View Full Version : Focal lengths of lenses

20-03-2011, 1:25am
Hi again!
Still trying to get my head around lenses. I know the focal length only refers to the distance between the lens and sensor so I get that. Also the focal length number refers to weather it is a Wide angle, standard or telephoto lens and these determine the field of view.
My problem is understanding which focal length to use for certain shots. Wide angle lenses are pretty straight forward and mostly obvious. But when it comes to standard & telephote lenses I cant quite grasp it. I have been reading DSLR Photography for Dummies. The writer says just by looking at a photo you should be able to determine the general focal length used and has a couple of examples of shots taken and asks you to guess and then gives you the answer. Needless to say I was totally off the mark. One of the shots was a close up from the chest up of a boy which I thought was taken at a focal length of around 55mm but was in fact taken at 170mm. So I assume the photographer had to be standing a fare distance away to be able to get that shot but how was I to know this? What I dont understand is how do you choose the correct lens, especially a standard lens, when a lot of the time you can change the field of view by physically moving further away or closer. Can someone explain to me in a simple way?
Thank you for you help!!!

20-03-2011, 7:53am
There's no simple way. I would agree with you its hard to sometime determine whether a long focal length or short is used on say a headshot but there are some characteristics to look for such as flattening the image where the subject pops out from the background with a larger focal length, and distortion.

As to what lens to use, well, that comes down to experience, typically for people I like 100-200mm range

20-03-2011, 8:11am
As kiwi says, practice, experience and experimentation. The best and only way to rally learn is to get your camera out, use it, and examine the results in detail on your computer. I think you are jumping ahead of yourself here, learn all the basics first, practice those, then start progressing. Learning about field of view etc down the track. Coming to grips with it all in a few months is impossible.

Start putting up your efforts on AP, the feedback you get will be invaluable. Put the dummies book down and go out and take photos.

20-03-2011, 8:29am
HI Manda.
In your introduction you said you were new to photography. You seem to be reading up on things and that's good. Do not worry too much if you don't get some ideas straight away. Photography is a field where you are always learning something. In the example about guessing the lens used I would not worry at all. The answers you are likely to get on this or any forum may not always make much sense at the time, but they will eventually.

It's important that you practice a fair bit using your equipment and enjoy what you're doing. You WILL pick up ideas and techniques doing just that. And you will be able to ask questions and understand the answers. For example, in one of your threads you asked about the "difference" between using two different lenses setting each to the same focal length. Have you tried doing just that? If you have, then that's good. If not, then there's a test that will answer one of your very own questions. (And one which many new photographers would no doubt ask.)

You can read references and lots of replies till you're blue in the face, but you can also do things with what you've got. It's just a suggestion, but what do you find when you try the two lenses like you said? Just pick the same scene/subject to photograph with each lens. Tell us your findings.
Cheers, Am.

(Edited in: Rick beat me to it:(

20-03-2011, 11:55am
Thanks so much for all your feedback, much appricated!!! And yes I do really need to start experimenting and I will do just that!
I am doing a online photography course at the moment and my first assignment is 10 completley different photography assignments in which I have the option of 8 different sized lenses to use, as many as I like on each assignment, and I have to explain why I nominated that particular lens for a particular assignment. These lenses are also being used on a full sized 35mm sensor. I own a 550d with a cropped sensor so it makes it hard to research with my camera as I have to crop factor my focal lengths to match the field of view on a full frame. So anyway that is why I asked the question I asked. Phewww hard work!

22-03-2011, 9:26pm
The writer says just by looking at a photo you should be able to determine the general focal length used and has a couple of examples of shots taken and asks you to guess and then gives you the answer.

That seems like a bit of a fallacy to me. A hint would be perspective distortion (http://en.wikipedia.org/wiki/Perspective_distortion_(photography)) - either accentuated 'looming' edges due to a wide view angle, or a 'flattening' of the image due to a narrow view angle. Then you have to guess what format film / sensor (s)he is using as well, in order to be able to work out an approximate focal length.

Too much guesswork for my liking.

23-03-2011, 10:54pm
Will have look shaunj. Thanks! :)

24-03-2011, 9:55pm
No worries! Hope it helps

25-03-2011, 9:20pm
G'day Manda

Lots of good advice above :th3: :th3:

I would presume that you have the 2 basic kit lenses that seem to come with many entry-level SLR cameras
If so, may I suggest that you do some qwik 'n dirty tests in your back yard ...

Grab a vase of flowers & set it up on a kitchen stool
Set camera to ISO-100, then Av mode and f8

Using the 18-55 lens set it to 24, move in close till the flowers fill the frame and take 1st pic
Change lens to 55, move back a bit till the image size is as close as poss to 1st pic, take 2nd pic

Change lens to 50-200 or 70-300 whatever you have ...
Set lens at about 100mm, move back till image size is much the same as before, take 3rd pic
Set lens to 200 ... etc etc ... take 4th pic

Examine all four images
Look at image separation between subject and background & get a feel for Depth of Field and Image Perspective via the differing focal lengths

When you have got this far, try it again with a live model perhaps &/or the flowers ... this time duplicate images at f4 then f8 then f16 and see the changes via f-stop changes
Regards, Phil

25-03-2011, 11:20pm
Thank you Phil, I will do just that!!! Will post my results for all to check out sometime soon then. :)