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Category: New Questions

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Photography Question 
Candace L. Carrillo

member since: 6/12/2008
 

F-stops for Lenses?


What does the f/stop mean on lenses? For example, if you have a 85mm-200mm f4 lens, does the f4 mean that f4 is the widest aperture you will be able to use with that lens? Thanks!

1/7/2009 4:29:45 PM

 
Pete H
BetterPhoto Member

member since: 8/9/2005
  Candace,

It is odd there are not TWO numbers on your zoom. If there is only the one f/stop number; it probably means at 200mm your widest F/stop is f/4.

Usually zooms have 2 numbers. For instance, the 18-200mm Nikkor is rated at 3.5/5.6 which means the widest aperture at 18mm is 3.5 and 5.6@200mm.
These are best case numbers.


Pete

1/7/2009 6:39:32 PM

 
John G. Clifford Jr
BetterPhoto Member

member since: 8/18/2005
  The f-stop, or aperture, is a number that indicates the relative size of the opening which light will pass through.

F-stops are relative, not absolute, because they depend on the focal length of the lens. The f-stop is really a ratio. For instance a 50mm lens with a maximum aperture of f/2 has a 25mm maximum opening for light to go through (50mm divided by 2 = 25mm). At f/4, the opening would be 12.5mm, at f/5.6 it would be 50/5.6, or 8.9mm. Equally obvious, a 200mm f/4 lens has a max opening of 50mm (200 divided by 4).

You can see that a 300/2.8 lens is going to be a lot larger around than a 300/4, and that a 70-200/3.5 lens will be larger in diameter than a 70-200/4.5 lens.

1/7/2009 7:50:45 PM

 
John G. Clifford Jr
BetterPhoto Member

member since: 8/18/2005
  Yes, the f-stop number given on the lens is the maximum (largest) aperture that the lens provides. For instance, a 70-200/4 has a maximum aperture of f/4, and a 70-200/3.5-4.5 has a maximum aperture at 70mm of f/3.5, decreasing to a maximum aperture of f/4.5 at 200mm.

1/8/2009 1:34:36 AM

 
Alan N. Marcus

member since: 3/4/2006
  Hi Candace
The camera lens acts like a slide projector only in reverse. Its job is to project an image of the outside world onto a screen at the back of the camera. This screen is the surface of the digital chip or film. How much light will be admitted is a function of the lens’s aperture.

This is a circular opening is like the pupil at in the center of our eye. The area surrounding the human pupil is called the Iris because it is colored (blue – green – brown etc.) named for the Greek goddess of the rainbow.

The camera’s aperture is also adjustable as to diameter. The mechanism is a series of overlapping thin metal leaves. This device is called an iris diaphragm. The large the diameter of the aperture the more light admitted.

In addition to the size of the aperture, the focal length of the lens also has a direct effect on how much light is admitted. Long focal lengths project a larger narrow angled image however they admit reduced amounts of light. Short focal lengths project a tiny wide-angle image however they admit oodles of light.

Both the aperture and the focal length must be taken into account when assessing how much light will be admitted. Knowing this amount is critical to setting the camera for correct exposure.

Thus aperture and focal length are intertwined. Only math can take the chaos away. The problem: with a world filled with lenses of all different diameters and focal lengths, how can we come up with a universal system that makes sense? Because the volume of light admitted in proportional to the working aperture divided into the focal length, a ratio best fills this bill. In English speaking countries we write this focal ratio as f/2, f/2.8, etc. The beautiful thing about the f/number system is; any lens functioning at the same f/number as any other lens, regardless of size, admits the same quantity of light thus the chaos is remived.

By convention the number engraved on the lens barrel is the one that admits the most light. Since a zoom lens changes focal length a mechanical linkage must adjust the size of the aperture as the lens is zoomed. This linkage keeps the f/number constant through the various changes in focal length. Most zooms have more zoom than aperture meaning as the zoom approaches maximum the iris runs out of ability to enlarge. Thus two f/number values are engraved on the barrel. One is the f/number at maximum zoom, the other the effective f/number for reduced zoom focal lengths.

