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Old Jan 16, 2010, 8:26 AM   #1
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Default Greetings From North Carolina!

Hello, I'm Kevin Childress - just wanted to stop by this morning and introduce myself to the community. I've been looking for a forum-structured site to join so that I may learn more about general photography from others willing to share their experience. I've been digging through some of the forums and threads here and I think this is just the place for me!

My goal is to learn enough that I might consider myself an amateur photographer (I think I'm almost there). In the last couple of weeks I've made large gains in understanding the manual mode of my camera (Kodak Z1012 is). Today I can provide a basic explanation of aperture, shutter speed, exposure compensation, and ISO. But this is also where I'm having a bit of trouble ... that is understanding how an adjustment to one of these settings drives a proportionate change to another (or not). I've read material that explains examples like, "...a half-second exposure is one stop darker than a one-second exposure..." and, "...a 1/1000 exposure is three stops darker than a 1/125 exposure". But, something about these scales hasn't clicked in my brain yet and I would appreciate any coaching you could provide!

I look forward to seeing you around the forums!

Cheers!

Kevin
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Old Jan 16, 2010, 9:30 AM   #2
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Welcome to the forums Kevin.

Here's an explanation I've used before (copied from another forum post I made):

You have 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.

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 sensitive 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 image (which you control via the aperture opening size and shutter speed).

So, when you see someone mentioning a concept like a one stop darker exposure, they can mean using a shutter speed twice as fast for the same lighting, aperture and ISO speed. Or, they can mean using a one stop smaller aperture (higher f/stop number, as in going from f/4 to f/5.6) for the same lighting, ISO speed and shutter speed. Or, they can mean using a one stop lower ISO speed setting (for example, going from ISO 400 to ISO 200, decreasing the sensitivity by half) for the same lighting, aperture and shutter speed, etc. These variables all work together to insure a properly exposed image.

This exposure simulator may help you to understand it better.

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

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

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

Note that a good book on basic photography can also help you understand these concepts (and it doesn't have to be specific to digital, as the same exposure concepts apply to film).

P.S.

But, your meter can do the work for you in most lighting (although with very long exposures a meter may not help a lot).

If you're in Av (Aperture Value, a.k.a., Aperture Priority) mode and vary the aperture (your f/stop setting), 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). When you use Av mode, you select the desired aperture (usually via a control dial). Then, the camera selects the correct shutter speed for proper exposure, based on how it's metering the scene.

Note that you also have more than one metering mode (spot, center weighted, and multi-segment) with most cameras that will influence what it thinks is needed for proper exposure, based on the areas of the scene it's evaluating.

If the camera is metering the scene where you end up with a darker or brighter exposure than desired, you can use Exposure Compensation to get a brighter or darker exposure. If you use a -EV setting (needle to the left of center), you'll have a darker exposure than the camera would have used. It gives you a darker exposure by using a faster shutter speed with the selected Aperture.

If you use a +EV setting (needle to the right of center), you'll have a brighter exposure than the camera would have used. It gives you a brighter exposure by using a slower shutter speed with the selected Aperture.

If you use manual exposure (setting both the aperture and the shutter speed), that takes the place of Exposure Compensation because you're setting both. Just use the meter in your viewfinder to see how your settings are working (if the needle or display is left of center, you'll be exposing darker than the camera metered the scene). If it's to the right of center, you'll be exposing brighter than the camera measured the scene.

Also note that a variety of settings combinations will give you the same exposure. For example, if the lighting was the same and your iso speed is set the same, these combinations would all result in the same exposure:

1/800 second at f/2.8
1/400 second at f/4
1/200 second at f/5.6
1/100 second at f/8

But, you may want to use different aperture settings for depth of field purposes (or a different combination resulting in faster or slower shutter speeds for other purposes, like freezing a moving subject).
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Old Jan 16, 2010, 9:30 AM   #3
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Welcome aboard! You may want to try this link out. It will let you see the results as you play with various settings.

http://dryreading.com/camera/index.html
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