Extension Tubes and Long Lenses

by Kenneth R. McVay

Photographing small birds with long lenses can be one of the most challenging aspects of wildlife photography. Most photographers would like to fill the frame with the small bird however, doing so is impossible in many instances depending on the minimum focal distance (MFD) of the lens and the physical size of the bird. Lenses commonly used for photographing small birds are the 600mm big guns. These lenses have a MFD of about 20 feet and many times because of this limitation you cannot get a frame filling image of the little bird. The solution to the problem is to use extension tubes to decrease the MFD of the lens so that a closer approach and thus frame filling images of the subject.

The first thing you need to know when adding an extension tube to a long lens is the new MFD. This can be found easily by using an elementary form of the lens makers equation.

(n-1)(1/r1-1/r2) = 1/s + 1/s1

this can be simplified to

1/f = 1/s + 1/s1

Where

f is the lens focal length

s is the subject to lens distance

s1 is the lens to film plane distance

n is the index of refraction

r1 and r2 are the radius of curvature of the lens

Since I use Nikon equipment, The examples I present are for Nikon extension tubes and lenses. However, these examples can be carried over to any other brand of equipment. The following is a list of Nikon long lens MFD and extension tube lengths.

Selected lenses and extension tubes for examples.

Lens	MFD (ft)	MFD (meters)		Extension tube	Extension mm
Nikon 600mm AFS f4.0	20	6.096		PK-11	8mm
Nikon 500mm AFS f4.0	20	6.096		PK-12	14mm
Nikon 400mm AFS f2.8	11.4	3.47		PK-13	27.5mm
Nikon 300mm AFS f2.8	7.5	2.29
Nikon 300mm AFS f4.0	4.8	1.46
Nikon Sigma 800mm HSM f5.6	23	7.0

Taking the 600mm with an MFD of 6.1 meters as an example and working in mm we have

1/600 = 1/6096 + 1/s1

solving for s1 we have

s1 = 665.5

Now let us add a PK-13 with an extension of 27.5mm to the lens we now have a new lens to film plane distance

s1 = 665.5 + 27.5 = 693.0mm

We can now substitute this value for s1 into the equation and solve for s which will be our new MFD for this particular lens/extension tube combination.

1/600 = 1/s + 1/693.0

yields s = MFD = 4470.97mm or 4.47 meters with a feet equivalent of 14.67 feet.

With this lens and a PK-13 we can now get a whopping 5 feet closer to our subject and this my friend will yield a much larger subject in the frame image.

New calculated MFD for Nikon extension tubes for the given lenses

Lens	MFD (ft)	MFD w/PK-11 (ft)	MFD w/PK-12 (ft)	MFD w/PK-13 (ft)
Nikon 600mm AFS f4.0	20	18.04	16.82	14.67
Nikon 500mm AFS f4.0	20	17.21	15.62	13.0
Nikon 400mm AFS f2.8	11.4	10.06	9.26	7.91
Nikon 300mm AFS f2.8	7.5	6.52	5.96	5.01
Nikon 300mm AFS f4.0	4.8	4.44	4.22	3.8
Nikon Sigma 800mm HSM f5.6	23	21.53	20.56	18.71

Although not shown in the table, you can easily stack extension tubes to further cut down the MFD. The calculations would be the same and you would add the total extension of the stacked tubes in the first part of the calculation.

Anytime you add an extension tube you need to compensate the exposure. Therefore you need to know the exact compensation values to add to the exposure or you will end up underexposing the image.When adding Nikon extension tubes to many bodies, it is common to loose metering and you have to resort to other techniques for metering. I have no idea why Nikon does not give us auto extension tubes although Kenko does. With Kenko tubes you do not have to compensate as your camera will meter correctly with the tube in place. However, when using Nikon tubes is when the knowing the compensation values come in handy. Exposure compensation can be calculated by using the formula

C = (log10(1+ET/F)^2)/log10(2)

where

C is the exposure compensation

ET is the extension tube length in mm

F is the focal length of the lens in mm

Thus as an example, if we added a PK-13 to a 600mm lens, the exposure compensation would be

C = (log10(1+27.5/600)^2)/log10(2) = .13 stops

This value is negligible since most cameras will only compensate in 1/3 stops. However, if you stack tubes or use other length lenses then the compensation values can be significant. Following is a table that gives the compensation values for the Nikon PK series of tubes and all the combinations of the three tubes. These values were calculated by using the compensation equation above.

Exposure compensation values for the given extension tubes and lens focal lengths.

Focal length	PK-11	PK-12	PK-13	PK-11+PK-12	PK-11+PK-13	PK-12+PK-13	PK-11+PK-12+PK-13
800mm	0.03	0.05	0.10	0.08	0.13	0.15	0.17
600mm	0.04	0.07	0.13	0.10	0.17	0.19	0.23
500mm	0.05	0.08	0.15	0.12	0.20	0.23	0.27
400mm	0.06	0.10	0.19	0.15	0.25	0.28	0.34
300mm	0.08	0.13	0.25	0.20	0.32	0.37	0.44

From the above table you can see that exposure compensation is negligible for long focal length lenses with a few exceptions. When stacking tubes, this brings the compensation values very close to 1/3 a stop and close to 1/2 a stop for the 300mm with all three tubes stacked. This 1/3 a stop may not seem like much but if using slide film or a digital slr, this 1/3 stop could make all the difference in the world since they have a narrow latitude and dynamic range.

Don't let your photos go down then tubes! Use extension tubes so that you can decrease the distance between you and your subject. As a result, you will get larger frame filling subjects with really nice backgrounds.