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Focal Length vs. optical zoom





Tony The Tiger
I went to the camera store today to try to understand what the difference is between a 55-250 and 75-300 lens. The guy says that you can compare the optical zoom capabilities by dividing the focal length by 18, which is essentially 1x zoom. Thus 250 focal length is 250/18=13.89x optical zoom. 300 focal length is 300/18=16.67x optical zoom. Is this correct?
william
You really don't want to use the term "optical zoom" with dSLRs. Technically, the optical zoom of a lens is the maximum focal length divided the minimum focal length. (So 250/55 and 300/75 respectively). I think the reason the guy at the camera store said to divide by 18 is because that's the base focal length for the kit lens, so 13.89x and 16.67x are relative to the "zoom" at 18mm.

Optical zoom is used for point and shoot cameras because their base zooms (1x) are relatively consistent across models. The same can't be said about dSLRs because you can mount several hundred different types of lenses that have different focal lengths. Not to mention the "zoom" of one lens on one dSLR won't be the same as on another dSLR. For example, the "zoom" you get at 35mm from a cropped camera will be about the same as you get at 50mm on a full-frame camera.

Check this page out to get an understanding of focal length. Also note the relationships between focal length and field of view. Remember, there are two ways to "zoom", increasing the focal length (which crops the background), and walking forward (which keeps a wider view).
Tony The Tiger
william wrote:
You really don't want to use the term "optical zoom" with dSLRs. Technically, the optical zoom of a lens is the maximum focal length divided the minimum focal length. (So 250/55 and 300/75 respectively). I think the reason the guy at the camera store said to divide by 18 is because that's the base focal length for the kit lens, so 13.89x and 16.67x are relative to the "zoom" at 18mm.

Optical zoom is used for point and shoot cameras because their base zooms (1x) are relatively consistent across models. The same can't be said about dSLRs because you can mount several hundred different types of lenses that have different focal lengths. Not to mention the "zoom" of one lens on one dSLR won't be the same as on another dSLR. For example, the "zoom" you get at 35mm from a cropped camera will be about the same as you get at 50mm on a full-frame camera.

Check this page out to get an understanding of focal length. Also note the relationships between focal length and field of view. Remember, there are two ways to "zoom", increasing the focal length (which crops the background), and walking forward (which keeps a wider view).


So once I switch lenses from the 18-55mm to the 55-250mm, dividing by 18 is no longer appropriate as the guy at the camera store suggested it is. Does this mean that 55mm focal length on the 18-55mm takes a different shot than the 55mm on the 55-250mm lens?

At times, it seems like moving forward generates similar results to increasing the focal length on the kit lens. However, when I am sitting on the upper deck at Wrigley field with the 55-250 zooming seems totally different.
william
No, 55mm is the same zoom on both lenses. The camera store guy said to divide by 18 because he was using 18mm as the base 1x zoom. But you can also use 55mm as the 1x zoom. In theory, if you were to act as if the kit lens and telephoto lens were one lens (18-250mm), then the total optical zoom would be 250/18. But again, 18 is just a base decided by the minimum focal length. This is why you don't want to use the term optical zoom with SLRs because the base focal length is different on different lenses. This is why, technically, the optical zoom of a lens is the maximum focal length divided by the minimum, because there is no set minimum focal length.

And like I said, zooming in by changing the focal length is like cropping the photo, then zooming in. Walking forward is like zooming in, then cropping. The diagram on the link I referenced earlier really helps in explaining what focal length is doing. Basic optics, really.

Tony The Tiger
william wrote:
No, 55mm is the same zoom on both lenses. The camera store guy said to divide by 18 because he was using 18mm as the base 1x zoom. But you can also use 55mm as the 1x zoom. In theory, if you were to act as if the kit lens and telephoto lens were one lens (18-250mm), then the total optical zoom would be 250/18. But again, 18 is just a base decided by the minimum focal length. This is why you don't want to use the term optical zoom with SLRs because the base focal length is different on different lenses. This is why, technically, the optical zoom of a lens is the maximum focal length divided by the minimum, because there is no set minimum focal length.

And like I said, zooming in by changing the focal length is like cropping the photo, then zooming in. Walking forward is like zooming in, then cropping. The diagram on the link I referenced earlier really helps in explaining what focal length is doing. Basic optics, really.


As I think about the optical zoom, it seems to be a simplification. I believe that the lens is moving on a zoom lens rather than the CCD. If the lens and the subject were stationery. The focal length distance between the lens and the CCD would give an exact multiplier. However, It is the CCD and the subject that are stationery. For large distances between the subject and the lens this is a moot point. However, if we are standing over a flower and using our zoom lens, the zoom X factor is going to be imprecise at a level that matters.
siemens13
william wrote:
You really don't want to use the term "optical zoom" with dSLRs. Technically, the optical zoom of a lens is the maximum focal length divided the minimum focal length. (So 250/55 and 300/75 respectively). I think the reason the guy at the camera store said to divide by 18 is because that's the base focal length for the kit lens, so 13.89x and 16.67x are relative to the "zoom" at 18mm.

Optical zoom is used for point and shoot cameras because their base zooms (1x) are relatively consistent across models. The same can't be said about dSLRs because you can mount several hundred different types of lenses that have different focal lengths. Not to mention the "zoom" of one lens on one dSLR won't be the same as on another dSLR. For example, the "zoom" you get at 35mm from a cropped camera will be about the same as you get at 50mm on a full-frame camera.

Check this page out to get an understanding of focal length. Also note the relationships between focal length and field of view. Remember, there are two ways to "zoom", increasing the focal length (which crops the background), and walking forward (which keeps a wider view).

I also agree with you and appreciate you.
Tony The Tiger
siemens13 wrote:
I also agree with you and appreciate you.

You responded to another author and failed to acknowledge the mathematical technicality that I pointed out. Did you understand my point?
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