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Old Sep 21, 2004, 2:16 PM   #1
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The manual says the lens aperture ranges from f/2.8 (wide) to f/4.9 (Telephoto)

Exposure compensation is +-2 stops in 1/3 increments

If I start at f/2.8 and add 1/3 increments then I get

f/2.8 f/3.1 f/3.4 f/3.7 f/4.0 f/4.3 f/4.6 f/4.9

Q1. Is this what 1/3 increments mean?

Iunderstand the difference between f/2.8 and f/4.0 is 1 stop.

Q2. What does +-2 stops mean in relation to my camera. The reason I ask is that the digital information from the camera regarding a photograph says in Zoombroser EX that it was taken at Av (Aperture Value) 7.1 and Photoshop Elements 2 says F-stop 7.1. Where does 7.1 come from when the range is f/2.8 to f/4.9?









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Old Sep 21, 2004, 3:15 PM   #2
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alisam wrote:
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The manual says the lens aperture ranges from f/2.8 (wide) to f/4.9 (Telephoto)

Exposure compensation is +-2 stops in 1/3 increments

If I start at f/2.8 and add 1/3 increments then I get

f/2.8 f/3.1 f/3.4 f/3.7 f/4.0 f/4.3 f/4.6 f/4.9

Q1. Is this what 1/3 increments mean?

Iunderstand the difference between f/2.8 and f/4.0 is 1 stop.

Q2. What does +-2 stops mean in relation to my camera. The reason I ask is that the digital information from the camera regarding a photograph says in Zoombroser EX that it was taken at Av (Aperture Value) 7.1 and Photoshop Elements 2 says F-stop 7.1. Where does 7.1 come from when the range is f/2.8 to f/4.9?









f/2.8 to f/4.9 is the MAX aperture throughout the zoom range of your camera. The minimum aperture at any given focal length is f/8. That means at full wide angle, your camera ranges from f/2.8 to f/8. At full telephoto, the aperture range is f/4.9 to f.8.

Exposure compensation is simply an easy way to have the camera adjust based on various lighting conditions. +/- 2 stops in 1/3 stop increments means that you can underexpose a scene by 2 stops, 1.7 stops, 1.3 stops, 1 stop, 0.7 stops, 0.3 stops, and you can overexpose a scene by 0.3 stops, 0.7 stops, 1 stop, 1.3 stops, 1.7 stops, or 2 stops. When you use exposure compensation, your camera may adjust either the aperture OR the shutter speed (or both) to get you the right exposure based on your EV setting. It doesn't always just change the aperture.
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Old Sep 21, 2004, 5:40 PM   #3
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Aperture is a ratio, and is determined by dividing the focal length of the lens by the size of the iris opening.

With your model, the lens is rated at f/2.8-4.9. The f/2.8 rating is the maximum aperture (represented by the smallest f/stop value) when you're at full wide angle, and the f/4.9 rating is the maximum aperture when you're at full zoom. The size of the iris opening is really not changing with your model when it's using it's largest aperture (even though the f/stop value of the rating is, as more zoom is used, since it's a ratio).

For example,if you had a 50mm lens with an 18mm iris opening, then it's aperture would be f/2.8 (50/18~= 2.8 ). Yet, if the physical size of the iris opening (18mm in this example) did not change, and the lens was zoomed out to 100mm, then your aperture would become approximately f/5.6 (100/18=5.6).

As a general rule, metered aperture (amount of light reaching the film or sensor) and physical aperture ratio are roughly the same. That is, less light reaches the sensor through the lens when longer focal lengths are used (for a given physical size of the iris opening). Although, your model may be an exception.

From what I've read, the S400 model may not have an aperture that changes it's real size (speculation has it that it uses a neutral density filter to simulate two different apertures, with the values of these two apertures changing as more zoom is used). I suspect this is also true for the S500 (although I have no way to confirm it with either model).

As the previous poster mentioned, cameras can vary the shutter speed instead when using EV compensation. It's just a way to vary the exposure used from what the metering thinks is needed. With your camera, if you use EV compensation at full wide angle, chances are, the aperture value will not change (since it does not appear to have a true adjustable iris). Instead, it should vary the shutter speed (either faster or slower as more EV compensation is used).

