And another thing...
I have a canon 100-400mm focal length lens and it transformed my photography when I got it about 18 months ago. Up to that point I had a 200mm lens which was great but I could never get enough magnification for photographing birds. Then I found that the 100-400 was actually good for shooting butterflies because I could get reasonable shots without getting close and scaring the subject. BUT, I would find that often I couldn't get close enough, because the close focus distance was about 1.5m, and sometimes I needed to move away from the subject in order to focus on it. Also these long focal length lenses tend to have moderate apertures which can affect depth of field. It seems a little odd maybe that you have all this magnification available but you can't make use of it. Macro lenses produce large images of small things because they can focus so close to the lens and the subject takes up most of the field of view. So if we could make our long focal length lenses focus closer we could get bigger images and put our lens to better use?
I decided to investigate 2 cheap methods of making long focal length lenses focus closer and see if they were viable from a quality point of view, or would I need to invest in an expensive long focal length macro lens. The 2 methods are the use of extension tubes and additional screw in close-up lenses. I have a set of cheap plastic extension tubes which cost less than £20, and I bought a set of close-up add on lenses for about £25, just to see how bad things would get.
To do the tests I made a test subject which was a series of pins stuck into cardboard, with graph paper so I always had a reference distance. I found the closest distance at which I could focus using tubes and lenses and took a photo. I will present the photos for you to make your own judgements, but also measured the relative magnification. My lens can vary from 100mm to 400mm so I took photos at these extremes in order understand how the new composite lens system would behave.
You can spend a lot of money on robust, well made metal extension tubes, or you can spend a few quid on plastic ones, like these. At the closest focusing distance the lens is able to form an image on the camera sensor. At distances less than this the lens tries to form the image at some point behind the sensor, which looks out of focus on the sensor itself. The extension tube essentially moves the sensor back so that the closer objects can be focussed. The further back, the closer objects can be focussed. But be aware that distant objects cannot now be focused.
The photos for all the tubes and focal lengths are shown above. The top photo in each column shows the scene with no extension tube at a focal length of 400mm at the closest focus for comparison. As you might expect the longer the extension tube the greater the magnification we can achieve. I have plotted the closest focus distances (in cm) and the measured relative magnifications for focal lengths of 100mm and 400mm on graphs below. .
Probably the most surprising result is that you can get closer focus distance and resulting higher magnification with the smallest focal length of 100mm. Indeed at the extreme of using all the extension tubes the closest distance is only 14cm. At this point the system is quite possibly acting as a true macro lens. But it's not as simple as just inserting the extension tube and snapping away. You will find that in most cases the autofocus wont work, at least not until you have manually found focus by either changing distance to the subject or zooming in and out until you see a clear image. This takes some getting used to and can be a bit frustrating at first. Nevertheless the results are not too bad in terms of image quality even at the extreme end of magnification, and the depth of field is a several mm. Typically if am near a butterfly and I am too close to focus normally, I am likely to be around 1m away or so and I would probably be using the 31mm extension tube so as not to get too close but to get a bit better magnification. Depending on what the distance is I should be able to find a focus with the zoom.
Close up lenses.
I purchased a set of screw in close up lenses for around £25. They came in 4 strengths +1, +2, +4 and +10 with the strength representing dioptres, bigger numbers are stronger lenses with shorter focal lengths. I did the same tests as for the extension tubes with the results shown below. I'll admit that I did not have high expectations of this working well as they can only make the image quality worse. Multi element lenses are highly designed to remove distortions and chromatic aberrations and adding some random lens will degrade all that design.
Again the first shot in each column is with no additional lenses and with the focal length set to 400mm. The obvious things to notice are that you can get some high magnifications , especially with the 400mm setting, but equally in every case the image quality is poor, unusable really. Thats not the case with the 100mm setting. With the exception of the +10 lens the image quality was better than I expected, OK considering.
The noticeable things from the graphs are that the focal length setting of the lens does not affect the close focus distance you can get, its the same for 100mm and 400mm. However you can only really use it at the low end (I guess you could zoom in a little from 100mm) so if you want to get a better magnification than by not using the lens then you need the +2 or +4 lenses. It is pointless carrying around the extra weight in glass of the +10 lens. It may be a little restricting in how you use these lenses but the good news is that, generally speaking the autofocus works.
So there it is, with plenty of caveats, it is possible to use your big telescopic lens to take macro shots and you dont have to break the bank to make it work. Is it worth it? Well that depends... if you want assured image quality and ease of use then you will have to invest in the right lens. If you are prepared to be flexible and learn how to use the add-ons then what have you got to lose? It beast carrying around the extra weight of another lens.
The insect photos shown on this page were all taken using the 31mm extension tube with the Canon 100-400mm lens.