(This in an age when people can’t understand the concept of effective focal length and wonder why their lenses aren’t physically that long, and “professionals” declare there’s no such thing as depth-of-field or aperture sweet spot.)
The lens extender is a laughingly simple concept: stick a magnifying lens on the back of your regular lens and voilà – you have a longer focal length!
They were very popular back in the film SLR days because they were the cheap alternative to buying a medium telephoto to complete your ‘kit’: you bought the camera with its ‘normal’ 50mm lens, a wide-angle lens of usually 35mm, and finally either a 135mm telephoto or for a lot less money a 2X lens extender that magically transformed your 50mm into 100mm. (Hey you could buy the 35mm lens, a 1.5X extender, a 2X extender, and a 3X extender and save even more money. But I guarantee you wouldn’t like the results at 315mm equivalent.)
This works, after a fashion. But as simple as the concept is there are some obvious built-in flaws. Obvious if you understand the basics of lens design, that is. For one thing the added magnification also increases any flaw inherent in the base lens. More so than if the same flaws were present in a lens manufactured as a longer focal length to begin with. For another the extenders add in their own flaws, both in terms of the quality of the product and the inescapable problems of their functional design.
Let’s discuss those. To start with they ‘use up’ light. Everyone knows that as you move a lens away from the image plane the amount of light available there decreases. You can demonstrate this with an extension tube. The principle can be expressed as the difference between an f stop and a t stop; with the tube or extender the f stop remains constant (focal length divided by aperture diameter) but the light on the film or sensor is less intense so compensation must be made. You see the opposite effect with so-called “speed boosters” which in essence are the inverse of an extender (a concave lens as opposed to a convex one) because they concentrate the light on the image plane. (Note you also don’t see the compounding of other problems with the speed boosters because they operate in the opposite direction, as it were. I should also point out that the purpose of speed boosters is mainly to reverse the “crop factor” effect of using less-than-full-frame sensors.)
The other major problem is that extenders refocus light that has already been manipulated to fall on the image plane in a certain way. As such the effort that went in to edge-to-edge sharpness in the lens is now challenged by being redirected in new pathways. This gets complicated because how severe the affect is on the final image depends not only on the quality of the extender itself but on the design and quality of the original lens. Thus extenders work better with some lenses than they do with others. For best results you’d use a high quality lens and extender from the same manufacturer that were meant to be used together. Even then “best results” may not be as good as using a lens that is the equivalent focal length in the first place.
Amongst the selection of accessories I recently acquired were two lens extenders (see first picture). One is a 2X Royal brand (on the right) which I never heard of, and the other is a 2X-3X Vincor brand (center) which I also never heard of. It doesn’t matter much because not only did I already have a 2X Vivitar (on the left) of fairly good quality but I don’t bother to use even that one due to the poor results as outlined by the above description of extender function.
The thing is, that 2X-3X Vincor is intriguing. It has a dial on it which shows the f-stop equivalent in a little window for a given aperture. It is not a fixed chart, but a stop-by-stop conversion shown one at a time. Odd, that. Odder still is that I can not see any way of shifting between the two magnifications. The lens within is spring-loaded and moves back to front and pops back instantly, but there is no apparent ring or lever or whatever to bring about this effect for when it is in use on the camera; it can not be made to stay in 3X mode. Either I am missing something obvious or it is. I would think it should be the one twist ring, but that has no means of affecting the position of the glass inside. In fact the scale revealed through the window changes to a different one for the 3X compensation when you move the lens, but not the other way around. I’ve had it apart and can see no sign of what mechanism is supposed to be employed to bring about the change in magnification.
Not that it matters as I’ll never use it, I’m just curious as to how it was supposed to work.
(By the way I had a 1.5X extender for my Exakta equipment. It too saw little use.)