What extension tubes do.
Okay, this is my first photo how to thread in a while, so please bear with me.
Extension tubes are available in a variety of sizes for SLR cameras, there are 3rd party brands and there are ones made by the OEM, but they all have the same purpose, to reduce the minimum focus distance of a given lens. All of my examples are taken with the same camera, the same lens, and the same aperture, so that depth of field was not affected in that way. The camera is a 1D from Canon, the lens is a 70-200 2.8L with a minimum focus distance of 1.5m without any modifiers. Here is a sample of the absolute closest that I can get to my Milliapora without any modifiers:
Not to bad right? Wrong! Well, it is OK for an overall picture, it is in focus etc... but you can't really see any detail. How can you get closer? Well I could CROP the image in photoshop or lightroom, or any other of the many post processing programs that are available today. The biggest problem with cropping, is that you are reducing the percentage of pixels that are actually used on the subject. I know, that sounds weird, but think of it this way, if I were to crop that image in close, even though I used an 8MP camera to take the image, I may only be using 2-3MP on the cropped image. So what right? Nope. The fewer MP you have, the less detail you can keep. The best solution is to put more of the subject on the sensor. That is the advantage of the extension tubes, by reducing the minimum focus distance I can put more pixels on the subject. These next images were captured using increasingly more and more extension tubes. In order from least to most: 12mm Extension Tube 20mm Extension Tube 36mm Extension Tube
Okay, that is it for single tubes, how about stacking them? Yup!!! I can do that too! 48mm 56mm 68mm
So why wouldn't you use extension tubes all the time? Well there are some disadvantages to their use. One is the reduction in depth of field, you can see in these images how the background (and foreground) gets more and more out of focus as the length of extension tubes increases. Another disadvantage is that the longer the tube gets, the longer your shutter speeds need to be. Longer shutter speeds mean that you have to use a tripod, it also means that unless you are VERY good at panning, that you will not be using an extension tube for fish pictures.
Is there a way to get even closer? Yup! You can add a diopter to the front of the lens, this last image is captured with all 68mm of extension tubes, PLUS a 500D diopter on the front:
More on diopters later!
If you have any questions about extension tubes, please feel free to ask them in this thread.
Thanks for reading,