Macro - extension tubes vs reversal - how to choose?

Question

I do a fair bit of macro photography, indoors with good lighting, using either a focus rail or manual focus pull to image-stack.

I've always considered that using extension tubes for this gives me the most flexibility. With one set of tubes I can vary the tube length in seconds; I can swap lenses at will; the electronics line up to the lenses so I can still vary the aperture. I can't use auto-focus, but at these magnifications I don't think that would be any benefit.

So, having read a few posts on here about reversal & that it might give me a sharper image, among them Why does a reverse lens act like a macro lens? as an overview & What do I need for reversed-lens macro photography with an entry-level Nikon DSLR? for a comprehensive setup & equipment guide, & even What are the biggest differences between Reversal Rings, Extension Tubes and Macro Lenses? which really seems more concerned with why I should give up on both & get a dedicated macro lens.

I'm left wondering why I would go to all that effort to flip the lens round, investing in different mount adaptors for each lens I want to use, against simply continuing to do it the way I do, which I've become reasonably adept at.

Is the quality gain all that noticeable, even after stacking?

This is an example of about as small as I'm going, 2mm bud from an allium flower head, done with a nifty fifty f/1.4 at f/16 on 68mm extension, D5500, 4-layer stack [I've gone over 20 for larger objects]
I'm pretty happy with it.

^{This will click through to a ⅓ size image}

Answer 1

Normally a large object in front of the lens projects a small image behind it, onto the sensor. When you flip the lens, a "small" object "behind" the lens projects a "large" image in "front" onto the sensor. Basically, the magnification ratio is flipped. (I haven't done the math, so it may not be exactly like that.)

• The max extension I can get with tubes is about 40-45mm. With a 28/2.8 lens and extension tubes, I get about 1.4:1.

  

• With a lens reversal adapter and 15mm extension, I get about 2:1.

  

I don't do any real macro photography, and have no idea which is "better". Except for the ease of obtaining a greater magnification ratio, they seem about even.

• The exposure difference wasn't significant. (ISO 1000 vs ISO 800).

• The lines in the reversed-lens image look a little straighter. But how much does this matter when not photographing rulers?

• I have no way to test sharpness at this level. If you have good lenses, they'll probably be sharp enough. I shot my test images handheld and wide open. They'd be much better stopped down on a rail.

• As you've noted, image stacking and averaging improves image quality. It may not be possible to do significantly better even if image quality of the source images is improved.

It seems as if it would come down to:

• Convenience factors you've already mentioned, like how easy it is to attach bayonet mounts vs screw adapters onto filter threads and whether you can control the aperture.

• Whether you can get the magnification you need with the equipment you have.

Since you get results you like, there may be no need to change. But it might be worthwhile for you to try. The adapters are cheap enough to buy several to leave attached to the lenses you like to use, so there wouldn't be any more "effort" than it takes to swap lenses normally. You could get a lens reversal adapter that fits your current extension tubes to increase the flexibility of your current setup.

Answer 2

The reason for inverting the lens --- The typical camera lens is optimized to image a world where objects are spaced at different distances and to project this image on the flat surface of film or digital image sensor. Thus the rear of lens is optimized to work a flat surface. When doing macro work, most times the object being imaged is quite shallow or perhaps flat. Reversing the lens often improves resolution when the subject to lens distance is quite close.

Additionally, every lens has two cardinal points. These are the front and rear nodal points. Such distance is measured from the front nodal to object. Image distance is measured from the rear nodal to image plane. One might think these points are located somewhere near the center of the lens barrel. However their location is elusive, they can even be flipped with the rear nodal located far forward of center. In any event, flipping the lens generally elongates the back-focus distance; this action often results in an ample increase in magnification.

If a conventional lens is used for macro work, the f-numbers engraved on the lens barrel are invalided. This is because the f-numbers are derived by dividing the working aperture diameter into the focal length. The focal length is a value derived when the lens is imaging at infinity. When doing macro work, the back-focus distance is substituted for the focal length. The error a unity (magnification 1) is two f-stops. This phenomenon is called bellows factor. The formula to correct when doing close-up work is M +1 x M +1. The amount of correction for exposure due to bellows factor can be significant. If the exposure determination is via a through the lens metering system, bellows factor is automatically taken into account.

A macro lens is optimized for close-up work. The macro design is optimized to image a shallow subject. The macro lens overcomes bellows factor thus the f-numbers are truthful.

Will you see any improvement if you use a macro lens. Since you are already skilled at this task, it is unlikely that you will see much improvement if you invest in a macro lens. However, there is always that possibility that the lens you choose will be a real champion.