Nikon DX Lens and crop factor questions.

[Ivan S]

I am still very confused about Nikons DX lens's, I hope it can be cleared up.

Right, I understand that the DX lens is designed for the DX bodies.

It's the confusing 1.5x crop I am lost on, my kit lens is the 18-55 DX VR, now does this mean it is 18-55mm on my D3100.

As I read people say that the 50mm 1.8g would be a 75mm on a DX, fair enough, but what about the 35mm 1.8 DX, is that 35mm or 52mm on a DX?

 

[einaroh]

hi,

Your 18-55mm is 27-82.5mm (in 35mm/FX eq.) and your 35mm is 52.5mm (in 35mm/FX eq). You basically just multiply all lenses with the appropriate factor, in this case 1.5, to get what they would be for a lens on a 35mm/FX body.

 

[Big Nick]

The figures are all based around traditional 35mm film sizes

A DX sensor is 1.5 times smaller than the above hence the image it 'sees' is narrower than the above, this acts to increase the focal length

An FX is full size (or 'full frame' as its known) which 'sees' the whole image the same as a traditional 35mm film would

You still need to multiply the focal lengths by 1.5 each time even though its classed as a DX lens as the crop factor is not factored into the lens focal lengths quoted

Your kit lens is therefore really a 27-82.5mm when compared to traditional 35mm, a 70-300 is actually 105-450mm etc etc

You also need to take into account your viewfinder may not show 100% of the actual image you're going to get depending on what camera you're using!!

 

[antihero]

Obviously DX lenses are designed for crop bodies, and FX lenses for full frame. DX lenses are just designed to project and image onto a smaller sensor, so are cheaper to engineer and manufacture.

If you put your dx lens on an fx body, the image projected through the lens would not cover the sensor and you'd end up with vignetting. You would then have to crop the image down to remove the vignette, and then enlarge to achieve the original image size (in essence exactly what a crop body does.)

A 50mm lens on a crop body has the same effective field of view as a 75mm lens on a full frame body. Magnification only comes into it when outputting image sizes.

 

[redhed17]

Whatever lens you put on your camera is whatever focal length is on the lens, as focal length is derived from the distance between the lens and the image sensor/film, usually stated in millimetres. That doesn't change whether the lens is on a DX or FX camera. On a DX camera you are just getting a cropped portion of the image projected on to the sensor.

On a cropped (DX, APS-C) sensor camera you multiply the focal length on the lens by 1.5 (by 1.6 for Canon) to get what the equivalent focal length of the lens you would need to put on a FX camera to get the same view. So as you say, a 50mm on a DX camera gives the same view as a 75mm on a FX sensor camera would.

So a 35mm lens is a 35mm lens, but on a DX camera you add half the focal length, or multiply by 1.5, to get what the cropped view you are getting would be equivalent to on a FX camera, so 35mm x 1.5 = 52.2mm. Your 18-55mm lens gives you the equivalent view of 27-82.5mm lens on a FX camera. So even though your lens is designed for a DX camera, the focal length is what it is because of the design of the lens, and the distance of the lens to sensor distance or film plane.

I wouldn't worry too much about what is equivalent to what. :shrug: You just need to get used to the views you get on your camera with the lenses you have. Do you need something wider, or something more telephoto? :shrug: If you are thinking of getting a prime lens, then for most of the common focal lengths, 28mm, 35mm, and 50mm, you can check whether they are what you want on the kit lens by zooming to that focal length and seeing the view you get.

A lot of people say get the 50mm f1.8 as a good cheap prime lens, which is fine, but if you have experience of a film SLR or full frame sensor camera with a 50mm lens, then a 50mm lens on a DX camera may be too telephoto for some as it gives a view equivalent 75mm view on an FX camera. A 35mm lens may give them the 50mm view (on a film camera) they are used to, as as I wrote earlier, the view of a 35mm lens on a DX camera gives the equivalent view of 52.7mm lens on a FX camera, which is close to 50mm.

Sorry to waffle. :shrug: It used to confuse and worry me when I started with DX cameras, coming from a film SLR background, but you get used to the view of the various focal lengths on the DX cameras.


Edit; Just saw I took a bit too long to reply, and was beaten to it.

 

[Ivan S]

Now that's where my confusion lies, DX lenses being designed for crop sensors, ok, but why would they then go and give a wrong value to the focal length?

or am I missing something?

What happens to the same picture at the same focal length on each DX and FX using lenses designed for each, i.e a DX on a Dx body and an FX on a FX body both at say 50mm on the lens ring, what would happen on the picture, would they be the same?

 

[architectfadi]

Here is a good read

http://photographylife.com/dx-or-fx-for-sports-and-wildlife-photography

"1) The Myth of the DX Built-in 1.5x Teleconverter

A lot of people seem to be very confused about the effect of a crop sensor on the focal length of a lens. Stating that a crop sensor increases the focal length of the lens or acts as a teleconverter is completely wrong, since focal length is an optical attribute of a lens and has nothing to do with the camera. I talked about this in detail in my “Equivalent Focal Length” article that I published a while ago. Simply put, a DX sensor can never change the optical parameters of a lens, so if you are shooting with a 300mm lens, it stays as a 300mm lens no matter what camera you mount it on. The confusion of “equivalent focal length” comes from manufacturers that initially wanted to make people understand that the field of view on a cropped sensor camera is tighter than 35mm, because the image corners get chopped off. The word “equivalent” is only relative to 35mm film. So you cannot say that your 300mm lens becomes a 450mm lens on a DX body. It does not and never will. All you are doing, is you are taking an image from a 300mm lens, cropping it in the center area and magnifying that center with increased resolution."

 

[wyx087]

What happens to the same picture at the same focal length on each DX and FX using lenses designed for each, i.e a DX on a Dx body and an FX on a FX body both at say 50mm on the lens ring, what would happen on the picture, would they be the same?

No, they will not be the same.

DX means crop. It is a crop of what FX (full frame) sees. Meaning DX will only get a centre crop of FX shot at 50mm, equivalent to FX using 50*1.5 = 75mm lens.

The crop factor and equivalent focal length only means one thing and nothing else. It only affects the field of view. Every single property of the lens doesn't change. Because the physical properties of the lens doesn't change, lenses are still marked as they should.

To be honest, unless you came from 35mm background, don't worry about crop factor and equivalent focal length. Just shoot and memorise focal lengths you use, don't worry about what other people says.

 

[Ivan S]

Thanks people, I have a better understanding now, it effectively crops the image, I got that now, can't help but feel it is a bit of a mess though.

 

[Bristolian]

Thanks people, I have a better understanding now, it effectively crops the image, I got that now, can't help but feel it is a bit of a mess though.

People tie themselves in knots trying to make sense of this and I always wonder why they bother?

In practice, it matters not one jot what the manufacturer writes on the side of the lens as long as you have the one you need to get the shot you want.

 

[munkee]

Thanks people, I have a better understanding now, it effectively crops the image, I got that now, can't help but feel it is a bit of a mess though.

It is a bit confusing. At least though with Nikon DX and FX format sensors the maths is easy.