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I'm looking at getting a prime lens for my Nikon D50, I'm looking at the 50mm ƒ1.8 and the 35mm ƒ1.8 DX.

What I understand is that the 50mm will actually give me an equivalent focal length of 75mm, but am I right in thinking the focal length of the 35mm is 1.5x longer (52mm) even though it's a DX sensor?

Just wanting to understand this as best I can, don't want to spend out on a lens if it's longer than I imagined.

Many thanks!

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possible duplicate of Choosing a prime lens for APS-C: 50mm or 35mm? –  mattdm Feb 22 '11 at 4:15
    

4 Answers 4

up vote 5 down vote accepted

You are correct on both accounts, the 50mm will effectively have a 75mm Field of View (FoV) and the 35mm will effectively have a 52mm FoV.

The DX just means that the lens should only be used on a DX body, not that its 'a 35mm FoV on a DX body'.

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Excellent, many thanks! –  Ben Everard Feb 21 '11 at 22:06

You're mostly right. The 35mm will have an EFL of 52mm, just as your 50mm will have an EFL of 75mm. Everything gets multiplied by the 1.5x crop factor.

When "DX" is applied to the lens, it means that it's designed so that it only puts light onto an area the size of a DX sensor. That means that, if you put it on an FX (full-frame) sensor-bearing body, only portion of the photograph will have an actual picture on it; the rest will be dark.

In turn, many (all?) Nikon FX-based cameras have a "DX crop mode" where they shrink the size of the photo to avoid this. This effectively makes your camera a very expensive DX sensor, with the same crop factor/EFL/field of view.

Conversely, your 50mm non-DX lens is "wasting" light on a DX sensor. I don't think there's any real downside to this besides lens size/cost.

Don't let this influence you when deciding between these two lenses though. Think about what you're going to be shooting, and then perhaps cost. The 50mm is better for portraits; the 35mm will be better (IMO) for everything else due to its wider viewing angle. I find 35mm to be a good general-purpose length; 50mm often is too tight for anything but head shots. They're both fine lenses though.

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I got the 35mm for my D5000 just a few weeks ago. Best advice I received when I was trying to decide what I wanted was to restrict myself to each on the kit lens. Assuming you have the 18-55mm kit, restrict yourself for some period of time (say a week or two) to 35mm then switch to 50mm.

For me 50mm seemed a little bit tight for most of the shooting I do (I mostly wanted the option to shoot indoors in lower light situations than the kit lens was really capable of), so I got the 35mm. Some day I'll probably get the 50mm for portrait shooting, but in the mean time 35mm works fine for that purpose.

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Got me a 35mm f/2.0 Nikkor several years ago (before DSLRs), and I love it.

And one thing: there's no "equivalent focal length". It's still a 35mm, it's just a cropped image from a 35mm. So the image will look like an image taken by the same lens using any other camera, but cropped by about a third.

It's usually not a big difference, but things like DOF are impacted by this, as is telephoto compression (most important at more extreme focal lengths, obviously).

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