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  1. #1
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    Aperture / F-Stop (Lens & Camera Settings ?)

    Not sure if this can be easily explained, so even a link to a site that can explain it (like I'm a 6 year old) would be appreciated.

    There are many references to Aperture Values of the Lens and how this effects depth of filed......hopefully I got that part right.

    Where I become confused, is if the lens is capable of 3.8-5.6, what is the effect of changing aperture values within the camera outside these limits (or does the camera prevent you from setting outside these limits by "knowing" what the lens is capable of).

    There are many references how "IS" can effect the F-Stop and how extenders result in stopping down/up (whichever it is).....and so on.

    With my future acquisition of a DSLR, I need to start understanding these relationships and probably need to investigate some courses in the future.

    I realise that this is something that is probably learned over many years of experience....so if it's it is too complex.....then feel free to say so.

  2. #2
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    Here's a simple explanation- To create a picture light must expose film or hit a digital sensor. Too much light and the photo is over exposed (looks very white), too little light and the photo is under exposed (looks very dark).

    So the goal is to get a proper exposure. There are two primary ways to do this. One is by having the size of the shutter open very wide but for a short period of time. The other is to open the shutter a smaller amount but for a longer period of time.

    The size of the shutter opening is called the aperature size, the length of time the shutter is open is called shutter speed.

    Aperature sizes are measured in what is called f stops. The smaller the f stop the larger the opening. Lenses with smaller f stops (ability to open larger) are called faster lenses because they allow the shutter to be open for a much shorter period of time to get a proper exposure.

    When hand holding a camera it is difficult if not impossible to hold the camera still for longer shutter times which results in blurry pictures (this is why there are Image Stabilized lenses coming to market). So one reason a smaller f stop (larger openning) is used to when shooting in low light situations so that the shutter speed can be faster and still allow enough light in to properly expose the photo.

    Another reason that you would want a smaller f stop (larger openning) is to allow a shutter speed fast enough to freeze motion. Ex. a car driving by, a cyclist, a waterfall. With a fast shutter speed motion is stopped, with a slower speed the car may look like it is in motion or the water may look like it is blurring together. This is not bad if that is your desired result, but if you want to have a stopped motion picture then you need a fast shutter speed and if you have a fast shutter speed you need a larger aperature openning (smaller f stop)

    Depth of field refers to how much of a picture is in focus and how much is blurred. Consider two scenarios both taking a picture of someone in front of a landscape with a building in the background. In the first scenario you wish to have your person as the subject with the background blurred someone (or fully), the second you want the entire picture in focus- person and background. It all depends on what you consider your subject.

    A large aperature (small f-stop) will create a shallow depth of field. For example, with a 2.8 f stop your person may be in focus and the background blurred significantly. Increase the f stop to 3.5 (making the aperature smaller) and your background will come more into focus also. If you were to make the aperature really small like f/22 then the entire picture will be in focus. Consider f/22 almost like making your camera squint to see further.

    Most lenses have a sweet spot where they are most sharp which lies somewhere in the middle. These are best used when you don't care about depth of field such as when your subject is standing close to a wall and there isn't much distance difference between subject and background.

    The issue of the zoom lens you mentioned 3.8-5.6 behaves exactly as I have indicated above. However- what it is saying is that the largest you can open the aperature at the low end is to 3.8 (not as small as 2.8 so you won't be able to have the shutter as fast as a 2.8) and the shutter will only open as large as 5.6 at the telephoto end (again not as large an openning so the shutter faster shutter speed you can shoot at is much slower than at 2.8)

    With lots of light, this isn't a problem. In low light situations the fastest shutter speed you can obtain may be too slow to hand hold your camera without blurr. This is where Image Stabilization can help because it allows you to shoot a picture at a slower shutter speed w/o camera blur. (We are only talking about your hands blurring the picture- IS doesn't help if the subject is in motion)

    So why wouldn't everyone just buy lenses with larger aperature openings such as 2.8 or lower? $$$ All other things being equal it explains a why you can buy a $300 lens with the same focal lengths as a $1200 lens. How important is it? It is only important if you want the larger openning to get the smaller depth of field or when handholding a camera where the ability to keep the shutter open long enough without blurring is an issue.

    - You didn't ask but there is one other element to proper exposure- ISO. This is a value that represents film or a digital sensors sensitivity to light. ISO is measured as 100, 200, 400... 1600, 3200. As ISO is increased two things happen. First, for a given aperature the shutter time to expose a picture decreases (great for low light situations). Second, the picture can develop noise, a negative attribute that looks like grainyness. This usually takes place after ISO 400 and to a different extent based upon the particular camera. Noise can often be removed or reduced from a photo by using post processing software such as Noise Ninja.

    For an excellent book on this subject refer to Understanding Exposure by Bryan Peterson published by AMPHOTO.

    Hope this helps.

  3. #3
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    Quote Originally Posted by RichNY
    .......

    For an excellent book on this subject refer to Understanding Exposure by Bryan Peterson published by AMPHOTO.

    Hope this helps.
    Yes and No.

    I now better understand the limitations of the lens....but obviously need to read up on the in-depth aspects.

    For example, on my FZ-20 I have the ability to change Aperture Values manually.....but obviously not the lens.

    On a DSLR....there will always be the option to change the Aperture Value manually, but I'm not sure if this will be goverened by the Lens capability. If it is a 3.5-4.5 lens....will the camera prevent me from making settings outside these limitations....or will it do some recalculations and adjust shutter speed accordingly.

