Go Back   Steve's Digicams Forums > Digital Cameras (Point and Shoot) > Kodak

Reply
 
Thread Tools Search this Thread
Old May 19, 2008, 4:47 PM   #1
Junior Member
 
Transit Cop's Avatar
 
Join Date: May 2008
Posts: 9
Default

I was wondering if someone could give the breakdown on Fstops and ISO settings. What are they used for and what do they produce.

thanks


Transit Cop is offline   Reply With Quote
Sponsored Links
Old May 19, 2008, 6:02 PM   #2
Administrator
 
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378
Default

Although we have a lot of automation with newer camera models (both film and digital), the concepts of exposure still work the same way as they do with old manual only cameras.

You still have only 4 main variables to take into consideration for exposure (and I use the term "main" since there are a lot of nuances to how you measure the light (for example, your metering mode), as well as different film characteristics if shooting film, and camera settings if shooting digital for the desired tone/contrast curve within an image and more). Once you have a better idea of how these 4 variables work together to give you a properly exposed image, the other fancy features will make more sense. These variables are light, aperture, ISO speed and shutter speed.

Light is typically measured as EV for Exposure Value in Photography.

Aperture (which works similar to the pupils in your eyes, where you can open up the aperture iris wider to let in more light, or close it down to let in less light). If you let in more light with a wider aperture, you can expose the film or sensor faster. If you let in less light with a smaller opening, it takes longer to expose the film or sensor. Note that aperture is normally expressed as f/stop, which is a ratio between the focal length of the lens and the diameter of the aperture iris. So, smaller values represent a larger iris diameter.

With a prime (non zoom) lens, you will see one aperture listed (the widest available). But, you can still use smaller apertures (represented by higher f/stop numbers).

With a zoom lens, you usually see two apertures listed (the largest available aperture at wide angle zoom setting, and the largest available aperture at the full telephoto zoom position). When in between the widest and longest focal length of the lens, the largest available aperture will fall somewhere in between the apertures shown.

Many high quality zoom lenses can maintain a constant aperture throughout their zoom range (with f/2.8 being the most common). So, you'll only see one aperture listed for this type of lens (the widest available, since you have that aperture available at all focal lengths supported if desired). But, you can still set it to smaller apertures (higher f/stop numbers).

When you vary the aperture, you're controlling the iris in the lens (which like a pupil in your eye, can be opened up to let in more light or closed down to let less light in). So, this impacts the shutter speeds you'll need for proper exposure (since more or less light is getting through to the sensor). Aperture also impacts Depth of Field.

The aperture scale in one stop increments (with larger than f/1 apertures possible but very rare in lenses) goes f/1.0, f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11, f/16, f/22... With each one stop move to a smaller aperture (represented by higher f/stop numbers), you will need shutter speeds twice as long for proper exposure for the same lighting and ISO speed (only half the light gets through compared to a one stop larger aperture).

ISO speed. This is how sensitive the film or sensor is to light and is the same thing as the older ASA rating for film. The higher the ISO speed, the faster you can expose it (each time you double the ISO speed, you can use shutter speeds twice as fast for the same lighting and aperture.

Shutter Speed is how long the camera's shutter stays open to expose the film or sensor).

IOW, it all boils down to how senstive the film or sensor is to light (which you control via the ISO or ASA speed of the film you use with film, or the ISO speed settings you use with digital), and how much light you need to let it see to "expose" the iimage (which you control via the aperture opening size and shutter speed).

So, you've got lots of fancy features on newer cameras to automate what settings it uses, and let you vary it's behavior to expose an image darker or brighter than the camera's metering would normally expose it. But, it really boils down to the camera changing the same things you had to worry about with a strictly manual camera without a fancy metering system, Automatic Exposure modes, etc. These exposure calculators and simulators may help you understand it better.

http://www.robert-barrett.com/photo/...alculator.html

http://www.photonhead.com/simcam/shutteraperture.php

If you don't want to use manual exposure, you can use Exposure Compensation if you want a brighter or darker image compared to what the camera's metering would normally give you in the same conditions.

An example of when you may want to use a +EV setting is for a backlit subject, where the subject would normally be much darker than the rest of the image. Since the camera has a limited dynamic range, it doesn't know that you want the dark subject exposed properly (at the expense of the rest of the image). So, you can make the darker subject brighter for correct exposure (which might cause the rest of the scene to be overexposed some).

If your subject is much brighter than the rest of the image (for example, direct sunlight hitting your subject, even though most of the photo is in shadows), you may want to use a -EV setting for Exposure Compensation so that your subject is not overexposed (making the rest of the image darker, too).

The camera has a limited range of bright to dark that it can capture. So, it makes choices so that most of the iimage is correctly exposed, depending on your metering mode. Sometimes that may not be what you want. That's where exposure compensation comes in.

In manual mode, you're controlling the Exposure with your settings for aperture and shutter speed (and your meter will show you if the camera thinks they will result in under or overexposure, much in the same way it will show you if your Exposure Compensation settings in non-manual modes will expose brighter or darker).

Note that aperture also influences depth of field. See this handy calculator for more information about it:

http://www.dofmaster.com/dofjs.html

JimC is offline   Reply With Quote
Old May 19, 2008, 6:12 PM   #3
Administrator
 
Join Date: Jun 2003
Location: Savannah, GA (USA)
Posts: 22,378
Default

Shorter Answer:

If you are shooting in low light, you may need to increase your ISO speed (so that the camera can use a faster shutter speed to help prevent blur from camera shake or subject movement). A higher ISO speed will also increase noise levels (similar to how using higher ISO speed film will cause more grain in a photo).

With your type of camera (which has very good depth of field for a given subject framing and aperture), you'd normally just leave the aperture set relatively wide (smaller f/stop numbers) so that more light gets through helping to keep shutter speeds faster.

But, for macros (where you may have a very shallow depth of field), you may want to use a higher f/stop number (smaller aperture opening to get more depth of field (more of the subject in focus as you get further away from your focus point). This will mean slower shutter speeds for any given lighting and ISO speed. So, be careful to use a tripod if shutter speeds are too slow using that technique.

JimC is offline   Reply With Quote
Old Jun 1, 2008, 6:26 PM   #4
Junior Member
 
Transit Cop's Avatar
 
Join Date: May 2008
Posts: 9
Default

Brilliant explaination guys. I have been learning, slowly but surely



thanks again!
Transit Cop is offline   Reply With Quote
Old Jun 11, 2008, 6:50 PM   #5
Senior Member
 
bigdawg's Avatar
 
Join Date: Feb 2005
Location: Thach Alabama
Posts: 14,981
Default

Looks like JimC covered all bases before I got here!LOL
bigdawg is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 3:51 PM.