Aperture and F-stops

Christer · May 29, 2011

This maybe a stupid question... But I'm going to ask it anyway...

I recently bought the D7000 camera that comes with the 18-105mm lense. On the camera box, it sepcifies the lense "AF-S DX NIKKOR 18-105MM f/3,5-5,6G ED VR".

I took my camera out on a sunny day, set it to aperture priority to test how many f-stops my lens was capable of, it goes from 3,5 - 22... So my question is, what does f/3,5-5,6G mean?

Derrel · May 29, 2011

At the 105mm focal length, the maximum (widest, biggest, most light-admitting) aperture value is f/5.6.

The specification f/3.5 is valid with the lens at its shorter focal lengths only: as the lens is zoomed out to the longer settings, it will not admit as much light as it can when it is in its maximum wide-angle range.

The f/3.5~5.6 specification is what is referred to as a "variable maximum aperture", since the maximum,widest, most light-admitting aperture value is dependent upon the focal length of the lens. The variable maximum aperture design is one of the main reasons the 18-105 Nikkor is such a small-diameter, lightweight lens. By allowing the lens to pass a little bit less light as it is zoomed longer, it allows the engineers to make a MUCH more-compact, smaller-diameter,lighter, and less-costly lens.

Christer · May 29, 2011

Ok, thanks - I think I understand now.

Basically, it is the highest / lowest you can set it at max and min zoom..?
What sort of numbers would you be looking for, on a lens for portrait and landscape photography?

480sparky · May 29, 2011

Simply put, a lower f/stop number means:

1. you will have a brighter image to see in the viewfinder,
2. you can have a shallower depth of field if you want to,
3. have more control to create a pleasing bokeh in a portrait,
4. you spent more money,
5. the lens is larger and heavier than is 'slower' counterparts,
6. the lens will perform better in low-light situations.

Christer · May 29, 2011

A low f/stop is the measurement of how open the lens is set. Is it possible to achieve f/0? And is it safe to say, that the lower f/stop number your lens can achieve, the better it is..?

480sparky · May 29, 2011

Christer said:
..... Is it possible to achieve f/0?..........

That's a physical impossibility. The f-stop number is a function of the focal length divided by the internal diameter. The only way to end up with an f/0 lens is to have a 0mm focal length. Have an internal diameter of 0mm is useless and creates the mathematical formula f divided by 0, which is unsolvable.

Christer said:
................... And is it safe to say, that the lower f/stop number your lens can achieve, the better it is..?

Not always. You will always have chromatic abberation, pinchushioning/barreling, coma, vignetting and a host of other optical problems to deal with.

KmH · May 29, 2011

Christer said:
A low f/stop is the measurement of how open the lens is set. Is it possible to achieve f/0? And is it safe to say, that the lower f/stop number your lens can achieve, the better it is..?

As 480sparkey pointed out f/0 is unattainable.

It is more on the point of doing photography to think of lens aperture in terms of the size of the lens opening. F/2 is a larger lens opening than f/5.6 is, though 2 is a smaller number than 5.6 is. But f/2 is a fraction (OMG! Fractions), and 1/2 is a bigger/higher number than 1/5.6 is.

Since f/2 is a fraction, the f represents the focal length of the lens. The lens opening of a 50 mm lens set at f/2 is 25 mm wide - 50 mm divided by 2 = 25 mm. A 300 mm lens set to f/2 has a lens opening that is 150 mm wide.

It costs more to make a lens that can open to a wider aperture, because the lens and all the glass elements in it have to be physically larger to accommodate a larger maximum aperture.

In the case of a zoom lens, it costs more to make a zoom lens that doesn't vary it's maximum aperture as it is zoomed for the same reason, the lens hads to be physically bigger .

Your kit lens is a "variable aperture" zoom lens, which can be made less expensively and have a lower cost to the consumer.

Here is a link to an f/0.95 lens. It's expensive, and manual focus only: Leica 50mm f/0.95 Noctilux-M Aspherical Manual Focus 11-602 B&H

Christer · May 29, 2011

Ok, I understand that part now.
I've always thought of the f/# as being the area size (i'm not really sure of all the technical terms) of the "flaps" that come out (inside the lense) making a smaller hole of light that hits the sensor. Yeah, great description huh? If you don't understand what I mean let me know, and I'll try again to explain what I mean:lmao:

480sparky · May 29, 2011

Christer said:
Ok, I understand that part now.
I've always thought of the f/# as being the area size (i'm not really sure of all the technical terms) of the "flaps" that come out (inside the lense) making a smaller hole of light that hits the sensor. Yeah, great description huh? If you don't understand what I mean let me know, and I'll try again to explain what I mean:lmao:

No need to explain.... FYI, they're called aperture blades.

Christer · May 29, 2011

480sparky said:
Christer said:

Ok, I understand that part now.
I've always thought of the f/# as being the area size (i'm not really sure of all the technical terms) of the "flaps" that come out (inside the lense) making a smaller hole of light that hits the sensor. Yeah, great description huh? If you don't understand what I mean let me know, and I'll try again to explain what I mean:lmao:

Click to expand...

No need to explain.... FYI, they're called aperture blades.

As simple as that

Helen B · May 29, 2011

f/0.5 is the theoretical maximum aperture. I can explain why if you are interested...

By the way, the f-number is not calculated from the diameter of the aperture blades. It is calculated from the diameter of the image of the aperture blades when seen from the front of the lens. This is quite an important distinction when considering zoom lenses because it allows for the changing magnification caused by the movement of the lens elements in front of the aperture blades.

Best,
Helen

Josh66 · May 29, 2011

Christer said:
Ok, I understand that part now.
I've always thought of the f/# as being the area size (i'm not really sure of all the technical terms) of the "flaps" that come out (inside the lense) making a smaller hole of light that hits the sensor. Yeah, great description huh? If you don't understand what I mean let me know, and I'll try again to explain what I mean:lmao:

Actually, it IS the area. That's why f/5.6 is half of f/4 (instead of f/8 being half of f/4).

The f/# is the area of the opening (er - the image of the opening - thanks, Helen). Since it's the area of a circle, to double it you multiply by the square root of 2 (1.41421356).

BTW - most people call the 'flaps' blades. More blades = more round opening = better.

Dao · May 29, 2011

Helen B said:
f/0.5 is the theoretical maximum aperture. I can explain why if you are interested...

By the way, the f-number is not calculated from the diameter of the aperture blades. It is calculated from the diameter of the image of the aperture blades when seen from the front of the lens. This is quite an important distinction when considering zoom lenses because it allows for the changing magnification caused by the movement of the lens elements in front of the aperture blades.

Best,
Helen