Can someone explain lenses?

sineadw

I've gotten to grips with just about everything else technical, but I'm looking for a new lens and this is confusing me and I can't seem to find something that explains it clearly. I get the mm thing, but what's all this 1.8, f/4 etc business? If that's not a stupid/huge question :rolleyes: Does anyone have a link to a good article even?

Huggy Bear

try these, I'm sure there are better sites around ...
http://members.tripod.com/~Prophotoman/fstop.html
http://www.zlew.net/thoughts/?p=186
http://www.dcresource.com/forums/archive/index.php/t-5614.html

DotOrg

the f part of a lens, whether it be a 50mm f1.4 lens or a 70-200mm f2.8 lens refers to the size of the aperture, which is the size of the hole in the lens which the light passes through into your camera.

the bigger the hole (meaning the smaller the number eg f2.8) means more light gets in but the less of the picture will be in focus. so you can isolate your subject making the foreground and background out of focus easily

a smaller aperture (a bigger number like f5.6) means that every picture you take more of the shot will be in focus and you won't get as much background blur

read this:
http://photo.net/learn/making-photographs/exposure

mervifwdc

To see it for yourself, take the longest (mm) lens you have, and shoot the same picture of a thing that is a little bit away, with a background that is a good bit further away. Shoot it twice, once at the largest aperature you can (AV mode, lowest setting you can), and the same picture with the highest setting you can. Put the camera on a table or tripod to make sure it's the same shot, just different aperatures.

You will see the difference in a few things. particularly, the differect in the DOF, which covers how much is in focus, and the shutter speed.

A lens that can open up to f1.8 (for example) gives a tiny DOF, typically, a persons eyes can be in focus and their ears out of focus. This effect can be used to great effect. These types of lenses are known as fast lenses, mainly because the shutter speeds are very fast when using them, but I like to think it's also because your wallet empties faster when buying them :-)

Fajitas!

mervifwdc wrote:

but I like to think it's also because your wallet empties faster when buying them :-)

davmigil

http://en.wikipedia.org/wiki/F-number

The F-number refers to the widest aperature the lens will open to.

So if you had two imaginary identical lenses, one with F4 and one with F2.8 and you were taking a picture of the same scene:

At F4 your exposure meter tells you a shutter speed of 1/30 second is needed.
The next bigger F-Stop (or aperature size) in the sequence is 2.8. Which means it lets in twice as much light as F4. Thus because you are getting twice as much light you only need half as much time to expose the same picture. So at F2.8 the exposure meter will tell you a shutter speed of 1/60 is required.
If you had a F5.6 lens, it would let in half as much light as the F4 lens, it would require twice as long to expose the same picture, so you would need a shutter speed of 1/15

Aperature Shutter Speed
F2.8 1/60
F4 1/30
F5.6 1/15

Each stop on the aperature scale represents a doubling/halving of light, so you need to double/halve the shutter speed to get the same exposure.

So a smaller F stop (larger aperature) will for instance let you take a picture hand held, where a larger F stop (smaller aperature) might require a tripod.

However there is a trade off: The larger the aperature the smaller the depth of field. For instance if you take a picture of a flower in a field at 1.8, (most of) the flower will be in focus but the rest of the field will be blurred. The same picture at F22, the flower will be in focus, but so too will the rest of the field.

As a rule of thumb (but not always correct, read reviews etc.) the smaller the F number quoted the higher the quality of the glass and the higher the price.

Shiny

The iris in your eye works on the same principle.

Just look into a mirror in a dimly lit room.
You will see that your iris is really wide to allow
your eyes to get as much light as possible.

Then,

While looking at the mirror, switch on a light,
Zooom! you will see your iris get really small!!!

Camera's do the same(on Auto or Tv mode).:)

Huggy Bear

Came across this very interesting site:

http://www.photonhead.com/simcam/

Film and Digital Camera Simulator:

• Shutter and Aperture
• Film Speed
• Camera Shake

Pretty cool site :cool:

Oriel

Shiny wrote:

Tv mode

Only if you own a girly Canon.

sineadw

Ah ok - so its maximum aperture then. Thats what I was missing. Thanks!

So a 28mm 1:2.8 wouldn't be bad then..

Shiny

That lens is ok.
Some reviews say it is too soft at f2.8 and that
you may be better off with the 50mm f1.8.

http://www.fredmiranda.com/reviews/showproduct.php?product=23&sort=7&cat=2&page=1

I'm assuming you have a girly Canon.:)

sinecurea wrote:

Only if you own a girly Canon.

sineadw

No I have an ancient Cosina It came with a 50mm f2 but I want something wider as I'm feeling the restriction now.. Its a Ricoh XR I'm looking at. Just noticed its listed as having fungus though. I'm assuming that's a big no-no. Oh well...

Roen

sinecurea wrote:

Only if you own a girly Canon.

Is it called S mode on Nikon?

Fionn

i used to have fungus!! too

the meds cleared it up tho!

Simon201

And just to get trivial when it says 'f2' it means the actual diameter of the aperture will go twice into the focal length of the lens. So if you had a lens with a 50mm focal length and you put the aperture setting to f8, and you could measure the aperture, it would be about 6.75mm in diameter.

Shiny

Basically the picture below: