Alley in Pisa, Italy

by Lars Kotthoff

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54

An f-stop is kind of a combination of two terms. First off, f/N is generally the notation used to indicate the size of the diaphragm opening, or aperture, in a camera. Let me give a little detail about how that notation came about, before I go on to explain the meaning of a stop. Aperture Values and f/Stops Aperture openings are measured as fractions of ...


30

The aperture range on your lens only shows the maximum aperture for your lens at the extremes of the zoom range; i.e. f/3.5 at 18mm and f/5.6 at 55mm. There is nothing to stop you using a narrower aperture; remember a larger number is a smaller hole (the f number is the diameter of the hole as a fraction of the focal length).


20

This is a matter of some debate, and the truth is that you can pick up any number of photography books (or visit any number of photography websites) and read a variety of answers to this question, the most common of which tend to be: Focal Stop Focal Ratio Factorial System Fractional Stop Fractional Diameter Finestra Stop (Finestra is Italian for 'window') ...


20

The answer will most probably change in time. Current top cameras are said to capture around 10-11 stops at base ISO, less at higher ISOs, see DPReview tests of Nikon D3X for example. As a sidenote - you won't probably like the pictures that are processed to measure the maximum dynamic range, they'll simply lack contrast you'd expect from "normal" picture. ...


19

Although the relative aperture numbers — the ƒ stops — are the same regardless of format, the actual focal lengths of the lenses on small cameras are quite low: 5mm or 6mm at the wide end. That in turn means that the real aperture is small, which is why the diffraction limit kicks in sooner. The smaller format also means that depth of field is hugely ...


19

F-stops deal with doubling/halving the amount of light hitting the sensor. Everything revolves around twos. With the shutter speed, it's easy to understand, as you say. Every shutter f-stop is (roughly) half/double the amount of time as the previous one. Personally, I don't even bother paying attention to the numerator ("1/") part of the shutter speed; I've ...


18

The sweet spot of a lens is probably just as dependent upon the type of image capturing surface used as the lens itself. Both film and digital sensors have a limit of detail they can resolve (although large-format film has the tendency to capture FAR more detail than 35mm or digital sensors at much tighter apertures, around f/22.) Assuming you have a lens ...


18

You've hit the diffraction limit. That link has some amazing answers with a lot of detail, so I won't be redundant, but in short, once the aperture gets to be below certain physical size, diffraction causes inevitable blur. For your camera (and any other camera with an APS-C-sized sensor), the limit is a little beyond f/11. The amount of light let in ...


17

Well, one way of remembering the f-stop scale is to remember that every other value is a multiplication by two, or in more photographic terms...every four-fold jump in light availability is twice the f-stop number. As an example: Double-stops starting at the beginning: 1, 2, 4, 8, 16, 32, 64 Double-stops starting skipping the first stop: 1.4, 2.8, 5.6, 11.2 ...


16

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: 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 ...


15

Let's start with lenses at the same location, and then address the moving the longer lens farther away to get the same field of view. Lenses at the same location The 50mm f/1.4 lens has an effective aperture that's twice the diameter, and four times the area, of the 25mm f/1.4 lens. The 50mm will, therefore, collect four times as much light (four times as ...


14

Most DSLRs let you choose shutter speed and aperture at 1/3 of a stop difference (3 clicks of the dial to double or halve the amount of light), I'm not a camera designer but I would guess that since 1/3 of a stop is a small difference being able to set exact shutter speed isn't worth the extra electronics and software to support it. For aperture also add to ...


14

The f-stop is more directly relevant to photographers because it normalizes out the focal length. It then gives you a measure of how bright the image will be on the sensor relative to the scene brightness. For example, if a scene is well exposed with a 50 mm lens at f/8 and 1/200 second, then it will be well exposed with any other lens at f/8 and 1/200 ...


13

No, this is not the case. Aperture F stops are calculated on pupil size and focal length of the lens. From wikipedia In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture1) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil.2 It is a dimensionless number that is a ...


13

It's not linear because it's based on area rather than lines. Or, to put it another way, the scale isn't based on square roots. It's based on the actual math that represents the physical properties of the system, and it happens that exposure doubles when aperture size opens by a factor of the square root of 2. If you remember back to your elementary-school ...


