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01-28-2008, 08:29 PM   #1
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Focal Length ?

Still learning about camera functions, and have a basic question: What exactly is focal length? Is it the same as the "magnification" on the lens (35mm, 200mm, etc)?

Thanks in advance for answering what I know must be a very basic question!

01-28-2008, 09:15 PM   #2
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The smaller the number the wider the view, and the bigger the number the bigger the objects appear.

If you got the 18 - 55mm kit lens you might want to zoom it out, and observe the numbers to have a good understanding.
01-28-2008, 09:17 PM   #3
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Google is nice.
01-28-2008, 09:30 PM   #4
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There was a good image someone posted up showing what each focal length looked like.
Will see if i can dig it up.

OK ... here it is:

POP4 from Sydney posted this ages ago.



Last edited by Mechan1k; 01-28-2008 at 09:41 PM.
01-28-2008, 10:05 PM   #5
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QuoteOriginally posted by ldelude Quote
Still learning about camera functions, and have a basic question: What exactly is focal length? Is it the same as the "magnification" on the lens (35mm, 200mm, etc)?

Thanks in advance for answering what I know must be a very basic question!
focal length is the distace between the focusing lens element and the picture plane (film or digital sensor)

the greater that distance, the narrower your field of view and the more magniciation.
01-28-2008, 10:11 PM   #6
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In the old days of film cameras 50mm was the midpoint btwn wide angle & telephoto.
With Pentax DSLR this midpoint is shifted to ~33mm.

So ... longer than 33mm is telephoto with magnification being the multiple of 33 eg. 70mm is about 2X, 100mm = 3X, etc.
01-28-2008, 10:14 PM   #7
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QuoteOriginally posted by Kguru Quote
In the old days of film cameras 50mm was the midpoint btwn wide angle & telephoto.
With Pentax DSLR this midpoint is shifted to ~33mm.

So ... longer than 33mm is telephoto with magnification being the multiple of 33 eg. 70mm is about 2X, 100mm = 3X, etc.
please dont confuse the fella

all he asked for was "what is focal length"

let him digest that, then, if he has more questions, answer as they come

now this thread is gonna hit 2 pages with people talking about crop factors and textbook definitions of normal and telephoto

good job!
01-28-2008, 10:24 PM   #8
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QuoteOriginally posted by Gooshin Quote
please dont confuse the fella
all he asked for was "what is focal length"
He also asked whether it was the same as magnification.
In answering yes I should explain what the magnification is relative to, hence the 33mm midpoint.

Note that I deliberately avoided using the word crop in that post.

01-29-2008, 04:50 AM   #9
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I see lots of not so good answers here.

Strictly speaking, focal length is the distance between the lens and the focal point, i.e. the place where parallel rays of light will converge.

This means in practice the distance from lens to sensor, when an image is at infinity.

It is related to but not the same as magnification ratio.

Magnification ratio is the ratio of the distance between lens to film plane divided by lens to subject distance.

If you stand in one place, and the subject is in a second place, relatively far away (i.e. much further away than the focal length) then a lens with double the focal length will produce an image twice as big.
01-29-2008, 06:28 AM   #10
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QuoteOriginally posted by Mechan1k Quote
There was a good image someone posted up showing what each focal length looked like.
Will see if i can dig it up.
QuoteOriginally posted by Kguru Quote
In the old days of film cameras 50mm was the midpoint btwn wide angle & telephoto.
With Pentax DSLR this midpoint is shifted to ~33mm.

So ... longer than 33mm is telephoto with magnification being the multiple of 33 eg. 70mm is about 2X, 100mm = 3X, etc.
Thanks .. the image was helpful, I am generally a visual learner

Kguru, that was helpful in another way; I've been recently grasping the "hows/whys" of telephoto vs wide angle ... I thought I didn't want a wide angle, and the more I learn, the more I realize I want one!

I've been going to pbase and looking at photos, and noting some of the camera settings to learn how to get certain types of shots. I always see 'focal length' listed, and was always wondering if that meant (a) how close they were to the subject, or (b) the type of lens.

From all of your replies, I am learning it's more about (a) how close they were to the subject...?
01-29-2008, 10:04 AM   #11
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QuoteOriginally posted by ldelude Quote
Thanks .. the image was helpful, I am generally a visual learner

Kguru, that was helpful in another way; I've been recently grasping the "hows/whys" of telephoto vs wide angle ... I thought I didn't want a wide angle, and the more I learn, the more I realize I want one!

I've been going to pbase and looking at photos, and noting some of the camera settings to learn how to get certain types of shots. I always see 'focal length' listed, and was always wondering if that meant (a) how close they were to the subject, or (b) the type of lens.

From all of your replies, I am learning it's more about (a) how close they were to the subject...?
Not quite (a), but you're almost on the right track.

For a beginner, the biggest thing you need to realize is that different focal lengths affect your field of view (often referred to as FOV for short in photo forums). Yes, photo terms I know, but I can explain it

You have the same given rectangle (size of photo).
When you want to fit more surrounding scene into that picture, you will use a smaller focal length (aka, a wider angle, smaller focal length).
When you want to fit less surrounding scene and "zoom in" into an area, you will use a longer focal length (larger focal length numbers)

Lets refer back to our buddies image.


See that 70mm will only "see" that little rectangle in the center while 10mm will "see" a whole lot more area around it?

A lot of people actually mention focusing, but it's more an attribute to focal lengths as it's not something you will often need to consciously think about.
01-29-2008, 08:31 PM   #12
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For a visual understanding, go here

Tamron - Focal Length Comparison
01-30-2008, 12:27 AM   #13
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Focal Length is the measure of the distance from the center of the lens to the image plane. (to poorly quote and paraphrase Wikipedia)
Focal length - Wikipedia, the free encyclopedia

Focal Length is not changed by the size of the sensor - it is the focus distance of the lens - period....

If you have a lens that will focus a square that is 10mm high and wide on a sensor, that 10mm square will be 10mm with that lens whether the sensor is a 135 format (35mm film) AP-C digital sensor, 4x5 piece of film or 645 format film. The focal length of the lens does not change and the size of the focused image does not change - at the sensor level.

There are two methods used to change the focal length of a given lens/camera combination. Extension tubes - for macro work - increasing the distance between the center of the lens and the focus plain. Teleconverters - which are basically magnifying glasses that would make the 10mm by 10mm square focus and be 15mm by 15mm on the sensor given a 1.5 teleconverter.

The Elitist - formerly known as PDL
01-30-2008, 02:55 AM   #14
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QuoteOriginally posted by creampuff Quote
For a visual understanding, go here

Tamron - Focal Length Comparison
That link is excellent for a 'visual learner' to grasp what the various focal length lenses will accomplish. Kudos to the folks at Tamron for making that page.
01-30-2008, 06:58 PM   #15
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QuoteOriginally posted by creampuff Quote
For a visual understanding, go here

Tamron - Focal Length Comparison
This is absolutely fantastic! Thanks for tapping into what works best for me .. and thanks to everyone else for all of the information. Add it up, and it's great reference material.
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