• New Patreon Tier and Early Access Content available. If you would like to support AVForums, we now have a new Patreon Tier which gives you access to selected news, reviews and articles before they are available to the public. Read more.

Focal Lengths

sachin

Standard Member
Hi all,

I wonder if you can help me understand something. I've been reading about lenses and have come across the concept of focal lengths. I've seen that at the wide angle, lenses can have focal lengths of 35mm, 28mm, 24mm and 20mm for example. My question is, in technical terms, what do these numbers mean? What is the difference between 20mm and 35mm?

Thanks,

Sachin
 

shotokan101

Banned
Hi all,

I wonder if you can help me understand something. I've been reading about lenses and have come across the concept of focal lengths. I've seen that at the wide angle, lenses can have focal lengths of 35mm, 28mm, 24mm and 20mm for example. My question is, in technical terms, what do these numbers mean? What is the difference between 20mm and 35mm?

Thanks,

Sachin

See here :-

SLR Camera Simulator | Simulates a digital SLR camera

Jim
 

sachin

Standard Member
These simulators are great. I will definitely use them but I still wonder about the numbers. What do the numbers represent?
 

sachin

Standard Member
Actually I've decided I don't like the Nikon simulator - the other one is excellent though - much more like my Canon.:D
 

eduk

Distinguished Member
Actually I've decided I don't like the Nikon simulator - the other one is excellent though - much more like my Canon.:D

LOL. The Nikon one is useful for trying out focal lengths before buying a lens because it goes from 10 to 600mm. Perfect for seeing how an ultra-wide-angle or a long lens would be like.
 
Last edited:

eduk

Distinguished Member
Hi all,

I wonder if you can help me understand something. I've been reading about lenses and have come across the concept of focal lengths. I've seen that at the wide angle, lenses can have focal lengths of 35mm, 28mm, 24mm and 20mm for example. My question is, in technical terms, what do these numbers mean? What is the difference between 20mm and 35mm?

Thanks,

Sachin

The difference is 15mm :D

I see what you are asking, what does the mm mean?

Imagine taking a ruler and measuring the distance between the sensor in your camera and centre of the focal lens (usually the front bit of glass in your lens). That distance measured in mm is it.

It's a bit more complex than that though and is probably best described as being the distance that light has to travel from the centre of the focal lens to the sensor.
 

sachin

Standard Member
The difference is 15mm :D

Lol :D

I see what you are asking, what does the mm mean?

Imagine taking a ruler and measuring the distance between the sensor in your camera and centre of the focal lens (usually the front bit of glass in your lens). That distance measured in mm is it.

It's a bit more complex than that though and is probably best described as being the distance that light has to travel from the centre of the focal lens to the sensor.

I see, I think. Thanks.
 

Calzor Suzay

Active Member
Also to add to the confusion the same mm can be different FF Full Format or 'cropped' sensors, for example a canon 5dmk II is a FF camera so a 70-200mm lens will that amount, but if you say put that lens on a canon 7d because the sensor is smaller than FF it would in the case of the 7d times everything by 1.6 so the 70-200 would become 112-320mm, simples :)
 

ncmoody

Active Member
Also to add to the confusion the same mm can be different FF Full Format or 'cropped' sensors, for example a canon 5dmk II is a FF camera so a 70-200mm lens will that amount, but if you say put that lens on a canon 7d because the sensor is smaller than FF it would in the case of the 7d times everything by 1.6 so the 70-200 would become 112-320mm, simples :)

NO it does not BECOME a 112-329mm lens.

It acts on the camera as though you had put a 112-329mm lens.on a Full Frame camera.
 

eduk

Distinguished Member
NO it does not BECOME a 112-329mm lens.

It acts on the camera as though you had put a 112-329mm lens.on a Full Frame camera.

I think for the sake of simplicity it's fine to say so.
 
D

Deleted member 27989

Guest
that is why I like the Nikon simulator as I can choose my body and my lense and get it perfect without all these old school conversions.

I'm with ncmoody on this one.
 

weetsie

Well-known Member
No it is not.

i agree, i think its quite important that you understand that a focal length is a physical property of a lens and no camera sensor regardless of the size can change that.

feild of view is determined by focal length and sensor size, saying FOV and focal length are the same thing will only lead to much confusion later.
 

sachin

Standard Member
So to summarise, focal length is the distance from the end of the lens to the sensor and that does not change regardless of sensor size. The field of view does change depending on sensor size.

