Help me out with my DX / FX math for a minute

AaronJAnderson

Let's say I've get the Sigma 85mm f/1.4 - a full frame lens.

I put it on my APS-C sensor camera.

It has a 127mm field of view, correct? That's the same field of view if I have my DX tele set to 127mm, right? Just making sure I'm not crazy.

ziggy53

FX and DX lenses are both displaying actual focal lengths. The crop factor and resulting narrower FOV is simply the result of the smaller imager size, exactly the same as if you used a FF imager and cropped it to the same DX imager size.

A DX zoom lens displaying 127mm, as in your example, will have an equivalent FOV to a 190mm lens on a FF camera.

AaronJAnderson

Hmm... I halfway understand...

in simple terms:

If I take a picture with the 85mm attached to an APS-C camera, and a picture with my (crappy nikon) 55-200mm dx formatted lens, what focal length would the zoom have to be set at for the photos to be the "same"?

Grainbelt

AaronJAnderson wrote: »

If I take a picture with the 85mm attached to an APS-C camera, and a picture with my (crappy nikon) 55-200mm dx formatted lens, what focal length would the zoom have to be set at for the photos to be the "same"?

85mm

but they would both have an FOV equivalent to 127mm in FX, because your sensor has 'cropped' the image down by 1.5X

ziggy53

Grainbelt wrote: »

85mm

but they would both have an FOV equivalent to 127mm in FX, because your sensor has 'cropped' the image down by 1.5X

AaronJAnderson

That makes sense. Thanks guys.