A reversed lens is a good way to get more magnification (above 1:1) using standard lenses. A reversed lens is just as it sounds, the lens is mounted to the camera backwards. Now you might ask: Why should I mount a lens backwards? A 28mm lens is still a 28mm lens mounted forward or backward. That is true, but most SLR lenses will work significantly better at magnifications above 1:1 when reversed.
Strengths:
- Inexpensive if you already have a short focal length lens, just need a reversing ring.
- Lens can be from virtually any manufacturer.
Weaknesses:
- Lose automatic aperture control.
- Will generally need extension tubes or a bellows to vary the magnification.
A reverse mounted Nikon 55 mm micro lens.
The Proper lens orientation
SLR lenses are made to work best (less image aberrations) with a long distance from the object to the front of the lens and a short distance from the back of the lens to the detector. When shooting images at magnifications less than 1:1 this condition is met. When shooting images above 1:1 magnification, the distances reverse and the object distance is less than the image distance. To keep the proper lens orientation at magnifications greater than 1:1, the front of the lens should point at the detector and the back at the object. Therefore, a typical lens should be reversed to work at its best above 1:1. This will minimize lens aberrations and improve the high-magnification performance of most lenses.
The top image shows how dSLR lenses are made to work, long distance on the object side and shorter on the image side. To keep that lens orientation correct for the best image quality the lens should be reversed when used above 1:1 magnification (bottom image).
What lenses work for reversing?
You can use almost any lens reverse mounted although it will need to have filter threads to attach to a reversing ring. Since the lens is attached to the camera by the filter threads, most any brand can be used and it does not even have to be a macro lens. If you have any old wide angle lens that has a filter thread, you should be able to use it as a reversed lens.
Fitting a reversed lens
To reverse mount a lens you will need a reversing ring. These rings have a standard lens mount on one side that fits onto your specific camera body and a thread on the other side that will fit onto the filter threads of the lens. You will need to make sure to get one that is made for your camera's specific lens mount. Reversing rings are inexpensive and can be purchased for less than $15.
A Nikon br-2a reversing ring.
Reversing rings are not all equal as the quality varies. My Nikon BR-2a ring fits more snugly on my camera/bellows than a generic brand ring that I have. A snug fit is a good thing as it will allow less motion of the lens at the time of imaging. You should avoid the older Nikon BR-2 ring as it can damage the electrical contacts on digital and autofocus film camera bodies.
| Common Reversing Ring Sizes | |
| Lens thread size | Camera mount* |
| 49 mm | 1,3,4,6,7,8,9 |
| 52 mm | 1,2,3,4,5,6,7,8,9 |
| 55 mm | 1,2,3,4,5,6,7,8,9 |
| 58 mm | 1,3,5,8,9 |
| 62 mm | 1,3,5,8 |
| 67 mm | 1,3,5,6,8 |
| 72 mm | 1,3,5,8 |
| 77 mm | 1,3,5,8 |
*1: Nikon F, 2: Canon FD, 3: Canon EF (current), 4: Olympus OM, 5: Olympus 4/3, 6: Minolta MA/Sony (current), 7: Minolta MD, 8: Pentax K (current), 9: Pentax M42
The only other accessory that you may need is an adapter ring for the specific filter size of your lens. Most reversing rings come in standard filter sizes such as 52 mm and 55 mm, but they can be found in a wide variety of sizes. Most lenses come with filter sizes anywhere from 49 mm to 77 mm or more. If the lens filter threads are a larger diameter than the ring you will need a step-down ring. Conversely, if the threads on your lens are a smaller diameter than the ring you will need a step-up ring. These adapters will allow the two different filter sizes to connect securely. Step-up and step-down rings can be purchased for a few dollars on eBay and come in almost every variation of size that you might need.
If you can't find a specific size step-down ring that you need, you can use more than one to bridge the gap. Let's say that your lens has a 62 mm filter thread and your reversing ring has a 52 mm filter thread. You can generally find a 62-52 mm step-down ring that will allow them to connect. If you can't find this ring you could get a 62-55 mm step-down ring and a 55-52 mm step-down ring to do the job.
Since a reversed lens is mounted backwards, the normal camera/lens connection is lost. This means that the aperture of the lens will need to be operated manually. This can create problems when trying to use modern autofocus lenses in reversed format. Many new lenses (Nikon G, Canon EF for example) do not have an aperture ring and control the aperture from the camera body through a small metal slider on the base of the lens or entirely electronic in the case of Canon. This slider is spring loaded and commonly defaults to a closed aperture.
Using a lens without an aperture ring
I have a Nikon autofocus lens that does not include an aperture ring. The aperture on this lens defaults to a closed position when not connected to the camera. A fully closed aperture is not good for shooting images or for focusing. Thankfully, there is a ring out there made by Nikon that accomplishes this task on Nikon mount lenses. Canon EF lens apertures can't be controlled mechanically. Novoflex makes an automatic aperture control for reversed Canon lenses. This ring does provide full automatic aperture control, but is quite pricy to purchase. I have also heard that the aperture on Canon EF lenses can be set manually prior to reverse mounting. This can be accomplished by mounting the lens to the body in the usual fashion, setting the camera to Av mode and setting the aperture to the desired position on the camera body. If you push down the depth of field preview button while simultaneously removing the lens from the body, the aperture setting will be retained on the lens when removed. I have never tried this technique myself but have heard that it works well.
A Nikon br-6 aperture control ring.
The Nikon BR-6 ring attaches to the end of a reversed lens and allows aperture control manually or with a cable remote. This ring will keep the aperture open for focusing but is really made to work with a lens that has an aperture ring. Once the focus is set, with either a push of a button or a push of the cable remote the aperture will be closed down to the aperture setting that is preset on the aperture ring. This ring can be adapted for use with a newer lens that doesn't have an aperture ring by only pushing the cable release part-way. This will allow you to set a middle aperture at the time of shooting. Getting the exact aperture you want may be difficult, but you can get reasonably close.
