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I have read this,so I know what an f-stop is, but what does the number on printed the lens signify?

Is this is the maximum f-stop, the ideal, or something else?

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5 Answers 5

up vote 16 down vote accepted

It refers to the maximum f-stop (which is defined as the ratio of focal length to effective aperture diameter). Smaller number means larger opening and more light:

Example of relative aperture sizes

Source: Wikipedia

Some (possibly overgeneralized) examples:

  • f0.9 to f1.2 is exceptional
  • f1.4 is very fast, usually the pro primes with less than 100mm focal length have this aperture
  • f1.8 to f2.4 - still quite fast
  • f2.8 - good pro zooms and long pro teles usually have f2.8 constant maximum aperture (constant only applies to zooms, see below)
  • f4 - good consumer-prosumer zooms and very long pro teles usually have f4 constant maximum aperture
  • f4-5.6 - aperture range denotes variable aperture zooms, meaning you lose light when zooming in (70-300 f4-5.6 is f4 lens at 70mm and f5.6 lens at 300mm)

Read more about aperture from Wikipedia.

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f:0.9 to f:1.2 is not only exceptional, but also extremely expensive! –  stevenvh Sep 18 '10 at 14:50
    
f:0.9 is only exception when it comes to photography, the lenses used to etch silicon to make computer chips are typically f/0.55 which is 2 stops faster than f/1! Not only that but they're tack sharp wide open and need to be that fast to counteract diffraction! –  Matt Grum Sep 18 '10 at 22:15
    
It'd be better to say that the pro zooms have a constant maximum aperture, not a fixed aperture. Some lenses, like mirror lenses, actually literally have fixed apertures and cannot be stopped down. –  mattdm Dec 21 '10 at 16:50
    
@mattdm - you're right, changed. –  Karel Dec 21 '10 at 20:52
    
@Matt Grum - 1. What is the size (in mm) of these lenses? 2. No wonder a new fab costs billions and billions to build. –  ysap May 27 '11 at 14:31

Apart from the aperture numbers appearing on the aperture ring, prime lenses sometimes have an additional number of f-numbers printed on them, symmetrical about a central line, most often towards the body of the camera:

alt text

They indicate depth of field for a given aperture. For the lens in the picture the selected aperture is f/5.6. Follow the lines starting at the 5.6 at the bottom and you arrive at 7m and infinity resp. So here your depth of field goes from about 7m to infinity.
Note that depth of field increases with smaller apertures (larger numbers).

(Image: http://www.kenrockwell.com/)

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1  
It's worth noting that the DOF scale printed on lenses is only approximate, and that the depth in front of and behind the subject is almost never the same (for some subject distances the DOF extends more in front of the subject and for some distances it extends more behind the subject). –  Matt Grum Sep 18 '10 at 22:17
1  
@Matt: yes, the scale is only approximate, but this should be clear from the distance scale: from 5m to 10m is only a few mm, and it's no coincidence that this scale doesn't have markers. Also, DOF isn't an exact measure in itself, since it depends on how large you define your circle of confusion. –  stevenvh Sep 19 '10 at 6:51
    
@Matt Grum - while we are at it, it is worth mentioning that the DoF itself depends on the output format as well, so these numbers are merely a guideline, or maybe proportion factor. Once should know what is the underlying standard for these figures and extrapolate the numbers to his own needs. –  ysap May 27 '11 at 14:41
    
Oh, yeah, what @stevenvh basically said in the comment... –  ysap May 27 '11 at 14:42

So, yes, all good information in other answers. The f-stop or f-stop range printed on your lens is its maximum.

You mentioned the ideal aperture.

The maximum is good for allowing you to take photos with less light at shorter exposure times, and in many situations, where the subject or you are moving, it can give you the sharpest results. But not always. Landscapes, architecture, and still life are examples of situations where you will want something else.

  • If you want the most light as fast as you can get it, you want the maximum aperture for the lens at that zoom, for that exposure. Moving subjects, moving camera, low light without a tripod, etc.
  • If you want the ideal aperture for sharp photos, you probably want f/8 or f/11. A gross simplification is that your lens is probably sharpest there. This assumes you're either on a tripod or have enough light to keep the exposure time down below 1/60 or so.
  • If you want to incorporate a specific Depth of Field to a photo and use film or high-end equipment, there are semi-complex procedures for determining the right aperture. But for consumer digital camera setups, you still basically want f/8. From the linked article:

    If you are a beginner or just shooting a 35mm or digital camera then this article addresses issues which won't bother you at reasonable apertures. Just use a tripod and choose the smallest aperture you have if you need depth of field. Avoid apertures smaller than f/8 or f/11 on digital cameras.

So, the max is great but not always the best.

EDIT

Another consideration is that beyond f/16 or so (in 35mm and smaller formats) you get increasing diffraction. With larger formats, the ideal becomes smaller. Large format lens ideal aperture are more like f/22 and diffraction is more of a problem at f/45.

Some links:

FWIW, another rule of thumb i've heard (less often) is: ideal aperture is 2-3 stops below wide open.

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Why do you suggest avoiding f/stops lower than f/8 on digital cameras? I shoot with an f/1.4 prime often and have never noticed any problems...in fact I love the results. I'm curious. –  Rob Clement Sep 22 '10 at 4:56
    
@Rob - a wide-open f/1.4 prime is the right choice in many situations, and i use the same with good results. My answer is more to point out that the ideal aperture for most lenses to achieve the sharpest results is often f/8 - f/11, given a tripod, proper exposure, etc. In those conditions i'll start in that "ideal" range, then adjust it +/- based on desired DOF. –  bill weaver Sep 22 '10 at 10:34
    
@RobClement The quote said avoid apertures smaller than f/8 or f/11 on digital cameras. That would be apertures with a higher f-number such as f/16 or f/22, not ... f/stops lower than f/8... –  Michael Clark Feb 10 at 11:25

The number printed on the lens will be the maximum aperture (smallest numerically) of which that lens is capable. There really is no single "ideal" -- for example, as you stop down, the depth of field increases. Depending on what you want and what kind of picture you're taking, under what circumstances, etc., you might want to minimize, maximize, or carefully choose the depth of field (e.g., I want my subject as sharp as possible, and this book in the background blurred enough that the text is no longer readable, but still sharp enough that it's recognizable as text and a book).

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When printed on a lens the f-stop is usually expressed as a ratio with a colon (i.e. 1:1.4) rather than the more usual f/1.4, however it still refers to the maximum apparent aperture divided by the focal length.

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You probably mean Aperture and not Depth of Field? –  ysap May 27 '11 at 14:45
    
Yep, no idea what was going through my brain there... –  Matt Grum May 27 '11 at 14:53

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