This is an amateurish attempt to answer the question of whether an APSC system has a ‘brightness’ advantage over a FF system. To do this, let’s consider the case of capturing the same composition using a FF lens (say, 70-200mm F4) on an APSC and a FF body. We can then ask the question of whether at a specific F-stop, the APSC could get more light compared to the FF shot (adjusted to have the equivalent Focal Length and F-stop).
To set up the comparison, consider using 100mm with f5 on an apsc body vs 160mm with f8 on a FF body (canon’s crop factor is 1.6, so this set up will provide the same composition for the same subject distance). Below is a summary of these and other operating assumptions:
Same lens: say, 70-200mm F4
Equivalent composition, same subject distance, identical light intensity (all spectrum)
Same shutter speed of S and same Light Intensity of L (ISO can be changed to achieve the same exposure) – this is for a ‘fair’ comparison.
All light entering camera falls uniformly onto the image circle – this is inaccurate but a decent simplifying assumption.
The table below tabulates the various parameters as well as the total ‘amount’ (can think of this as the total number of photons across different frequencies) of light reaching the respective sensors.
| Focal Length | F-stop / Diameter | Aperture Area | Amount of light entering body | Sensor area / image circle* |
| APSC Body | 100mm | F5 / 20mm | 0.00126 | L x S x 0.00126 = T | 23% |
| FF Body | 160mm | F8 / 20mm | 0.00126 | L x S x 0.00126 = T | 59% |
*The FF lens has an image circle with a minimum radius of 21.6mm (canon’s ff sensor size is 36x24mm). It means that the sensor covers an area approximately 59% of the image circle whereas an APSC sensor would cover 23% of this an FF image circle.
Based on the scenario above, the APSC body, when using a FF lens, would actually receive ‘less’ light than an FF body, so it does not quite have the ‘brightness’ advantage.
What about using an APSC lens? This is hard to compare accurately with a FF lens without the optical details of the lenses. However, an APSC lens would have a smaller image circle, and, in general, is likely to have smaller lens elements, which broadly translate into a reduced ability to ‘collect’ light. If we ignore this, and assume that the amount of light collected is simply dependent on the aperture size and shutter speed, and all of it falls uniformly across the image circle, then the total amount of light reaching the APSC sensor would also be about 59% of that reaching its image circle. All else being the same (mostly not quite true), then there is still no real ‘brightness’ advantage for an APSC body, even with an APSC lens operating ideally.
Having said all this, it ultimately depends on personal preferences with how the image looks. As long as one is happy with it, then use the system that best meet one's preferences. For me, I use APSC to save weight and cost. However, with the RF mount, the weight 'disadvantage' of FF is reduced considerably. There is still the pricing, of course, so one good lens at a time and mix in with EF lenses work for me.
