DX and Full frame lens naming

Chris R-UK said:

Chris R-UK wrote:

You are mixing up actual focal lengths with "equivalent" focal lengths.

The focal length doesn't change. The 70-200 lens is 70-20mm whether it is mounted on an FX or a DX camera. The field of view will be different but the focal length is unchanged. The same is true of th 17-55 DX lens. If mounted on an FX camera (which is possible with Nikon) it's focal length is still 17-55 mm but it only covers a portion of the sensor so the image is cropped.

Click to expand...

Correction. The image isn't cropped but is has lower resolution.

Chris R-UK said:

The missing focal length range is that between the 55mm of the 17-55 and the 70mm of the 70-200, i.e. 55-70mm.

Click to expand...

Thanks Chris, already thought it was like this, but wanted to be sure.

The focal lengths are the true physical focal lengths of the lenses.

So each lens will be that focal length on whatever system it is on.

An FX lens just means that a lens covers the image circle of the sensor. DX lenses are not designed to cover the full image circle of an FX sensor. That's the only difference.

So you will have a gap from 55-70. You can crop the 55 is that suits you.

You might consider a Tamron or Sigma 17-50 f2.8 and a Sigma 50-150 f2.8.

The Sigma 50-150 f2.8 is designed to give the same field of view on a DX camera as a 70-200 f2.8 would on an FX camera. It's also about half the weight of a 70-200 f2.8.

Regardless of DX or FX, lenses will behave the same on the camera you want to buy. A 17-55mm on a DX camera will be equivalent to a 25.5-82.5mm lens on a FX camera, and the 70-200mm will be like 105-300mm. So you would have a hole between 82.5 and 105...not a big deal.

The nominal price difference between the Nikon 17-55 and the Sigma 17-50 is not that big; it is much bigger with the 70-200 for both these lenses. That's why I am also considering the Nikon 70-200 F4. I am going to buy everything in January in Hong Kong, so the F4 will be available then. Have been considering the 50-150 as well, but since I am using a 18-200 on my D80 now, think i will miss the extra length.

It is cropped because DX lenses have a smaller circle than FX lenses. Because it is cropped it has lower resolution.