# What do the F and f numbers written on lens mean?

On this particular lens rim is written "F:2.6 f=3.8mm". What does it mean? Focal point 3.8 mm and aperture 1/2.6 of the focal point ???

• I don't see that other question answering this one, at least not obviously. First, this labeling is different on the front ring of the lens than it often is in listed lens names (which that question is about), and second, this isn't a brand covered by the detailed answers. – mattdm Aug 17 '18 at 13:44

The `F 2.6` is the f-stop, "speed" or how light sensitive the lens is. A lower number means more light can hit the sensor.
A lower number also means you can easier get the background out of focus.

The `f=3.8 mm` is the focal length of the lens. This is the number that describes if it's a wide angle lens (like a GoPro) or a telephoto lens (like sports and wildlife photographers use).

In this case 3.8 mm needs to be multiplied with the crop factor of the sensor if you want to get the "35 mm equivalent". After googling I see that the 35mm equivalent is a 37 mm lens.

Also note that the camera only has digital zoom. The result will be, let's just say, not what you hoped for if you use the digital zoom.
My advice is to look for something with optical zoom. It's a massive difference. Even a used old camera is probably better, if you ever need the zoom. If you never ever need the zoom than this is probably fine.

It seems their marketing department needs to learn how the focal length works, since this diagram shows a longer focal length as wider. That is backwards.

`F:###` (in this case `F:2.6`) gives the maximum "focal ratio" or aperture stop for the lens. A lower number means more light-gathering ability. Read What is aperture, and how does it affect my photographs? for more on this. You will also commonly see this written on a lens as `1:###`, like `1:2.8` or `1:1.4`. Some lenses give a range, like `1:3.5-5.6` — this means that over the zoom range of the lens, the maximum aperture changes. (It doesn't give the minimum aperture. See this for more if you're curious.)

`f=###mm` (in this case `f=3.8mm`) gives the focal length of the lens. In combination with the sensor size, this determines the field of view for the lens. See What is focal length and how does it affect my photos? for details on focal length. This is also often just written as ###mm, like `50mm` or `35mm`. A range here (like `24-70mm`) indicates a zoom lens. Note that even if the lens is marketed using "35mm equivalent" terminology, lenses are almost always labeled with their true focal length.

F:2.6 f=3.8mm engravings on the lens -- what they mean. The camera lens is a copycat of the human eye. It gathers light rays stemming from object. As these rays traverse the lens their path is changed. The lens causes these rays to travel taking on a converging path. Now they project an image of the outside world onto the surface a digital image senor (or film) located inside the the rear of the camera. The shape of the lens regulates the distance lens to projected image. We call this distance the “focal length”. This lens has a focal length of 3.8mm (about 1/7 of an inch) about the height of three stacked pennies.

The focal length is the most important fact about a lens. It reveals the properties of the lens such as how big images of objects will be and as well as the angular field of view. In other words, does the lens operate as a wide-angle, or telephoto, or does it deliver a “normal” view of the world. These are significant facts because if we fit a lens with a focal length about equal the corner-to-corner measure of the image sensor, the view resulting will be “normal”. Meaning the view matches what we see with our own eyes. If we mount a shorter lens, the view delivered will be “wide-angle”. If we mount a longer lens, the view will be magnified (telephoto).

F:2.6 is a ratio. In math, a ratio is written using the : (colon symbol). A ratio is used to show the relationship between two numbers. As an example – in your class there are 12 boys and 14 girls. To compare we can write this as 12:14. In optical jargon, F stands for “focal ratio”. This value compares the focal length to its diameter. We know the focal length is 3.8mm. We know the focal ratio is 2.6. Now we can calculate the diameter. It is 3.8 ÷ 2.6 = 1.46mm. We can’t reap much from the diameter be we can gain much from the focal ratio which is 2.6.

By convention we write focal ratio as f/2.6. This value compares the bigness of the image projected by this lens to other lenses. The focal ratio (f-number) is universal in that any lens, regardless of focal length or working diameter, will project an image that is reasonably the same as to brightness as any other lens set to the same focal ratio. Photographers place great value on the f-number setting of a lens. Such a system allows them to set their cameras so the exposure used will yield a satisfactory image i.e. not too dark / not to light. The f-number has many other uses. You would do well if you read up on camera lenses, particularly on focal length and f-number settings.