Quick & Dirty Guide To What The Numbers Mean

[kja]

After speaking with some new dslr camera users I thought I'd whip up a very quick and dirty guide to what all those numbers mean on lenses.

We'll use these lenses as our examples: 18-200mm f/3.5-6.3 and 50 1.8

The first numbers in a lens (18-200 and 50) tells you the focal length. When a range is mentioned (18-200) it means that you are looking at a zoom lens. Focal length is, essentially, the reach of your lens. Focal length determines your field of view - how much of a scene will be captured. The smaller this number is, the wider the scene; the larger this number, the farther away your subject can be and still fill the frame*

Here's a breakdown of focal lengths - all are approximates, just to get you in the ballpark:

ultra wide angles = 4.5mm to about 20mm - great for landscapes and scenes; generally not great for portraits as distortion will occur

wide angles = about 20mm to 35mm - great for landscapes, scenes, interior shots, careful use of the widest of these can produce cool portraits but it's a tough technique to master as they still have a distortion thing happening.

normal range = 35mm to about 85mm - great for portrait type shooting; the smaller this number the more of the surrounding scene you can get in your lens; the larger this number, the farther away from your subject you can be and still fill the frame; extremely versatile lenses

telephoto (this does NOT mean zooms, necessarily) = 85mm - 300mm - usually for distance shooting but also can do killer portraits

supertelephoto (this does NOT mean zooms, necessarily) = 300mm - 800mm+ - almost under the specialty lens category (which I'm not touching on in this post) - especially for things like wildlife etc; and you thought some of the others were expensive, whooooeeee!

Obviously some zooms will cross two or even three of these ranges, they are usually referred to as super zooms (our 18-200mm example is a super zoom) and there are several out that do a very nice job.

The second number in a lens description (f3.5-6.3 and 1.8 in our examples) indicates the maximum aperture. Now stay with me, it's really not that hard...

The aperture of your lens is the hole that light comes through. You can control that hole with your aperture setting and all lenses will stop down - that is, make the hole smaller. The number only tells us how wide open we can make the hole to let in the most light possible with that lens. It's actually a fraction so the BIGGER the number in the description, the SMALLER the hole that can let light in is. And photography is all about light so a lens with a smaller number is more desirable (usually and to a point) and thus, heavier and more expensive and almost always referred to as "better". This does not mean that lenses with big number suck.

As we know, everything in photography is a trade off so as we have a smaller number in the description (larger aperture more light) we also lose depth of field. Depth of Field or DOF is how much of your subject is in the plane of focus - with these fast lenses it can literally be a couple of mm's only. It can make it very hard to nail focus on a static subject much less something that is moving.

What we gain from a shallower DOF is subject separation - the background and other non-essential elements in the frame become blurred out.

So, why do some zoom lenses have a range of apertures (f3.5-6.3 in our example). This is called a variable aperture and is very common on many zooms. Basically it means when you have your lens set at the widest focal length (18mm in our zoom example) then the aperture can also open all the way up to that smallest number (3.5 in our example). When we use the zoom feature that maximum aperture will get smaller - so at 200mm in our zoom example, our maximum aperture will be 6.3 not 3.5).

What does that really means? In a nutshell - you'll need more light for the longer end of the zoom. You may need to add an external light source, slow your shutter speed down and/or increase your ISO (all things we can talk about in future if anyone is interested or doesn't quite get it). Easy.

There are zooms out there with fixed apertures - such as Canon's stellar 17-55 f2.8 IS or the 70-200 f4. This means that no matter what focal length you shoot at, the maximum aperture stays the same. You will pay more for these lenses and they'll be heavier/bulkier but when you know you're going to need as much light as possible at all focal lengths, they are certainly worth the price tag. (BTW the Canon 70-200 f4 is a killer lens at a really nice price point, if you can live without the extra stops of light of the 2.8 (much pricier) version.)

I hope this helps some out there. Fire away with questions if you have them.

*gross oversimplification, but this post is intended to be a rudimentary introduction.

[altheau]

Thankyou for taking the time to put this together, it is appreciated, I always get so lost with it all

[Lab lady]

;)

We talked about lenses last night at the photography course I'm doing and my head was spinning by the time i got home so your timing with this is perfect.

I'm sure i will have some questions at some point but for now you have helped my decipher what some of my scribble notes are from last night.

[redangel]

Great info to share

[Guest belgian.blue]

You're the bestest kja

I'm going to print that out at work tomorrow and I should be a lens pro by 3pm tomorrow and defiantly by Sunday! But please no quizzes

[Kirislin]

great explanation.

this might go well as a companion peice to yours. I hope you dont mind me posting it.

http://photography-on-the.net/forum/showthread.php?t=414088

[Firestone]

thanks so much. May I save that please??

[MrRedDog]

Great post, Thank you

[kja]

Thanks for the great reception - I'm happy it helped some!

Kirs - anything that helps others is welcome

Of course you can save it, print it, whatever helps you the most!!

[Frodo's mum]

thank you so much for taking the time to post this great info kja

[carebear]

thanks for explaining it so simply. while i kinda understood some of it, there was a lot i didn't know before. greatly appreciated as i want to get back into my photography.

[Lyla]

Thanks Kja and Kirs! Hoarding all this info, lets hope the brain retains it!

[kittikat]

Thanks Kja & Kirislin

I am researching for my next lense so this info is great for me to see what

I may need etc....or not doing properly with what I have :D