DX Lens Focal Lengths

[HeldInTheMoment]

OK, first I feel very silly for asking this here...but it's been driving me nuts. I totally understand that a FX 70-200mm f/2.8 Nikon lens is more of a 105-300mm focal length range on a DX camera.

But are the DX lens still multiplied as well? For example, the 35mm f.18 DX Lens from Nikon...is that really 35mm on the DX camera or is it more like 50mm due to the 1.5x conversion?

 

[astroNikon]

The lens is designed as a standalone part.
So a 50mm lens will work as a 50mm lens on any camera.
It's the sensor size of the camera body that causes the mathematical issue of what part of the image circle the sensor captures, which gives it a multiple of a fake "zoom" of the image.

So a 50mm lens is a 50mm lens no matter what.

I did see some nikon 1 lenses which also had stamped the cropped version.

 

[Rob99]

OK, first I feel very silly for asking this here...but it's been driving me nuts. I totally understand that a FX 70-200mm f/2.8 Nikon lens is more of a 105-300mm focal length range on a DX camera.

But are the DX lens still multiplied as well? For example, the 35mm f.18 DX Lens from Nikon...is that really 35mm on the DX camera or is it more like 50mm due to the 1.5x conversion?

The focal length doesn't change on any lens, a 70-200 is a 70-200 and a 50 is a 50. What changes is the field of view on a crop sensor.

 

[astroNikon]

This may make more sense

The lens focal length is what it is. 50mm is a 50mm.

You have to first understand what size of sensor you are buying in the camera.

The size of the sensor determines the "crop"
35mm FullFrame sensor = 1 to 1
APS-C sensor = 1.5x (or 1.6x) to 1
other smaller sizes increase that amount ==>https://lensvid.com/wp-content/uploads/2014/01/Sensors-size-01-01.jpg
as below - crop factor is the 4th row.

You then take that crop factor, such as 1.5x for APS-C and multiple it to the lens focal length.
So a 18-55 on a APS-C sensor would
18 x 1.5 - 55 x 1.5 = 27 - 82.5 in perceived field of view

 

[HeldInTheMoment]

Thanks for the replies, I totally understand the difference between FX and DX (APC-S) as well that a lens focal length is the focal length which does not change.

The question was, does a 35mm DX Lens mean it is 35mm on a DX camera? If not, why have the 35mm DX model lens when it is the same focal length as the FX anyway??

 

[astroNikon]

a 35mm lens is a 35mm lens.
DX or FX
you have to do the sensor size multiple for it's "true focal length", or Field of View.

The difference of a DX vs FX lens is the image circle that it creates for the sensor size.

An FX lens has wider glass in it which creates a larger image on the back side for the larger sensor.

Keep in mind, there are various sensor sizes out there not just DX and FX.

 

[HeldInTheMoment]

Thank You, that is what I was trying to understand. From what I read online and self-taught, I was getting a little confused...lol

 

[jaomul]

It doesn't matter if it is an fx or dx lens, the focal lengths remains the same, a 35mm dx lens will give the same view as a 35mm fx lens on your d7100

 

[Solarflare]

The lens does not care what sensor you use. It always does exactly the same. The focal length is perfectly fixed.

The only difference between a 35mm DX lens and a 35mm FX lens is that the FX lens has a larger image circle. Otherwise the DX sensor will see the exact same thing through both lenses, while the FX sensor will see dark corners when you use it with a mere DX lens.

What changes is the EQUIVALENT focal length. Meaning a DX sensor will show you with a 35mm lens the same image that a FX sensor will show you with a 52.5mm lens.

 

[goob4114]

The answer to your question is that the 35mm lens will give you the equivalent FIELD OF VIEW of a 52-53mm lens on a full frame.

Crop factors are never applied to lenses. They are applied to sensors only. A 35mm lens is a 35mm lens is a 35mm lens is a 35mm lens. It is the distance from the optics (the glass) to the sensor when in focus. This is the same regardless of the body the lens is attached to. This distance is 35mm for a 35mm lens, and the image circle the lens creates is the same no matter what camera the lens is attached to.

Think of it like this :

Imagine you have an old slide projector - projecting an image on the wall. Now, take a normal 4 x 5 note card and tape it to the center of the image on the wall - and trace the image. Now, remove the note card and replace it with a regular sheet of printing paper (it's larger obviously). Also in the center. Now, trace your image.

In this scenario, the projector is the equivalent of your lens, and the note card a printing paper are your sensors. Did anything about the lens change between your two traces? No. It was the exact same lens projecting the exact same image circle over the exact same distance. Everything about it was exactly the same. The difference was simply the size of the medium you traced the image on - or the size of your sensor. Now, suppose you wanted to make your two 'drawings' the same exact size. You would need to blow up the one on the note card to be the size of the printing paper - and you'd have an image with a narrower field of view. That's what a crop sensor does.

 

[Derrel]

Thanks for the replies, I totally understand the difference between FX and DX (APC-S) as well that a lens focal length is the focal length which does not change.

The question was, does a 35mm DX Lens mean it is 35mm on a DX camera? If not, why have the 35mm DX model lens when it is the same focal length as the FX anyway??

First off, there are not all that many DX lenses. DX is Nikon's very own terminology, and most of their lineup of lenses covers 24x36mm. But to answer the question, why have a DX model lens when there is also a lens that covers FX? PRICE POINT is the main answer. Followed by "expected type of customer/user."

Nikon itself for example has never made a single "FX Nikkor"...not one, single lens made by Nikon ever has been listed as an FX...just so we're clear on that. Anyway...Nikon does make a few lenses in both DX versions, and in full-frame-capable versions thatr share focal length--but which are very low-cost in their DX version, and in some cases, pretty expensive in their full-frame versions.

Case in point, the 35mm f/1.8 DX-Nikkor, and the 35/2 AF-D, 35mm f/1.8 Nikkor, and also the 35mm f/1.4 AF-S G Nikkor versions. Four different 35mm Nikkor lens models, of different ages. One lens is light, plastic-y, and has a number of very significant imaging flaws...but it is under $200 brand-new...and it is in the lineup to attract casual users, beginners, and as part of a system, a sort of brand check-mark box item. Canon has its second-tier 50mm f/1.8 as its noob-attractor", cheapie 50....Nikon has its second-tier 35mm f/ .8 DX to attract beginners who want a prime lens, and want one that costs very few dollars. Both Canon and Nikon also have high-grade, fairly expensive 35mm and 50mm lenses, for working pros,serious enthusiasts, the kind of lens that is as good as it can be, and which will last for 20 years or more.

Moving up....the 85mm DX Nikkor macro lens is affordable, small, and light. Nikon's older 85mm f/2.8 Tilt Shift, and the newer 85mm f/2.8 TS-E macro/tilt-shift: two massive, heavy, HIGH-grade lenses retailing in the 1.5k range....one for lightweight DX body users...the other extreme aimed at very technically astute shooters, and for very exacting uses.

PRICE POINT is the main answer. Followed by "expected type of customer/user." Same goes with the 70-200/2.8 versus the f/4 70-200...

 

[KmH]

A 35 mm FX lens will fully illuminate both an FX and a DX sensor.

A 35 mm DX lens will not fully illuminate an FX image sensor.
The 35 mm DX lens will fully illuminate the DX size center portion of an FX image sensor.

Nikon FX camera bodies are set by default to detect when a DX lens is mounted, and then only use the central DX size portion of the FX image sensor.

That DX lens detect setting can be turned off, and some DX zoom lenses can come quite close to fully illuminating an FX image sensor when zoomed to an appropriate segment of their zoom range.