# 18-55mm lens what does it mean?



## soap266 (Oct 17, 2011)

Im saving up for my first dslr. Im trying to learn what I can first. From what I understand this is the range of the lens. But I don't fully understand that. For my 35mm camera I usually use a 50mm lens. Would a higher mm lens mean that my subject seems closer and more narrow? Or do I have it backwards?


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## 480sparky (Oct 17, 2011)

You have it correct.  The higher the number, the narrower the field of view and therefore the 'closer' your subject appears in the viewfinder.


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## analog.universe (Oct 17, 2011)

This article gives a nice quick overview: Focal Length: Optical: Glossary: Learn: Digital Photography Review


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## D-B-J (Oct 17, 2011)

I'm assuming you are buying a crop sensor (Canon Rebel or Nikon D5000 for example), which means that there is a 1.5x crop on the nikon, and a 1.6x on the canon.  So a 50mm lens on the nikon would be a 75mm lens on your 35mm camera.  A higher mm means your subject will appear closer, whereas a low mm number means you get a very wide angle. 

Play with this. 

Nikon | Imaging Products | NIKKOR Lenses Simulator


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## soap266 (Oct 17, 2011)

Djb you are correct im very much debating between the reble t2i and a d5100. Thank you all for the explanations.


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## D-B-J (Oct 17, 2011)

soap266 said:


> Djb you are correct im very much debating between the reble t2i and a d5100. Thank you all for the explanations.



Haha what a lucky guess!  You are welcome.


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## KmH (Oct 17, 2011)

D-B-J said:


> So a 50mm lens on the nikon would be a 75mm lens on your 35mm camera.


The 50 mm will be a 50 mm on both cameras.

When the 50 mm is mounted on a camera having an APS-C sized *1.5x *crop image sensor the field-of-view (FoV), the 50 mm lens delivers will be equivelent to the FoV a 75 mm lens would give mounted on a camera having a full frame (135 format, or 35 mm) image sensor. (50 mm x *1.5* = 75 mm)

The 50 mm mounted on a camera having an APS-C sized *1.6x* crop image sensor the field-of-view (FoV) delivered would be  equivelent to the FoV an 80 mm lens would give mounted on a full frame. (50 mm x *1.6* = 80 mm)


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## DiskoJoe (Oct 18, 2011)

KmH said:


> D-B-J said:
> 
> 
> > So a 50mm lens on the nikon would be a 75mm lens on your 35mm camera.
> ...



Thats what he just said. sheesh.


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## KmH (Oct 18, 2011)

No, he had it exactly backwards.


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## Tiberius47 (Oct 18, 2011)

I think it's safe to say that if someone says, "On a camera with a 1.5 crop factor, a 50mm lens will be the same as a 75mm lens" what they mean is that the field of view will be the same.

Can we stop getting hung up on the precise wording?  At the end of the day, we all know what is meant, okay?


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## 480sparky (Oct 18, 2011)

Tiberius47 said:


> ..............Can we stop getting hung up on the precise wording?  At the end of the day, we all know what is meant, okay?



Fine.


Until tomorrow, when there's someone else who _doesn't_ know what it means.


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## analog.universe (Oct 19, 2011)

It is important (in order to properly understand perspective and depth of field) to distinguish between a 50mm lens having the same field of view as 75mm, and a 50mm lens acting like a 75mm lens.  The latter never happens.  a 75mm lens has a way of rendering perspective and dof that a 50 will never emulate, even if you mount it to a crop sensor that gives you 1.5 times tighter field of view.  It's best to think of the crop as actually that, picture taking the shot with a full frame camera and then cropping the edges off.  This explanation gives a much better understanding than imagining which lens on full frame would look like the lens in question on crop.  The answer is no lens on full frame looks like any lens on crop, even if certain combinations share the same field of view.


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## D-B-J (Oct 19, 2011)

KmH said:


> D-B-J said:
> 
> 
> > So a 50mm lens on the nikon would be a 75mm lens on your 35mm camera.
> ...



I said that.  The 50mm on a DX body is the same as a 75mm on a FX body.  Sheesh


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## KmH (Oct 19, 2011)

A 50 mm on a DX body _*isn't*_ the same as a 75 mm on an FX body. http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm

And the first time you did not say "The 50mm on a DX body is the same as a 75mm on a FX body.
You said "So a 50mm lens on the nikon would be a 75mm lens on your 35mm camera.", also an incorrect statement.


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## Tiberius47 (Oct 19, 2011)

analog.universe said:


> a 75mm lens has a way of rendering perspective and dof that a 50 will never emulate, even if you mount it to a crop sensor that gives you 1.5 times tighter field of view.



Could you clarify that, preferably with examples?

Let's say I have a tripod with a crop camera on it.  I put a 50mm lens on that camera and take a picture.  Then I put a full frame camera on that same tripod, put a 75mm lens on that camera and take a photo of the exact same thing, there really isn't going to be much difference between the two shots.  The perspective certainly won't cahnge, because that depends entirely on the position of the camera.


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## analog.universe (Oct 19, 2011)

Tiberius47 said:


> analog.universe said:
> 
> 
> > a 75mm lens has a way of rendering perspective and dof that a 50 will never emulate, even if you mount it to a crop sensor that gives you 1.5 times tighter field of view.
> ...



In your example, provided both shots were taken at the same aperture, the full frame with a 75 will have a shorter depth of field, despite having the same perspective as the cropped 50.  In practice it's helpful to associate "50 on crop means 75 on full frame" because people who've shot full frame have a feel for the field of view that a 75 gets you, but that doesn't address the dof issue.  At equal apertures, equal subject distances, and focal length _increased_ on full frame to match the field of view on crop, the full frame will have _decreased_ dof.


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