NEX Focal Reducer for E-Mount Lenses?

Started Apr 4, 2014 | Questions thread
David Wyman
David Wyman Regular Member • Posts: 484
Re: NEX Focal Reducer for E-Mount Lenses?

DtEW wrote:

Sleepdance wrote:

Ahh, I see. Thanks for clarifying that they're only useful for full frame lenses on cropped sensors.

Since they are designed for APS-C, does that mean that the SEL1855 and SELP1650 are not 1.5x cropped? I've always assumed that they were still 1.5 x 18mm when set at 18mm for example.

It would be best to lose the idea of "crop".

The focal length (FL) of a lens is an absolute term. It is a literal dimension.

It is only in the course of trying to describe the angular-width-of-view (better known as FoV, or field of view) it produces that we apply what is basically a relational factor (this "crop factor") to convert the lesser-familiar-to-most FoV of a FL on an APS-C-format sensor... to the more-familiar-to-most FoVs of 35mm film (or "FF" for "full frame").

So the SEL1855 and SEL1650PZ are not "1.5x cropped". They are APS-C lenses (which produce APS-C-sized light circles). The only reason you would calculate in the 1.5x relational factor is if you want to describe the width perspective in a way most people would understand.

But to avoid confusion between native vs. post-relational numbers, you want to say "18mm APS-C" or "27mm equivalent FF FoV".

That was a good explanation, but it was a mouthful, DtEW.

The shorter answer might be we consider it's the sensor that's cropped. That is, an APS-C sensor is smaller - cropped - than the full-frame sensor.

And while a focal length is a focal length is a focal length (more or less the distance it takes to focus an image from the lens the sensor), lens bodies can differ in size because (smaller) lenses for cropped sensors don't have to throw as large a circle of light as (larger) lenses do that have to cover larger sensors.

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