# Determining Appropriate Focal Length

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• MxwllsPersuasns
In summary: If the diode laser you have doesn't have a correction lens, you can make one by soldering a small lens to the front of the diode. Assuming the object is infinitely far away, the equation 1/ƒ = 1/μ or focal length would be the same as the distance from the lens to the image.In summary, the focal length should be the same as the distance from the lens to the image.
MxwllsPersuasns
So I'm working on a project where we're deciding a new lens to use for our laser diode. I need to determine the appropriate focal length based on the equation: 1/ƒ = 1/μ + 1/σ where ƒ is our focal length, μ is the distance from the lens to image and σ is distance from lens to object. For the purposes of our project we've determined we can assume that our object is infinitely far away and thus we have 1/ƒ = 1/μ or our focal length should be the same as the distance from our lens to our image.

Now I just need some help finding that distance. I was looking on the schematic diagram for our laser diode and wanted to determine that distance. I imagined the distance from the "image" to the lens for a laser (as opposed to a camera) would be from the point of emission of the laser to the lens. I've attached a picture of this schematic, if anyone can help me find the distance σ I would truly appreciate it.

** The little distance indicator that I drew in pencil on the diagram is where I believed the distance σ would be. But there's no way to determine this distance from the info given.

Sorry but I coulnd't understand the phisical situation.

You have a laser a lens and a plane where the image will be formed, right? Where should the object be?

Yes we have laser light coming out of a laser diode (what I interpret to be the "image" -- please correct me if I am wrong) which then passes through and gets refracted by a lens and which then travels through a crystal and through a beamsplitter into photo-detectors. Now we can assume the object that the laser is hitting is far enough away such that we can neglect the term associated with it. This then means the equation 1/ƒ = 1/σ tells us the appropriate focal length will be the length from the "image" (again, where the laser is emitted, I believe) to the lens.

I need to find that distance.

From the LASER spec sheet, look up the beam diameter and the beam divergence angle. From these, calculate where a point would have to be to match the listed values. Note that the beam from a bare diode laser is elliptical, not circular; i.e. there is a lot of astigmatism. Some, not all, prepackaged diode lasers have a built-in correction lens to yield a somewhat circular beam.

## 1. What is focal length?

Focal length is the distance between the lens of a camera and the image sensor when the subject is in focus. It is typically measured in millimeters (mm) and determines the magnification and angle of view of the image.

## 2. How do you determine the appropriate focal length for a photo?

The appropriate focal length for a photo depends on the subject, the desired composition, and the type of camera being used. Generally, a shorter focal length (e.g. 35mm) is better for capturing a wide angle view, while a longer focal length (e.g. 200mm) is better for close-up shots. It is also important to consider the crop factor of the camera, as this will affect the effective focal length.

## 3. What is the difference between a fixed and zoom focal length?

A fixed focal length, also known as a prime lens, has a fixed focal length and cannot be adjusted. A zoom lens, on the other hand, has a range of focal lengths that can be adjusted by the photographer. Prime lenses are generally sharper and faster (allowing for a larger aperture) than zoom lenses, but zoom lenses offer more versatility in terms of composition.

## 4. How does focal length affect depth of field?

Focal length is one of the factors that affects depth of field, or the area of the photo that is in focus. A longer focal length will result in a shallower depth of field, while a shorter focal length will result in a deeper depth of field. This is why portrait photographers often use longer focal lengths to achieve a shallow depth of field and blur the background.

## 5. Can I change the focal length of my smartphone camera?

Some smartphone cameras have a fixed focal length, meaning it cannot be changed. However, some newer models have multiple lenses with different focal lengths that can be switched between, allowing for more versatility in composition. It is important to check the specifications of your specific smartphone camera to see if it has this feature.

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