Alley in Pisa, Italy

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I am using a Sony W520 camera. The specifications mention Focal Length (f= mm) 4.5-22.5. What number should I take? The lower value or the higher? And if not both of them, then how to calculate actual focal length without knowing distance?

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What do you mean "without knowing distance"? It sounds like you might be thinking that this means the distance to your subject (perhaps minimum focus distance?). That's unrelated. Overall, it would really help if you could explain what you're trying to do at a higher level. –  mattdm Dec 26 '12 at 13:49

3 Answers 3

This is a zoom camera - the lens move and changes focal length - it can be 4.5mm, it can be 22.5mm and it can be any value in between.

When the camera is most zoomed out it's 4.5mm and when it's most zoomed in it's 22.5mm (and when its somewhere in the middle its somewhere in the middle).

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then how to caculate somewhere middele value without knowing the distance? –  swapna Dec 26 '12 at 11:39
    
@swapna - the camera can be at any focal length in the range, it can be 4.5 or 4.6 or 4.7 or 4.75 ... up to 22.5, there is no value to calculate the camera's focal length is whatever the photographer choose for that picture. you didn't tell us why you need the focal length, I suspect you are trying to do something in a way that's just not relevant to zoom cameras, if you tell us what you are trying to do maybe we can help you. –  Nir Dec 26 '12 at 12:37
    
Pedantic note: most point and shoot cameras have stepped zooms, so it's not quite true that it can by any focal length. The Fuji and Olympus cameras I've used have a large number of steps, but Canon P&S cameras tend to be limited to steps corresponding to traditional 35mm-equivalent focal lengths (24mm, 35mm, 50mm, 85mm) without letting you stop in between. –  mattdm Dec 26 '12 at 13:55
    
@mattdm - you are right - the point I was trying to make is that you can't calculate a single focal length for a zoom camera and that swapna is probably trying to do something in a way that is irrelevant for a zoom - and in the comment that we may be able to help him accomplish whatever he is trying to do in another way if he tells us what it is –  Nir Dec 26 '12 at 14:08
    
@Nir Yeah. I'm puzzled too. –  mattdm Dec 26 '12 at 14:11

This is not response to your question, but it is response to question you should have asked.

First of, these numbers indicate that your camera has 5x zoom (22.5 / 4.5 = 5). Second, considering that size of your camera sensor is roughly 6.2x4.65mm these numbers mean that lens of an analog 35mm film camera (or full frame digital) that gives same range is ~25-125mm.

Focal length of 50mm gives "normal" FOV approximate to what you see with your eyes, so finally, numbers from your camera mean that it goes from half of normal FOV (think of it as 2x zoomed-out) to 2.5x normal when fully zoomed in.

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First, I assume by your question that what you want is to have the correct focal length so that all of what you are shooting in the picture. Please correct me if this is not what you wanted.

As Nir said, this is a zoom camera - and honestly, the focal length cannot be calculated by the distance of the camera from the subject (at least I don't know such a way).

I would follow these steps -

Choose a comfortable location to stand/sit, point the camera at the what you want to shoot, zoom out completely, and see if you have everything you want in the frame. If you don't have all of what you want to shoot (that is, some of it is partially or totally outside the frame), think about moving further from your subject. If there is not much space, consider changing your photo shoot location to a different place where you have the distance between you and your subject to fit in all of your subject.

If, on the other hand, you have more than what you need that you see in the frame, zoom in a little bit, check if you have everything in the frame. Repeat until necessary.

Hope that helps.

Edit: If you still want to know the focal length after you found the best focusing distance, you can either look at the live view screen - switch between your display modes (I am not sure how that works on on your camera), OR you can find the info as below once you import the photo onto a computer -

  • if you are on a PC, you can right-click the image, open the properties, then Summary tab and Advanced button (this is on XP, I believe other Windows OS should be similar), here you would see the Focal Length along with other details.
  • if you are on a Mac, you can right-click the image, open the Get Info, then More Info, here you would see the Focal Length along with other details.
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