Understanding zoom

OGroteKoning

OGroteKoning

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I did some research into the "10x zoom" thing on video and point and shoot cameras. And it would seem that it is like the PMPO measurement in the music world = pretty useless except that it does carry some value in terms of the specific glass in the specific camera/video recorder.

As I understand it, it merely means that you a 50-200mm lens will have a 4x zoom and a 200-400mm lens is only 2x zoom.

This would mean that if you have a point and shoot camera with a 20x optical zoom and one that is 15x zoom, that the 20x zoom might not give you a bigger zoom. It could in fact give you a smaller zoom than the 15x if the widest part off the lens is smaller than that of the 15x zoom.

Is my understanding correct?
 
Without knowing at least one of the focal lengths the zoom factor really doesn't indicate much.

I ask my video friends about their glass all the time and all they seem to ever know is the zoom factor.
 
OGroteKoning said:
Is my understanding correct?
Click to expand...

Yes. My experience with video cams is that the shortest focal length is the most important one anyway, for my applications, I should add.

Nothing worse than shooting indoors, and not being able to get the framing you wanted.

On the long side, I found that the zooming in too much is a sure way to overwhelm the IS, unless you are using a tripod, of course.

Just one minor detail: the zoom ratio is the FOV at the shortest focal length divided by the FOV at the longest focal length (not the ratio of the actual focal lengths). You will have to know the sensor size to calculate FOV, though.
 
fvdbergh said:
Yes. My experience with video cams is that the shortest focal length is the most important one anyway, for my applications, I should add.

Nothing worse than shooting indoors, and not being able to get the framing you wanted.

On the long side, I found that the zooming in too much is a sure way to overwhelm the IS, unless you are using a tripod, of course.

Just one minor detail: the zoom ratio is the FOV at the shortest focal length divided by the FOV at the longest focal length (not the ratio of the actual focal lengths). You will have to know the sensor size to calculate FOV, though.
Click to expand...

So the ratio is still kinda useless. How do I convert this info to 35mm equivalent? Because only then can we really put it into some form of perspective in terms of SLR lenses.
 
Yup.

The 'x' zoom always starts at the widest.

So a 15x zoom could either be a 24-360mm, 28-420mm or even 18-270mm. Quite a difference! Luckily though, they generally a give a 35mm rating when buying compacts
 
OGroteKoning said:
So the ratio is still kinda useless. How do I convert this info to 35mm equivalent? Because only then can we really put it into some form of perspective in terms of SLR lenses.
Click to expand...

As Dolby said, they usually give you the 35 mm equivalent focal lengths. If not, then you still need the sensor dimensions. With the dimensions, you can pop them into the last calculator on this page --- just enter FF dimensions for camera 2, and your video sensor dimensions for camera 1.

You can obtain the actual sensor size from the stated sensor size (e.g., 1/1.7" -> 7.5x5.7 mm) using the info found here:http://en.wikipedia.org/wiki/Image_sensor_format. There is a diagram with common sizes somewhere on that page.
 
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bwana said:
Manufacturers often do that for you

Canon PowerShot SX280
http://www.usa.canon.com/cusa/consu...tal_cameras/powershot_sx280_hs#Specifications
Focal Length: 4.5 (W) - 90.0 (T) mm (35mm film equivalent: 25-500mm)
Click to expand...

What is the relationship between the focal length and the 35mm film equivalent? A quick calc shows 5.555555
But here, the relations is quite different - Focal Length: 2.8-89.6mm 35mm Film Equivalent: Optical Zoom: 38.5-1232mm. The calc shows 13.75

What is the 5.555 and 13.75?
 
OGroteKoning said:
What is the relationship between the focal length and the 35mm film equivalent? A quick calc shows 5.555555
But here, the relations is quite different - Focal Length: 2.8-89.6mm 35mm Film Equivalent: Optical Zoom: 38.5-1232mm. The calc shows 13.75

What is the 5.555 and 13.75?
Click to expand...

That is why you should use the FOV, not the actual focal length. The ratio in FOV is the same for both pairs of focal lengths.
 
