Whenever you see a specification for a microscope objective, you will likely see the term NA, the numerical aperture value. Many people, particularly beginners in extreme macro photography, ignore or pay less attention to this parameter of an objective, and mainly focus on magnification and (eventually) working distance, but in actuality, the value of NA plays very important role.
What does this NA specification for an objective mean? According to Wiki, “Numerical aperture is commonly used in microscopy to describe the acceptance cone of an objective (and hence its light-gathering ability and resolution)”. That might sound a bit too general for some, so here is the specifics:
NA = n*sin(θ)
where the θ is objective angular aperture, or another way to put it according to Wiki, θ is the maximal half-angle of the cone of light that can enter or exit the objective. The n is the refractive index, which is always 1 for air, so with air as ambient medium,
NA = sin(θ)
OK, this seems a bit simplified and since the sine function is an increasing function when θ is between 0 and 90 degrees (note, it can never exceeds or even equal to 90 degrees), the bigger the θ, the higher the NA value is.
Why does it matter to us as end user of an objective? As described in Wiki, the NA value determines the “light gathering ability and resolution”. So the first benefit of larger NA value is the light gathering ability, the larger the NA, the more light can be let in and less exposure time for a given ISO. Cool, this is very similar to the concept of aperture in general photography, the f-number.
Also, it turns out the higher the NA value, the more resolving power an objective can produce, hence more resolution. For detailed theoretical discussion, here is a link. So for the same magnification, if one objective has higher NA value than another one, it will have more resolving power and image shot with it will be sharper and have more details than the other one.
So we have established the understanding that the higher the NA value for an objective, the more light it can let it and the higher resolving power it has. Cool! Now lets get to the point: taking advantage of higher magnification objective for lower magnification work.
If you are shopping for an objective, you will notice one thing that it seems the higher the magnification of an objective, the higher NA value it has. Why is this? Well, the higher the magnification, the more resolving power is needed to provide details (beside letting in more lights), otherwise, the phenomenon of Empty Magnification will occur — the size of an objective might be bigger in an image, but it does not provide any more details than an objective with less magnification but with same NA value. For this reason, objectives with higher magnification usually have high NA value and thus have high resolving power.
Now, since NA = sin(θ) (air medium), it means once an objective is constructed, its NA value does not change. This is interesting because, from my other blog, we know that with infinite objectives, their actual magnification change if different tube lens is used. So, it is possible to use a higher magnification objective on a shorter tube lens to do low magnification work, though sometimes with vignetting. Why do this? Because the higher magnification objective normally has higher NA value and thus higher resolving power, with this configuration, we can obtain sharper images with more details (whilst suffer vignetting a bit).
Conclusion — this trick has been used by some highly experienced extreme macro photographers and experts in this field. I have seen many doing this with 135mm (short) telephoto lens or Raynox 250 as tube lens. And one of the astonishing example is this post.