To further explain, you have an f4 lens with possibly a min aperture of f22 and I have an f1.8 lens with a minimum aperture of f22. We have the same cameras and the light, in a cool location, is beginning to fail as the day moves into evening. We set the ISO to what we want (the same) and we want our shutter speed to be the same too, because there is slight motion of people walking inside this location, at a normal pace.

Say we have a 250th of a sec. shutter and our aperture is f8 because we want the depth of field (more range of in focus things in the scene, from closer to us to furthest away). As the light levels inside drop, we need to change settings. We would like to keep the shutter where it is for the speed and to help prevent camera shake, if hand holding. So, we must open the aperture to achieve a correct exposure for the light, if we want to keep shooting. We drop down to f5.6 together.

Then the light gets dimmer so we must open up the aperture again, so we go down to f4 and keep shooting. Then the sun gets lower and the light gets worse, so we need to change settings again to get a correct exposure. I change down to f3.5 but you are stuck with your maximum aperture lens of f4, so now you need to do one of two things to still get correct exposure. You need to lower your shutter speed, because you are stuck at f4 but with that you may get some motion blur from the slower shutter speed. Or you may change your ISO setting, which means your images will start to get noisier/more grainy as you keep shooting.

The light gets worse still, so I change my aperture down to f2.8 and keep shooting but you have to make more decisions. Go even slower with your shutter speed or change ISO and get even more noise in your images, the choice is yours. Now the lights gets even lower so I go to my max. of f1.8 and keep shooting, you are now well into a possible situation, due to your slow f4 lens, whereby you may no longer be able to keep shooting, because the shutter will be too slow, or your ISO will give you unacceptably noisy images.

This gives you an idea of why a faster lens, that is, a lens with a lower max. aperture, is beneficial when shooting in questionable lighting conditions. There is nothing wrong with your f4 lens when we both have lots of light available to us, but when the light begins to drop off, that is when we need that better/faster lens with a lower max. f stop. Hope that made sense to you.

Thank you very much, fzk. I'm using an Olympus Micro 4/3 so my crop factor is 2. I believe your Canon has a crop of 1.6. I try to shoot in bright sun light for these kind of shots. I was in Aperture priority. My setting: ISO 1000, focal length 210mm (420mm FF equiv.), f/4.5 (since I'm only about 13ft. away the dof is pretty shallow and I usually shoot between f/5.6 and f/7.1, not sure why I was at f/4.5. However, the path between the branch and the feeder is in the focus plane and the bird is small so I got away with it.). The exposure compensation was set at -0.3ev. This speeds up the shutter speed a little and I simple increase the exposure in post. The shutter speed was 1/3200. I normally would want only 1/2000 or 1/2500. I suspect the sun brightened and I didn't adjust. Had I realized that I would have shot at f/6.3.

For these kinds of shots the Olympus has an advantage in that it is mirrorless and has both a mechanical shutter and an electronic shutter. With the electronic shutter it can shoot at 60 fps although I was at 30 fps. In addition it has a Pro Capture mode which uses the electronic shutter (no moving parts). When you push the button half-way down to focus it starts taking photos and stores them in a buffer. You can hold it in that mode for as long as you want and it will keep the last 30 photos. When you push the button all the way down it starts taking photos the normal way, but you also have the 30 shots prior. This is great for this type of shot. I knew the bird was going to fly to the feeder. I just didn't know when. Since my sequence is only 0.2 seconds long most of the photos, except maybe the last, were taken before I had time to push the shutter button down. They were stored prior to that in the buffer and transferred to the SD card.