To a lot of students who do not have a photography background F-stops are very mysterious. The F-Stop is the actual opening of the lens. The aperture is the device controlling the opening inside the lens. The aperture size is controlled by the F-Stop setting on the camera for the lens. The F-Stops make no sense logically. The smaller the F-stop the larger the size of the actual aperture of the lens. Meaning, F 5.6 is a larger opening, letting way more light into the lens, onto your digital camera CCD (the digital chip that has replaced film) than F-16. So if you are in a darker environment you need to “open up” your lens to a wider F-stop. F-16 would be used in a bright light situation – less light is needed so your F-stop is now smaller, letting less light into your camera. The higher the F-stop number the smaller the actual aperture is, exact opposite of what you would think.
Think of it like this – if you are in a darker environment you have less light, you need a smaller number aperture, like F-5.6. Then if there’s more light in the scene you need a higher number aperture like F-16.
So here’s something else you have to know about F-stops. The smaller the F-stop the smaller the depth of field you have. That actually makes more sense. Depth of field ends up being how much in the shot is in focus from front to back of your shot. I’m personally a huge fan of using a small F-stop. I use what’s called selected focus to force the viewer to look at what I want them to look at in my pictures. Our eyes naturally go to what’s in focus in a shot, generally speaking.
Here is an example of two shots. The first shot is what I personally like. It is very selected focus. This was shot at F 5.6. Only a few strawberries are in focus. Then I go all the way to F-16 where you can see a lot more is in focus in this shot than the one I started with. You have so much control of what an image looks like just from choosing your F-stop.
The shot on the left was shot at F-5.6 and the shot on the right was shot at F-16
Notice as the F-stop changes your shutter speed has to change accordingly to allow for the difference in the light you now need for the F-stop you want. I am not shooting on any automatic settings. I want total control of the exposure so I am manually setting the camera to what I think looks the best. So as I “close down” my F-stop I have to slow down my shutter speed allowing more light to come into the camera. The shutter speed is much faster at F5.6 and much slower at F-16 in this strawberry scene.
Please keep in mind this is a personal preference. There is no right or wrong choice with selecting your depth of field. You have to look at your shot and decide what you what the viewer to really focus on. If you are shooting a huge table full of wonderful food, I’d try to get as much of that in focus as I could and would try to use an F-stop around 16.
Hopefully this sheds some light on why I keep harping on F-stops in class and in all your emails.