Closeup: Choices, Choices and More Choices
Generally speaking, adding extension works better on shorter focal length lenses, while diopters work better on longer lenses. To understand why, I'm afraid I'm going to have to introduce a few formulas. If math was never your strong suit, relax. The formulas aren't that complicated, and you can always come back to this page later if you want to gloss over them on first pass. Truth be told, I'm only going to be going over the basics. If you're a serious macro junky there are even more formulas out there just a Google away.
Extension
To calculate magnification due to extension, use the following formula:
Total Extension Focal Length 
= Magnification 
For example, it would take 100mm of extension on a lens with a 100mm focal length to achieve life size (one hundred divided by one hundred equals 1x). On a 200mm lens, that same 100mm of extension would only get you half life size (one hundred divided by two hundred = 1/2x). To get to life size, you'd need 200mm of extension (two hundred divided by two hundred). As you can see, on longer lenses, you rapidly reach the point where you just can't add enough extension to achieve macro sizes. It's easy to add enough extension to a 50mm lens to reach life size, but all but impossible to do so at 300mm: you simply can't stack enough bellows and extension tubes together to reach the needed length.
You will lose light when using extension tubes as well. The more extension, the more light lost.
Diopters
To figure out magnification from the use of a diopter, we first have to determine its focal length:
1000 mm Diopter Strength 
= fl of Diopter 
I've covered this before, but as you can see, the Nikon 3T and 5T which each have a strength of 1.5 diopters turn out to have a focal length of 666mm (1000 divided by 1.5). The 2.9 diopter strength 4T and 6T each have a focal length of around 345mm (1000 divided by 2.9). You can then determine magnification with another formula:
fl of Prime Lens fl of Stacked Lens 
= Magnification 
So, putting a 6T on top of a 300mm lens will give you just shy of life size (300 divided by 345). This same 6T stacked on a 50mm lens would only net you around one seventh life size. As you can see, the effect of a diopter increases as does the focal length of the main lens used. Shorter lenses will give you less magnification, while longer lenses will give you more.
Diopters involve the addition of extra layers of glass so they may marginally affect image quality, but on the bright side they do not cost you any light loss. Remember not to use the cheaper singleelement closeup lenses though.
Stacked Lenses
For all practical purposes, a stacked lens is functioning in exactly the same way as a diopter does. To determine magnification, use the second formula given above for diopters. Thus, a 50mm lens stacked on top of a 200mm lens will give you roughly a whopping four times life size (200 divided by 50). My trusty 28mm f/2.8 stacked atop the same 200mm lens gives nearly six times life size (200 divided by 28). These formulas assume the main lens is focused at infinity so you should be able to get slightly higher magnification. Since your focus range will be so limited though, actual gains will likely be modest. Also, as mentioned last week, not all combinations will give good results, but if you try out the lenses you already own, you may just find a winner of a combination.
Mixing and Matching
No reason you can't use both extension tubes and diopters. Part of the fun is trying different combinations. Heck, while you're at it, throw in a teleconverter and see what happens. If you're shooting digital these days, experimenting provides instant feedback and no cost.
With very few exceptions, if you can fit it together, you can try shooting with things configured that way. The most notable exception for Nikon shooters is the difference between the PK11 and PK11a extension tube. Both are the same size, but the newer PK11a has a small notch cut into the bayonet mount to prevent shorting out the contacts on autofocus cameras. To be safe, never use the older PK11. Even older preAI tubes also could cause problems.
You'll also find that some things that seem like they ought to work just don't. Some teleconverters won't fit since the rear element of many macro lenses sticks out too far to mate properly. Severe vignetting could also result from some stacked lens combinations, so don't expect everthing to work.
It's probably best to find a few setups that will work for you and not go completely overboard. Take an inventory of what you already own and factor in what you might run out and buy after reading all this, and take things one step at a time. If you can already do life size one or more different ways, you really don't need even more. Get something else instead. Even if you do go a bit crazy, just don't do this:
Next week we'll start looking at various macro techniques and issues.
