Zoom confusion

[Sarmad]

I am really confused about zooming in DSLR's. In P&S's the ability of zoom is defined by X's, like 10X zoom, 15X zoom etc. In DSLR's, it is described as 18-55mm, 70-200mm. The first thing I want to know about is that if a 18-55mm lens provides 3X zoom, what will a 70-200mm provide? How to convert DSLR zoom units to P&S zoom unit?
Second confusion is, Can we zoom using a lens that has fixed focal length? like Nikkor 35mm f1.8, 85mm, 50mm.

 

[Solarflare]

The "10x zoom, 15x zoom" etc are not very precise. They do not specify what range exactly is covered. Thus usually people in more serious discussions specify what the focal length range would be on a small format camera (also called 35mm fild or full frame). They do it even for p&s.

Thats why online discussions about p&s cameras often contain wild arguments if the camera in question offers only 28mm, or 24mm, or even 21mm as the lower end of their range

A zoom of 14-42mm is something completely different than a zoom 70-210mm, even if both are 3x zooms, but they dont share a single focal length.

Also, DSLRs have a much higher image quality than p&s cameras and thus one usually only uses zooms with not much more than a 3x range, sometimes even less. The largest focal length area you can get with Nikon is the 18-300mm DX, which is equal to 28-450mm on small format.

Thus, with DSLRs, its even more important than with p&s to know what focal lengths they actually cover.

Besides - some companies, such as Leica, even only offer almost exclusively fixed focal lengths for their cameras. My p&s also has a fixed focal length. Thats done in order to get maximum image quality.

 

[KmH]

DSLR cameras allow for lens interchangeability, so the zoom range can be changed by changing a lens. By stating the zoom in focal length mm values, photographers get an indication of the lenses field-of-view (FoV).
A 3x zoom short focal length lens has a much wider FoV than a 3x zoom mid-telephoto lens.

P&S camera lenses are not interchangeable, and often the focal lengths and apertures stated for the lens are 'equivilent' values and not actual values. Further, many P&S cameras use a combination of physical lens zoom and software zoom.

Fixed focal length lens are zoomed by you moving closer to, or further from, your subject. Fixed focal length lenses area also known as 'prime' lenses, but no, their focal length cannot be changed (zoomed).

 

[ptl2010]

I'll try to explain this the best I can.

First off you don't really need to know the zoom range, like 3x, 10x, 15x, etc unless you're looking at a point and shoot. What you have to learn when it comes to dslrs is what focal lengths are best in what situations.

The focal length of a lens is measured in mm, and is usually labeled like 18-55, 55-250mm, 70-200mm, 50mm, 85mm,etc. When its a number like 18-55mm, that means the lens can zoom from 18-55. If its one number, like 50mm, that means its a fixed focal lenght, which is called a prime lens. This number measures the distance of the lens from the back of the camera. The shorter the distance (lower the number) the wider the field of view, the longer the distance (larger the number) the narrower the field of view becomes.

To convert this mm idea to the P&S variety, on your camera you should have a zoom switch. one side should have a T for telephoto, and a W for wide. Wide means wide angle of view, which to a dslr would be a smaller mm number on the lens. Telephoto means a narrower field of view, which to a dslr would mean a larger mm number. IF you hold the switch to W until it can't zoom out anymore, that's the widest field of view the lens has, which would be the lowest number on a dslr lens. If you hold it on T till it stops, that's the narrowest field of view, which is the biggest number on the lens.

When you have a lens that starts with a smaller number like 18 or 28, its a wide angle zoom. If you have a lens that starts with a larger number like 55 or 70, its a telephoto zoom. If the zoom starts with a small number ilke 18 and ends with a really large number like 135 or 200, it's called a superzoom because there is such a big difference between the shortest and longest focal lengths possible. Most point and shoots are like superzooms, they have a really big focal range

 

[480sparky41 40]

To me, the 'x' number is just a mass-marketing gimmick. Used to make a '10x' lens somehow better than a '5x' zoom to the uneducated buying public.

