What you need as glass (Part VIII - Macro)

July 20th, 2011

This is part VIII of a series of what glass you will need. Each part will discuss a certain lens type and its applications.

Any lens can double as a macro with close-up lenses and extension tubes, BUT most normal lenses don’t  perform best at close focus and adding close-ups/extensions will push macro lenses even further. Short macro lenses often don’t go to 1:1 magnification, they are close focus lenses for exaggerating perspective. Useful focal lengths are 60-200mm. 1:1 magnification is always reached at four times the focal length, so do the math what working distance you get and how to do lighting (At 1:1 you loose 2 stops that have to be taken into account as well for the diffraction limit). All macro lenses are good, a smooth manual focus ring is a big help. So you should get a 200mm macro? Yes, if you are really serious about it and can wait that some renewed version comes out. Nikkor 105/2.8VR and Tamron 60/2 are exceptional lenses at the moment. The VR is useless in macro range, btw. -the back and forth movements can’t be corrected by VR and AF will give up as well.

Extensions & Bellows

Close-up lenses work by reducing the focal length, as the mechanics stay the same the lens becomes short-sighted.

Extensions attack the problem from the other side, it shifts the optics as if it could focus closer - of course it can’t focus close anymore.

You can compute very precisely the effect with Bill Claff’s Simple Close-up Calculator.  This of course works as well for bellows which are basically deluxe extension rings.

When doing these calculations you’ll see that extensions will work best with short focal lengths and close-ups will work best on longer lenses.

Before you go to eBay to get a bellow: There is a caveat. Bellows have a minimal extension ~40mm (and with some you need an additional spacer to stay clear of the flash housing). This can be too much so that the focal point will be within the lens .You can mitigate the problem by a reversal ring that lets you mount the lens the other way around.

Lens reversal

If you have paid a bit attention in school you might have noticed that it is odd to have a 18mm lens mounted 40+mm away from the sensor. All short focals for DSLRs are retro-focus lenses. They are constructed by combining a tele and a wide lens at some distance so that the optical axis will lie behind the lens. This explains why wide angles are so long (physically) and why it is so hard to achieve large apertures ( and also why such lenses on range finders perform better) .

When reverse mounting you virtually work with a lens in front of the physical lens. A cheap way to have some macro.

Male-male rings are difficult to find but allow for something really cool: You reverse mount a wide angle in front of a tele. When both lenses are set to infinity the magnification is the ratio of the focal length.  You can try doing this  by hand-holding (free-lensing) with nothing than a tele and a wide angle, here an extreme example of 17mm on a 210mm - magnification ~ 12:1


Leave a Reply