Nikon DX, FX, lenses and bodies

[kalfa]

Hi all,
I am going to buy a Nikon D90 and I am trying to understand how things works.

My biggest doubt is about using DX and FX lenses in a DX body.

Sorry for the quite long post!

Let's suppose to have a FX and a DX body and two lenses both of the same length, again one DX and one FX.

- What do I see in the viewfinder using a FX body and a FX lens?
- And if I use FX body and DX lens?

My understanding:
FX body +FX lens, the exact image I will obtain is what I'll see in the viewfinder.

FX body + DX-mode ON with the DX lens, the sensor will surround the resulting image with a black area, I am not sure what I will see in the viewfinder, my wild guess is the black frame is seen in the viewfinder as well.


- What do I see in the viewfinder using a DX body and a DX lens?
- And if I use DX body and FX lens?

My understanding is absolutely no difference in any of the above cases.
What happens is that the image projected on the sensor plane actually "exceed" the sensor size in the FX lens case, but the part of the image "cropped" out of the sensor actually is not perceived, since the viewfinder will see exactly the image captured by the sensor.

My wild guess, is that the mirror projecting to the prism is DX-sized, so the exceeding image is actually lost by the mirror too.
Is it right?

My initial fear was that using a FX lens I actually will see a larger image in the viewfinder than what I will actually obtain on the sensor.

Further question about optic concepts
I understand that on a DX body, a 18-105mm FX lens will result as I had something like a 27-155mm, due to the view angle getting narrower because of the crop factor on a DX sensor.
Shooting at 105mm, optically I am using the lens at 105mm, but I will obtain shoots appearing "closer", as it were 155mm.

What does it change on a 18-105mm DX lens? Will it result as a 18-105mm (instead of the approx 27-155mm)? Which means I am actually using a "smaller" lens in 35mm/FX terms?
I am not getting what I am buying, buying a DX lens.

I've read of people suggesting of investing on FX lenses used on DX bodies, since when a FX body will be affordable I will already have FX lenses to use with it. DX lenses would limit my future FX body. It makes sense to me.

So, what does it change, technically, using a DX? Why did Nikon invent it?

thank you for your patience ,
C.

[Dave Humphries]

Hi kalfa, and welcome to the CiC forums from me.

Hi all,

I am going to buy a Nikon D90 and I am trying to understand how things works.

My biggest doubt is about using DX and FX lenses in a DX body.

Good choice and don't worry.

Let's suppose to have a FX and a DX body and two lenses both of the same length, again one DX and one FX.

- What do I see in the viewfinder using a FX body and a FX lens?
- And if I use FX body and DX lens?

My understanding:
FX body + FX lens, the exact image I will obtain is what I'll see in the viewfinder.

FX body + DX-mode ON with the DX lens, the sensor will surround the resulting image with a black area, I am not sure what I will see in the viewfinder, my wild guess is the black frame is seen in the viewfinder as well.

FX body + FX lens = Yes, what you see is what you get

FX body + DX lens = Since the viewfinder is optical, I doubt you'll see a difference, but I expect there is a guideline box that is switched on that superimposes a DX sized box on the image though.

- What do I see in the viewfinder using a DX body and a DX lens?
- And if I use DX body and FX lens?

My understanding is absolutely no difference in any of the above cases.
What happens is that the image projected on the sensor plane actually "exceed" the sensor size in the FX lens case, but the part of the image "cropped" out of the sensor actually is not perceived, since the viewfinder will see exactly the image captured by the sensor.

My wild guess, is that the mirror projecting to the prism is DX-sized, so the exceeding image is actually lost by the mirror too.
Is it right?

My initial fear was that using a FX lens I actually will see a larger image in the viewfinder than what I will actually obtain on the sensor.

Yes, yes and no, your initial fear is unfounded.

Further question about optic concepts
I understand that on a DX body, a 18-105mm FX lens will result as I had something like a 27-155mm, due to the view angle getting narrower because of the crop factor on a DX sensor.
Shooting at 105mm, optically I am using the lens at 105mm, but I will obtain shoots appearing "closer", as it were 155mm.

Yes

What does it change on a 18-105mm DX lens? Will it result as a 18-105mm (instead of the approx 27-155mm)? Which means I am actually using a "smaller" lens in 35mm/FX terms?
I am not getting what I am buying, buying a DX lens.

No, the focal length is the focal length and doesn't change; i.e. they don't use a crop factor on it, which is why we have to when using non FX sensor cameras.

I've read of people suggesting of investing on FX lenses used on DX bodies, since when a FX body will be affordable I will already have FX lenses to use with it. DX lenses would limit my future FX body. It makes sense to me.

This is true, it is investing for the future.

So, what does it change, technically, using a DX? Why did Nikon invent it?

It is cheaper to produce DX sized sensors than FX, so makes the cameras more affordable - i.e. they sell more!

Sorry for the quite long post!

Yeah, it was a bit

Cheers,

[sleist]

Hi kalfa,

I have the D90 and it's a great camera. I use a mix of FX and DX lenses on it. In my case, I am not buying FX lenses in preparation for a possible move to FX body. In fact I plan on staying with DX with my next upgrade in a year or so (the D300s replacement). Whether you go with FX of DX depends a lot on what you plan to shoot. The DX crop factor changes the field of view of a lens by a factor of 1.5. This would make a 50mm lens produce an image that would require a 75mm lens to produce on an FX or 35mm film body. This is good news for anyone that needs extra reach with their zooms as the 300mm f/4 will give the FOV of a 450mm on DX. Unfortunately, what you gain at the long end, you also lose at the wide end so my 11-16mm ultrawide is more like a 16-24mm wide angle on DX - not a problem for me, but others might feel differently. The above is true regardless of whether the lens is DX or FX.

In many cases, DX lenses will not work well on an FX body because the image circle is not big enough to cover the FX sensor leaving you with varying degrees of dark corners. That's not always the case though and I know some reviewers (i.e. Thom Hogan) that will mention if the lens will be at all usable on an FX body.

I've seen it stated in a number of places that there is no real benefit of creating DX telephoto zooms. However, as Dave mentions above, DX lenses can be less costly on the wider side of things. The Nikon 35mm f/1.8 DX goes for around $200. The Sigma 30mm f/1.4 DX is about $450 (faster glass). The Nikon 35mm f/1.4 (FX) goes for about $2000.

Another thing to consider when using FX lenses on a DX body is that the DX sensor will acquire the image from the center portion of the lens which is usually the sharpest. My Sigma 50mm f/1.4 is a FX lens that is known for great center sharpness, but soft corners on FX bodies. DX users of this lens get to avoid some of this softness as the sensor is covered mostly by the center portion of the lens.

[kalfa]

Hi Dave and Steve,
thank you for your answers!

No, the focal length is the focal length and doesn't change; i.e. they don't use a crop factor on it, which is why we have to when using non FX sensor cameras.

Another thing to consider when using FX lenses on a DX body is that the DX sensor will acquire the image from the center portion of the lens which is usually the sharpest

shortly:

- from an image size point of view there's no difference in using a DX or a FX lens on a DX body: we still have to apply the crop factor and the resulting angle of view, field of view and co. will be modified by it.

- from an image quality point of view, all the image aberration problems with the external area of the lens on a DX are still there at some degrees, while on a FX lens a good part of it is cut out

Did I get it right? It seems to make sense in my head.