find the

How to calculate focal length of a lens

oh hello photography fans and welcome to

another episode of our classic cameras

in classic photo gear


Hey Dude they said result I want to show

you guys and girls how to do some

calculations when it comes to some

lenses few years ago I built this big ol

locked for my camera out of some two by

fours and I propped the lens of it now

when it comes to certain lenses they

they don't have any markings on it they

don't have the aperture markings so you

don't know what aperture and a lens may

be or they don't have a focal length

marked on it either this this lens does

but I want to show you how to calculate

an aperture and focal length of the lens

very easily you all you need is a piece

of paper tape measure something to write


a calculator perhaps unless you're

really good at math and you can do these

calculations so crazy so let me get

ready and I'll restart the camera I'll

show you how that's done so so the first

thing we need to do is determine the

focal length of the lens and ideally

this is done and on in a room where you

have a lot of natural light coming in so

you can so the light that's hitting the

lens is pretty bright so in this give

them in the basement so what I did I put

this super super bright 500 watt light

right outside their room and I'm gonna

try to project the image on the wall

right here somewhere

and you'll see the the reflector being

projected so here it is you can see the

reflector being projected now what you

do is you you can see the image gets

blurry and the sharp and blurry as you

move the lens in and out away or closer

to the subject and once you see the

image is in sharp focus you take your

tape measure and you touch the which in

this case

focal plane make sure your lens isn't

sharp focus

just about here and you take the

measurement the measurement in this case

is 20 inches so you mark that

measurement on the piece of paper next

thing that you need to do is you need to

measure the diameter of the lens at its

opening I'm gonna deter I'm gonna try to

determine the aperture and the lens wide

open so what I'm gonna do take my tape

measure and measure the not the outside

last barrel the last but the inner

element there the actual glass so that

gives me roughly okay that gives me 1.75

inches so 20 and my wide open lens is at

1.75 inches you can use millimeters

noodles call sticks fingers anything and

now I'm using my 21st century device so

20 inches from the nodal point of the

lens a bloody hand I forgot to mention

what a nodal point of the lenses the

nodal point of the lens the nodal points

of the lens is a point in the lens at

which the rays of light come in from

your subject or reflective of your

subject converge into one single point

that area in the lense is usually

located at the diaphragm or at the

aperture some old lenses of apertures in

front so it's basically the just the

center you would say of the lens but

normally this is located at the aperture

so when you measure the distance you

measure from the subject or from the

reflection from the focal plane to the

normal point in my case the normal point

of the lens is right about here it's

about 2/3 from the front of the lens

that's the note so once you have the

focal length of the lens and once you

have the diameter of the lens all you

need to do is calculate it so given the

fact that I suck at math I'm going to

use this nifty calculator built into

money communication device so I do 20

inches and you have to use the same

units if you're using noodles the length

of the neural you have to use the same

you can't use 20 inches divided by the

by the one-third of a noodle it's not

gonna work you have to use inches inches

noodles noodles call cold lengths of

pans length of pens whatever the

thickness of your eyebrows whatever you

want to do the units have to be the same

obviously so 20 inches divided it by

1.75 equals eleven point four to eight

five seven one four to six what the hell

you just look at the first number number

11 so this divided by that is roughly 11

given the bag 11.4 you just just get

just doing that so that means this lens

is aperture is f11 YY open

but now is it I know it is because it's

marked on it now where is this useful

it's useful if you want to calculate the

lens that doesn't have any markings on

it or in case of these old Kodak box

cameras you have your lens you have your

focal plane but apertures are not marked

on these damn things so what do you want

to do is what I do because it's helpful

for me to know which aperture I'm sure

usually these things have about 2

aperture settings let's see one this wow

this is advanced ok this one has 3 D so

focal white in this case you don't have

to calculate the focal length of the

lens because you already know it well

how do you know it you know it because

this thing was built so that it focuses

at certain predetermined distance and in

this case is this is where the film goes

so what I'm gonna do I'm gonna fix it

first come on you go alright so what I'm

gonna do is measure the focal length of

the lens I want to find the normal point

of the lens so in this case is in this

case is this lens doesn't really have a

boy does have aperture but it sit right

in front of the lens so I'm gonna

calculate from roughly the middle of the

lens so right here the middle of the

lens and the focal plane comes out to be

a five point two five so five point two

five inches so I know that now I need to

determine the lens at its opening that I

want to use so I'm gonna

use it at wide open first so I'm gonna

take my tape measure and then measure

the diameter of the opening in this case

is 3/8 of an inch so 3/8 of an inch is

0.375 so I have that let's calculate it

using this communication device five

point two five inches divide it by point

three seven five that gives me aperture

of f-14 which I kind of run run up to

f-16 so f-16 this is about correct if

you law and sugars at that sixteen yes

it's gonna work

so now let's determine the narrower

aperture ah this one is 1/8 of an inch

roughly so 1/8 of an inch equals 1/4


so let's calculate it wearing on the

focal plane of the lens a focal length

of the lens so we're going to calculate

5 point 2 5 divided by 0.125

that gives us a patrol app for TE which

I would probably guess it an f32 so I

would say and given the fact that this

camera has three apertures okay I'll

determined and it might be off I have

half a stop I'll calculate f-16 a

wide-open f-22 this is very simple

sometimes useful sometimes may not be

useful if you find this useful give me

thumbs up if you didn't find this useful

also give me thumbs up because I need


so ladies and gentlemen I hope you

enjoyed this video I hope you now know

how to calculate the focal length of the

lens and the aperture of the lens as

well very easy very simple no fuss stay

tuned for more videos subscribe now more

videos coming you can see I started

digging out my collection from the bags

so I started putting here arranging

nicely and I got Soviets and Germans and

some Japs I'm gonna try to build like a

shelving unit here maybe little more up

anyways stay tuned

keep shooting film keep the film alive