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Old Oct 21, 2007, 6:59 AM   #1
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Hi again, after reading my manual and getting even more confused than I was in the first place I thought I would be brave and post up another question for you more experienced people to help out on.I am doing an online course and this weeks task is to "freeze the movement" or take pictures of traffic with the lights streaking. I am using a Fuji S9600 and do have a tripod although I do feel a bit self concious about taking it out in public . I know that I have to have a slow shutter speed so am assuming that I put the camera to "S" mode and adjust it there , do I also change the ISO speed or is that done automatically? I seem to have developed a form of photo dyslexia as I keep getting muddled as to whether I want a high number or low number If there is a section on this already in this forum could someone point me in the right direction? or give me a brief explanation of what I should be doing Thanks for your help
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Old Oct 21, 2007, 7:43 AM   #2
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You'll probably have no choice but to use a slower shutter speed if you want to capture streaking headlights (light at night will require slower shutter speeds if you don't want to bump up the ISO speed and introduce a lot of noise).

The higher the ISO speed, the more sensitive the camera is to light. So, you'll get faster shutter speeds as ISO speeds are increased for the same lighting and aperture setting.

I'd keep the iso speed set low to keep noise down for the streaking headlights type shots (since you'll need slower shutter speeds anyway), and use a tripod (otherwise, you'll get blur from camera shake).

You may want to stop down the aperture a tad, too (smaller f/stop numbers) for better depth of field and sharpness (which will also mean slower shutter speeds). I'd probably use manual exposure and take some test shots until I got the desired result.

Yes, you could use Shutter Priority and set the camera to the desired shutter speed and let it pick the aperture. But, if you pick a shutter speed that's too slow for the smallest available aperture (higher f/stop numbers) you can get overexposed (too bright images). If you pick a shutter speed that's too fast for the widest available aperture the lens supports, you'll get underexposed images (too dark).

So, the capabilities of the lens used is one consideration to keep in mind. If you want to go with longer exposures, the metering accuracy can also suffer (so, sometimes manual is easier for those types of shots).

A book on basic photography (film or digital) can probably help you understand the basics of exposure. But, once you understand the relationship between light, aperture, iso speed (a.k.a., film speed) and shutter speed, it will start to make sense.

I found an older post with some information on variables that make up exposure and more. So, I copied most of it to this one below. It may be a bit more than you want to know right this minute though. I'd check out the links to the exposure calculators below and that would probably help more.

Warning -- long winded post. ;-)

If you try to use a shutter speed that's too fast for the lighting, ISO speed and aperture, you'll get underexposed images (too dark).

If you try to use a shutter speed that's too slow for the ISO speed lighting and aperture, you'll get overexposed images (too bright). ;-)

So, you can't just use anything you want to. Use the camera's meter to let you know if your settings will result in under or overexposure if you want to use manual exposure.

The camera has to keep the shutter open long enough to expose the film or sensor and the same concepts apply to film or digital.

The meter in your viewfinder is designed to let you know if your settings are going to result in a darker or brighter exposure, compared to the way the camera's metering thinks it should be exposed (and the metering mode and what you meter on will impact what the camera thinks is needed).

When you are shooting in non-manual modes, this meter is used for a feature called Exposure Compensation. If you set the camera so that the pointer is higher than zero, it will take a brighter exposure than the camera would have used by default. If it's set to a -EV value, it will expose it darker than the metering thinks is needed.

If you're shooting in Manual Exposure mode, there is no Exposure Compensation (because your aperture and shutter speed settings take the place of it since you're controlling both of these variables, although most cameras have a meter that shows you how your settings are impacting exposure).

Exposure Compensation lets you alter the way a camera's autoexposure/metering algorithms expose an image (brighten or darken it compared to the way the camera metered the scene). It's one of my most frequently used settings on most cameras.

A +EV value gives you a brighter exposure. The camera uses a slower shutter speed and/or larger aperture (smaller f/stop number) to get a brighter exposure, compared to what the camera's autoexposure/metering algorithms would have selected.

A -EV value gives you a darker exposure. The camera uses a faster shutter speed and/or smaller aperture (higher f/stop number) to get a darker exposure, compared to what the camera's autoexposure/metering algorithms would have selected.

If you're in Av Mode (Aperture Priority) and use Exposure Compensation, the camera will vary the shutter speed (since you're setting the aperture). If you're using Tv (Shutter Priority) mode and use Exposure Compensation, the camera will vary the Aperture (since you're controlling the shutter speed).

If you're in Auto (or other similar modes), the camera may vary aperture or shutter speed when you use Exposure Compensation. In low light, since your aperture is already wide open, it varies shutter speed if you use a -EV setting.

If you're shooting in Manual Exposure mode, there is no Exposure Compensation (because your aperture and shutter speed settings take the place of it since you're controlling both).

This is the concept you need to become familar with, so I'll make it bold:

Correct Exposure comes down to the amount of light, the ISO speed, the shutter speed, and the aperture. A variety of combinations will produce identical exposure.

These exposure calculators and simulators may help you understand it better, too:

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

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

You 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 if you're not using manual exposure.

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).

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.

1. Light (typically measured as EV for Exposure Value in Photography).

2. 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.

3. 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.

4. Shutter Speed (this 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.

So, Exposure Compensation is just another tool you have to work with, allowing you to expose an image brighter or darker than the camera's metering would normally expose it when using non-manual modes, and it varies shutter speed and/or aperture to accomplish that.

With manual exposure, you're controlling those variables (and your viewfinder scale for the metering will let you know if your setttings are exposing brighter or darker than the camera's metering thinks is needed).
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Old Oct 21, 2007, 10:34 AM   #3
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thanks Jim, I'll read through your post and see if I can manage to change the settings correctly. I experimented with changing the iso settings on a trip to the park and got disaterous results! all the pictures were far too dark and VERY grainy. When I uploaded them to iphoto something really weird happened - all the images looked stretched you know like the funny mirrors in a fairground???? I've no idea why this would happen?
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Old Oct 21, 2007, 10:44 AM   #4
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I don't know about the stretched part, unless the software was trying to resize them and wasn't maintaining the original aspect ratio (ratio of width to height).

As for being too dark, that means that the shutter speed was too fast for the lighting, aperture and ISO speed, resulting in underexposure.



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