Close-up lenses (also called diopters) are a good way to gain a little extra magnification in your images. These are small lenses that fit onto the end of an existing lens. They often come in sets and can be stacked for a cumulative effect. They do require a filter thread to work. Filter threads are pretty much universally present with SLR lenses, but are less common with point and shoot cameras. There are magnetic adapters that will allow these lenses to be mounted on point and shoot cameras without filter threads (attaches to the end of the existing lens or the camera body). Close-up lenses will provide a modest increase in magnification and can be purchased for anywhere from a few dollars to around $150.
- Good way to add a modest amount of magnification
- No change of effective aperture
- Not great for adding a lot of magnification
- Need to match the size to the lens you are mounting it on, you may need adapters
- Moderate working distance loss
- Most effective at low magnification
Close-up lenses work by shortening the focal length of the lens which will allow for closer focusing and more magnification. The advantage of adding magnification this way is that the close-up lens doesn't affect the effective aperture of the lens. Most other methods of adding magnification cause the effective aperture to increase which will negatively affect the resulting resolution capabilities of the lens. Since these lenses don't cause the effective aperture to rise (worsen), a sufficiently high-quality lens you will get the added magnification with very similar image quality. I find this to be a win-win situation for adding modest amounts of magnification.
How do close-up lenses work?
Close-up lenses work in a couple of different ways to increase the magnification and potentially the resolution of a lens. They primarily work by shortening the focal length of the lens. The amount of extension has not changed (actually there is a slight change as the principal planes will move a little with the added lens, but we'll ignore that). A shorter focal length with the same extension causes the image to be more magnified.
The added glass of the close-up lens does tend to cause increased aberrations within the system, potentially decreasing the resolution capabilities of the lens it is being added to. This increase in aberrations is at least partially counteracted by the benefits of adding one of these lenses. Lens aberrations become a bigger issue as the power of the lens is increased or if the focal length of the lens being modified is increased. So, longer focal length macro lenses and higher power close-up lenses will tend to make the image quality worse.
A big benefit of using an close-up lens is that the image magnification increases but the effective aperture doesn't change. With most other methods, increasing the magnification will cause the effective aperture to also increase. Increased effective aperture means more diffraction and less image quality. Close-up lenses only affect the amount of glass in front of the aperture, i.e. only the entrance pupil will change. The exit pupil size and its distance from the detector are unchanged. That means that the effective aperture is completely unaffected.
In order to get increased magnification at the same effective aperture, the f/number of the lens needs to decrease. The f/number decreases because the focal length shortens, while the entrance pupil is slightly enlarges. The end result is that the resolution you will see at the detector is similar while the magnification increases. That means that you should be able to resolve smaller details on your images with close-up lens than you would have by just increasing the magnification by the use of increased extension.
Again, this improvement is all predicated on the assumption that the lens is of a high-quality and that the power of the close-up lens is not too high for the lens it is being added to. Either of these situations leads to increased lens aberrations and a potential decrease in image quality.
The power of a close-up lens is measured in diopters (D), the same unit of measurement that eyeglasses are measured. The diopter measurement can be converted into a focal length by dividing 1000 by the power in diopters. A +4D lens will have a focal length of 250 mm.
The amount of magnification achieved with close-up lenses tends to be modest and varies with the focal length of lens and the power of the close-up lens. Short focal length lenses will require more power to get the same added magnification when compared to a longer focal length lens. A +4 diopter lens will provide more additional magnification on a 105 mm lens than on a 50 mm lens. Conversely, it is easier to use more powerful close-up lenses on shorter focal length lens. Thus, close-up lenses for short focal length lenses tend to be in the +1 to +8 diopter range while those made for longer focal length lenses tend to be in the +1 to +3 diopter range.
The resulting minimum magnification of the combo will be the power of the close-up lens divided by the power of the camera lens. The maximum working distance of the combo will be the focal length of the close-up lens.
Example: If I am adding a +4D lens onto a 100 mm camera lens (+10D), the resulting minimum image magnification (macro lens focused at infinity) will be 4/10 or 0.4:1 at a working distance of 250 mm. The combo will have a focal length of about 70 mm (4 + 10 = +14D = 1000/14 = about 70 mm). The maximum magnification will be somewhat more since the lens is focused at infinity for the above calculations. If the lens initially focused to a maximum magnification of 1:2, it will now be able to focus to about 1:1 with the close-up lens.
|Nikon 105 mm micro||Maximum Magnification||Working distance|
|No close-up||0.51:1||274 mm|
The power of the lenses that are available for purchase are typically in the +1 to +10 diopter range, although the best quality lenses are typically found with powers ranging from +1 to +4 (although Raynox makes a very good +8). I believe this limitation to the lower power levels is related to the relatively simple construction of these lenses and the associated difficulty with controlling lens aberrations at higher power levels. This concept is borne out with the fact that most standard camera lenses have four or more elements while close-up lenses typically have one or two, sometimes 3. Close-up lenses are best used in moderation as adding too much power to any lens will create a large increase in image aberrations and thus negatively affect the image quality.