Basically, one stop means that you'll need exposures either twice as long (with -1 EV set), or twice as fast (with +1 EV set) for a given lighting condition and aperture. Since it's adjustable in 1/3 stop increments, then you'll get shutter speeds in between these values.

For cameras that do adjust aperture, the aperture scale (in one stop increments) goes 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 larger f/stop numbers), you will need shutter speeds twice as long for proper exposure.

Here is achart you can use to get anidea of the shutter speeds required for any EV and Aperture (but make sure to use your camera's metering, as lighting can vary -- this is only to give you an idea of how it works). It's based on ISO 100. So, each time double the ISO speed, you can use shutter speeds twice as fast:

http://home.earthlink.net/~terryleedawson/dcnotes/tables.htm

Aperture also impacts Depth of Field. Thelarger the aperture (represented by smallerf/stop numbers), and the closer you are to your subject (focus distance), and the longer your focal length (amount of zoom used), the less depth of field you will have (less of the scene in focus, as you get further away from your focus point).

Again, from what I have read, the aperture values in your model may not *really* impact depth of field (it's probably using the same "wide open" value of f/2.8 at wide angle, with the values changing based on the amount of zoom used; or in better light, using a neutral density filter to simulate a smaller aperture at wide angle, with the value changing based on the amount of zoom used. I have no way to confirm this though.

Here is an online depth of field calculator. Plug in your camera model, then change focal length, aperture and focus distance to see what impact aperture has. Note that you must use the actual (versus 35mm equivalent) focal length of the lens for computing depth of field (you'll see a link to a table with information for popular cameras).

This focal length is much shorter than the 35mm Equivalent Focal Lengthon most Digital Camera models (since thesensor is much smaller than 35mm film, the focal length can be shorter):

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

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Old Sep 21, 2004, 8:31 PM   #4
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Zal

Where did you get from or work out that the minimum aperture at any given focal length is f/8?

Since you mentioned f/8, I have been looking at various web sites and have come across an extrabits of information that I did not see listed in the Specifications section of the manual.

f/number: f/2.8 to f/7.1 (Wide) and f/4.9 to f/13.0 (Tele)

Since I mentioned F-stop 7.1 in my first post AND the fact that the photograph was taken in very sunny conditions of ahotel (white) and pool area (light blue). Does this mean that camera was letting the least amount of light that it could (since it was very sunny)? Also since the photograph was taken at F-stop 7.1 then could I have compensated by +-2 stops?

Have I put 1 and 1 together and got 3?


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Old Sep 21, 2004, 9:01 PM   #5
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alisam wrote:
Quote:
Zal

Where did you get from or work out that the minimum aperture at any given focal length is f/8?

Since you mentioned f/8, I have been looking at various web sites and have come across an extrabits of information that I did not see listed in the Specifications section of the manual.

f/number: f/2.8 to f/7.1 (Wide) and f/4.9 to f/13.0 (Tele)

Since I mentioned F-stop 7.1 in my first post AND the fact that the photograph was taken in very sunny conditions of ahotel (white) and pool area (light blue). Does this mean that camera was letting the least amount of light that it could (since it was very sunny)? Also since the photograph was taken at F-stop 7.1 then could I have compensated by +-2 stops?

Have I put 1 and 1 together and got 3?
He probably assumed that f/8 was about the smallest aperture it would use, because this is typical for cameras with short focal lengths (your lens is only 7.2-22.mm, to give it a 35mm equivalent focal range of approximately 36-108mm).

When you go much smaller than f/8 with a lens this small, you get refraction problems. That's another reason why it appears that your model is using a Neutral Density Filter to simulate aperture (rather than having an iris that really varies the physical size of the opening).

Chances are, at any given focal length, you won't be able to get more than 2 apertures that are being used by the camera. Whereas, a camera with a multi-blade iris would have manyaperture choices for any given focal length.

The aperture design is one reason why you don't have control over the aperture in your model.

