DSLR lens "resolving power"

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DSLR lens "resolving power"

Post by Beatsy »

Is there a formula to calculate the resolving power of a DSLR macro lens at (say) minimum focus distance? Or any distance? I assume such a formula would cater for the wavelength of light too - much like the formulae for microscope objective resolution (without the need for a condenser term).

I thought I could simply google an answer for this, but everything I found seems to descend into discussion of sensor resolution instead. Not quite what I'm after - I want to calculate a resolution at the subject side of the optics (in microns).

Thanks in advance...

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Post by rjlittlefield »

The simplest method I know is to calculate the subject-side NA based on effective aperture and magnification, then use the standard resolution formulas based on NA.

The key facts you need are that

1. NA = 1/(2*f_eff), where NA and f_eff = effective f-number are on the same side of the lens.

2. NA_subject = magnification * NA_camera

Then you can use the formulas copied at http://www.photomacrography.net/forum/v ... 831#124831 to find the cutoff frequency, compute MTF at any spatial frequency, and so on.

I'm a bit short of time this evening, so I'll skip providing an example. Please let me know if any of this is unclear or looks wrong.


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Post by Beatsy »

I didn't think of converting to N.A. first. Numbers come out in the ballpark I was expecting, so most likely all correct. Thanks Rik.

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Post by Stevie »

You can google Dawes limit .

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Post by Mendel314 »

Any calculation will give you a theoretical maximum. You will almost never approach this for the following reasons.

Lenses are designed for optimum performance at a certain magnification ratio, which is usually when focused at infinity. For macro lenses, it is usually for some reproduction ratio between infinity focus and minimum focus. Moving away from the design point will reduce the actual achievable resolution. Moreover, every lens falls within a range of spec considered acceptable by the manufacturer. They are almost never perfect. Furthermore, DSLR lenses are exercises in compromise, balancing performance over a range of use, thus even when used at their optimum reproduction ratio, they are below what would be theoretically achievable, since designers must choose which aberrations are most important to correct over this range, and will thus never eliminate them all. Finally, the calculated NA may differ from the observed NA, since at macro focusing distances, it is entirely possible that only a central portion of a lens is in use, and thus the angle between the extreme rays and the lens axis is actually smaller than predicted by a theoretical f/stop value.

The above reasons are why dedicated lenses like microscope objectives, enlarger lenses, and microfilm lenses will (almost) always outperform dslr lenses.

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