I have found that using a cable release with the BR-6 ring is helpful. Cable releases come in several different configurations and only one of them will fit the BR-6 ring. When you read the BR-6 manual it suggests a couple of remotes that work well with it. These will work but they are double remotes. These were made for older film cameras - one cable sets the aperture and one pushes the shutter release. The shutter release isn't necessary (or even mechanical anymore) for any newer cameras. Remotes on most digital SLR's are now either electrical or infrared. I did find that the Nikon AR-3 remote works well with this ring and is only a single cable. The AR-3 remote has a locking feature so that if you fully or partly push the plunger, it will stay where you push it to. This allows you to set the aperture and then shoot the image.
A Nikon ar-3 remote cable
To work the BR-6/AR-3 combination with a modern lens without an aperture ring you just push the plunger until you get to a mid aperture and leave it at this position. This turns out to be about half way for my 18-70 zoom. This combination allows the 18-70 zoom to be easily used in a reverse configuration. The nice part about using a zoom lens is that the magnification is variable. When reverse mounted directly onto the camera, the 18-70 can produce magnifications from about 1.6:1 to 4.5:1 without any additional help. With a couple of 25 mm extension tubes the magnification range can extend all the way to about 7.4:1 (50 mm extension + lens at 18 mm and close focus setting).
I was pleasantly surprised when I realized that I could cover almost all of the range from 1:2 to 7:1 with an 18-70 mm zoom lens and a few accessories. The picture quality is generally quite acceptable.
Why does my short focal length SLR lens produce more magnification when reversed?
Short focal length SLR lenses work very well for high magnification imaging when reversed because they produce more magnification and provide better working distance than when they are mounted normally. They provide these extra benefits because of their design.
The principal planes of a retrofocus lens are typically behind the lens when mounted normally (top image). This situation is reversed when the lens is reversed. When reversed, the principal planes are now in front of the lens (bottom image). The rear principal plane (P') is now farther from the detector than it was when mounted normally. The extra distance means that the lens is working with more extension. More extension means more magnification, often significantly more. The shorter the focal length of the lens, the more extra magnification it produces when reversed.
The second effect of having the principal planes in the front of the lens is that there will be more working distance at all magnifications. Imaging from 1:1 and up will occur with object distances from 1-2 focal lengths from the front principal plane (P). Since the front principal plane is now in front of the lens, the working distance can be more than 1-2 focal lengths in front of the lens. With the same lens mounted normally, that same area of 1-2 focal lengths in front of the lens will be partly or totally inside the lens.
The amount of additional magnification that reversed lens produces varies by the focal length of the lens being reversed. When the previously mentioned 28 mm lens is mounted in reverse on the SLR it produces a minimum magnification of about 2:1. My 55 mm macro lens produces slightly more magnification when it is used reversed as opposed to normal mounting. My 105 mm lens produce significantly less magnification when used reversed. This is because longer focal length lenses tend to have a more telephoto construction, the principal planes are relatively in the front of the lens when normally mounted. This is the opposite of short focal length lenses.
| Lens | Mag normal mount | Mag reverse mount |
| 28 mm | 0.25 : 1 | 2.1 : 1 |
| 55 mm | 0.5 : 1 | 0.9 : 1 |
| 105 mm | 0.5 : 1 | Won't focus without extra extension |
Shooting Images with reversed lenses
Reversed lenses can be used in a variety of ways. They can be mounted directly onto the camera body, mounted with an extension tube or bellows and even mounted with a teleconverter. They can be used in almost any way that a normal lens would be used as long as you have the necessary adapters.
Shooting images with a reversed lens is similar to using any other lens except that the aperture is manual and the focus may or may not work. Some camera bodies will properly expose the image in all shooting modes (more expensive Nikon dSLR's) and some will have difficulty in all but manual (M) mode when shooting with a reversed lens (less expensive Nikon dSLR's). Focusing with the focus ring has a variable effect. If the lens focuses primarily with extension (lens get longer as the focus is made closer), the focus will be best set by adjusting the working distance (moving the object or camera). Adjusting the focus ring on these lenses won't significantly affect the focus and will just tend to decrease the available working distance. This is because the end of the lens extends toward the coin while the optics are relatively stationary. If the camera has internal focusing (doesn't lengthen with focusing) it should be able to focus normally when reversed.
Since the aperture is manual you will need to make sure that the aperture is wide open or close to wide open for focusing. That means setting it to its lowest number manually or by using an aperture control ring such as the Nikon BR-6. Prior to shooting the aperture will need to be closed down to the appropriate setting. I prefer f4-f8 which is where most lenses are at their sharpest. This can be done by turning the aperture ring to the number you wish or by using the aperture control ring with or without a cable release.
Below is a table showing the magnification that can be expected with reverse mounted lenses with extension, either from an extension tube or bellows. Shorter lenses than a 28 mm can be used such as a 20 or 24 mm. The extremely wide angle lenses below 20 mm commonly don't have filter rings and have protruding front elements that won't work in a reversed mounting.
| Lens | No extension | 25 mm (extension tube) | 50 mm (bellows) | 90 mm (bellows) | 130 mm (bellows) |
| 28 mm | 2.1:1 | 3.1:1 | 3.9:1 | 5.2:1 | 6.8:1 |
| 55 mm | 0.9:1 | 1.4:1 | 1.8:1 | 2.5:1 | 3.3:1 |
Image from a reversed 55 mm micro lens, mounted directly to the camera (0.9:1 magnification)