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Ok, here's my doubt. Suppose I want to get a certain exposure for an image. Keeping the ISO constant, there are two tentative possibilities here:

  1. Put the camera at f 1.4 (say) and at 1/1600 shutter speed (suppose).
  2. Put aperture at f 8.0 (say) and shutter speed at 1/50 (suppose).

Both would get me the exact same exposure. Yes, for the latter, I may have to use a tripod. But the question being, what would be the difference in the image thus obtained? I can fairly interchange the settings for the same shot (theoretically). Or can't I?

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Note that f/1.8 is 4.3 stops from f/8, but ¹/₂₀₀th of a second is only two stops faster than ¹/₅₀th. So, these two examples will not give you the same exposure — the second will be considerably darker. –  mattdm Jan 26 '11 at 20:53
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Also, not that this question is inherently bad, but you can find a lot more on the topic simply by clicking on the links for the [exposure], [aperture], and [shutter-speed] tags. –  mattdm Jan 26 '11 at 21:07
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Thanks. But I only supposed those values. Just for conveying the idea behind the question :) –  Rish Jan 26 '11 at 21:08
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If you put the first one to f/1.4 and make the shutter speed 1/1600th of a second, it's both technically correct and conveys the idea. :) –  mattdm Jan 26 '11 at 21:14
    
Got it, and it's Done! –  Rish Jan 27 '11 at 16:09
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5 Answers 5

up vote 12 down vote accepted

Different shutter speeds have an obvious different effect: more motion-blur. (That includes both subject motion and blur from any motion of the camera itself.)

Different apertures also produce different results; most notably that depth of field increases as you stop down. So, f/8 gives you a much deeper in-focus area than f/1.8. This effect is lessened on a smaller sensor (or, if for some reason you just crop out the center part of the image and blow it up), to the point where it's basically a non-factor on most point & shoot cameras.

But there are other effects of changing as well: lens sharpness, contrast, and vignetting characteristics change, usually improving significantly when you stop down a bit. (For a certain look, though, that technical improvement may not, in fact, be what you want.)

That's assuming perfect reciprocity — the idea that aperture and shutter speed really are perfectly interchangeable for exposure. With film that's not strictly true as you get to extremes — see this question on reciprocity failure. But for digital, it's not meaningfully a factor.

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The biggest difference by adjusting the aperture (f-stop) is that the depth of field changes, so whilst the exposure might stay the same, by setting the aperture to f/8.0 you'll get much more of the scene in apparent focus.

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I am not upvoting this correct answer only b/c it is partial and assumes a static scene. –  ysap Jan 26 '11 at 21:42
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If you shot at f4 and 1/200, the exposure would be the same as f8 at 1/50. However your images may not be identical for a number of reasons:

  1. Depth of field (as mentioned by Rowland Shaw's answer)
  2. Movement of the camera or subject will be more pronounced at the slower shutter speed
  3. Most lenses (especially cheaper ones) may be sharper at the narrower apertures.
  4. Shadows will be noisier for the longer exposure
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1.8->2->2.8->4->5.6->8 = ~4Ev shift in aperture You have to compensate it by shuter speed on the same number of stops in order to get same exposure i.e. 1/200->1/100->1/50->1/25->1/12->1/10

So f/1.8 and 1/200s have the same exposure as f/8 and 1/10

Actualy you can choose from any of the following: f/2 and 1/200, f/2.8 and 1/100, f/4 and 1/50, f/5.6 and 1/25, f/8 and 1/12 (probably this be 1/10) ... - all these pairs gives you the same exposure, you can choose any of them,

if you need shallow DOF - you'll choose pair with most open aperture(f/2 and 1/200), if you need wide DOF - you'll choose pair with more closed aperture (f/8 and 1/10), if you need to freeze movement - you'll chose pair with faster shutter speed (f/2 and 1/200) you probably can't steady handheld camera with f/8 and 1/12 settings

And yes aperture values are the same for different cameras/lenses combinations(if these calibrated properly). You can point two different cameras to some target(wall for example), measure exposure - they should show you the same exposure values.

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1/12th to 1/10th? –  mattdm Jan 26 '11 at 21:19
    
adding 1/3ev which we get by moving from nonstandard f/1.8 to standard f/2(-1/3ev). –  Aleksandr Reznik Jan 27 '11 at 14:51
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One thing which hasn't been mentioned in the other answers is that dynamic range often varies with the ISO setting.

More specifically, the dynamic range of the shot tends to decrease as ISO increases. So, for example, there tends to be more dynamic range at ISO 100 than at ISO 400.

Some interesting graphs here, showing range against ISO for various cameras.

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Dynamic range in digital sensors is directly related to the signal to noise ratio. Amplifying the signal by 2 (1 stop EV) will for all intents and purposes halve the SNR (30dB SNR would be rated at 27dB after a doubling in ISO). –  Nick Bedford Jan 27 '11 at 9:46
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