# 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

aclo wrote:

JimKasson wrote:

Leonard Shepherd wrote:

JimKasson wrote:

I am not sure what formula you’re talking about. Please state it.

The usual one - image resolution equals 1/sensor resolution plus 1/image resolution.

That is a loosely phrased (especially with image resolution on both sides of the equation!) approximation to another approximation, I believe. The other approximation is System MTF = Lens MTF * Camera MTF.

Your loosely phrased one doesn't work for extinction resolution.

Neither of those say anything about aliasing, which is what happens when the lens is too sharp for the sensor.

Jim

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.

Though I like the simple mathematical argument, I guess there's a practical question here - what does rL>>rS mean? Even a 24 MP FX sensor has approx. 160 pixels/mm, approx. 80 lp/mm if you want. If ">>" means "5x" or "10x", then you will struggle to find a lens that can resolve that much, and below that, it may be hard to argue that the other component can be entirely neglected.

A while ago, I came up with the following chart for lens vs. system resolution for various sensor pixel counts (using the formulae detailed here:  https://photo.blogoverflow.com/2012/06/the-realities-of-resolution/ ):

Things would appear to break down when approaching and exceeding the 24 MP FX sensor's pixel pitch because I would expect aliasing to do something to these curves, but I am not sure how that would be incorporated here. Maybe Jim can weigh in?

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