Nikon 85mm 1.8 FX lens on a DX Body focal length?
Hi, I was thinking about buying a portrait lens, and saw the 85mm 1.8, and tried the 85mm DX EQUIVALENT focal length on my 55-200mm DX lens. So I set my 55-200mm lens to 127mm, is what I'm looking at on my 55-200mm lens exactly what I'm going to be looking at if I buy the 85mm lens for my DX body? Or will I be looking at the 85mm focal length from my 55-200mm DX lens on the FX 85mm prime? I'm confused!
You only need to multiply the 85 mm focal length by 1.5 to get the equivalent field of view looking through your DX body
Here is a a link what will visually show you what various lens lengths see using a DX camera body
You should set your 55-200mm lens to 85mm if you want to see what that lens is like.
If you had a FX body, you would use 127mm if you wanted to see what 85mm looked like on a Dx body.
There's no difference in focal length between FX and DX lenses or bodies. This has been a subject of great confusion, and fueled by the notion that there's a different focal length for Dx, which there's not.
To visualize the difference, think "crop" just as you would in photoshop. In reality, all that changes is the angle of view. The only time you may need to think "equivalent focal length" is when determining what the same lens would look like on a FX vs. DX body.
In that regard, the equivalent view from a FX camera would in a DX body would be 1.5x. In other words, a 85mm lens on a FX body would look like a 127mm lens on a DX body.
This may be confusing still, so here are some summary points.
- A 85mm lens is a 85mm lens, whether used on a Fx or Dx body.
- A Dx camera crops the angle of view - in other words, it only uses the center of the lens.
- A Dx lens is smaller in diameter as the outer part of the lens is not needed.
- A 85mm Dx lens (if it existed) is the same focal length as a 85mm Fx lens.
- Crop factors are only useful when comparing equivalent views between Dx and Dx cameras.
- Another way to get an equivalent view on a Dx camera is to backup.
- Backing up does increase the focal point, so it also increases the DoF.
I have a D7100 (cropped DSLR) and a 85mm f/1.8. The 85mm focal length is the best portrait lens, regardless of whether it be used for a Dx or Fx body. Using the 85mm produces sharp images, and suffers no compression or perspective distortion.
Yes, you have to backup a bit when using it on a Dx camera, and the DoF is slightly deeper, but not noticeable. I have even been able to get great bokeh using my 85mm f/1.8 on my Nikon V1 (2.7x crop factor)
While many people discuss equivalent focal lengths, I believe too much emphasis is put on it, and it is technically incorrect. Here is why.
The angle of view changes when using a cropped camera, not the true focal length. The sensor to lens center distance is identical regardless of whether using a Fx or Dx camera.
And, if the focal length were to increase, the DoF should decrease. However, exactly the opposite happens. The reason is you have to backup with a Dx camera to obtain the identical photo - and backing up increases the focus point - which increases DoF.
A far better thought process for a Dx camera should be to consider cropping in a similar fashion as you would in post-processing, or simply the notion you have to backup. That keeps confusion down.