DSLR Zoom Lens

[mmm]

Hi all

I have a basic question about the zoom in DSLR lenses.

In the fixed lens in point & shoot digital cameras, the zoom is normally given in an X-number (eg. 3X or 4X, etc...).

But for the DSLR lenses, it is given in lengths of mm (eg. 18-55 or 18-200 mm). Could anyone clarify the difference between the two and how much is for example 18-200 mm in the X-number zoom.

Thanks.

[Rhys]

Hi all

I have a basic question about the zoom in DSLR lenses.

In the fixed lens in point & shoot digital cameras, the zoom is normally given in an X-number (eg. 3X or 4X, etc...).

But for the DSLR lenses, it is given in lengths of mm (eg. 18-55 or 18-200 mm). Could anyone clarify the difference between the two and how much is for example 18-200 mm in the X-number zoom.

Thanks.

Because compacts and zoom compacts have differing sizes of sensor, the zoom ratio is given rather than the focal length. It's also a lot simpler for consumers to judge comparative strengths by the times factor rather than by focal length which varies depending on sensor size.

An 18-200 zoom would be 200/18 (approx 11x).

[Sintares]

Lenses are usually quoted as the length they would be when used on a slr which had a fixed sensor size ( the film ).

Compacts have a range of sensor sizes and the lenses reach is typically given as X10 or X12 etc which makes it easy for the salesman to impress clueless noobies, and also as the slr equivalent reach in mm.

Dslrs also have different sized sensors, usually this is noted as a crop factor (or sometimes as a focal length multiplier) eg x1.6 or x1.5 etc , so a 50-500mm lens attached to a x1.5 crop factor dslr would have the field of view of a 75-750mm lens on a slr.

[TenD]

Hi all

I have a basic question about the zoom in DSLR lenses. In the fixed lens in point & shoot digital cameras, the zoom is normally given in an X-number (eg. 3X or 4X, etc...).Actually the zoom lenses in compacts are expressed in focal lengths also, read the specs. of any of them. They are however also expressed as 3x or 4x too, that is pure marketing which in actual use it doesn't really mean anything, except you can tell what cameras will have more range, from that particular cameras smallest focal length to it's longest focal length.
Focal length is simply the distance the front element is from the film plane. Because of the architecture of many lenses this number isn't exactly that distance but an equivalent of that distance. Focal length is expressed in mm gets really complicated when you start changing formats, most numbers today are referenced to a 35mm film camera: 40-50mm is considered "normal" anything less than that is considered wide angle and anything more than that is considered telephoto. So the numbers are given as an equivalent to 35mm. Medium format users have always had to do calculations, a 50mm which is considered a normal lens on a 35mm film camera, or a short telephoto lens on a digital SLR, is a wide angle on a medium format camera, because of the size of their film plane.
So, a 3x lens is might be kind of useless for a point and shoot if the wide end starts at 100mm: that would be a 100-300mm lens, 100x3=300. But a 3x zoom that starts at 36mm might be somewhat useful, that would be 36-108mm. The x number is simply the ratio between the smallest focal length and the largest focal length, totally arbitrary, all depending on where you started from.


But for the DSLR lenses, it is given in lengths of mm (eg. 18-55 or 18-200 mm). Could anyone clarify the difference between the two and how much is for example 18-200 mm in the X-number zoom.
An 18-200mm lens would be about an 11x zoom.

[mmm]

Rhys, Sintares and TenD; thanks for the great information.

But I have a question, what about the DSLR lenses that do not have a zoom range like the (Sigma 50mm f/2.8), what is the general use of them? and how this one differ from (Tamron 17-50mm f/2.8)? The later is a zoom lens.

