# question on lenses( newbie question)

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#### kyudave

##### Guest
hi, very newbie here

wanna know how to understand the lenses
(eg. EF-S 18-55mm f/3.5-5.6 and EF 28-105mm f/3.5-4.5 II USM)

18 - 55 mm = zoom ?
f/3.5-5.6 = ???
II USM = ???

thks.

#### SzennyBoy

##### New Member
18 - 55 mm = zoom - correct
f/3.5-5.6 = variable maximum or widest aperture - capable of f/3.5 at 18mm and f/5.6 at 55mm
II USM = Ultra Sonic Motor - Canon's silent focus system on the lens. Nikon's equvalent are the AFS lenses.

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#### kyudave

##### Guest
thks...

18 - 55 is he zoom, then how do I calculate this zoom equivalent to how many X zoom in prosumer / consumer camera?

thks

#### SzennyBoy

##### New Member
kyudave said:
thks...

18 - 55 is he zoom, then how do I calculate this zoom equivalent to how many X zoom in prosumer / consumer camera?

thks
huh?!? what do mean by "how many X zoom"? The zoom length is defined by the focal length range of the lens, i.e. 18mm to 55mm in the above example. The camera body is only the "box" that houses the film (or memory card in the case of a digital body) and does not dictate the zoom capability of the lens.

#### AReality

##### Senior Member
Typical consumer cameras starts the zoom at 35mm.

Dun be confused with "35mm camera" or "35mm equivalent focal length". The former means that the camera uses a film of size 35mm.(those widely used films.) The latter is more technical. Won't go into that yet.

So for a normal 35mm camera, with starting focal length at 35mm(widely available type), a 3x zoom camera will have focal length from 35mm to 105mm (note: 105/35 = 3).

Also note that:
For a normal 35mm camera, with starting focal length at 28mm(may be a bit more expensive), a 3x zoom camera will have focal length from 28mm to 84mm (note: 84/28 = 3).

Thus the more professional way of telling pple how much zoom your camera/lens have is to quote the focal length millimetres. Pls note that the larger the number, the "further" you can see.

In the case of the Canon 300D, the sensor size is not as large as the normal film(35mm film) everyone uses. it's slightly smaller. Thus if you look at it from another way, the image is actually cropped. That's where the "cropping factor" comes in. The 300D has a cropping factor of 1.6. The cropping factor differs from camera to camera. depending on the sensor/film size.

Thus when pple say "35mm equivalent focal length", he has already taken the cropping factor into account. So the 35mm equivalent focal length of the EF-S 18-55mm is from 28.8mm to 88mm.( 18x1.6=28.8, 55x1.6=88)

#### jasphotography

##### New Member
kyudave said:
thks...

18 - 55 is he zoom, then how do I calculate this zoom equivalent to how many X zoom in prosumer / consumer camera?

thks
do you mean X zoom as in 55mm/18mm = 3X zoom?

If yes, then the 18-55mm lens is a 3X zoom lens...

#### jasphotography

##### New Member
Dammit, AReality beat me to it by split second.... ;( :bsmilie:

AReality said:
Typical consumer cameras starts the zoom at 35mm.

Dun be confused with "35mm camera" or "35mm equivalent focal length". The former means that the camera uses a film of size 35mm.(those widely used films.) The latter is more technical. Won't go into that yet.

So for a normal 35mm camera, with starting focal length at 35mm(widely available type), a 3x zoom camera will have focal length from 35mm to 105mm (note: 105/35 = 3).

Also note that:
For a normal 35mm camera, with starting focal length at 28mm(may be a bit more expensive), a 3x zoom camera will have focal length from 28mm to 84mm (note: 84/28 = 3).

Thus the more professional way of telling pple how much zoom your camera/lens have is to quote the focal length millimetres. Pls note that the larger the number, the "further" you can see.

In the case of the Canon 300D, the sensor size is not as large as the normal film(35mm film) everyone uses. it's slightly smaller. Thus if you look at it from another way, the image is actually cropped. That's where the "cropping factor" comes in. The 300D has a cropping factor of 1.6. The cropping factor differs from camera to camera. depending on the sensor/film size.

Thus when pple say "35mm equivalent focal length", he has already taken the cropping factor into account. So the 35mm equivalent focal length of the EF-S 18-55mm is from 28.8mm to 88mm.( 18x1.6=28.8, 55x1.6=88)

#### AReality

##### Senior Member
jasphotography said:
Dammit, AReality beat me to it by split second.... ;( :bsmilie:

:blah: :blah:

In office on friday mood...

#### ST1100

##### Senior Member
II USM = ???

The II means this is version two of this particular lens. Newer versions not necc better - may just be cheaper.

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thks lots

#### jasphotography

##### New Member
AReality said:
:blah: :blah:
right, very funny right? Don't let me see you or know who you are for I am going to skin you alive! :faint:

hmmmm... ha, who say moderator has no sense of humour! :bsmilie:

#### Prismatic

##### Senior Member
Just to summarised AReality's long post.

How many times zoom equal largest focal length divided by shortest focal length.
So 18-55mm equals 3x zoom
However, 28-85mm also equals 3x zoom.
75-300m equals 4x zoom.

So when buying a digital camera, in order to appreciate the zoom, you have to know the minimum focal length of your camera.

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#### kyudave

##### Guest
still have 1 question.

let say I have EF 28-200mm f/3.5-5.6 USM

do I need to buy other lense let say EF 28-135mm f/3.5-5.6 IS USM

which is actually the range is inside EF 28-200mm f/3.5-5.6 USM .

thks

#### Prismatic

##### Senior Member
kyudave said:
still have 1 question.

let say I have EF 28-200mm f/3.5-5.6 USM

do I need to buy other lense let say EF 28-135mm f/3.5-5.6 IS USM

which is actually the range is inside EF 28-200mm f/3.5-5.6 USM .

thks
This will depends on your needs then.
Although the 28-200 is convenient and cover a wide range, some people may find that the optical quality of the lens is compromised. The sharpness of the lense is compromised in order to fit such wide focal length in and the extreme ends of the lenses is not really that useful as distortions usually set in.

Also on the 28-135, there's the IS system, which allows shooting at lower shutter speed as the lens eliminates handshake during picture taking.

Whether you need to buy more lens that overlaps one another depends on how you value sharpness, convenience and money.

#### ST1100

##### Senior Member
kyudave said:
still have 1 question.

let say I have EF 28-200mm f/3.5-5.6 USM

do I need to buy other lense let say EF 28-135mm f/3.5-5.6 IS USM

which is actually the range is inside EF 28-200mm f/3.5-5.6 USM .

thks
i hate to blow your mind, but do you know there are lenses that don't even zoom? They are called 'primes', and come in standard flavours, from 28-200, they would be 28, 35, 50, 85, 105, 135, and 200. So your 28-200 would actually replace more than 7 or 8 lenses.

Primes tend to be sharper, lighter and cheaper than zooms.

#### nemesis32

##### Senior Member
For info only.... the EFS18-55 mm lens can only be used for EOS 300D. Hence, anyone who just purchase the EOS 300D kit shd not sell away their wide-angle lens as the 18-55mm lens cannot be used on other camera....

I am a Nikon user and i find it to be rather stupid... pardon me.

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#### Camm

##### New Member
Prismatic said:
Just to summarised AReality's long post.

How many times zoom equal largest focal length divided by shortest focal length.
So 18-55mm equals 3x zoom
However, 28-85mm also equals 3x zoom.
75-300m equals 4x zoom.

So when buying a digital camera, in order to appreciate the zoom, you have to know the minimum focal length of your camera.
For 18-55mm, is 18 the minimum focal length? Y is min focal length important?
So comparing 18-55 and 28-85, which one is better?

#### Prismatic

##### Senior Member
Camm said:
For 18-55mm, is 18 the minimum focal length? Y is min focal length important?
So comparing 18-55 and 28-85, which one is better?
There's no exact definition of 'better' in this case. It's just a difference of angle of view. For a 18mm min focal length, you will be able to fit a wider angle of view into your picture than the 28mm. However, the trade-off is you have an expanded perspective. That's why, lense with focal length shorter than 28mm are called wide-angle lenses.

For comparision, on the 35mm film standard, a normal person's angle of view is about 92 degs horizontally. The angle of view of one eye is about 45 degs or so (rough guide) which is just about the angle of view of a standard 50mm lens.

A 28mm lens typical gives 65 deg.
A 20mm lens gives 84 deg.
While a extremely wide angle lens like 14mm will give 104 deg.

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