Need help understanding zoom and lenses?

[tnelson42345]

I really do not understand optical zoom. First of all how to you calculate the optical zoom? Do you divide the distance of the lenses? ex 380mm/38mm = 10 times optical zoom? If that is correct why do the telephoto lenses cost so much more when many do zoom any more? I assume I am wrong in the dividing part :)

I assume focal length has something to do with this.

I understand that some lenses have a different purposes, I mostly just do not understand how the zoom is any different on most lenses.

Also I see and hear the word "noise" with lenses. What is this?

If someone could clear up the concept of optical zoom and lenses I would be very greatful.

[coldrain]

Focal ranges are referring to facal ranges as we know them from 35mm film photo cameras. With them a 50mm lens is a "normal" lens, 80-135mm is portrait slight tele range, 80 and up is considered tele range, 35 and down considered wide angle.

A digital compact that touts 36-420mm focal range of course does not actually have a lens with 36-420mm focal range. That would mean that the lens would/could actually extend around 40cm at least, that is over one foot.
The sensor of these cameras is a LOT smaller than what a 35mm film's size is. That is where the crop factor of the sensor comes in.

So, with this crop factor they calculate what the actual field of view of the camera/lens is in comparison to the field of view of 35mm camera lenses. And then they come up with the 36-420mm numbers.

So... what is important to know when you see figures like that, and want to know what they mean, you have to have an understanding of field of view with 35mm full frame, and the different focal lengths.
To get a better understanding of what those figures mean, play around here:
http://www.tamroneurope.com/flc.htm

You can see the field of view on a 35mm full frame SLR and of a digital APS-C size sensor DSLR.

The "optical zoom range" quoted in the camera marketing does not say anything about what the lens actually shows, it just says what you actually figured out: 380/38 = 10.

Consider this: a Sigma 50-500 lens has BIG tele range, and no wide angle range.
A Sigma 18-200 has a medium telerange and a medium wide angle range.

Yet the "zoom range" of the 50-500 = 10x, the zoom range of the 18-200 = 11x.
The "10x" and "11x" figures say nothing about what the lens actually will be capable of.

So it is best to get used to what the milimeters mean, so you can leave the "12x zoom range" blurb for what it is, marketing blah.

[ReF]

Also I see and hear the word "noise" with lenses. What is this?

noise as in it's loud when auto focusing - a problem on some slr lenses. some lenses have motors that are quiet and fast.

most all in one cameras are also a bit noisy when zooming

[tnelson42345]

"The sensor of these cameras is a LOT smaller than what a 35mm film's size is. That is where the crop factor of the sensor comes in. "


Does this mean you are losing quality as far as megapixels go? Or just are not zooming as much as you are being told?

Thank you very much for the responses

[coldrain]

No, it means the lens can be a lot smaller, not "420mm" for instance, but giving the equivalent of 420mm on a 35mm film camera with a much smaller lens.

And yes, smaller sensors give less image quaility due to smaller photo diodes on the sensor.

[cwphoto]

I really do not understand optical zoom. First of all how to you calculate the optical zoom? Do you divide the distance of the lenses? ex 380mm/38mm = 10 times optical zoom? If that is correct why do the telephoto lenses cost so much more when many do zoom any more? I assume I am wrong in the dividing part :)

I assume focal length has something to do with this.

I understand that some lenses have a different purposes, I mostly just do not understand how the zoom is any different on most lenses.

Also I see and hear the word "noise" with lenses. What is this?

If someone could clear up the concept of optical zoom and lenses I would be very greatful.

Focal length is 'rough and dirty' the length of the lens, it controls the magnification of your subject. With respect to the imager size it also controls the angle of view.

As Coldy says, a 500mm lens provides ten times magnification as a 50mm lens, hence the '10x' tag.

Some telephoto lenses offer things that a zoom lens doesn't, such as optical quality, lens-speed, focusing-speed etc - helping to explain why a non-zoom lens is often more expensive than a zoom lens.:)

[BowerR64]

Do they use the "crop factor" at the wide or telephoto side?

[Ray Schnoor]

Do they use the "crop factor" at the wide or telephoto side?
The crop factor is used at all focal lengths of a lens. For example on a 1.5x crop factor camera (APS-C), a 70-200mm lens is equivalent to a 105-300mm lens on a full frame 35mm camera.

Ray.