OGroteKoning said:
What is the relationship between the focal length and the 35mm film equivalent? A quick calc shows 5.555555
But here, the relations is quite different - Focal Length: 2.8-89.6mm 35mm Film Equivalent: Optical Zoom: 38.5-1232mm. The calc shows 13.75

What is the 5.555 and 13.75?
Click to expand...
I suspect it's the result of different sensor sizes.
 
fvdbergh said:
As Dolby said, they usually give you the 35 mm equivalent focal lengths. If not, then you still need the sensor dimensions. With the dimensions, you can pop them into the last calculator on this page --- just enter FF dimensions for camera 2, and your video sensor dimensions for camera 1.

You can obtain the actual sensor size from the stated sensor size (e.g., 1/1.7" -> 7.5x5.7 mm) using the info found here:http://en.wikipedia.org/wiki/Image_sensor_format. There is a diagram with common sizes somewhere on that page.
Click to expand...
Aaah ... okay. So it is much more complicated than just getting a factor and applying it.
fvdbergh said:
That is why you should use the FOV, not the actual focal length. The ratio in FOV is the same for both pairs of focal lengths.
Click to expand...
Still studying the links you gave. Thanks
bwana said:
I suspect it's the result of different sensor sizes.
Click to expand...
Yup, that is part of it as I understand
.
.
.
/Still checking out the links of @fvdbergh
 
OGroteKoning said:
What is the relationship between the focal length and the 35mm film equivalent? A quick calc shows 5.555555
But here, the relations is quite different - Focal Length: 2.8-89.6mm 35mm Film Equivalent: Optical Zoom: 38.5-1232mm. The calc shows 13.75

What is the 5.555 and 13.75?
Click to expand...

The SX280 has a 1/2.3" sensor, which is 5.76 mm wide, or a crop factor of 6.25 (if you must). Plugging it into the FOV calculator (http://www.tawbaware.com/maxlyons/calc.htm) tells you that the 4.5 mm focal length has a horizontal FOV of 65.2 degrees.

Converting all the values:
4.5 mm @ 1/2.3" -> 65.2 degrees
25 mm @ 35 mm -> 71.5 degrees
90 mm @ 1/2.3" -> 3.7 degrees
500 mm @ 35 mm -> 4.1 degrees

So that means the 35 mm equivalent zoom ratio is 71.5/4.1 ~ 17.4x zoom, and the SX280 zoom ratio is 65.2/3.7 ~ 17.4x zoom.

The minor discrepancies (SX280 is claimed at 20x zoom) arise from details like aspect ratio, as well as minor variations in claimed focal length versus actual focal length, and minor sensor size differences (1/2.3" is only roughly 5.76 mm wide, can vary depending on manufacturer)
 
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Thanks guys - the easiest is obviously the 35mm equivalent given by manufacturers. But at least it is clear now that looking at 15x or 60x is kinda meaningless when comparing the zoom on these equipment
 
OGroteKoning said:
This would mean that if you have a point and shoot camera with a 20x optical zoom and one that is 15x zoom, that the 20x zoom might not give you a bigger zoom. It could in fact give you a smaller zoom than the 15x if the widest part off the lens is smaller than that of the 15x zoom.
Click to expand...

Just to add to this, it's worth pointing out that your post beatuifully demonstrates one of the problems with terminology. "Zoom" is popularly (but incorrectly, IMHO) used to mean "telephoto", for example my colleague who keeps referring to my 200mm prime as a zoom lens.

"Zoom" in the context of photography means one thing, and one thing only: to move smoothly from one FL/FOV to another.

"smoothly" is a key word there. You get cameras (mostly compacts in the 1990s and earlier) that have multiple FOVs but do not have zoom capabilities, i.e. they have two or more lenses that you can select between.

Also note the difference between focal length (FL) and field of view (FOV). Digital zoom alters the FOV without changing the FL.

Zoom does not mean a long focal length (or narrow FOV). The Nikon 12-24mm is a zoom lens in every sense of the word (and one of the finest too), yet even on the long end, you still have to watch your feet or they'll end up in the frame. Ok, that's somewhat exaggerated, but you get the picture - it's not a tele lens by any stretch of the imagination.
 
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