Recently I read a review of the Canon G3 that has a lens that goes from 24mm to 600mm, my ‘zoom’ grouse immediately came into play. While I am happy with the image quality and flexibility of my Canon 700D, I am still to overcome the fact that it still goes only till 250mm, while my older Kodak could reach up to 380mm.
A part of the review mentions that the picture goes a bit dark at the longer end of the lens (higher zoom). This aspect of lenses using a larger f/stop (smaller aperture) has always been a puzzle for me. Based on what I have read on the Internet, the Panasonic Lumix FZ1000 has a constant f2.8, but is quite expensive. Apparently, maintaining a large aperture on longer mm lenses is hard and very expensive.
Some time later, I thought I got the answer. Here is my explanation for the higher f/stop on longer zoom. Let me know if it is correct.
The reason for a higher f/stop is due to plain simple physics of light. When the aperture is small (large f/stop), light focuses properly on the sensor. When the aperture is large, light from a wider area can enter the camera and its focus will fall short of the sensor, unless a lot of management is done in the camera.