The natural questions are:

1. What then is field of view?
2. Why does it change depending on sensor size?
3. What determines the multiplication factor used? For instance, in our example we used 1.6.

Thanks,

Sachin
 

eduk

Distinguished Member
No it is not.

The focal length does not change only the effect.

Semantics. To say that a 70-200 would become 112-320mm is fine for the sake of simplicity. I guess you fail to understand what that means though, no matter.
 

eduk

Distinguished Member
So to summarise, focal length is the distance from the end of the lens to the sensor and that does not change regardless of sensor size. The field of view does change depending on sensor size.

The natural questions are:

1. What then is field of view?
2. Why does it change depending on sensor size?
3. What determines the multiplication factor used? For instance, in our example we used 1.6.

Thanks,

Sachin


1.
Field of view - Wikipedia, the free encyclopedia

2.
Digital photography - Wikipedia, the free encyclopedia

3.
Image sensor format - Wikipedia, the free encyclopedia
 
D

Deleted member 27989

Guest
that nikon lens simulator gives a very good visual example of the effect for fov.
 

Faldrax

Well-known Member
So to summarise, focal length is the distance from the end of the lens to the sensor and that does not change regardless of sensor size. The field of view does change depending on sensor size.

The natural questions are:

1. What then is field of view?
2. Why does it change depending on sensor size?
3. What determines the multiplication factor used? For instance, in our example we used 1.6.

Thanks,

Sachin

It's slightly more complex than that, as the focal length is actually the distance from sensor to the centre of a single element lens (such as a magnifying glass) that would give the same magnification as the actual multi-element lens. IE You cannot simply use a ruler to measure the focal length of a lens.

For field of view, imagine a line from the camera to centre of the subject. The field of view is the angle from this line that is also in view in the image.
So a telephoto lens will have a small field of view (narrow angle), a 'wide angle' lens has a wide field of view (wide angle!).

This is normally expressed in terms of a 35mm sensor (or pre-digital, piece of film). If you instead have a smaller sensor, then you only 'see' part of that image - in effect you have narrowed the angle of the field of view.

Since (for a given sensor size) the field of view of a lens is determined by it's focal length, this reduction in field of view ('crop') due to reduced sensor size is equivalent to the field of view that would have been seen with a longer focal length lens on the 'full frame' sensor.

NB: It is only field of view that changes - the actual focal length (and magnification) does not change.

The multiplication factor ('Crop Factor') is calculated from the relative sizes of the sensors.
 

ncmoody

Active Member
Semantics. To say that a 70-200 would become 112-320mm is fine for the sake of simplicity. I guess you fail to understand what that means though, no matter.

I did not fail to understand

I suggest you read weetsie's comment above.
 

sachin

Standard Member
It's slightly more complex than that, as the focal length is actually the distance from sensor to the centre of a single element lens (such as a magnifying glass) that would give the same magnification as the actual multi-element lens. IE You cannot simply use a ruler to measure the focal length of a lens.

For field of view, imagine a line from the camera to centre of the subject. The field of view is the angle from this line that is also in view in the image.
So a telephoto lens will have a small field of view (narrow angle), a 'wide angle' lens has a wide field of view (wide angle!).

This is normally expressed in terms of a 35mm sensor (or pre-digital, piece of film). If you instead have a smaller sensor, then you only 'see' part of that image - in effect you have narrowed the angle of the field of view.

Since (for a given sensor size) the field of view of a lens is determined by it's focal length, this reduction in field of view ('crop') due to reduced sensor size is equivalent to the field of view that would have been seen with a longer focal length lens on the 'full frame' sensor.

NB: It is only field of view that changes - the actual focal length (and magnification) does not change.

The multiplication factor ('Crop Factor') is calculated from the relative sizes of the sensors.

That was almost poetic. That's exactly what I was asking and the explanation was brilliant. Thanks Faldrax.
 

The latest video from AVForums

Fidelity in Motion's David Mackenzie talks about his work on disc encoding & the future of Blu-ray
Subscribe to our YouTube channel

Full fat HDMI teeshirts

Support AVForums with Patreon

Top Bottom