Lastly, some of the zoom lenses also have the following notation: 28-105mm f/3.5-4.5, but some other zoome lenses don't have the f-stop range?

thanks again;
mmm

[TenD]

A lens with only one focal length such as a 50mm f/1.8 is generally called a prime lens. A prime lens cannot zoom in or out, but due to the simplicity of it's design it's usually faster(can take photos in lower light), a lot more compact(less elements, no zoom mechanism), and will generally have better image quality at it's focal length than a zoom lens with that particular focal length included in it's range. Primes usually cost a lot less than zoom lenses too, they aren't as convenient as a zoom because you have to either move closer or farther from your subject for framing, or change lenses more often.
A lens with a fixed aperture(that fixed number is the widest aperture-the widest aperture allows the most light in)has a wide enough front element to allow the aperture to be the same on the longest focal length as it is on the shortest focal length within it's zoom range. This makes for a very large front element, this is expensive to manufacture, and makes the lens a lot bigger and heavier. When you zoom in on something you basically make the focal length(remember the distance from the front element to the film plane)longer. So think of it like this: on the short end of the zoom it's like looking through a toilet paper tube, then you zoom in(increase focal length)and it's like looking through a paper towel roll, to your eye(the film plane)the hole looks smaller through the paper towel roll than it does through the toilet paper roll, the smaller hole lets in less light, so effectively your aperture(lens opening)is smaller. So on a lens that costs less and is more compact it will usually have a variable aperture, the front element is large enough to be f/3.5 at 17mm but may not be large enough to gather that much light at 55mm so it's max aperture goes down to f/4.5. A fixed aperture lens kind of starts backwards, it makes the lens opening large enough at the long end of the zoom range for say f/2.8 and the short end of the zoom range is limited by the lens design.

[DonSchap]

"TenD", my old friend... had you broken this up a little more, it would be a skosh more "readible".

Great information... it's all there... but it just needs to be re-organized into smaller sections of information, a bit. ;)

Such as (rough outline... then the meat):

Fixed Focal-length lenses aka "Primes" (ie, 35mm, 50mm, 85mm, 200mm)

Zoom Lenses (ie, 18-200mm, 11-18mm, 24-70mm)

Constant Aperture (ie, f/2.8)

Variable Aperture (ie, f/2.8-4.5)

For future efforts... just works a little better... and really gets the point across quickly :)

[TenD]

Your posts with their multiple fonts and smilies and colors all over the place are some of the most annoying to look at and unreadable posts here. I didn't realize I was getting graded on my writing skills. This is a photo forum isn't it? If it's English class, I apologize. Factually correct, usually, creatively written, maybe not. I'll take the facts.

[Rhys]

Rhys, Sintares and TenD; thanks for the great information.

But I have a question, what about the DSLR lenses that do not have a zoom range like the (Sigma 50mm f/2.8), what is the general use of them? and how this one differ from (Tamron 17-50mm f/2.8)? The later is a zoom lens.

Lastly, some of the zoom lenses also have the following notation: 28-105mm f/3.5-4.5, but some other zoome lenses don't have the f-stop range?

thanks again;
mmm

Prime lenses - those that don't give a zoom range - are used for several reasons:
1. Higher optical quality. All zooms no matter how good are always a compromise.
2. Faster apertures. The fastest commonly available zoom is f2.8. Faster than that and there's a weight penalty. With a prime lens there's less glass so it can be faster. The fastest common prime is about f1.4 although f0.95 lenses have been made.

Prime lenses are fixed focal length. A Tamron 17-50 will give you in one lens the equivalent of 17, 24, 28, 35 and 50mm primes. The benefit of posessing such closely related primes however is debatable. Personally I found with 35mm that my 28-80 and 70-210 zooms could be replaced entirely by 28, 50, 85, 135 and 200mm lenses. That gave me higher quality and faster apertures.

f3.5-f4.5 means that the effective f-stop reduces as the lens is extended. At the wide end the lens will be f3.5 but at the long end it will be f4.5 (and gradations thereof along the range). A single f number lens such as the Tamron 28-75 f2.8 is a constant f2.8 from 28 to 75 with no light lost. Consequently the 28-75 has a wider front element and is heavier.

