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

Started 11 months ago | Discussions thread
Re: A picture is worth a 1000 words
2

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.

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, 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?

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