Close-up lenses can be stacked owing to the filter threads that are generally seen on both sides of these lenses. The resulting power is just the two powers added together. A +1 and a +3 stacked are a +4 total power. If you do stack these lenses, it is commonly suggested that you place the higher power lens closer to the camera. In general, I would avoid stacking these lenses.
|Nikon 3T, 5T||+1.5D|
|Nikon 4T, 6T||+3D|
Close-up lenses come in a variety of prices and quality. I have found that as with many other products, you get what you pay for with these lenses. The cheaper lenses tend to be single element lenses which can work fine if of sufficient quality. The more expensive lenses tend to have 2 or 3 elements that allow them to be corrected for color and spherical aberration.
In my own testing of these lenses, I bought a variety of lenses all with a +4D power – a cheap one ($20 for a set of 4 different lenses), a mid-price ($25 from B&H Photo), and an expensive one ($75 Canon 250D). When I started to test these lenses my gut feeling was that price would directly relate to performance. I found that to be mostly a true statement although the relationship isn’t linear.
As expected, the Canon 250D is a outstanding performer as far as my testing went. It didn’t significantly decrease the performance of any shorter focal length lens (less than 100 mm) I tried it with, despite the additional magnification. This is virtually free magnification in that you really don’t pay for it with any significant resolution drop as is seen with most other techniques. The mid-price +4D lens form B&H photo only showed a very slight drop in performance compared to the 250D which is likely not visible on an image. As a contrast, the cheap +4D lens I bought with a group of 3 other lenses for $20 on eBay performed relatively poorly and had a large drop in sharpness. The sharpness that it gave was less than no close-up lens at all (see last column of table below). The cheap lens did show a mild improvement in resolution over no lens at all, but the drop in sharpness counteracts a lot of the usefulness of the slight improvement in resolution.
|Nikon 55 mm micro||Mag||MTF50 (lp/ph)||Sharp pixel size|
|No close-up lens||0.52:1||1332||22.5 µm|
|Expensive +4||0.74:1||1368||15.7 µm|
|Mid-price + 4||0.75:1||1186||17.7 µm|
|Cheap + 4||0.76:1||742||27.8 µm|
My advice with close-up lenses is to spend a little money on one and avoid the bargain basement ones.
Using a close-up lens:
The Canon 250D (+4 diopters) close-up lens is made for shorter focal length lenses. I find that this lens works extremely well with my short focal length lenses. Less powerful lenses, such as the Canon 500D (+2D), will work better with longer focal length macro lenses. The 250D works marginally for my 105 mm lens and not well at all for my 200 mm lens. It will generally provide about a 0.1 to 0.5 increase in the magnification factor, depending upon the lens and the magnification you are working at. This relatively mild increase makes it more effective at lower magnification ratios.
Close-up lenses will give a little additional magnification in the image but will also significantly drop the working distance to achieve that improvement. The working distance is the distance from the front of the lens to the object being imaged.
The far focus of the lens will be impaired by the close-up lens. The lens will only focus to the focal length of the close-up lens when the focus is set to infinity. The far focus limit for any lens with a +4d lens (250 mm focal length) on it will be 250 mm. The near focus limit will be significantly closer, and with it, increased image magnification.
Close-up lenses do not work as well for many point and shoot cameras because of the loss of working distance. Some point and shoot cameras can shoot fairly high magnification images but they have to shoot these images from extremely close to the subject. If you are already close to the subject, you can’t afford to lose any more working distance (and still get good lighting).
Close-up lenses are also most useful at low magnification. Adding a +4D lens to a 105mm macro lens will allow it to go from a maximum of 0.5:1 up to 1:1. That's a doubling of the magnification. Adding the same +4D lens to a 10mm lens working at 10:1 will increase the magnification from 10:1 to 10.5:1. The increase in magnification is about the same, but the percent increase in magnification is only about 5% compared to about 100% in the first case.
Reversing an close-up macro lens
When using a close-up lens it is sometimes advantageous to use the lens reversed – mounted backward. This is a similar technique to coupled lenses as discussed in chapter 15. To reverse mount a close-up lens you will need a macro coupler. This is a ring with male filter threads on both sides. One side of the ring screws into the lens and one into the front of the close-up lens. This is mainly of use when you want to use a higher powered lens with a longer focal length macro lens.
If I put my Canon 250D +4 close-up lens onto my 105 mm macro lens I find it difficult to focus the image. The image through the viewfinder is very fuzzy (lens at f/4, wide open for focusing). The images taken with this combination are considerably better when the lens is stopped down to f/8 and are quite acceptable for sharpness. The problem is mainly that it is difficult to focus properly. I have found that using the DOF preview button on the camera body is helpful to getting a good focus setting in this situation.
Using that same close-up lens reversed makes a huge difference in the clarity of the image when the aperture is wide open for focusing. Of note, there is an increase in peripheral lens aberrations with the center of the field sharp and the edges a little fuzzier. This is most evident when focusing and does improve somewhat when the lens is stopped down to f/8 – although still worse than when the close-up lens is mounted normally.
|Nikon 105 mm micro||Normal mount (MTF50)||Reverse mount (MTF50)|
When the images are analyzed further, the sharpness in the center is a little better when the lens is reversed rather than non-reversed. The end result is that you get a better image in the viewfinder with the lens reversed and slightly improved sharpness in the center of the field. The payback comes in the slightly decreased sharpness on the periphery of the image.