What Is An F-Stop and Why Is It So Important And How To Use It

To a lot of students starting out, F‐stops are very mysterious and take a little bit of time to understand.

So what is an F-stop? The F-Stop controls two things. First is how much light is allowed to enter the lens to take your picture – your exposure. The second thing it controls is your Depth of Field.

#1 – Using F-stop for exposure

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 your camera.

I’m not going to bore you with the math behind F-stops. The smaller the F‐stop number (see diagram above), the larger the size of the actual aperture of the lens.

Meaning, F 2.8 is a larger opening, letting way more light into the lens, and onto your digital camera’s image sensor (the digital chip that creates your image). This means that F-2.8 lets in way more light than F‐11.

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, which is the exact opposite of what you would think.

Think of it like this – if you are in a darker environment, you have less light, and 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.

#2 Using F-stop for Depth of Field

Ok, so now for the second thing the F-stop controls at the same time, depth of field.

Depth of field is simply, how much in your shot is in focus VS out of focus – how much in the shot is in focus from front to back of your shot. So, if you have everything in focus in your shot, that’s a long, or large depth of field.

If you have very few things in focus, then that is called shallow depth of field. This is also called “Selected Focus”. Meaning, only a small area is selected to be in focus.

 

The shot on the left was shot at F5.6, and the shot on the right was shot at F16 with a 100mm macro lens. The focal length of the lens will affect the depth of field that the F-stops will yield. The longer the lens, the more shallow your depth of field will be at the same f-stop of a wider lens.

So a 50mm lens will look much more in focus at F5.6 than the 100mm macro lens.

Here is what I always say in class. “The smaller the F‐stop number, the smaller the depth of field you have, and the larger the f-stop number, the larger your DOF is”.

I’m personally a huge fan of using a small F‐stop. Many food shooters love this actually. You see it all the time. I use this 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.

The first shot on the left above is what I personally like. It is very selected focus. This was shot at F 5.6. Only the olive oil dish is 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.

I want you to notice something here. In the F-16 image to the left, you can actually see there is text on the cream colored prop on the left side? This is distracting.

Using selected focus can make a potentially busy background much softer, helping to direct attention to your food. After all, that’s what you want your peeps to be looking at, right? Your beautiful food, and not some white prop with text on it that we are trying to figure out what it says.

Always remember, your viewers will look at the brightest thing in the shot, or text, or what’s in focus. This happens without thinking about it.

Do this, just look up food photography on google images and with each shot, what is the first thing you noticed? If it wasn’t the food, that’s probably not a good thing if that was what the shot was supposed to be about.

Please note: When shooting with your camera in the Manual mode, as the F‐stop changes in your picture, you have to change your shutter speed accordingly to allow for the difference in the light you now need for the F‐stop you want.

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 the olive scene above.

Please keep in mind that 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 want 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.

For overhead shots – I want everything in focus, so I will use an f-stop around F8 or F11, and if there are really tall items in it, I might even shoot at F16.

Now, for all my jobs in the studio, my cameras are on manual settings, no automatic settings used at all. When I am on a farm, I will program my 5D Mark III a custom setting where the shutter speed will never go below 1/250th of a second, the ISO is also on auto, then I set the F-stop to what ever I want.

So there you have it, F-stops in a little nutshell. Let me know if you have any questions in the comments below.