12

This is really simple when you think about it. The additional element changes the focal length of the lens, without changing the apparent size of the aperture. That means that the relative size of the aperture decreases, so the f number does in fact actually change. (If this is unclear to you, see the bit about f numbers in this other answer. This is also ...


11

Most digital cameras use a 10 to 14-bit A/D converter, and so their theoretical maximum dynamic range is 10-14 stops. However, this high bit depth only helps minimize image posterization since total dynamic range is usually limited by noise levels. Similar to how a high bit depth image does not necessarily mean that image contains more ...


11

Needless complexity. This isn't a technical challenge, it's a usability one. Sure the mechanical tooling could be adjusted to do that, and in some high end cell phone cameras you do see some truly bizarre shutter speeds to adapt to their limited range of aperture... but WHY? What technical advantage would it present you to be able to do 1/19th instead of ...


11

The pupil (aperture opening) area is proportional to the square of the focal length (at the same f-stop). So 105mm being about twice the focal length of the 50mm, it would need 4x the pupil to be f/1.2. In other words f/1.2, or any f-stop, doesn't correspond to a fixed diameter - it increases for larger focal lengths. That also assumes both lenses gather ...


10

I think "sweet spot" is a rather poorly-defined term in general use, and in fact you'll see some people talk about the sweet-spot of a lens with regard to the sharpest aperture setting, and others talking about the sweet spot of a lens' image circle (e.g. using a full-frame 35mm lens on a cropped-sensor DSLR). You cannot generalize and say "50mm primes have ...


10

False. Going from f/2 to f/2.8 will halve the light; going from f/2 to f/4 will quarter it.


10

Dynamic range is not measured in f-stops, it is measured in stops. A stop is often used to refer to a change that doubles the value or, in the case of cameras, the amount of light. Changing the aperture by one f-stop doubles to amount of light allowed in, so in the case of aperture, a stop is an f-stop. Similarly, cutting the shutter speed in half is a ...


9

ISO Bracketing This will create images with different amounts of noise in them. I call this grain, others just call it noise. Depending on the ISO range that you are using, you may or may not cause issues by doing this. Higher in the range, if you combine different areas of a ISO bracketed image you will see different noise profiles throughout and that may ...


8

edit Totally misunderstood the question, which was whether an L lens at the same f/ number transmits an extra stop of light by virtue of lower dispersion etc., the answer is no it won't be anything like a full stop, but there is probably a small advantage. Our friends over at DXO mark test transmission of lenses, though only wide open, and they don't have a ...


8

With primes I always put a page of text up on the wall, put my camera on a tripod with a remote trigger (the self timer works too) and take a couple of photos at each major f-stop: 2.8, 4, 5.6, 8, 11, 16, 20 and then I compare them for sharpness at the center, edges, and corners. You'll see that there's a range that's sharpest and I use a label maker and ...


8

Yes, it's a physical limitation due to the size of the sensor (and thus the size of the individual photosites) that means that diffraction becomes a limiting factor sooner than it does with DSLRs, hence the aperture doesn't go smaller than f/8. Older compacts do have a larger range, my old Fuji Finepix s602 went down to f/11 see also: What is a ...


8

It's convenient for shutter speed, aperture, and ISO to operate in stops, because then it's easy to interchange them. Minus one stop of shutter means plus one stop of aperture or ISO, for example. Using fractional stops adds some versatility, but at the expense of having to cycle through more options. My DSLR lets me choose whether I want operate in third or ...


8

In this context, a "stop" refers to exposure duration. As an approximation, the distance a handheld camera moves during an exposure is directly proportional to the duration of the exposure: double the time the shutter is open and you double the movement. That (again approximately) doubles the amount of blurring in the image. Normally, to achieve an ...


7

There is an easy trick: "measure" for yourself: take your camera. Take your camera switch to manual mode close the aperture or short time until the picture of some uniform surface is pitch black Now open the aperture one full stop (2-3 clicks depending on cam) or double time Take another picture Repeat from step 4 until your picture is all white count the ...



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