Extension Tube Question

Does anyone know how to calculate the max/min focus distances when using extension tubes? I'm having trouble figuring out what combination of tubes would yield the best results. I'm trying to shoot a hummingbird from about 10feet with a 1.3 crop body and a 500.

I assume that you at using a Canon 500/4, given the 1.3x crop ?
That being the case the Canon 12 mm extension tube will get you down to 3.85 M, and the 25 mm extension tube will get you down to 3.39 M. This is detailed in the instruction manual here http://gdlp01.c-wss.com/gds/0/030000347 ... usm-en.pdf

http://www.cambridgeincolour.com/tutori ... loseup.htm

has a minimum distance calculator in this article. See if your lens data sheet has native magnification data. If not then use one of these methods from here:

http://www.pentaxforums.com/forums/phot ... lator.html

and I quote from Newarts response;

"1) Lens specifications will often state magnification (see mfg's data on-line.)

2) Take a photo of a ruler and figure out the magnification:
Magnification = (Millimeters shown on photo)/(sensor width)

Sensor width = 23.6mm for K-x

3) Use the manufacturer's closest focus distance (distance from sensor to subject) - on the lens barrel or the smallest distance on the focusing ring. You can estimate magnification, M, using closest focus distance, D, and focal length, F:

M ~ (D/2F-1) - SquareRoot((D/2F-1)^2 -1)

Example: a 28mm lens has a closest focus distance of 5.5" (140mm) so D/2F = 2.5
M ~ (2.5-1) - SquareRoot( (2.5-1)^2 -1) =0.38X

Dave" (end quote)

I used the Canon EF 500mmL f4 as a test which has a closest distance of 3.7 meters. From the formula D/2F = 3.7/(2x.5) = 3.7
(D/2F - 1) = 3.7 - 1 = 2.7
(D/2F-1)^2 - 1 = 7.29 -1 = 6.29
SqRoot 6.29 approx 2.5
Filling in formula components results 2.7-2.5 = 0.2. This is the FF magnification. Your 1.3 crop factor changes the native magnification to .26.

Plugging in to the calculator: NM=.26 F=500mm

For a 12mm tube I got minimum distance of 2.9 meters (9.5 feet)
25mm I got 2.76 meters (9 feet)
36mm I got 2.67 meters (8.8 feet)

I hope I did it correct. It is close to the Nikon data below.

For this Nikon Lens http://www.nikonusa.com/Nikon-Products/ ... -TechSpecs I use .14 x 1.3=.18 for the native magnification and the results:

12mm tube = 11.6 feet
25mm tube = 10.8 feet
36mm tube = 10.2 feet

If you use the manufacturer's data and it is based on a full frame sensor then multiply the native magnification by your crop multiplier.

Now, you'll notice not much difference in the minimum distance for your 500. That is because the extension tube is a low difference in the ratio of the tube length to focal length.

The spec minimum distance is 12.4 feet on the Nikon. In other words on this long 500mm focal length you do not get much difference, less than 2 feet.

Don't forget you lose f/stops a bit but for this small difference I don't think you'd have much difference.

For both of these telephoto lenses filters are drop in so unless they have a huge close-up filter that does not apply but on that same page I linked you'll see a calculator for close-up diopter front lens mounting results that are more effective.

For baited hummers at a feeder I could get pretty close with slow moves. Once I had a red cap on and the hummer investigated it.

If your lens is an f/4 you might get better results with a 1.4x teleconverter. and less FL or same FL and an increase in minimum distance.

Edit: I see in the time I tracked down the calculations a .pdf was found for the Canon lens. However since Nikon gave the native magnification direct that section should be correct.

Thanks, that is exactly what I was looking for. I'm using a Canon 1D Mark IV with a 500/4.

I should have divided the FF NM, not multiplied it, by the CF. Then it matched. Oh, well.
Does the extension tube table take into consideration the crop factor? Is 1.3 the the crop factor of a full frame camera? In the lens spec section it states the maximum magnification is .12x for the 500. If I plug .12x1.3 = .156 into that calculator for 12 mm tube I come up with 3960mm minimum distance against the manual's 3856. Close enough for me.

Anyway I have the 300 f/4L and I found the table in that manual. So thanks for the similar link.

I don't believe crop factor affects max magnification, or minimum focusing distance. Happy to be corrected if I've got any of this wrong though.

Minimum focusing distance is an optical function of the lens, and how close you can focus and project an image to the sensor plane (which remains where it is regardless of crop factor), the crop factor only relates to how much of the image circle is sampled, think of it as a mask overlaid over a full size sensor.

The maximum magnification relates to the size of image projected onto the sensor plane, ie at 0.15x a 1" object will be projected at 0.15" on the sensor plane, and in the old days would have taken up 0.15" on the slide.
Now with digital we don't have the same fixed references at output, in that we have different crop factors & different pixel densities so it is harder to visualise what this magnification figure means on the monitor, but our 1" object will still project at 0.15" on the sensor plane, and use 0.15" worth of pixels, however many that may be on that particular sensor. The crop factor will simply vary the amount of background around the object.

Technically you are correct Colin. What crop factor does do is affect angle of view, not magnification. And of course crop factor can not affect min focus distance. But by adding an extension tube and physically moving the lens elements farther from the sensor plane, the minimum focus distance and magnification are changed.

I don't mean to hijack the thread but do extension tubes reduce the light hitting the sensor? Since there isn't any glass being added I didn't think it did, but an article in Outdoor Photographer suggested that they do (this was brought up in the context of teleconverters and although I don't have the issue in front of me, if memory serves me correctly, the article states that extension tubes reduce light much as TCs do).

Since you are effectively making the lens longer but not increasing the size of the front element, the effective aperture is reduced. For very long lenses like super telephotos, this is almost negligible but for a short lens it can be significant. The formula is pretty easy:
New aperture = Old aperture *(lens focal length + Extension)/lens focal length

So if you add a 25mm tube to a 500 f/4 you get 4*(525/500) = f/4.2 or about 1/6 of a stop loss
But if you add that same 25mm tube to a 50mm f/1.4 lens you get 1.4*(75/50) = f/2.1 or more than a stop loss

It's a bit different with teleconverters since you are changing the optical formula resuting in a much longer focal length, yet the front element isn't scaling up to compensate. Aperture is basically the focal length divided by the diameter of the front element. So on a 500mm f/4 lens, the front element is 125mm in diameter but now you add a 1.4x making it a 700mm lens yet the front element is still 125mm. 700/125 = 5.6!

Martin 095 wrote:I don't mean to hijack the thread but do extension tubes reduce the light hitting the sensor? ...

An easy way to remember how this works is that light intensity falls off with the square of distance. If the distance from lens to sensor is increased by 40% then the area of the incident area has doubled (1.4 X 1.4 = 2) and so the incident light intensity decreases by a factor of 2, or 1 f-stop.

Thanks for the explanation guys – had I thought about it enough, I would have answered my own question. I should have known this.

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