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ISO vs Shutter Speed vs Aperture
NewCreation517
Registered Users Posts: 78 Big grins
I'm sure this is a relatively basic topic, and I apologize for taking up valuable forum space with it. But for the life of me, I can't seem to find a "Light Settings for Dummies" anywhere! Could someone point me in the right direction in discovering when to use what in terms of these three variables (ISO vs Shutter Speed vs Aperture)? :scratch Thanks so much!
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Think of ISO as film speed - how much or how little light is required to properly expose an image on film or a silicon sensor. ISOs typically run from a low of 50 ( very slow - requires lots of light) to 1600 or 3200 ( very fast - requires a lot less light) An ISO of 200 requires one half the light for exposure that an ISO of 100 requires. An ISO of 400 needs 1/4th the light that ISO 100 requires for a proper exposed image.
An aperture is how large an opening there is in the iris diaphragm of a lens. The larger the iris diaphram opening, the more light that can enter a camera. The convention for describing aperrtures is f stops and they run from f1.4 - f2 - f2.8 - f4 - f5.6 - f8 - f11 - f16 etc Each f stop is one half the size ( lens frontal area ) of the one below it. f2 admits twice as much light as f2.8 for instance. f4 admits one half the light that f2.8 admits.
The halving/doubling of light via an apertures is convenient, because shutter speeds are set up in the same ratios 1/30th 1/60th 1/125th 1/250th 1/500th 1/1000th
Thus you can set a camera at f16 at 1/100th sec and then change the aperture to f11 at 1/200th and not change the exposure on the film. Or you can go to f8 at 1/400th.
Here are three links about ISO, aperture and shutter speed for futher reading
http://en.wikipedia.org/wiki/Aperture
http://en.wikipedia.org/wiki/Film_speed
http://en.wikipedia.org/wiki/Shutter_speed
I wrote a little blurb about setting exposure outoors without a light meter here
Moderator of the Technique Forum and Finishing School on Dgrin
Reading is good, but Andy says reading is hard.
Moderator of the Technique Forum and Finishing School on Dgrin
Think of ISO, Aperture and Shutter speed as three sides of a triangle.
If you want to keep a high shutter speed, ISO and Aperture are the two
parameters you have to work with. If you want to work in low light and
with high ISO, shutter speed and aperture are your tools. Pick one side
of the triangle and the other two are what you have in the way of
adjustment.
It's an over simplification but you get the idea.
All three control how much light you get, but the difference is in the side effects.
I see the triangle this way:
Shutter speed controls how much you stop action.
Aperture controls how much depth of field you get.
ISO controls how much detail and noise you get.
If you require a specific shutter speed, you must use an aperture that can get you that shutter speed or higher (if you want to stop action) or lower (if you want to blur it).
If you require a specific depth of field, you must use a shutter speed that can get you that aperture or higher (if you want more depth of field) or lower (if you want less depth of field).
If in the previous two cases, the other variable can't be adjusted far enough to reach a correct exposure, you must turn to ISO and pay the price in noise. For example, if you need 1/60th of a second to stop action but opening the aperture as far as it will go still underexposes the picture, you have to turn up the ISO. In some cases, all three hit their limits and you either give up some quality, change your requirements, or abandon the shot (or change your lens, or add more light).
If there is activity like games, I choose shutter priority. If we're doing worship or teaching, I choose aperture priority - the students aren't moving around much so I can afford a slower shutter speed. I usually shoot everything indoors at ISO400 to get a decent background lighting with the regular facility lights we have - that is just my preference. I use external flash to highlight faces and folks that are more than 12 feet away - which is most of the time.
Its useful to use your camera to tell you what settings it thinks are good in auto mode, then tailor your settings to what you want to capture. By using your camera as a light meter, knowing the principles that Pathfinder noted is really key - everything is basically one half or twice as much. Try to memorize the f-stops and that will free you up to tailor your shots more to your liking - the ISO and shutter speed stuff is easier to remember.
- Mike
IR Modified Sony F717
http://2H2OPhoto.smugmug.com
What is aperture?
Aperture is the relative size of how wide the lens opens when the shutter is released. It is one of the factors that determines how much light enters the camera. The other factor is shutter speed, which determines how long the shutter stays open.
Here is the f-stop scale
f/1
f/1.4
f/2
f/2.8
f/4
f/5.6
f/8
f/11
f/16
f/22
f/32
f/45
f/64
Each step on the scale lets in half as much light as the previous step. So f/1.4 lets in half as much light as f/1, f/2 lets in half as much light as f/1.4 and so on.
You can remember the f-stop scale by remembering that the first two numbers are 1 and 1.4. All the others are doubling of those two numbers.
How does aperture relate to shutter speed?
Well, suppose you have a correct exposure at f/4 and 1/100 sec. If you decreased your aperture to f/5.6 what shutter speed would you need to get the same exposure? f/5.6 lets in half as much light as f/4. That means you have to leave the shutter open twice as long to get the same exposure. So at f/5.6 you would need to set the shutter speed to 1/50 sec to get the same exposure.
Suppose on the other hand you want to go from f/4, 1/100s to 1/200 sec. What f-stop would you need? 1/200 sec is half the exposure of 1/100 sec. That means you have to let in twice as much light to get the same exposure. So at 1/200sec you have to increase your aperture to f/2.8
Why would you choose one shutter speed over another, or one f/stop over another?
