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Old Aug 8, 2010, 5:16 AM   #1
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Default Zoom & Focal Length

Hi.

I'm trying to do an exercise which requires writing down settings with corresponding focal lengths. My P&S camera does not have focal lengths written on it but I do know that it is 25mm to 600mm equivalent and that it has 24x zoom.

Does this mean 1x zoom = 25mm and 24x zoom = 600mm?

Is the relationship linear as shown below where I've filled in the gaps? Each increase in 1x zoom = another 25mm?

x mm
1 25
2 50
3 75
4 100
5 125
6 150
7 175
8 200
9 225
10 250
11 275
12 300
13 325
14 350
15 375
16 400
17 425
18 450
19 475
20 500
21 525
22 550
23 575
24 600

Cheers,

Dean
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Old Aug 8, 2010, 5:22 AM   #2
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Well that is kinda how it works.

If the lens starts at 20mm, a 10x will reach out to 200mm. So if you know the starting range of a lens, and they say how times zoom, you can figure out the end range, by multiplying the start range and the zoom factor they say it is. 5x, 12x, 24x or what ever.
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Old Aug 8, 2010, 7:06 AM   #3
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The zoom number is simply the ratio of the shortest focal length to the longest, and has nothing to do with the actual focal length. Your lens has a zoom range of from 25mm to 600mm (in 35mm equivalent focal lengths) which gives it a zoom ratio of 24X. But a different camera may not be able to zoom as wide as yours. It may only go as wide as 35mm, but it may go longer, as far as 840mm, and still have the same 24X zoom ratio. So the two cameras would have the same 24X zoom ratio, yet have very different zoom ranges.

The zoom ratio is a relative measure of the amount of zoom range a lens has, not an absolute measure of its zoom range. First, you're trying to take that relative measure and turn it into an absoulte measure. Yes, 100mm is 4 time as long as 25mm, but 600mm is also 4 times as long as 150mm. Second, there's no such thing as 1X Zoom. What you're trying to do is substitute one system of measurement for another, and it doesn't work that way.

The EXIF data in every one of your photos already contains the 35mm equivalent focal length. If you want to compare the focal lengths you used to take your photos, you can use that information, and it will be more precise than what you're trying to do here.
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Old Aug 8, 2010, 7:54 AM   #4
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Thanks for the explanation guys.

I compared the EXIF data for some and they are around the mark I guess.

x mm
1 25 (25)
2 50 (52)
3 75 (78)
4 100 (107)
5 125
6 150
7 175
8 200
9 225
10 250 (264)
11 275
12 300
13 325
14 350
15 375 (396)
16 400
17 425
18 450
19 475
20 500 (513)
21 525
22 550
23 575
24 600 (600)

Cheers,

Dean
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Old Aug 9, 2010, 1:32 AM   #5
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There is danger and confusion here. Your 24x zoom is NOT equivalent to a 25-600 zoom on 135/FF, except for angle/field of view (AoV, FoV). Perspective and depth-of-field (DOF) are NOWHERE near the same between a P&S and a larger-frame camera. This "35mm equivalence" is well-established marketing hype, not reality. Your camera's zoom is probably something on the order of 5-120mm. Especially in the wide-to-short-tele range, say 5-30mm (the 'equivalent' of 25-150mm on FF, or 16-100mm on APS-C) the difference is tremendous. Short focal lengths have very thick DOF; almost everything in the image is in-focus, sharp. Focal lengths over 40mm, no matter what format or frame size, have much thinner DOF unless they're stopped down to a small aperture.

I mention this because I've read so many complaints from people new to dSLRs who are sorely disappointed the pictures aren't as good as they got from their old P&S. "Why isn't this picture I shot at 100mm crisp and clear? What is wrong?" Then they blame their kit lens, and start looking to buy a replacement lens... which makes the lensmakers and sellers very happy. The problem is, you can't expect a 100mm lens to behave like its P&S 'equivalent'. Focus is trickier and more critical; you need to stop-down the aperture and use a slower shutter to get anywhere near the DOF; it's just a different world.

[/rant]
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Old Aug 9, 2010, 10:26 AM   #6
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Quote:
Originally Posted by RioRico View Post
There is danger and confusion here. Your 24x zoom is NOT equivalent to a 25-600 zoom on 135/FF, except for angle/field of view (AoV, FoV). Perspective and depth-of-field (DOF) are NOWHERE near the same between a P&S and a larger-frame camera.
Are you quite sure that perspective is not essentially equivalent? My understanding is that that is a geometric property that should track the field of view, but I may well be mistaken.
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Old Aug 9, 2010, 10:30 AM   #7
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Thanks for the explanation Rio. There's a lot to absorb.

It's pretty complicated learning the ins and out of photography. Pretty rewarding when you start to put some of the stuff you read about into practice though - like trying to blur a background out by using different aperture settings.

Cheers,

Dean
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Old Aug 10, 2010, 8:28 PM   #8
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Quote:
Originally Posted by tclune View Post
Are you quite sure that perspective is not essentially equivalent? My understanding is that that is a geometric property that should track the field of view, but I may well be mistaken.
Alas, I was wrong. That happens occasionally. Perspective is entirely a function of lens-to-subject distance. If I stand in one place holding a zoom camera, and shoot the same subject using different focal lengths, the pictures will all show the same perspective. Perspective changes if I change position.
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