In what situations you can use a high iso value and a short shutter time, or use a low iso value and a long shutter time? What will be the difference between two pictures of exactly the same scene, when you take one picture with a high iso value and a short shutter time and another picture with a low iso value and a long shutter time?
It is a question about limits:
- What ISO is acceptable to you with that camera?
- What shutter speed will give you sharp results at that focal length?
Too high ISO makes a photo noisy. Too slow a shutter speed will make the whole frame blurry. If you don't have subject motion in mind, you can raise the ISO to make sure shutter speed is fast enough. Otherwise, set shutter speed to something that gives the desired amount of blur and balance ISO with aperture to your liking, considering the resulting depth of field.
"What will be the difference" can perhaps get complicated with excessive detail, of perhaps limited interest and general use.
But "what situations" is perhaps the most basic principle of beginning photography, the Most Important and Very First Thing we must learn (with regard to using any adjustable camera). If we have not yet learned this, then we absolutely don't know nuthin' about photography yet.
It is well covered many places on the internet, often under the title of "Exposure Triangle" (Google search term). That's not a great name, but it simply refers to the three factors of exposure, which are shutter speed, aperture f/stop, and ISO sensitivity. We do of course need a combination giving proper exposure in the current scene situation, however then many combinations of the three can give the same exposure. We know to pick our choices depending on what the specific scene requires (there is no one general answer).
We would use fast enough shutter speed to stop motion in the scene, to prevent blur. Or maybe sometimes intentionally a slower shutter speed to create a little motion blur. Many scenes involve no motion, in which case we can give preference to one of the other two factors.
We would use a stopped down aperture (towards f/16) to create more depth of field, a larger zone of acceptable focus sharpness. For example, a scenic landscape often needs depth of field extending to infinity, but maybe also back to real close, which is a struggle to achieve. Or some scenes have only minimal depth range, and we can give preference to one of the other two factors.
We might sometimes open the aperture wide (towards f/1.4) to intentionally limit the zone of sharpness (Depth of Field), to intentionally blur the background for example.
The photographer aims for the goal he wants to achieve.
When possible in the situation, we would normally try to use a low ISO for less digital noise in the image. But a low light level might require greater ISO sensitivity to achieve any usable shutter speed or aperture. We would use a high ISO when the lower ISO just wasn't feasible, when it won't otherwise accumulate sufficient exposure. There really isn't any advantage of high ISO for its sake alone.
Proper exposure is selecting the best compromise choice of all three properties, best for the situation in front of the camera at the instant. We use the settings that the scene needs.
The fully automatic compact or phone camera does not require we know anything about photography. Anyone can push the button, and the camera does what it does. The basic exposure might be often OK in an average scene with sufficient light, with no special requirements, but it doesn't take much to exceed the camera's standard solution. The automation can find a choice to get an exposed picture, but it has absolutely no clue about the needs of the specific situation. That's what the photographer learns to do.
1). 'high iso value and a short shutter time' can be used when you need to freeze motion (short shutter time) along with inadequate lighting conditions (high iso). Eg: capturing sports activities.
2). 'low iso value and a long shutter time' can be used in vice versa conditions. Eg: capturing the flowing river, waterfall in sunlight.
Correct exposure is intertwined with the intensity of the illumination playing on the scene and the sensitivity (ISO) of film or digital sensor. Based on the above the chip logic (or camera operator) select an appropriate f-number (aperture) and shutter speed combination. The word “combination” is central to this question because there are umpteen combinations that fill the requirement to deliver a “correct exposure”.
For example – f/8 @ 1/100 of a second delivers the same exposing light energy as f/5.6 @ 1/200 as does f/4 @ 1/400 as does f/2.8 @ 1/800 as does f/2 @ 1/1600 as does f/1.4 @ 1/3200. The fact that these various exposure combinations are equivalent is called reciprocity or mutual dependence such as equal work for equal pay etc.
Should you elevate the ISO, this allows faster shutter speeds and or smaller working diameter aperture setting. Conversely, if the ISO is lowered, this obliges the use of slower shutter speed and lager aperture diameters.
As you know, we generally choose a lowered ISO because such a setting will yield a more faithful image. Why is this? If photographic film is used, higher ISO generally induces what is called “grain”. This is a clumping together of the metallic silver or dye that comprise the image. This limits the amount of enlargement that the film can tolerate. Conversely, digital images display “noise” which is perceived as a lack of uniformity called “granularity”.
Why noise when we up the ISO? The image projected by the lens is quire dim thus the photo receptor sites on the image chip receive a feeble amount of light energy. This energy induces an electrical charge within the photo site. The change is meager and thus it must be amplified to make it useful. The higher the ISO setting, the more the charge will be amplified. When a signal is amplified we always get some static intertwined with the good signal Thus tuning up the ISO yield more and more static. This is similar to turning up the volume on a radio, ultimately static results. In digital photography we call this static “noise”.
The nature of the beast is: We shoot with low ISO when we can. We up the ISO when the light is feeble or we need superfast shutter to freeze action or tiny working diameters to grand expanded depth of field. The bottom line is all these phenomenon are interconnected. The skilled photographer uses these distinct different settings to enhance and gain a faithful image.
