Can you explain lens focal length?

[FishyBiz]

Ive been doing some heavy searching on the net and cant seem to find anything. Or at least anything I can understand. I need someone to "dumb it down" for me.
I understand what the "F" on the lens stands for and I understand that the "mm" is basically how far the lens is from the camera but I dont understand what that means when choosing a lens.

Can someone help me? :hammer:

 

[BlueCorn]

Focal length is measure in milimeters, (mm), I'll forego the scientific explanation but it basically is a measure of lens magnification. The larger the number, the higher the magnification of an object, at the same distance.

Low numbers, 10-30ish are generally considered to be wide angle lenses. On a full frame digital or 35mm film camera 50mm is considered to be a "normal" lens. i.e. it provides roughly the same field of view as the human eye. Up from there are telephoto and super-telephoto lenses.

The "f" numbers refer to the lenses aperture (whole the light goes through). The lower the number, the wider the aperture and the more light it allows in.

 

[BlueCorn]

Wide angle (24mm):

Telephoto (400mm):

 

[Recty]

I think the easiest way to put it is that focal length is like a zoom.

Low focal length lenses, like say a Canon 10-22, will be very zoomed out. You can stand in a room and take a picture at 10mm and get almost the entire room in the shot.

A large focal length, like say a 400mm lens, is exactly the opposite. It it zoomed way in. You could stand in a room and take a picture and the only thing you would see is half of the picture hanging on the opposite wall.

I dont know if that helps or not.

EDIT - Well, Beerguy beat me to it and sounded better explaining it too

 

[FishyBiz]

You guys are great.
So let me ask this: Why is a macro lens 100mm and not 10mm?

 

[BlueCorn]

A macro lens doesn't have to be 100mm. There are 50mm, 150mm, 180mm, etc. macro lenses.

What makes macro lenses special is that they have a shortened minimum focusing distance. While a standard 100mm lens might have a minimum focusing distance of 5' a 100mm macro lens has a minimum focusing distance of about 1'

Since you're able to be closer, you get more apparent magnification.

The reason why the 100mm range is so frequently recommended here is that it provides a decent working distance from your subject in an aquarium.

 

[FishyBiz]

Right now I'm using a Canon 50D and a Tamron 18-270mm and it sucks for taking pics of my 14 gallon nano.
I just rented the Canon 100mm IS to try out.

 

[BlueCorn]

Right. The Tammy 18-270 is a pretty decent consumer lens but it's not a macro. Even though many zoom (i.e. adjustable focal length) lenses may have the word "Macro" listed on them, they aren't truly macro lenses.

True macro lenses can do 1:1, or "life sized", or better magnification. What that means is that the size of the image projected on your camera sensor is the same as the object in real life.

 

[FishyBiz]

So then what would be the purpose of me getting say a standard 70mm lens if the one I have now could function at 70mm?

 

[BlueCorn]

The lens that you have now functions as a 70mm lens, as well as every focal length between 18 and 270.

It is not a macro lens.

 

[FishyBiz]

Right. But Im saying why would I buy a regular 70mm? Not a macro.
And thanks for putting up with a noobie. :rollface:

 

[BlueCorn]

Macro lenses are generally slower to focus. There's a trade-off with everything.

 

[dendronepthya]

The other tradeoff is you lose roughly a stop going from a regular lens to a macro lens. A 100mm macro is usually a 2.8 aperture lens. A 100mm regular lens is a 2.0 aperture lens.

 

["Umm fish?"]

But, macro lenses will also have great quality glass. When shooting macros, autofocus is not a huge issue (I usually leave mine off), so the lens makers aren't going to increase the cost of the lens by huge amounts just to give you a fast autofocus.

With macro lenses, the smaller the focal length the closer to the front of the lens you have to be to get 1:1. I have a 65mm macro and the distance to the subject is about 4 inches, far too close to the lens for it to be terribly useful for aquariums when you also have to factor in the thickness of the glass. So, I also have the 100mm macro and that places the subject a good deal farther from the front of the lens at 1:1.

Like everything else, it's a trade-off. The 100mm lets you shoot from a decently-workable distance from your subject without adding all the cost of going up to the 180mm macro. (There are also compositional framing and subject isolation from background issues with changing focal lengths, but working distance and price are likely the main motivations for most aquarists' decision to go with ~100mm.) But if I spent more time shooting insects, I would likely try to move up to the 180mm for the additional working distance.