# A few lens question

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#### chimaerax

##### Guest
1. What does f/1.8 and other f/ things mean?
2. Point and shoot cameras have the optical zoom like 6x written somewhere. How do we know the magnification from the lens' focal length?
3. What does shooting wide open means?
4. How do I vary the dof?
5. What are primes?
6. How do u tell a picture is sharp or unsharp? Any samples for comparision? How sharp is sharp?

#### coke21

##### New Member
chimaerax said:
1. What does f/1.8 and other f/ things mean?
2. Point and shoot cameras have the optical zoom like 6x written somewhere. How do we know the magnification from the lens' focal length?
3. What does shooting wide open means?
4. How do I vary the dof?
5. What are primes?
1. The f/value refers to the aperature size. The smaller the number refers to the bigger the aperature size and the larger the number refers to a smaller aperature size. One way of remembering this is 1/f(value).

3. shooting wide open refers to shooting at the largest aperature size ie the smallest f/value
4. You can vary the DOF by changing the focal length and at the same time varying the f/value. Generally the smaller the f/value, the shallower the DOF. eg DOF at f1.8 will be shallower then f/8

#### espn

##### Deregistered
1) f/xx is aperture, do a search on google or read up dpreview's glossary.

2) lens minimum length (eg 35), to it's max (105) take 105 / 35 = 3x zoom. 6x = 35 x 6 (assuming the format is 35mm)

3) Wide open means the aperture is set to the largest value and shots are taken. ie: set 50mm at f/1.8 and shoot. Wide open (assuming the lens largest is f/1.8)

4) DOF is controlled by the aperture and zoom play around

5) Primes are lenses with fixed focal lengths, they cannot change focal length, eg 28-105 are called zoom lens. 24, 35, 50, 300, 400 are all fixed focal lengths, thus called primes

6) sharp and unsharp hmm I don't have pictures for comparison, all my pictures are not sharp :embrass: Maybe the others can advise..

**these are what I understand to my own definitions, if I'm wrong, please correct me. Thanks

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#### chimaerax

##### Guest
Wow thanks for the fast replies. Now its clearer
Is there any reason why people will choose primes over zoom lens?
So a lower f/value means a faster lens? What does a faster lens do? (sorry seems stupid but i cant find the answers.)

#### espn

##### Deregistered
Primes are reputed and proven to be sharper in quality then zoom lens. That's why some people prefer primes.

#### coke21

##### New Member
chimaerax said:
Wow thanks for the fast replies. Now its clearer
Is there any reason why people will choose primes over zoom lens?
So a lower f/value means a faster lens? What does a faster lens do? (sorry seems stupid but i cant find the answers.)
Usually prime lenses have a lower f/value and thus...makes it "faster" It just means that the lens allows the user to take a lower light levels cos the aperature is bigger. So for example a 50mm f1.4 is faster then a 50mm f1.8.

There are many kinds of lenses out there...some zoom, some primes. Zoom lenses usually have variable f/values. eg 28-105mm f3.5-f4.5 Meaning the smallest acheivble aperature at 28mm is f3.5 and at 105mm f4.5.

Some zooms have constant f/values through out the variable focal lengths. These lenses tend to be very much more expensive

#### Ah Pao

##### Senior Member
For your question #2, about zoom factor, the focal length of your lens should be written on the rim of your lens. Because different digicams use different sensor sizes, their focal length tends to be different, but will produce the same zoom factor.

However, this does not apply to DSLR lenses as their focal length is based on the 35mm film format. DSLRs usually have a focal length multiplier (or crop factor, from the way I see it) that helps convert the 35mm format's focal length to your particular DSLR's actual focal length.

In fact, the lens rim may tell you a lot more information, like the lens' largest apeture size(s), make/model and perhaps the country of manufacture.

Finally, to reiterate what espn has said,
Longest focal length / Shortest focal length = Zoom factor.

#### mpenza

##### Senior Member
primes are generally cheaper than zooms too.

#### frisky

##### Deregistered
espn said:
2) lens minimum length (eg 35), to it's max (105) take 105 / 35 = 3x zoom. 6x = 35 x 6 (assuming the format is 35mm)
My understanding is that the 6x/3x/10x has nothing to do with 35mm.
So how does the 35 x 6 comes by?

Should be assuming the minimum zoom is 35mm (equivalent) right?

#### majere2sg

##### Senior Member
chimaerax said:
2. Point and shoot cameras have the optical zoom like 6x written somewhere. How do we know the magnification from the lens' focal length?
You don't really have to bother abt optical zoom, just need to know abt the minimum and maximum focal length.. etc.. 70-200mm.
a 10x optical zoom on one camera may not be equalivent to another 10x on another lens.. eg.. a 28-280mm (10x) p&s camera is certainly not equalivent to a 50-500mm (10x) sigma lens.

#### espn

##### Deregistered
frisky: Yep, that is correct, thanks for pointing that out

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