Try your camera at a set focal length (amount of zoom used), in both poor light, and very bright light (and lighting levels in between). Then, see if you can make it choose more than 2 aperture choices (chances are, it can't). It's possible that they changed the design to use 2 real aperture + a NDF (for a total of 3 effective apertures at any given zoom setting). But, this would be the most I would expect.

A true multi-blade iris would have very fine control of aperture (many choices for a given zoom setting).




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Old Sep 21, 2004, 9:32 PM   #6
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alisam wrote:
Quote:
Zal

Where did you get from or work out that the minimum aperture at any given focal length is f/8?
Quote:
Like Jim said, f/8 is usually the smallest aperture on small cameras like yours.
Quote:
Since you mentioned f/8, I have been looking at various web sites and have come across an extrabits of information that I did not see listed in the Specifications section of the manual.

f/number: f/2.8 to f/7.1 (Wide) and f/4.9 to f/13.0 (Tele)

Since I mentioned F-stop 7.1 in my first post AND the fact that the photograph was taken in very sunny conditions of ahotel (white) and pool area (light blue). Does this mean that camera was letting the least amount of light that it could (since it was very sunny)?
Quote:
Well, you can always use a shorter shutter speed to let in even less light.
Quote:
Also since the photograph was taken at F-stop 7.1 then could I have compensated by +-2 stops?
Quote:
Sure, by changing your shutter speed by 2 stops
Quote:

Have I put 1 and 1 together and got 3?
Quote:
I think you might be getting there :-)
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Old Sep 21, 2004, 9:36 PM   #7
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Actually now that I just reread everything, it occurred to me that your camera might not give you control over aperture and shutter speed values. EV compensation might be all you have--in that case, my post above doesn't really apply to your camera, sorry.:?
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Old Sep 21, 2004, 9:59 PM   #8
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Alisam:

I wouldn't worry too much about how it works. Your camera is not designed to give youcontrol over Aperture. You'd need a model with manual exposure and/or aperture priority for that. Canon does not include these features in it's smaller models like your S500.

Basically, you've got an Exposure Compensation feature that let's you vary the exposure for a given scene. If you really want to figure out how it works, take some test photos in a given lighting level, at a set focal length (amount of zoom used), then use the Exposure Compensation at different settings and take some photos.

Then, review the EXIF information in the photos to see what settings it used for them. If you don't have an image editor that lets you see this information, download irfanview from http://www.irfanview.com (it's free). Make sure to download the free plugins, too. You'll see the camera settings used for an image under "Image, Information, EXIF"

Chances are, it's simply using a faster shutter speed when you have it set for -EV; and a slower shutter speed when you set it to a +EV Value.

You really don't need control over the Aperturein a model with a lens this small. You've got tremendous Depth of Field anyway (even if it's shooting with the Aperture "wide open"), thanks to the very short actual focal length of the lens.

The exposure compensation feature is designed to let you override the settings the camera's meteringthinks is correct for proper exposure of a scene.

You have a limited dynamic range with a camera (the ability to capture both very bright and very dark areas in the same image). So, sometimes you need to make a choice of which areas are properly exposed, and which ones may be left either underexposed or overexposed; if the scene is not evenly lit.

An example of when you may want to use +EV with Exposure Compensation is for a backlit subject. If you use the default matrix (multi-segment) metering, the camera takes the entire image frame into consideration when determining the best aperture/shutter speed combination to use.

Since the backlit subject is much darker than the rest of the frame, you can set Exposure Compensation to a +EV setting to make the image brighter (so that your subject is properly exposed). This can have the effect of overexposing the rest of the image; but the camera gives you the choice. Using +EV usually forces the camera to use a slower shutter speed (although it may also use a larger aperture, depending on the conditions, focal length, etc.).

An example of when you may want to use -EV Compensation is when your subject is much brighter than the rest of the scene. So, if the camera tried to properly expose the entire scene (to make sure shadow areas are brighter), your subject could be overexposed. When -EV Compensation is used, the camera normally picks a faster shutter speed than it would normally use (or in some cases a smaller aperture).

Use of Exposure Compensation allows you to adjust the brightness of the scene (overriding what the camera thinks should be used), while still taking advantage of it's metering and exposure algorithms.
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