakage currents between them decreases, thus reducing the noise. And the larger the sensor, the larger the individual pixels can be. This is even more important because the larger a pixel is, the more photons it can capture during a given exposure. More photons mean a larger useful signal and therefore a better signal-to-noise ratio, while the noise potential of the residual electronics remains the same. This results in an optimal pixel size between 8 and 9 µm for sensors in full 35 mm format.

Why don’t manufacturers follow the maxim of the largest possible pixels for all models? In order to market high megapixel figures more attractively than a good signal-to-noise ratio, which only a handful of occasional photographers understand, manufacturers today strive to achieve the largest possible number of pixels per chip. To achieve this, either the entire sensor must grow or the individual pixels must become smaller. – After all, sensor size is equal to the number of pixels multiplied by their size. The larger the sensors, however, the lower the yield of usable specimens in production and the more expensive the production becomes. Furthermore, a larger sensor also requires larger optics and a generally larger camera body, which tends to appeal less to potential buyers. For these reasons, we now see up to 12 million pixels on a 1/1.8″ sensor in digital point-and-shoot cameras, although this results in pixel sizes well below 2 µm and all the associated drawbacks. At the other end of the scale are digital SLR cameras with full-frame sensors and pixel sizes between 6 and 9 µm, which almost exploit what is theoretically possible.

If you have a choice between two models with different sensor sizes, such as 5 megapixels on a 1/2.5″ chip for model A or a 1/1.8″ chip for model B, and you are buying a digital camera within your budget and equipment expectations, you should choose the first model, as it is likely to produce better quality images.

Next

Main Contrast

Previous The bit width of the A/D conversion