...Proud owner of a Nikon D5600....
Because of sensor size a 150mm will operate as a ??mm.... Also is there a formula to use?
Yes, there's a formula.
Here's a little secret.... It really doesn't matter.
All that matters is how any given focal length works on YOUR camera. Who cares about how it would work on anyone else's?
All you really need to know is what's compatible. In your case, it's both DX and FX lenses in Nikon F-mount... which is literally many millions.
You simply need to know what focal lengths are wide, standard, telephoto on YOUR camera.
- 4mm to 8mm are fisheye, extremely wide, but uncorrected so that straight lines will tend to be strongly curved.
- 10mm to 15mm are ultrawide, more optically corrected than fisheye, but still some distortion, exaggeration.
- 16mm to 21mm wide angle
- 22mm to 38mm standard
- 40mm to 90mm short telephoto
- 100mm to 200mm moderate telephoto
- 250mm and up, powerful telephoto
It doesn't matter if the lenses are DX "crop sensor" or FX "full frame" designs. Focal length is focal length, regardless of the format it used upon. Changing formats does affect how any given focal length "behaves". But unless you are planning to switch formats and want to replicate the angle of view of your lenses on your old format, it really doesn't matter.
If you still want the formula, it's the "lens factor" comparing a DX camera like your D5600 to a full frame/FX such as the Nikon D750, you either multiply or divide by 1.5X.
To understand what lens you'd need to buy for an FX camera, if you wanted one that will "act like" a 21mmm did on your DX camera, 21 times
1.5 = 31.5.... and the closest typical lenses are either a 28mm (slight wider) or a 35mm (slightly less wide).
Conversely, if you were looking for a lens to use on your DX camera that would "act like" an 85mm lens someone else is using on their FX camera, 85 divided by
1.5 = 56.66... and there are 55mm and 56mm lenses available that would give the results you want.
But if you are just looking at lenses to use on your current camera or another camera with the same sensor format, it really doesn't matter.