Experiments with Close-up Lenses and Extension Tubes: How much close-up lenses and extension tubes enlarge a photographic image and the price you pay for this.
If you want to take close-up pictures but can't afford a true macro lens, a cheaper option is to use close-up lenses, extension tubes, or a combination of both. This page investigates how much magnification these attachments provide.
It does so by taking pictures of a small grid of black lines with a rectangle outlined in green in the center that's the same size as the 22.5 by 15mm sensor in my Canon EOS 20D camera. By measuring how much of the frame the rectangle takes up, the magnification, or macro factor, can be determined. Measurements of working distance (distance from the front of the lens to the object), and depth of field were also recorded.
The camera was mounted on a macro rack so that accurate measurements could be made down to 0.2 mm. Since my interest is in macro or near macro photography (macro being defined as when the size of the image of the object on the sensor is that same as the size of the object) all of the following tests were accomplished with my 18-200 lens set to 200mm and F8. The following thumbnails are too small and compressed to show that in each case the focus was sharp and set to the midpoint of the depth of field. They also de-emphasize the amount of blurring, color, and curvature the lenses introduced. The grid was illuminated from behind so that the amount of light entering the lens was not reduced when the lens was positioned very close to it.
Here's what the test grid looks like through the 200mm lens without any attachments. Each square in the grid is 0.058 inches on a side. They can't be seen clearly because they are so small and regular that digital effects (called moire) prevent them from properly displayed. The following picture is a close up of the grid to prove the squares really are there:
(I colored the green in by hand, which is why it looks so irregular.)
The 200mm lens by itself has a working distance from the front of the lens to the object of 6.5 inches and a depth of in-focus field of 4mm. (I apologize for the mixed units. All my tape measurers are in inches while the lens sizes and position on the rack mount are in millimeters.) The magnification is .29, in other words the image projected on the sensor is 29-percent as large as the size of the actual object. The term magnification seems odd being used when it isn't enlarging the image as most of us are used to, but is still valid. In a sense, this is like looking through a binocular or telescope backwards: the image is being made smaller than the object.
First up is an inexpensive ($80.00 in 2005) set of +1, +2, and +4 diopter close-up lenses by Hoya.
These screw onto the front of the main camera lens and act like magnifying lenses. They can be used individually or stacked to provide +1, +2, +3, +4, +5, +6, and +7 diopters of magnification. (My problem is that I don't know what a "diopter of magnification" means.)
The 1 diopter lens provides a magnification of .32, a working distance of 5.5 inches and a depth of field (DOF) of 3mm.
The 2 diopter lens provides a magnification of .38, a working distance of 4.75 inches and a DOF of 2mm.
The 3 diopter lens (actually the 1 and 2 diopter lenses stacked together) provides a magnification of .42, a working distance of 4.25 inches and a DOF of 1.5mm. I began to notice a very slight curvature bowing the very outside line inward at this magnification. It wouldn't effect anything near the center.
The 4 diopter lens provides a magnification of .47, a working distance of 4 inches and a DOF of 1.5mm. At this point the outside line was starting to look slightly out of focus and there was a hint of color.
The 5 diopter lens stack provides a magnification of .51, a working distance of 3.375 inches and a DOF of .75mm.
The 6 diopter lens stack provides a magnification of .56, a working distance of 3.184 inches and a DOF of .5mm.
The 7 diopter lens stack provides a magnification of .59, a working distance of 3 inches and a DOF of .3mm. At this magnification the outer three lines were very blurred and the the color in the outermost block was obvious. The amount of inward bowing was also easy to see. However, the center half of the frame was still clear.
Next let's look at a Canon 500D close up lens ($65.00), which is nothing more than a better corrected version of the lenses above:
It provided a magnification of .38, a working distance of 4.5 inches and a DOF of 2.5MM. Comparing these numbers to the Hoya set suggests the Canon is a 2 diopter lens.
Stacking the Canon 500d lens on top of the 7 diopter stack of Hoya lenses to create a 9 diopter set provides a magnification factor of .67, a working distance of 2.125 inches, and a DOF of .25mm. The outer five blocks are blurred and color has crept in for the outer 2 blocks. Curvature is obvious. Here's what it looks like:
And next we'll try a Canon 25mm extension tube ($70.00), which fits between the main lens and the body of the camera.
The extension tube provides a magnification of .67, a working distance of 1 inch and a DOF of .25mm. The image is darker than with the close-up lenses and appears slightly bowed outward, opposite of the close-up lenses. (This bowing is greatly enhanced by the downsizing of the image for this page for some reason.) There is no blurring or color in the outer edges of the frame.
Now let's try combining the +7 diopter stack of Hoya lenses and the tube extension lens:
This system provides a magnification of .71, a working distance of 0.375 inches, and a DOF of .2mm. The image was quite good, much better than the 7 diopter stack alone. There was very little color and blurring at the edges and no noticeable bowing inward or outward. The big problems were the loss of light cause by the extension tube and extremely short working distance.
For the last test I tried combining all the close-up lenses and the extension tube but the working distance was shorter than the front ring of the outside lens.
What I learned from these experiments:
1. While close-up lenses and extension tubes provide some increase in magnification, they do so at the expense of loss of sharpness at the edge, greatly reduced working distances and very small depth of fields.
2. Because of the distortions, these attachments are probably only good for small objects that fill the middle half of the frame.
3. The best combination was the 25mm extension tube with the 7 diopter Hoya lens stack. It provided the greatest magnification with the least distortion, though the small working distance and depth of field will make getting a good photo tricky.
4. The Canon 500D 2-diopter close-up lens didn't appear to provide images that were any better than the Hoya 2-diopter lens, and the Canon lens cost three times as much.
5. It's odd that the extension tube, which provides a magnification by itself of .67, and the Hoya 7-diopter stack, which by itself has a magnification of .59, when combined only provide a magnification of .71.
Pictures of grids are interesting, but how do the contents of this page translate into the real world?
To answer this question I took some pictures of a very small (0.24-inches across) flower called sweet alyssum. Here's what it looks like in actual size, assuming your monitor is set to the standard 72ppi default:
(A very small, plain flower with a magnificent fragrance.)
The next picture was cropped from an image taken using the 200mm lens without any attachments. The image was not up or down-sampled or resized for this page.
Upsampling to 300 dpi enables this image to be printed out up to a 5-inch tall size and still look okay, not great but okay.
I took the next picture with the 7-diopter Hoya stack and 25mm tube extension to increase the magnification from 0.29 to 0.71. This increased it's size on the camera's sensor by a factor of 2.5. More sensor coverage means more pixels means more detail captured. As with the first photo it has not been up or down-sampled.
It's easy to see that the attachments help capture much more detail. This picture, when resized to 8-inches across and upsampled to 300 dpi, can print out at the scale of an 8x10-inch print and look very good, excepting the poor quality of my picture-taking technique.
To see the difference yourself, copy both files to your computer, upsample both to 300dpi and print them out at the 8x10-inch size. The picture taken with the attachments will come out far clearer.
Final conclusion: After working with close-up lenses and extension tubes I now understand that a dedicated macro lens with a magnification of 1.0 that provides a long working distance, and a flat, distortion free field with a greater depth of field is well worth the couple of hundred dollars more than the cost of the extension tube and Hoya set. (Unless you can get them for free, and even then it's questionable if it's a bargain.)
Having said that, I found close-up lenses and extension tubes of some use in extreme macro photography. For more infomation about this please visit my CANON MP-E 65 mm 5x MACRO LENS GUIDE page.
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