Nx 50-150 is it really F4.2

Milondon15 wrote:

I have just bought the Samsung 50-150 for a great price but my excitement was slightly dampened after finding that F2.8 lens on APS-C is really equivalent to F4.2 full frame lens. Is this true?

Your lens is an f/2.8 lens, and it gives the same exposure (sensor illumination) as an f/2.8 lens on FF (so you would use the same ISO and shutter speed settings). But it will give the same depth of field and (given similar sensor technology) same total image noise as a f/4.2 lens on FF. Why? Read on...

The "f-number" (focal ratio) of a lens is its focal length divided by its aperture:

N=f/D, where

N is focal ratio ("f-number") [dimensionless],
f is focal length [unit: mm], and
D is aperture [unit: mm].

Your APS-C lens' aperture varies between 50 mm/2.8 ≈ 18 mm and 150 mm/2.8 ≈ 54 mm over its zoom range.

For the same angle of view (AOV) on FF you would use a 75-225 mm lens. Given a focal ratio of 2.8, its aperture would vary between 27 and 80 mm over its zoom range.

Given the same subject distance and framing (AOV), the only thing determining how much light is gathered from the subject (and thus final image noise) is the size of the aperture (and this also determines the depth of field). To collect the SAME amount of light, you need the SAME aperture size, regardless of sensor size.

A lens with the same AOV and same light gathering capability as a 75-225/2.8 mm lens on FF would need the same aperture size but a shorter focal length: 50-150/1.9 (50/1.9 ≈ 26 mm, 150/1.9 ≈ 79 mm). It would be approximately a stop faster than the FF lens, and you would use one stop lower ISO on the APS-C camera than with the equivalent lens on the FF.

"One stop lower ISO? But then the APS-C image would be lower in noise!" No! Check the dpreview ISO comparisons: comparing contemporary cameras, the APS-C cameras have about the same noise as a FF camera at one step higher ISO! Edit: Link. Use Low light, Print resolution (otherwise you'll only be comparing sensor resolutions), and RAW. That's because ISO have nothing to do with noise, only with exposure, and at the same exposure (sensor illumination), the APS-C will collect a smaller amount of light since its area is smaller, and noise has everything to do with total amount of light collected. Note that sensor size does not determine noise - only lens aperture does, but you need a faster lens with a smaller sensor to collect the same amount of light from the same scene.

This is true for micro four-thirds as well; they have about a two stop difference in noise compared to FF, but will again give the same amount of image noise given a fair comparison, i.e. with the same total amount of light.