About NANOHA F number
In NANOHA, the open F number is indicated as F11. This is large number. On the other hand, for example, Canon MP-E65mm F2.8 is indicated as F2.8. So you may think it is much brighter lens than NANOHA. But it is not true. The meanings of F number are different in these lnses. In fact, NANOHA is brighter.
F number is calculated at infinity position. If you focus closer, the light to the image sensor is reduced and "Effective F number" get larger. In ordinary lenses, you don't have to think about this point so seriously. But, in macro lenses, it is not unignorable.
These are the Effective F numbers of Canon MP-E65mm F2.8 in every magnifications.
|Magnification||Effective F number|
Canon MP-E65mm F2.8 can take pictures only in the magnification range from 1x to 5x. It means you cannot get actual F2.8 brightness at anytime. At x5 magnification, the Effective F number is larger than F16.
On the other hand, In NANOHA, the Effective F number is F11 at x5 magnification. We indicate Effective F number at x5 magnification. NANOHA is brighter than Canon MP-E65mm F2.8 .
The small F number looks good. But, thinking about the fact Effective F number is much useful for ISO and shutter speed setting, we indicate Effective F number on NANOHA.