Three main factors here. To summarize:
- Shutter speed - faster will "freeze" motion, slower will show motion blur. Depends on what you are looking for. Often you want specific items in a scene blurred while other items are crisp - such as a ball player swinging a bat - and so you need to find the "right" exposure for the effect you want. Also note that there is motion of the camera - how steady your hands and stance are - which will show up more in a slow shutter speed, although image stabilization will help with that aspect of low-shutter-speed blur.
- Aperture - smaller number ("wider aperture" or "larger aperture" means items out of focus will be more blurred. Since focus is a non-linear gradient, this means that with a smaller f/# you will perceive less of the image as "in focus". Larger number will generally bring more and more of the image in focus, and hide any focus issues, although at a point you start hitting diffraction issues which make everything (even at the focal point) more blurry. Selective focus (the non-focus areas will often be referred to as 'bokeh') is a key tool in drawing the viewer's eye where the photographer wants to draw it, so smaller f-stops (larger apertures) are often sought after so long as the full subject is kept within the depth of field. Focus in general is a whole different topic to discuss.
- ISO - sensitivity of the hardware sensor, or "gain". A lower ISO is less sensitive to light fluctuations, and so will show less "noise". A higher ISO is more sensitive to light fluctuations, and so will have more "noise". Note that there is often an "expanded ISO", which means that the sensor will be at its most sensitive (or least sensitive for a lower expanded ISO) and then the signal will be amplified/suppressed after the sensor and before any JPEG compression.
These three all work in concert to give the correct exposure. While exposure, too, can be an artistic choice, generally you want to properly expose a picture so that the subjects are properly colored and lit. To increase exposure, one would:
- Increase shutter time (also increasing motion blur)
- Decrease the f/stop number (also decreasing the selective focus / increasing perceived depth of field)
- Increase ISO (also increasing noise)
To decrease, do the opposite.
Which of these you choose is entirely up to you, as it is an artistic choice of the photographer which effect they want, and which effect they want to compromise on to get proper exposure. The amount of each change is generally spoken of as "stops", where a single stop is a doubling of the light (1.4x of the aperture f/# since that is a linear measure while amount of light is an area flux; 2x the shutter duration, 2x the ISO). But, just knowing which direction to pull each to increase/decrease exposure is the first and most important thing.
Also note that your camera likely has settings which will keep all but one of these set while still getting the proper exposure via metering. In a Canon the modes to let each factor solely determine the exposure are:
- Shutter only - Av (Aperture priority) with ISO set
- Aperture only - Tv (Time priority) with ISO set
- ISO only - M (Manual) with Aperture and Shutter set, Auto ISO (on newer bodies)
You can also do the opposite, set one and let the other two compensate:
- Set Shutter - Tv with auto-ISO
- Set Aperture - Av with auto-ISO (often want to set a minimum shutter speed too)
- Set ISO - P with ISO set
Or, of course, have all of them manually set:
- M (Manual) with set ISO, Aperture, and Shutter
(in which case you are on your own with respect to exposure, but this is really important for fleeting non-meterable scenes like fireworks)
In terms of post-processing, it is generally regarded as much easier to compensate for a higher ISO in post-processing using a noise reduction algorithm, than to compensate for camera or subject motion blur or lost focus (or even not-selective-enough focus). So that is often the first "compromise" to make to increase a too-dark exposure. Getting the ISO right in-camera - even if that means more noise - is always better than increasing the exposure in post-processing (because the gain is applied at the sensor less noise is introduced while the signal is low), although some cameras are approaching "ISO invariance" which allows any "ISO gain" to be applied entirely in post.
That said, if your subjects are all standing motionless, there is no reason to have a 1/1000th shutter speed, so decreasing the shutter speed to match the subject (and your camera-holding abilities) is always a "free" compromise to make and should be done before lowering ISO or compromising on selective focus.
Nobody ever CHOOSES to use a high ISO. You should always use the lowest ISO possible.. it's your get out of jail free card to allow you to use the aperture and shutter speed combination of your choice - but even though camera ISO performance improves, by definition the clearest cleanest shots still happen at lowest (native) ISO possible.
On shutter speed unless you're going for some effect around movement, or unless you have to sync with a flash, then it's some basic rules of thumb:
- Your shutter speed should be at least the same (or 1.5 times on crop sensor camera) as the focal length of your lens to avoid camera shake. e.g. if you are at the long end of a 70-200mm zoom, at the 200mm end, on a full frame camera you should have at least 1/200th of a second, and on crop sensor at least 1/300th. Now image stabilization messes with this rule of thumb, but it's still a general rule of thumb.
- Secondly if what you are shooting is moving quickly, then you need a faster shutter speed to "freeze" it.. eg if you're up close and personal with a sports event you'll want a shutter speed of 1/500th or more or you'll get movement blur.
So, for example, if you're shooting basketball you may want your aperture at about f5.6 to help keep things in focus and get some context around the player. You'll want 1/500th of a second to freeze the action.. and the ISO will likely have to be boosted so that's all possible.. say 3200 in a crappy gymnasium or 1600 in stadium lighting.
If on the other hand this is an outside game in the sun, you might only need ISO 200 because the sun is lighting things nicely, and you might even need to go up to a FASTER shutter speed because at lowest ISO there's too much light getting in at 1/500th.