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I have come across feature called longer exposures. How it effects photography?

When we have to consider these features?

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3  
Based on the two questions you recently asked on the site today, I would strongly recommend reading this question and it's answer: What are the first few photography books someone should read?. I especially like "The Digital Photography Book" by Scott Kelby. –  dpollitt Apr 26 '13 at 14:06
    
Did we answer your question? ^^ –  Olivier Dulac Dec 5 '13 at 18:32

5 Answers 5

Why 8s or 30s?

Longer exposure allow you to:

  • get more light to reach the captor (night, etc)
  • have nice blurry effects (waterfalls, etc) (in that case, usually you need a dark ND filter to compensate, so that during that long exposure you don't overexpose). For exemple: photograph of buildings, using the maximum ND filter available, will make people/cars "disappear", sometimes "completely".
  • On the downside it will probably also induce more noise. For this, most pro cameras, and some (many?) non-pro too, offer some kind of "long exposure noise reduction" to compensate, offering a great reduction but it can't be 'magic'. This feature usually work by taking, after the picture, another picture with the shutter closed (a "dark frame") and with the same exposure time. That 2nd picture will be used internally to find where the noise builds up. Then the first picture is corrected using that data. But it does take twice as long (so you have to wait to take the next picture...)

Shorter exposure will allow you to:

  • Limit movements (those clouds in the background will stay sharper, because they moved less)
  • have a different blur effect (waterfalls, etc)
  • keep noise in check
  • save some time! (maybe those shorter 8s can help you setup the longer ones, but you'll need to compute the new exposure parameters to compensate for the time difference)

A camera that offers a 30s max exposure time limit will also be able to do shorter ones, but the opposite is of course not true (as the limit, by definition, limits the exposure time offered by the camera). That limit on exposure time is actually the limit time the camera allows itself to open for. If you use Tv mode, you'll only be able to go that long. If Av mode, it can only go up to that limit to compensate the exposure, etc. (At the opposite on the time scale, there is also a minimum time, which depends havily on the camera build).

However, you can often also "bypass" the long-exposure limit : some cameras offer another mode where you tell it when to close the shutter, either by pressing another time, or by holding the button pressed (and the shutter opened) until the end of the exposure time you want. [The latter is called B or Bulb mode, on Canon EOS 5d mark ii, for example]. This allows for even greater exposure times (and I don't really know its limitation, apart from noise buildup, the risk the captor may suffer if the subject is bright, the battery life which you'll need in order to save the final picture, and movements of the subject(s), or of unwanted things passing by during those long exposures... And, if not using a remote control, your own ability to keep the button pressed, if your camera is using the "keep shutter open until depressed" mode).

(interrestingly, on the short-exposure limit side: you shouldn't be able to bypass this limit, as it depends on the camera shutter speed, etc. But in some cases, you can cheat that by using specialized very-short burst flashes, and a low light environment, having your subject only illuminated for the flash duration... Sometime used to photograph water drops, bullets going through something, etc)

There is probably many more to add in each, I'll update if I can (or maybe someone will do one of those 1200+ point answer instead ^^)

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Welcome to photo.stackexchange.com. Thanks for a great answer. –  AJ Henderson Apr 26 '13 at 15:58
    
@AJHenderson Thanks for a great welcome :) (But I fear my answer is just part of what a great answer would be... I'll try to update it, or I'll gladly +1 those who will provide a better one) –  Olivier Dulac Apr 26 '13 at 16:04
    
Brevity can be as useful as detail, especially in broad topics where more focused questions would better address specific topics. Either way, for a first post, it's well worth welcoming you to the community. –  AJ Henderson Apr 26 '13 at 16:33
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You write: 'most pro cameras offer some kind of "long exposure noise reduction" to compensate' If you refer to dark-frame substraction, then not only pro cameras do that. I noticed that my Pansonic Lumix DMC-FZ5 seems to do it as well. After taking a picture with, say, two seconds exposure time, it closes the shutter for another two seconds, almost certainly to create a dark frame. –  feklee Apr 28 '13 at 18:16
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@feklee: thanks for pointing it out, I didn't see that my comment on this seemed to exclude non-pro cameras (it was unintended. I did have a lumix too, an older one, and it too allowed some kind of dark-frame substraction ^^) –  Olivier Dulac Apr 29 '13 at 11:25

You'll want to use longer exposures at night in conjunction with a tripod. It will help you capture more of what little light is available and give you clearer pictures.

