# Thread: Aperture and Focal length

1. ## Aperture and Focal length

Aperture for a lenese is say 2.0 to 8.0 right?

then there is a f value associate with the lense.. that is the focal length?

how does both work in relating to each other?

2. Originally Posted by Swakoo
Aperture for a lenese is say 2.0 to 8.0 right?

then there is a f value associate with the lense.. that is the focal length?

how does both work in relating to each other?
Wah, welcome to photography.
To make it simple, aperture is always f-number, eg f2 or f8 or etc.
Focal length is always number-mm, eg 50mm or 200mm or etc.

When you buy a lens, you always specifiy the aperture (f-number), and the focal length (number-mm).
If you want to buy a zoom lens, then it is a bit more complicated.

3. Originally Posted by Swakoo
Aperture for a lenese is say 2.0 to 8.0 right?

then there is a f value associate with the lense.. that is the focal length?

how does both work in relating to each other?
The focal length and the f/stop are lens dependent. For prime lens, the f/stop range from 1.8 to 22 for lets say a Nikkor 50mm. For zoom lenses, the f/stop range will be different for different focal lengths.

From your example of 2.0 to 8.0, are you using a digital cam?

http://www.tpub.com/content/photography/14209/

4. In layman's terms:
Aperture decides how much light is allowed to pass through the lens. A smaller number means more light can pass. Eg F2 pass more light than F8.
Focal length decides how far/near your subject can focus onto the film/CCD. (in theory it is actually the physical distance between the lens front and the focal plane at the back of the camera.)

5. Originally Posted by Astin
In layman's terms:
Aperture decides how much light is allowed to pass through the lens. A smaller number means more light can pass. Eg F2 pass more light than F8.
Focal length decides how far/near your subject can focus onto the film/CCD. (in theory it is actually the physical distance between the lens front and the focal plane at the back of the camera.)

thanks
haha yours is the literal sense of it..
my is the fiction sense of it

so that means.. the f value of the lense is refering to the aperture... and it changes according to the zoom lense?

6. Originally Posted by Swakoo
thanks
haha yours is the literal sense of it..
my is the fiction sense of it

so that means.. the f value of the lense is refering to the aperture... and it changes according to the zoom lense?
Yes that is correct. On zoom lens it usually has a range for the f/stop. For example for a Nikkor 28-105 the f/stop is 3.5 to 4.5. That means that at 28mm the smallest f/stop is 3.5 and at 105mm it is 4.5.

7. Originally Posted by Swakoo
thanks
haha yours is the literal sense of it..
my is the fiction sense of it

so that means.. the f value of the lense is refering to the aperture... and it changes according to the zoom lense?
yes, "f" value refers to the aperture.

As for it changing according to the zoom, that depends. There are zoom lens models in which the aperture remains constant throughout the entire zoom range. e.g. 28-80mm f2.8 or a 70-210 f2.8, the maximum aperture achievable throughout the entire zoom range is f2.8. These are expensive and heavy pieces of glass. Then there are those whose MAXIMUM aperture vary accross the range, e.g. 28-80mm f3.5-5.6, this means that at 28mm (lens at it's widest) the maximum aperture you can achieve is f3.5, and at 80mm (lens at it's longest) the maximum aperture is f5.6. These lenses are cheaper and not as heavy as the former.

8. Originally Posted by Swakoo
so that means.. the f value of the lense is refering to the aperture... and it changes according to the zoom lense?
The f-number does not change according to the zoom lens. It is a number to let you know how much light can pass through.
So in layman term again, f2 on 50mm can pass the same amount of light as f2 on 200mm.

The only problem is, for the "cheap" zoom lens, they cannot maintain a constant f2 aperture on the whole zoom range, so when you zoom closer, (eg 50mm) you can achieve a bigger aperture (smaller f-number), then when you zoom further out, (eg 200mm) you can only achieve a smaller aperture (bigger f-number). This is due to the design of the optics inside the zoom (physics), nothing to do with the mathematics of aperture.

I know this is confusing, and take some time to get used to. But after you get used to it, you will never forget.

9. The f-number is defined as:

Focal length divided by the diameter of the aperture.

For a lens with an aperture diameter of 28mm and a focal length of 50mm, the f-number is thus 50/28 = 1.8.

The aperture diameter is adjustable, so usually the maximum aperture (minimum f-number) is quited as the specs for the lens.

For zoom lenses, it is easier and cheaper to design aperture mechanism that does not change the aperture diameter as the lens' focal length is changed (zoomed in or out), such a lens will have a varying minimum f-number. So when you see a lens specified as 35~135mm f/3.5~4.5, it means that at 35mm focal length zoom position, the f-number is 3.5, and at 135mm focal length zoom position, the f-number is 4.5.

Zoom lenses with constant maximum aperture are usually more costly, as the aperture mechanism is more complex such that the aperture diameter is varied across the zoom range (without changing the setting on the aperture ring) to maintain a constant f number.

10. Originally Posted by roygoh
The f-number is defined as:

Focal length divided by the diameter of the aperture.

For a lens with an aperture diameter of 28mm and a focal length of 50mm, the f-number is thus 50/28 = 1.8.

The aperture diameter is adjustable, so usually the maximum aperture (minimum f-number) is quited as the specs for the lens.

For zoom lenses, it is easier and cheaper to design aperture mechanism that does not change the aperture diameter as the lens' focal length is changed (zoomed in or out), such a lens will have a varying minimum f-number. So when you see a lens specified as 35~135mm f/3.5~4.5, it means that at 35mm focal length zoom position, the f-number is 3.5, and at 135mm focal length zoom position, the f-number is 4.5.

Zoom lenses with constant maximum aperture are usually more costly, as the aperture mechanism is more complex such that the aperture diameter is varied across the zoom range (without changing the setting on the aperture ring) to maintain a constant f number.

Thanks for the full-length explanation.

11. Originally Posted by iFly
Thanks for the full-length explanation.
You're welcomed!

12. woooo... i see
thanks!

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