[DonSchap]

Your posts with their multiple fonts and smilies and colors all over the place are some of the most annoying to look at and unreadable posts here. I didn't realize I was getting graded on my writing skills. This is a photo forum isn't it? If it's English class, I apologize. Factually correct, usually, creatively written, maybe not. I'll take the facts.

Obviously, that didn't help out much. Sorry for drawing to it your attention... the comment seems to be of no value to you... and have a wonderful day. :)

[murrays]

Your posts with their multiple fonts and smilies and colors all over the place are some of the most annoying to look at and unreadable posts here. I didn't realize I was getting graded on my writing skills. This is a photo forum isn't it? If it's English class, I apologize. Factually correct, usually, creatively written, maybe not. I'll take the facts.

+1

-murray

[TenD]

Obviously, that didn't help out much. Sorry for drawing to it your attention... the comment seems to be of no value to you... and have a wonderful day. :)
I am fully aware that I have no outline writing skills, I didn't need to have that drawn to my attention. I have the ability to write that way but it takes me hours to organize my thoughts in that manner. I am not going to take hours to post to a BB, if I was writing a book, maybe. I hope Rhys' post was more to you liking.

[Razr]

Hi all

I have a basic question about the zoom in DSLR lenses.

In the fixed lens in point & shoot digital cameras, the zoom is normally given in an X-number (eg. 3X or 4X, etc...).To arrive at "X", divide the short focal length into the long focal length: for one of my cameras that is 6mm divided into 72mm (36 divided into 432mm in 35mm equivalency), both equaling 12X (the zoom ratio).

But for the DSLR lenses, it is given in lengths of mm (eg. 18-55 or 18-200 mm).18mm into 55mm = 3.05 or 3X zoom ratio. 18mm into 200mm = 11.11 X or 11X zoom ratio.
We SLR folks want to be seen as somewhat sophisticated (superior), so we don't talk about plebian facts like "zoom ratios".
Could anyone clarify the difference between the two and how much is for example 18-200 mm in the X-number zoom.
Thanks.11.11 (11X).
Also, SLR lenses usually don't have such prodigious zoom ratios as 10 or 12 X.
**Sensor sizes don't have diddly to do with focal lengths, not when your OP specifically addressed zoom range.

[DonSchap]

I am fully aware that I have no outline writing skills, I didn't need to have that drawn to my attention. I have the ability to write that way but it takes me hours to organize my thoughts in that manner. I am not going to take hours to post to a BB, if I was writing a book, maybe. I hope Rhys' post was more to you liking.


You know, that statement seems rather pointless. I simply suggested a better method to present your thoughts... because you have excellent coverage on the issue. This would lend itself to impact and understanding... but no, you would rather squabble about someone asking for improvement and insult me, too. Here's a buck... want coffee with it? You're not going to make my Christmas list if you keep this up. Geez Louise. :rolleyes:

It took me two-to-three minutes to prepare a simple outline covering the items the OP asked about. Heck, you had the blanks already filled in.

Accchhh, whatever! (he said in frustration...) I need to focus... on something else. Have a happy... :cool:

[krzkrzkrz]

"TenD", my old friend... had you broken this up a little more, it would be a skosh more "readible".

Great information... it's all there... but it just needs to be re-organized into smaller sections of information, a bit. ;)

Such as (rough outline... then the meat):

Fixed Focal-length lenses aka "Primes" (ie, 35mm, 50mm, 85mm, 200mm)

Zoom Lenses (ie, 18-200mm, 11-18mm, 24-70mm)

Constant Aperture (ie, f/2.8)

Variable Aperture (ie, f/2.8-4.5)

For future efforts... just works a little better... and really gets the point across quickly :)

I agree with this. TenD, you got the knowledge, however it was just a bit difficult trying to read everything on ONE paragraph :) Just my 2 cents for future posts.

Looking forward to them

[Razr]

I am distressed by how many threads are hijacked by personal pissing matches.