How do I change a player's FOV (Field of View)?

You can modify the .FieldOfView property of Camera. To inherit this number from a TextBox, we can tie a listener to the FocusLost signal. By using it’s EnterPressed parameter, we can detect when someone presses return—simultaneously escaping the TextBox—and take the Text.

We have to ensure two things: a). That the Text is purely numerical, and that this number remains within a range of 70-120. This is because these are the maximum & minimum FOV degrees that we can set. To do this, we can use math.clamp() to pressure the number into being in this range.

local Player = game:GetService("Players").LocalPlayer
local Camera = workspace.CurrentCamera

local TextBox = script.Parent

TextBox.FocusLost:Connect(function(Return)
	if not (Return) then return end
	if (TextBox.Text:match("^%d+$")) then
		local Field = tonumber(TextBox.Text)
		Camera.FieldOfView = math.clamp(Field, 70, 120)
	end
end)

If TextBox.Text:match("^%d+$") confuses you, my apologies. I am simply exercising my string manipulation as I saw this as a good opportunity to do so. For a short explanation, the function is comparing a "pattern" to the string :match() is being called on. %d+ or "digits" you could say, is what I look for. By using the two anchor points ^ and $, I say that I want to compare digits from the start to the end of the string.

If that is still difficult, you can simplify the conditional with:

if (tonumber(TextBox.Text)) then
    --// Code
end

This will try to convert the string to a number, if there are any characters beside numerical, this conversion will fail, causing the if-statement to reject it.

If this helps, don’t forget to accept this answer!