# What F means in lenses and how to find focus distance [duplicate]

I've found some camera sensors with very low resolution to use with electronics. Camera have 150x150px and can mount different lenses type: F2.7, F4.0, F6.0.

I want to ask what F means and if I can use this value to found distance at which the object I'm pointing is in focus.

• Also photo.stackexchange.com/questions/49/… (and a tons other) May 30, 2016 at 10:46
• Plug in your values here to determine at which distance your object is in focus for your chosen f-stop. May 30, 2016 at 13:23
• Welcome to the Photo.stackexchange. While you're here don't forget to look at the reviews, take our tour, and visit the help pages. I hope your stay here is informative, helpful, and FUN.
– Stan
May 30, 2016 at 15:39
• May 30, 2016 at 15:52
• @Stan I don't know what "I think the OP was edited into an existing answer" means. May 30, 2016 at 21:35

First, I believe that the lens numbers you give refers to the "speed" of a lens. I'll begin with that and finish with what I think is your actual situation to solve.

Different lens focal lengths have different "speeds" or the ability to gather light expressed as an "f number."

Each lens has its maximum opening marked on the rim of the lens. When you see two different lenses with two different f numbers, the smaller the number (f/2.7 versus f/4.0) the more light can pass through it. Thus, in this example, the lens marked as f/2.7 is a "faster" lens than the f/4.0 one. It lets more light in at its maximum/widest aperture.

That said, the f/# of a lens has no direct, practical relationship with the sharpness or the point where the lens has its optimal focus. There are many factors that affect sharpness that have to do with the lens, light sensitive material or sensor, and the handling of the image data during and after the exposure.

Your problem has to do with finding "optimal focus" for your on-board sensor

Without knowing more about your system components, to find the optimal focus, the focal length must be known or determined. When the focal length is known, placing the sensor this distance from the rear element of the lens will give you a system that will focus at infinity. This distance is the minimum distance from the sensor to the rear element of the lens. As the object gets closer to the lens, the point of best focus gets further from the rear element of the lens. There is a point where the distances are equal. When the object is twice the focal length from the front element, the object image is also at twice the focal length from the rear element. The actual distances can be calculated by using the lens-maker's formulas found at Wikipedia.

• This is right, and helpful — I'm not sure why it's downvoted (except maybe as an expression of frustration with the question overall). I also don't understand why you really want to soften your statement to "I believe..." when it's definitely correct, but, hey, I guess that's up to you. :) May 30, 2016 at 21:37
• @mattdm - I wasn't sure about the intent of the message so I ventured a guess. I can't KNOW what people are really implying by their questions. Some are not clear. Some are confused. Then, there's the rock tumbler action of my brain.
– Stan
May 30, 2016 at 21:49
• Yeah, but we do know that F numbers refer to the speed of the lens. The chance of these lenses being labeled with with F numbers which mean something else is vanishingly small. May 30, 2016 at 21:54
• @mattdm - Hindsight is 20-20.
– Stan
May 30, 2016 at 22:01

Success at making a good picture has many ingredients. Getting the exposure “correct” is surely one key factor. As you know, the camera lens collects the light reflected off the subject. This light transverses the lens in such a way that an image of the outside world is projected on a screen inside the camera. It is this projected image that is recorded by the camera. The brightness of this projected image must be within certain limits. If it is too dim, an under-exposure results and the resulting image will be too dark. If the projected image is too bright, an over-exposure results and the picture will turn out too light.

How can the photographer or the automation built into the camera control the brightness of the image projected by the lens? The modern camera has a built-in light-measuring sensor. Armed with this information, the camera automatically or the photographer manually makes an adjustment. The result is a correct exposure, not too dark, not too bright.

There are several ingredients (adjustments) that can be manipulated to get the exposure correct. The main one is the working diameter of the lens. You see, the lens acts like a light funnel in that it gathers light. The greater the diameter of the lens, the more light it captures. The camera lens has a built in adjustable aperture that changes the working diameter of the lens. This action mimics the human eye. Our pupil in our eye does the same thing.

Now, matters pertaining to the lens are a little complicated. The brightness of the projected lens has another key ingredient. This is the focal length of the lens. When the lens is made, the distance the image-forming rays travel from lens to screen must be determined. This is called the focal length. Long focal lengths magnify like a telescope, short focal lengths deliver a wide-angle view. Now the brightness of the projected image is intertwined. We are talking working diameter (aperture) intertwined with focal length.

Because there are zillions of cameras, each with lenses that are different, it is nearly impossible to set the lens to always deliver the correct projected image brightness.

Math to the rescue: If we divide the focal length of the lens by the working diameter, we get a value we call the focal ratio. This number takes into account the two intertwined values. The result is a single number that tells us or the camera’s automation how to set the working diameter of the lens.

When we adjust the working diameter to suit the brightness of the scene, we are adjusting the focal ratio. We just say f-number or f/number or just “F” for short.

The f-numbers are a number set that allows the camera to be adjusted by doubling or halving the light energy projected on the screen inside the camera.

Here is the f-number set from brightest to dimmest:

1 – 1.4 – 2 – 2.8 – 4 – 5.6 – 8 – 11 – 16 – 22

Each going right is its neighbor on the left multiplied by 1.4. This number set pertains to the geometry of circles. Multiplying or dividing the diameter of any circle by 1.4 computes a revised circle with a capture area change of 2X.

We use the f-number to help set our lens for correct exposure. We cannot use the f-number to help us determine the distance camera to subject.