# Thread: what does 70mm practically mean?

1. ## what does 70mm practically mean?

Hi all, I've got a question. How much more magnification do one get between 50mm to 70mm to 200mm and above? From what i read, for a flim slr every 100mm is about taking 9-10 yards (3 metres) nearer? (pls correct me if i'm wrong) why i'm asking this is because i need to explain to my "finance minister" the need to buy a longer lense ha.....

2. My recommendation is to do a visual mockup with Photoshop...show how the crop gets tighter at different focal lengths.

If not, here a Flash animation on Canon's website to show how a picture changes with varying focal lengths: http://www.canon.com.sg/index.cfm?fu...psnhints#focal

If you're interested how mathematically FOV and focal length relates, here's a pretty good article: http://www.vision1.com/systems/fovmath.shtml

3. If you want to be analytical, just get your secondary school physic book and I'm sure you can produce a white paper on it.

However, one of the best I have seen (my limited experience) for knowing the effect of different focal length is the photographs from the Nikon lens catalogue - not sure if it is on-line. I got the printed version at the nikon centre.

If it is within your means, sometimes in such hobby, you dun need and you cannot justify your actions.

4. every time you double the focal length, eg, from 50mm to 100mm, the area is reduced to 25% that of original, ie 50mm in this example.

5. Originally Posted by Ah Pao
My recommendation is to do a visual mockup with Photoshop...show how the crop gets tighter at different focal lengths.

If not, here a Flash animation on Canon's website to show how a picture changes with varying focal lengths: http://www.canon.com.sg/index.cfm?fu...psnhints#focal

If you're interested how mathematically FOV and focal length relates, here's a pretty good article: http://www.vision1.com/systems/fovmath.shtml
thanks i saw that already but my "finance minister" say just go nearer lor...

6. ptwong,

You and your finance minister make a very cute couple!!!!

BiG BeRD

7. Originally Posted by denniskee
every time you double the focal length, eg, from 50mm to 100mm, the area is reduced to 25% that of original, ie 50mm in this example.
I didn't know that, thanks for sharing.

so does it mean that if would be 25% bigger also? For example if i take a pic of a 50cent coin at 50mm at a fix distance, so if i use a 100mm it would be 25% bigger? if i take it at 85mm how?

sorry guys my physics cannot make it ah so please bear with me.

8. ptwong, tell you finance minister that

you will catch the bird flying in mid air for her
you will pluck the flower growing on the cliff for her
you will take the tiger teeth and present to her
you will big her closer to the rainbow

and when she ask you to go nearer, pls and pls go nearer to her first before going nearer to your subject...

9. wah..
chu liu xiang..

10. Originally Posted by ptwong
I didn't know that, thanks for sharing.

so does it mean that if would be 25% bigger also? For example if i take a pic of a 50cent coin at 50mm at a fix distance, so if i use a 100mm it would be 25% bigger? if i take it at 85mm how?

sorry guys my physics cannot make it ah so please bear with me.
It will be 400% bigger from 50mm to 100mm. The area changes by a factor of 4 when the focal length changes by a factor of 2.

11. An easier way to check fov, dof etc

http://members.rogers.com/jul.loke/jlcalc.html

12. ha....i'll try it out but her reply will more likely would be....

you will catch the bird flying in mid air for her
I too fat will fall and squash the bird

you will pluck the flower growing on the cliff for her
I too fat will fall and squash the tiger

you will take the tiger teeth and present to her
I too fat will fall and squash the tiger

you will big her closer to the rainbow
I too fat will fall and squash her

ha....and what makes you think my finance minister is female?

13. Originally Posted by ptwong
Hi all, I've got a question. How much more magnification do one get between 50mm to 70mm to 200mm and above? From what i read, for a flim slr every 100mm is about taking 9-10 yards (3 metres) nearer? (pls correct me if i'm wrong) why i'm asking this is because i need to explain to my "finance minister" the need to buy a longer lense ha.....
What is "focal length?"

The "focal length" of a lens is the distance between the optical center of the lens and the place where it focuses its image. For conventional cameras, the place where the image is formed is the film plane. For digital cameras, it is the imaging sensor, usually a CCD array.

To make it easier think about this, we can pretend that the rays of light pass through the optical center on their way to the film plane or the imaging sensor. In fact, this is exactly how a pinhole camera works.

Click here to see the image for detail

14. Ok, if she replies this way then you may say:

"I too fat to move nearer."

Why I think yr finance minister is a she:

A man will say yes to a lady request to bring her heart nearer to his, whereas
A lady will say no to a man to bring his feeling closer to hers!

Originally Posted by ptwong
ha....i'll try it out but her reply will more likely would be....

you will catch the bird flying in mid air for her
I too fat will fall and squash the bird

you will pluck the flower growing on the cliff for her
I too fat will fall and squash the tiger

you will take the tiger teeth and present to her
I too fat will fall and squash the tiger

you will big her closer to the rainbow
I too fat will fall and squash her

ha....and what makes you think my finance minister is female?

15. Originally Posted by ptwong
I didn't know that, thanks for sharing.

so does it mean that if would be 25% bigger also? For example if i take a pic of a 50cent coin at 50mm at a fix distance, so if i use a 100mm it would be 25% bigger? if i take it at 85mm how?

sorry guys my physics cannot make it ah so please bear with me.
each time your focal length is multiplied by 2, the area will decrease by 2^2 = 4 times, aka become one-quarter of original. effectively, this means, if your focal length is multipled by 3, your area will become 1/9th of the original.

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