The photographer with a low budget may look at a macro lens and say “Woa! That’s a bit steep.” There are cheaper alternatives for taking macro or macro like images without the expense of a specialized macro lens. One such option is a set of extension tubes. These tubes fit between your lens and your camera body and make it possible to focus on objects much closer to the lens.
I hear a lot of people trying extension tubes for the first time complain that they can’t get anything in focus. Often this is due to not knowing just how close their lens has to be to the object they want to photograph.
This post outlines a procedure for getting you to a point where you know what the focal range is for any given tube and lens combination. All of the examples shown are with one particular lens and tube set up and should not be taken as the final answer for your lenses, camera, and tubes. You will want to do something similar to this procedure with all of the combinations you want to use in the field.
To start this test I am using a Nikon D300s camera body with a 36mm Vello extension tube and an old Nikon manual lens – specifically a Nikkor 50mm f/2.0 prime lens. Here’s the lens and tube.
For a target I used a ruler with 1/32 inch markings. I placed the ruler on a diagonal to my camera position. I carefully placed the zero end of the ruler on the line that would be the front of my camera lens, and placed the other end of the 16 inch ruler 8 inches from the front of the lens. With this set up I can simply divide any reading I get by 2 to get the actual camera to subject distance.
Here’s the target set up.
Here’s the camera and lens set up to shoot the target.
A word about the front of the lens position. In the first image above you might have noticed that no filter was attached to the lens and that the actual front glass element is recessed some distance into the lens barrel. It is possible to get a small subject into that space but it may be difficult to light it once there and you may have a filter attached. Therefore, for ease of measurement and consistency with what I might encounter in the field, I did all my distance tests relative to the front edge of the lens.
For each lens/tube combination I did four test shots. Two with the lens diaphragm wide open (f/2.0 on this lens) and two with the lens stopped all the way down (f/16.0 with this lens.) Each set of two shots included one with the focus set at infinity and one with the lens focus set as close as it will go.
Starting at f/2.0 and infinity I moved the camera left and right along the base line until I found a ruler mark in focus in the center of the field of view. Without shifting the camera position I changed the f stop to f/16.0. After the infinity set of shots I refocused at the near limit and again moved the camera to find a line in focus and then did the f/2.0 and f/16.0 images.
Here are the resulting images:
f/2.0 at infinity
f/16.0 at infinity
f/2.0 at near focus limit
f/16.0 at near focus limit
Remember that because of the ruler angle all of its readings should be halved. So f/2.0 at infinity for this lens and tube focuses about 3.25 inches away and the depth of field is all of about 1/8 inch. Switching to f/16.0 doesn’t change the focus distance but increases depth of filed to about 1/2 inch. Meanwhile f/2.0 at the near focus limit focuses at about 2.75 inches and has a depth of field only marginally more than f/2.0 at infinity. At this point the astute reader might notice that in the near focus limit examples the focus point is not centered in the field of view and that at f/16.0 the near limit of the depth of field is out of the frame! There are two solutions to this error in shooting. Reshoot with the focus point centered or change the ruler angle to get more of it’s length in the image. If you do the latter be sure to use an angle that will make the math easy!
If you do this test, and find that no matter where you slide the camera, you can’t find anything in focus, you may be using a tube that is too long for the lens. The shorter the focal length of the lens the shorter the tube length that will work. A 36mm tube on an 18mm lens will move the focus point inside of the lens! I would bet that you don’t want to try and position your subject inside the lens |:-)>
This lens, camera, and tube combination has a focus distance from 2.75 inches to 3.25 inches. Yes that all of 1/2 inch! If I put my subject outside of that range and I will never find focus. Depth of filed ranges from 1/8 to 1/2 inch. If my subject is over half an inch long I won’t be able to get all of it in focus. Knowing these limitations makes me better prepared to pick subjects to shoot.
One last example. You might think that if you put your ruler running straight away from the lens that the math is eliminated. Well that presents its own problems.
Something like the above you say.
There is one little stretch of the yard stick in focus – but as you can see, you can’t see the readings! Of course you could always reach out while looking through the viewfinder and put your finger down on the spot in focus. This will work but will not be anywhere near as accurate.