If you use it during the day, you'll see blur effects for things in motion. The longer the exposure, the longer the blur "streak." I can't think of anything I'd use a 30s shutter speed for during the day though. 1-8 seconds are nice for waterfalls and such.

edit: At those shutter speeds, you'll want to use a tripod during the day too.

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so I have to consider average exposure,So that I can use it for both? Or which is best 30s or 8s.So that i can adjust for both –  stefun Apr 26 '13 at 13:43
    
Sorry, I'm not understanding your question... –  Ivan Apr 26 '13 at 13:46
    
Now say two cameras one with 30s and another with 8sec longer explosure.Which is best? –  stefun Apr 26 '13 at 13:48
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Neither is "best". Different shutter speeds have different uses. –  Philip Kendall Apr 26 '13 at 14:00
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But, a camera that is limited to 8s as the longest exposure can't do what one without that limitation can. The camera with a limit of 30s can always do shorter exposures too. –  mattdm Apr 26 '13 at 14:08

What you are referring to is shutter-speed. It can range from fractions of a second to hours. Most large-sensor cameras offer a range from around 1/4000s to 30s. The longer the time, the more light gets in.

At some point, usually over a second or so, people call it a long exposure. There is no point at which it becomes long but there is a point at which you need a tripod, even before it is a long exposure. That point depends on the camera and lens too. Anything slower than 1/15s will almost always need a tripod, so with 8 and 30s, you absolutely need one.

You should familiarize yourself with the Exposure-Triangle because one cannot simply use a longer shutter-speed without offsetting it somehow. Otherwise, the image will over-expose.

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So which is best 30sec or 8sec.? –  stefun Apr 26 '13 at 13:54
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@stefun - Which is best, driving at 60kph or 100kpmh? It depends on the road! Same with ALL exposure parameters, it depends the scene. –  Itai Apr 26 '13 at 13:58
    
Nothing is best. It is art, and subjects and situations differ. –  dpollitt Apr 26 '13 at 14:05
    
Okay.I want to re-frame question. Camera gives max 30 sec longer exposure will it be able to exposure 8 sec. But 8 sec cannot give 30 sec exposure.Am I right? –  stefun Apr 26 '13 at 14:13
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@stefun - Correct. The max and min shutter-speeds are limits. The camera can exposure at points between the limits in 1/3 or 1/2 EV stops. If the max is 30s, it will also do 8s, 10s, 12s, 15s, 20s, 25s (exact values depend on the camera). –  Itai Apr 26 '13 at 14:16

While we've mostly changed from film to digital sensors, a photograph is still formed by the summation of the photons that hit pixels (or phosphors in film) over the period of time during which it is being developed. A long exposure gives more time for photons to hit the sensor and thus produces a "brighter" image. If the shutter is open for 3 times the length, then 3 times as many photons can register on the sensor.

Unfortunately however, digital sensors are not a perfect system, some amount of noise occurs in the system which looks like photons hitting when no photons actually arrived. The longer the image is exposed, the longer this noise has to accumulate as well and thus the noisier the image gets. Long exposure noise reduction tries to figure out which of these supposed photons is an actual photon and which is actually noise, but it can't always do a perfect job, so you still do get more noise.

Otherwise, there isn't anything particularly special about a short versus a long exposure until you get in to really fast exposures. It's still simply how long the sensor is exposed to light entering the lens during which it is collecting information about the photons hitting each pixel of the sensor.

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Concerning the comment by the author "Now say two cameras one with 30s and another with 8sec longer explosure. Which is best?"

In case you are really comparing two camera models, check if a camera has a BULB mode. This allows to use an arbitrary exposure time by manually stopping the exposure (best using a remote control to avoid camera shake). So a camera with max. 8s shutter speed and BULB mode is more flexible than a camera with 30s maximum shutter speed without the BULB mode.

The use cases for these exposure times have been mentioned already. I'd like to add that you will need to invest in a strong ND filter to achieve these shutter speeds, otherwise you can only use them in very dark conditions. To use them during daylight, you need a ND3 (10 stops, equivalent to 1000x exposure time) filter. They are not cheap and not many vendors make them.

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