Suppose you're shooting a moving subject. If you want to freeze the action you'd choose a fast shutter speed, but you'd have to open up the aperture to let in more light. However, a wider aperture also leads to a shallower depth of field. You've probably seen sports photos of athletes frozen in mid air with the the background out of focus. Fast shutter speed and wide aperture will do that.
If you want to increase your depth of field you'd choose a smaller aperture and a slower shutter speed.
How do you know when you have the right combination of aperture and shutter speed?
1. You could take a test shot, review and adjust. Look at the histogram:
1.1 if the histogram is too far to the left it means your exposure is too dark. Open up the aperture or use a slower shutter speed.
1.2. if the histogram is too far to the right it means you exposure is too bright. Close down the aperture or use a faster shutter speed.
2. You could use the camera's built-in light meter. Your camera will tell you when it thinks you have the right exposure. See your camera's user manual for instructions on how to use the light meter.
How does ISO relate to all this?
ISO sets your camera's sensitivity to light. ISO 200 is twice as sensitive to light as ISO 100; ISO 400 is twice as sensitive to light as ISO 200 and so on.
Suppose you have a proper exposure at ISO 100, f/4.0 and 1/100sec. If you set the camera to ISO 200 you can set the camera to f/5.6 & 1/100 sec, or f/4.0 and 1/200 sec an get the same exposure.
Why would you change the ISO setting?
Suppose you're in a low light situation. You've got the lens at it's widest setting and the shutter speed as slow as you'd dare take it (any slower and the picture will turn out blury because of movement, etc), but your exposure is still too dark. You can boost the ISO to get a better exposure.
Be warned, higher ISO means lower saturation and more noise.
Hope this helps.
www.dkoyanagi.com
www.flickr.com/photos/dkoyanagi/
Mastering your Digital SLR by Chris Weston
http://www.amazon.com/Mastering-Your-Digital-SLR-Camera/dp/2940378037/ref=pd_bbs_2/002-8481545-8490455?ie=UTF8&s=books&qid=1182390443&sr=8-2
A bit technical, but would answer all of the questions you've asked, plus provides a fly-by of terminology and practice.
The Digital Photography Book, by Scott Kelby
http://www.amazon.com/Digital-Photography-Book-Scott-Kelby/dp/032147404X/ref=pd_bbs_sr_1/002-8481545-8490455?ie=UTF8&s=books&qid=1182390553&sr=1-1
An easy read, includes things like 'recipies' for shots. Extremely helpful, a great starter book
Hope that helps!
http://midnightblue.smugmug.com
Canon
If you really want to understand exposures, purchase a Kodak Color Separation and Gray Scale - http://cameras.pricegrabber.com/enlargers-accessories/m/4820504/search=kodak%20gray%20scale/st=query/ -- and learn to meter and capture it shooting in Manual mode, so that you can see the entire 20 steps of the grey scale - if you can do this, you will have a very accurate exposure, like this one I did for an evaluation of various methods of white balance.
If you prefer you can make a gray scale in Photoshop by creating a Gradient from Black to White, and then posterizing it with a number of steps, and then printing it on an inkjet printer.
Moderator of the Technique Forum and Finishing School on Dgrin
So this is my first post on D-Grin (I'm SUCH a beginner) and I'm hoping to get an answer to this most basic, but most annoying problem. I understand (at least in theory) the relationship between ISO/shutter speed & aperture. I shoot with a Canon xti and often use AV (aperture priority) mode because controlling depth of field is my priority when taking pet portraits. After setting the aperture I want, I use the in-camera meter and adjust the shutter speed until a "good" exposure (neither over nor under) is indicated on the read-out. Once taken, the image displayed on the LCD looks good - BUT when I get the images onto the computer, they often are just too dark and my day of shooting has been for no good result. I hate to re-process every shot in post-production (I use the GIMP, because we run Linux here), it's tiresome and the results are not necessarily good. Sooo, what do I do when the LCD image doesn't really tell me which of my shots is over-exposed and which under-exposed so I can adjust there at the time?
Keep the histogram to the right, but not so much you are blowing out highlight pixels.
Using a reflective light meter, as in an SLR, means that you have to be aware of what you are metering off of. The meter assumes you are metering off a neutral gray field. If you meter off a white dog, and set the exposure to match what your meter says, your image of the dog will be gray and under exposed. Like wise, if you shoot a black dog, and zero your meter, your image will be gray and over exposed, because they meter assumes a gray field.
When shooting white dogs, snow, white dresses, you must add + exposure compensation to properly expose the white dog. Likewise, if you are shooting a black dog, then you will need to dial in - exposure compensation, to keep the dog from appearing gray in the final image.
Note my previous post with the color and gray scales all properly displayed - that image is from a jpg right out of the camera with zero post processing. To do that requires an exposure within 1/3 of a stop or the gray scale will be shifted up or down. My exposures were measured with an incident meter ( Sekonic 358 ), but sunny 16 would have given the same exposure values for the shots done out of doors in sunlight.
Moderator of the Technique Forum and Finishing School on Dgrin
Thanks Pathfinder, for suggesting that I check the histogram. I've figured out where it is and have a general idea of what the histogram for a properly exposed image should look like but after checking around on Canon's website, I wasn't able to find any further explanation of its use. Could you direct me to an additional resource? Chimp
http://www.luminous-landscape.com/tutorials/understanding-series/understandexposure.shtml
http://www.luminous-landscape.com/tutorials/understanding-series/understanding-histograms.shtml
Moderator of the Technique Forum and Finishing School on Dgrin