 # Ephotozine Review: Z 100-400/4.5-5.6 S VR

Started Jan 25, 2022 | Discussions thread
Re: A picture is worth a 1000 words

JimKasson wrote:

aclo wrote:

Even if this was an exact relation, it relates the system resolution to those of the lens and the sensor. And it's consistent with saying the lens can outresolve the sensor: if the lens resolution is rL, the sensor resolution is rS and the total system resolution is rT then we have 1/rT=1/rL+1/rS. If say rS is much larger than the rL, so the sensor outresolves the lens, ie rS>>rL, then 1/rS<<1/rL so 1/rT=1/rS+1/rL is approximately equal to 1/rL or in other words rT=rL, ie, the lens resolution limits the total resolution (or, increasing the sensor resolution won't increase the system resolution). I'm not sure how this equation implies a lens can't outresolve a sensor, the equation itself tells you that if one of the rL or rS is much larger than the other then the lower one determines the overall resolution.

Define “resolution” as you’re using it above, please. But the above ignores the sampling frequency, doesn’t it? And the sampling frequency is key to knowing whether the sensor is out resolved by the lens.

I can't, as I don't know where the relation came from and am not defending the equation. I am saying that the qualitative picture that equation gives is that if the "resolution" of one of (lens, sensor) is much smaller than the other then that limits the total "resolution," no matter how much you increase the other. So I am unsure why it would be at odds with saying one of the two components can "outresolve" the other, as I interpret "outresolve" as "have much more resolution which then becomes useless as it does not contribute any more to the system resolution".

I guess this approximation comes from System MTF = Lens MTF * Camera MTF and using that the MTF is (as far as I can tell) the FT of the image of a delta function,

The MTF is the absolute value of the FT of the point spread function, which is indeed the system response to a Delta function. The absolute value part is why multiplying MTFs is an approximation.

and that the MTF's width is roughly inversely proportional to the width of the image of the delta fn then it sort of makes sense?

One thing to remember is that sensor MTF is not determined only by pixel pitch. The sampling window is important.

Right, true, it depends on many implementation details. Thanks, I'd never thought about this in any depth.

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