Nobody said this stuff is easy!

Alan Marcus (marginal technical gobbledygook)

1/8/2009 8:09:30 AM

 
Rich Collins
BetterPhoto Member

member since: 6/24/2005
 
 
 
What does the f/stop mean on lenses? For example, if you have a 85mm-200mm f4 lens, does the f4 mean that f4 is the widest aperture you will be able to use with that lens? Thanks!

Yes Candice you have it exactly right. The lower the F number on a lens the more light-gathering ability, meaning, if the number is low (say 2.8 or below) it is termed a fast lens, because less time is needed to capture enough light for a proper exposure.

And accordingly the cost of the lens follows its speed. One is often tempted to purchase the lower quality lenses as they cost less, but what is sacrificed is hand-holding, or the need for stronger light. Which I think is why we are reminded to buy the best lens we can afford.

So in particular your Nikkor lens may be all that you want it to be, but if shooting a concert or indoor sports or rodeo, or even low light sunsets with lots of clouds, it may necessitate at minimum a tripod. This is why as photographers we end with an ever-increasingly costly bank of lenses and another reason why ebay is so popular. :=]

If you double the f-number, increase the exposure time by a factor of four. You can see by the image that with each increase in F number, fully 1/2 of the light will enter the lens, hit the sensor, produce an image. This does not make a larger F number inferior, (to a point) as it is commonly held that somewhere in the mid-range of a zoom lies a lens ability to capture it's very best image.

1/13/2009 8:48:40 AM

 
Rich Collins
BetterPhoto Member

member since: 6/24/2005
  What does the f/stop mean on lenses? For example, if you have a 85mm-200mm f4 lens, does the f4 mean that f4 is the widest aperture you will be able to use with that lens? Thanks!

Yes Candice you have it exactly right. The lower the F number on a lens the more light-gathering ability, meaning, if the number is low (say 2.8 or below) it is termed a fast lens, because less time is needed to capture enough light for a proper exposure.

And accordingly the cost of the lens follows its speed. One is often tempted to purchase the lower quality lenses as they cost less, but what is sacrificed is hand-holding, or the need for stronger light. Which I think is why we are reminded to buy the best lens we can afford.

So in particular your Nikkor lens may be all that you want it to be, but if shooting a concert or indoor sports or rodeo, or even low light sunsets with lots of clouds, it may necessitate at minimum a tripod. This is why as photographers we end with an ever-increasingly costly bank of lenses and another reason why ebay is so popular. :=]

If you double the f-number, increase the exposure time by a factor of four. You can see by the image that with each increase in F number, fully 1/2 of the light will enter the lens, hit the sensor, produce an image. This does not make a larger F number inferior, (to a point) as it is commonly held that somewhere in the mid-range of a zoom lies a lens ability to capture it's very best image.

Here is a link to an image which demonstrates that phenomenom, though as it was created outside of this forum's ability to plant an .svg you can C&P to view online at Wikipedia.

http://en.wikipedia.org/wiki/File:Aperture_diagram.svg

1/13/2009 8:58:00 AM

 
Rich Collins
BetterPhoto Member

member since: 6/24/2005
  Sorry for the double post but I cannot delete. Perhaps Jim will take care of this.

1/13/2009 8:59:27 AM

 
W.  Smith VIII
BetterPhoto Member

member since: 1/5/2009
 
This board's software is about 25 years overdue for an upgrade. It is archaic.

1/13/2009 9:08:20 AM

 
Rich Collins
BetterPhoto Member

member since: 6/24/2005
  No doubt about that W.S. but it is a small business & I'm sure they'll change when affordability rears it's pretty little head. Know of any that doesn't cost much? I could ask a frined who is into that stuff.

Are you open to this Jim?

1/13/2009 3:36:21 PM

 
Rich Collins
BetterPhoto Member

member since: 6/24/2005
  Let me just add this which I ran across today unrelated to this thread: http://www.invisionpower.com/community/board/purchase.html

$300 but it is a very up-to-date Forum app.

1/13/2009 5:17:48 PM

 

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