 

[boborone]

One thing I haven't seen anyone hit on that might help OP is a focal length of about 50mm is what the human eye sees. So you can judge from that the distance the lens can see.

 

[KmH]

That refers to field-of-view, and is a true approximation only when the 50 mm lens is mounted on a camera that has a full frame image sensor.

 

[AlexanderB]

Can we zoom using a lens that has fixed focal length? like Nikkor 35mm f1.8, 85mm, 50mm.

No. Zoom is the ability to change focal length. So by the definition answer is No.

 

[TCampbell]

Can we zoom using a lens that has fixed focal length? like Nikkor 35mm f1.8, 85mm, 50mm.

No. Zoom is the ability to change focal length. So by the definition answer is No.

We call it "zoom with your feet". When you walk closer... the subject gets larger. ;-)

 

[ph0enix]

The max. focal length divided by the min. focal length gives you the zoom ratio. So for a 70-200 lens it's 200/70 = ~2.86 (~3X). For an 18-55, it's 55/18 = ~3.06 (~3X). A 28-300 lens has a zoom ratio of over 10X. High zoom ratios are not desirable in terms of image quality. Some bridge cameras have zoom ratios of over 40X which is very useful but a lot of IQ is lost because of it.
Fixed focal length or prime lenses don't have zoom but usually provide superior IQ over zoom lenses.

 

[480sparky41 40]

To further muddy the water, some P&Ss add 'digital' zoom to the mix. It's the same as cropping the image, either in-camera or in post. So the box may scream 50x zoom!, but it's really a 20x optical zoom and the rest is done by tossing out pixels.

 

[table1349]

Can we zoom using a lens that has fixed focal length? like Nikkor 35mm f1.8, 85mm, 50mm.

No. Zoom is the ability to change focal length. So by the definition answer is No.

We call it "zoom with your feet". When you walk closer... the subject gets larger. ;-)

Wait.......... what................you mean if I am say using an 85mm lens to take head and shoulder portraits and decide that I want to take a couple full body shots all I have to do is back up a couple of steps or so and use the same lens? I don't have to buy a whole new lens and call it a full body shot lens??? Well ain't that handier than a pocket on a shirt. :mrgreen: Next thing ya know there gonna start making cameras that you don't have to buy film for.

 

[Kolia]

Wait.......... what................you mean if I am say using an 85mm lens to take head and shoulder portraits and decide that I want to take a couple full body shots all I have to do is back up a couple of steps or so and use the same lens? I don't have to buy a whole new lens and call it a full body shot lens??? Well ain't that handier than a pocket on a shirt. :mrgreen: Next thing ya know there gonna start making cameras that you don't have to buy film for.

A lens might be cheaper than a 50 feet extension to your studio tho...

 

[480sparky41 40]

Wait.......... what................you mean if I am say using an 85mm lens to take head and shoulder portraits and decide that I want to take a couple full body shots all I have to do is back up a couple of steps or so and use the same lens? I don't have to buy a whole new lens and call it a full body shot lens??? Well ain't that handier than a pocket on a shirt. :mrgreen: Next thing ya know there gonna start making cameras that you don't have to buy film for.

A lens might be cheaper than a 50 feet extension to your studio tho...

Or walking backwards off a cli

 

[milinda]

The max. focal length divided by the min. focal length gives you the zoom ratio. So for a 70-200 lens it's 200/70 = ~2.86 (~3X). For an 18-55, it's 55/18 = ~3.06 (~3X). A 28-300 lens has a zoom ratio of over 10X. High zoom ratios are not desirable in terms of image quality. Some bridge cameras have zoom ratios of over 40X which is very useful but a lot of IQ is lost because of it.
Fixed focal length or prime lenses don't have zoom but usually provide superior IQ over zoom lenses.

Exactly the correct answer. in P&S this X factor is given for the attraction. Since lot many don't understand this, they think having a big X factor makes it a big zoom camera. IF you have a lens 5-100mm, in P&S its 20X. But it cannot zoom as 70-200mm lens as given in your example which is 2.86X. And more the X factor less the quality of the image