  4. #4
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    Quote Originally Posted by Honest Gaza
    For example, on my FZ-20 I have the ability to change Aperture Values manually.....but obviously not the lens.

    On a DSLR....there will always be the option to change the Aperture Value manually, but I'm not sure if this will be goverened by the Lens capability. If it is a 3.5-4.5 lens....will the camera prevent me from making settings outside these limitations....or will it do some recalculations and adjust shutter speed accordingly.
    I'm not that familiar with the FZ-20 so I pulled up this review. Is this your camera? http://www.dpreview.com/reviews/panasonicfz20/page9.asp

    If so, it says that it is an f/2.8 lens not a 3.5-4.5. Where are you getting those figures from?

    Assuming that this is your camera with a f/2.8 lens at all zoom ranges, then your logic is correct- 2.8 would be the largest aperature openning and you couldn't open as wide as 1.4 for example. If you were to find that for a given light condition at the largest aperature your camera could set that the shutter speed was still too low to shoot the picture that would be an example of where you would up the ISO setting. If you still couldn't get an acceptable shutter speed you would up the ISO setting again and repeat the process until you could. In the worst case scenario you would find that your ISO were high enough to cause lots of noise, but then again the alternative would be not taking the picture!

    Note- if this is in fact your camera this review states that you will start noticing visible noise at ISO levels above 100 so you should plan on using software like Noise Ninja to clean up your photos before printing them.

    -To answer your specific questions on whether the camera will prevent you from setting outside of its aperature range: Yes, with a 3.5 max lens you won't find a 2.8 or 1.4 f stop to even dial it to!

    Will it do some calculations and adjust the shutter accordingly- Yes it will increase the time the shutter is open if the aperature setting isn't as big- but that doesn't mean you will be able to hold the camera steady enough for the increased shutter time to get a clear picture. A tripod will help here as it can hold the camera steady for long periods of time.

  5. #5
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    Quote Originally Posted by Honest Gaza
    Yes and No.

    I now better understand the limitations of the lens....but obviously need to read up on the in-depth aspects.

    For example, on my FZ-20 I have the ability to change Aperture Values manually.....but obviously not the lens.

    On a DSLR....there will always be the option to change the Aperture Value manually, but I'm not sure if this will be goverened by the Lens capability. If it is a 3.5-4.5 lens....will the camera prevent me from making settings outside these limitations....or will it do some recalculations and adjust shutter speed accordingly.
    You have to realize that the 3.5-4.5 is the widest aperture possible throughout the zoom of the lens. When the lens is it's widest you can use a minimum (widest) aperture of f/3.5 when the zoom is around half-way to fully telephoto you can use a minimum aperture of f/4.5. No matter what focal length you are at you can "stop down" on the aperture (make the f/# higher) and you will have greater DOF, the camera will not let you select an aperture wider than the lens permits.
    Ken
    Canon dSLR User

  6. #6
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    Quote Originally Posted by RichNY
    I'm not that familiar with the FZ-20 so I pulled up this review. Is this your camera? http://www.dpreview.com/reviews/panasonicfz20/page9.asp

    If so, it says that it is an f/2.8 lens not a 3.5-4.5. Where are you getting those figures from?

    .
    Yes, that is the camera I currently have, and am aware it is an f/2.8.

    My reference to a 3.5-4.5 was merely an example of a lens that I plucked out of the air. Various contributors to this forum (other threads) have put forward suggestions of useful lenses to purchase and I wanted to better understand the "limitations/effects" before purchasing lenses for a DSLR.

  7. #7
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    Quote Originally Posted by ktixx
    ..... No matter what focal length you are at you can "stop down" on the aperture (make the f/# higher) and you will have greater DOF, the camera will not let you select an aperture wider than the lens permits.
    Ken
    This is the type of reference that I am referring to...."stop down"

    Knowing when to "stop down" I guess determines if you understand the concept or not.

  8. #8
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    Using prices for Nikon lenses from Ritz (10-15% higher than you can buy elsewhere) you'd have to invest the following to get the focal range of your camera at 2.8

    Nikon 28-70mm f/2.8 ED-IF AF-S Zoom-Nikkor $1599
    Nikon 70-200 F/2.8 ED-IF AF-S VR Lens $1799

    Now while nobody was suggesting buying this quality glass, if you are wanting to practice with non-flash low light photography you are going to want a faster lens (just like you've got on your P&S)

    Again, I'm not trying to prove my position is right, just explaining the logic behind my suggestion.

  9. #9
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    Yep.....and that's why my wife won't be reading this thread (or seeing the receipts)

  10. #10
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    Quote Originally Posted by Honest Gaza
    This is the type of reference that I am referring to...."stop down"

    Knowing when to "stop down" I guess determines if you understand the concept or not.
    with a point and shoot you have a lot more DOF then DSLR's because of the smaller sensor. Normally, you "stop down" or use a smaller aperture when you want the background, foreground and focus point in focus. If you use a very wide aperture with a DSLR only the focal point will be in focus. An example would be a bush a few feet in front of a person on a bench and people a few feet behind the bench. If you were to focus on the subject on the bench with f/2.8 aperture only the focus point would be in focus, the bush and the people behind the bench would be out of focus. If you stopped down to (say) f/10 then the bush in front of the bench as well as the people behind the bench will all be in focus (or more in focus then f/2.8)

    To simplify - stop down when you want the entire frame to be in focus, use a wide aperture when you want the background/foreground out of focus.

    Ken
    Canon dSLR User

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