Extension tubes

I have a Nikon 500mm F/4 P that will focus down to 20 feet. If I want to shoot a subject at 10 or 15 feet, is there a formula for determining the size extension tube I will need?

There is a formula, but I don't know it, unfortunately.

 

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In practice, though, with my 600/4, which also focuses down to about 20 ft (6 meters), I find a 25mm (1 inch) extension tube is adequate 95% of the time. I somewhat recently bought a 12mm tube to add to the 25 (giving me 37mm, i.e. 1.5 inches) when I want to focus closer. This is usually only true with the smallest and friendliest songbirds or shorebirds.

 

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I think you'll find yourself often using a 1.4x extender plus an inch or so extension to shoot songbirds. Getting a bit closer than 20 feet from one is usually pretty simple, but getting 10 feet from one often isn't, so 500 or 600 still leaves you just a bit short in many cases.

 

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I'm a Canon dude, but I believe Dan Smith uses a PK13 with his 600/4. If that's not the right designation perhaps he or another Nikon user can help you.

Yes there is.

 

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You can use the lens equation which is

 

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<b>(1/f) = (1/v) + (1/u)</b>

 

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where <b>f</b> is the focal length,<b> v</b> is the distance from the lens to the subject and <b> u</b> is the distance from the lens to the film to the film.

To make life easy assume a simple (single element) lens. Use the

same units for all these variables (typically you would work in mm).

 

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At 20ft (6096mm), you calculate that the distance to the film

for a 500mm lens will be 544.6mm. If you

add a 25mm extension tube, it will become 569.7mm. Putting this

value back into the formula, you will calculate

that the focus is now at 4088mm or 13.4ft. You can also

calculate the maximum focus distance. Normally this would

be infinity, when the distance to the film would be 500mm.

If you have a 25mm extension, this becomes 525mm, which when

you work through the formula gives you a maximum focus distance

of 34.5 ft.

 

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In reality the situation is complicated by the fact that camera

lenses are not simple lenses and you have to define the "position"

of the lens via nodal points etc. However,

the simple lens approximation is good enough for practical use.

Only one way to be sure. Put a PK extension tube on your lens & try it. I use PK23,s on the 600 and have used 2 on occasion. With one it works well & with 2 is does so also. With one I can fill the frame with a marsh wren so your 500 should be able to focus at 15 feet. But, go to the camera store, try one and use a tape measure to see just how close it works. Also try a small carved bird or mouse, set it up & see just how big it is in the viewfinder. That will give you real world sizing & is always (for me) preferable to charts & graphs.

Also, if you try this at a camera store or with a friends extension tube you can shoot a frame or two so you will have a good reference as to just how much you really get on the slide. Good luck.

First, the good news is that I believe that the 500/4P focuses to 16 feet, not 20. The confusion comes from the fact that the numeral "20" is the smallest one engraved into the focusing collar. However, the collar turns a bit past the 20. I think even the little pamphlet that comes with the lens says 20...but try it! Four feet is one heck of a difference!

 

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I'll skip the answers about the tubes because there are many good ones already here. My favorite way to use this lens is to put a PK11a (8mm) tube on it (yes, it's tiny, but for a reason) and then attach a TC14b. The short tube between the converter and the lens, plus the 1.4x factor, allows good sized images of closer small subjects plus enough far focusing distance to get subjects a bit further away without fiddling around detaching the tube.

 

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As I recall, the working range seemed to go out to 50 yards or so (if you'd like, I can make a more definitive test.) You can't get that kind of far-focusing distance with a larger tube like a PK13 or a PN11, plus you still get a good image size up close as if you had a longer tube (advantaging the property of the converter that focusing distances do not change.)