Focal Length & Minimum shooting distance

Started Sep 15, 2010 | Discussions
macca1978 Regular Member • Posts: 248
Focal Length & Minimum shooting distance

Morning all!

Just need to clear up a few things that are confusing me.

when a lens say is classed as 18-55mm....what exactly does that mean?..what is the 18mm?

I thought it was the minimum shooting distance, i.e the closest you can get to a subject and for it to be in focus is 18mm but I believe that is not the case...

so what is the difference between focal length and minimum shooting distance?..

If I have an 18-200mm lens...what does the 200mm mean?..200mm from the lense to the sensor when in full telephoto or minimum shooting distance is 200mm?

any help clearing this up for me would be greatly appreciated

thanks

baloo_buc Forum Pro • Posts: 10,465
Re: Focal Length & Minimum shooting distance

You need a lot of reading. Minimum shooting distance is different from the focal length.

Focal length relates with the angle of view. 30 mm in most dSLRs (Nikon, Sony and Pentax) will give you something similar with what your eyes can see (of course the camera works totally different than your eyes but it's a good equivalence). So 18 mm is wider than the eye normal view and 55 is narrower than the eye normal view. The narrower the angle of view the less objects will enter the frame (with 18 mm you can have in the frame let's say 20 trees and with 55 mm just 3).

The minimum shooting distance is the minimum distance between the focus plane and the sensor plane. The longer the lens the longer that distance. On very short lenses (like 18-55 mm) it's around 0.28 m while with long lenses (like 70-300 mm) it's about 1,5 m.
--
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OP macca1978 Regular Member • Posts: 248
Re: Focal Length & Minimum shooting distance

Thanks, that makes it a bit clearer.

So now I understand what the focal length is actually doing, what does the actaully numbers refer to, i.e how does 18mm differ from 200mm?..does the 18mm refer to how far away the lens is to the sensor?

Also, with a 18-200mm lens, out of the box with the lens not zoomed out i assume the focal length is 18mm, and only when the lens is at full telephoto zoom it reaches is focal length of 200mm?

is there a calculation to measure minimum shooting distance based on focal length or are the two completley seperate and it depends on other factors like sensor size etc?

baloo_buc Forum Pro • Posts: 10,465
Re: Focal Length & Minimum shooting distance

macca1978 wrote:

Thanks, that makes it a bit clearer.

So now I understand what the focal length is actually doing, what does the actaully numbers refer to, i.e how does 18mm differ from 200mm?..does the 18mm refer to how far away the lens is to the sensor?

18 mm or 200 mm are the focal lengths thus the distance from the lens equivalent center to the sensor. I don't have 200 mm but I can give you some samples:
18 mm:

200 mm:

Also, with a 18-200mm lens, out of the box with the lens not zoomed out i assume the focal length is 18mm, and only when the lens is at full telephoto zoom it reaches is focal length of 200mm?

Yes.

is there a calculation to measure minimum shooting distance based on focal length or are the two completley seperate and it depends on other factors like sensor size etc?

No. It is mentioned in lens specification.
--
Victor
Bucuresti, Romania
http://s106.photobucket.com/albums/m268/victor_petcu/
http://picasaweb.google.com/teodor.nitica/
http://picasaweb.google.com/vpreallize/
http://www.flickr.com/photos/v_petcu/

 baloo_buc's gear list:baloo_buc's gear list
Nikon D750 Nikon D7200 Fujifilm X-T20 Nikon AF-S DX Nikkor 16-85mm f/3.5-5.6G ED VR Tokina AT-X Pro 12-24mm f/4 DX II +18 more
James Cafferty Veteran Member • Posts: 5,364
Re: Focal Length & Minimum shooting distance

macca1978 wrote:

when a lens say is classed as 18-55mm....what exactly does that mean?..what is the 18mm?

If I have an 18-200mm lens...what does the 200mm mean?..200mm from the lense to the sensor when in full telephoto or minimum shooting distance is 200mm?

Use this interactive guide. Move the bottom slider to 18mm then to 200mm and you will see the range covered by a zoom listed as 18-200mm. Move the slider to 500mm and you will see why that lens has limited use for most people. (Thanks! Tamron.)

http://www.tamron.com/lenses/learning_center/tools/focal-length-comparison.php

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All the best,
Jim

Photographers take pictures, not cameras.

Cedarhill Veteran Member • Posts: 4,672
Minimum focus distance . . .

. . . is not related to minimum focus distance. Both these lens properties are highly dependant on the design of the lens are are only loosley related. As an example, take a look at two lenses that have a very similar focal length. The Nikon 85mm f/1.8 portrait lens has a minimum focus distance of 0.85 meters and is not suitable for very close up photography. The Tamron 80mm f/2.8 macro lens has a minimum focus distance of 0.29 meters and is capable of producing an image on the sensor that is the same size as the subject matter.

OP macca1978 Regular Member • Posts: 248
Re: Minimum focus distance . . .

Guys, really appreciate all your help, and am pretty sure I know whats what now!

thanks again!

Mike_PEAT Forum Pro • Posts: 13,344
Use DPR's LEARN tab to learn all this and more...

macca1978 wrote:

when a lens say is classed as 18-55mm....what exactly does that mean?..what is the 18mm?

DPReview has a section (under the LEARN tab) that teaches this stuff and more.

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