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by kiwijohn on Fri Nov 10, 2017 1:55 pm
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Hi everyone,
I visited one of our local offshore bird reserves today with my 600mm telephoto to capture some images of our smallest NZ bird - the Rifleman (weighing in at a mere 6.0 grams).
I used a single 36mm extension tube with the lens to see what benefit could be gained (mainly in terms of shortening its minimum focus distance).
The rifleman bird I had in mind tends to use a favourite perching twig in the forest and is thus very predictable in terms of location and distance from the path. The perch is well inside the normal focus range of the 600mm, but I wanted to test the effect of adding the 36mm ext. tube on lens performance.
 
I noted the following details:
 
Infinity focus is now approx. at 50 ft (15m)
 
Closest focus is now at approx. at 10 ft (3m) instead of 16.4 ft (5m)
 
E.J and Dick Ludwig are right about minimal light loss, as the camera display tells me the lens still goes as wide as f4 so no change there. (Have you noticed how an f2.8 macro suddenly becomes an f5.6 max aperture when you add extension tubes?)
 
Chris Ross is right, photographing the same branch with and without the 36mm extension tube reveals a slight magnification (measured on the computer screen) of 110.5% with the extension tube as might be expected. (So a 600mm lens becomes 662mm)
 
I have heavily cropped the photo attached, but did not see any bad vignetting on the original RAW file (although the forest was pretty dark on this occasion making it hard to tell). Exposure was f8 at 1/60 ISO 200, with TTL BL flash set at -3 stops underexposure, later recovered in ACR.
 
Interestingly, the combination seems to be spot on focus wise (see photo to see if you agree)
Previously, without the ext. tube, focus had been pretty much spot on/very slightly front focus (with the original fine adjustments for this camera body).
 
I have two fine adjustment settings stored on this D810 camera body (one for the 600mm alone and a second for the 600 plus a 1.7x TC).
A suggestion was made by E.J. to store a third (600 + Ex. Tube).
 
Question – If I do, how does the camera body KNOW whether it is the 600mm alone or the 600 + ext. tube? As the Kenko ext. tubes do not contain any “intelligent chips” announcing its presence to the camera (presumably)?
 
Taking Joe Smith’s advice, I resisted the temptation to go the whole 9 yards and slap the full set of tubes on, as the camera/flash combo weighs nearly 2kg and I had visions of the Kenko tubes breaking and sending the body/flash combo crashing earthwards.
 
Hope this report isn’t too wordy and you find it interesting!
 
John Sibley

Image
 

by E.J. Peiker on Fri Nov 10, 2017 3:19 pm
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The camera will always say the aperture of the lens even if you add a thousand mm of extension as the electronics are just a straight pass through with no recalculation.  The meter however will read a slightly different exposure but again, on a 600mm you would need to add a ton of extension before there is any light loss that actually would require a shutter speed change.

I didn't say to "store" a third value, I said to determine a third value ;)  The camera doesn't know the difference between the 600 and 600 + extension tube so you would manually have to change the AFFT value in the camera when you add a tube so you need to just know what that value is.  A TC on the other hand has electronics in it that tell the camera that it is now a different lens so it can store that as a separate value.  Since the extension tube has no real electronics in it, just wires that translate the lens info through the tube to the camera body it doe not see that as a different optic.
 

by kiwijohn on Fri Nov 10, 2017 5:08 pm
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Thanks E.J.
I will have to do some research on how to manually recall an AFFT setting when I use the Ext. tube.
Normally when I change from my 300mm tele to the 600mm tele the camera does the different AFFT adjustment for me automatically without me having to do anything.
 

by E.J. Peiker on Fri Nov 10, 2017 5:44 pm
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You just need to know the number, mount the lens and extension tube, go into the Auto Focus Fine Tune menu item and change the number. Then change it back when you take off the extension tube and remount the lens without it and change the value back. The lens has to be mounted to change the value. If you are lucky, the number will be the same or close enough with and without the extension tube that this won't be necessary.
 

by kiwijohn on Sat Nov 11, 2017 8:40 pm
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Thats great. Thank you. I think I'm am in luck, because the addition of the ext. tube has not visibly affected the focus calibration of the lens.
Most of my bird photography in NZ is in the 6' to 50' range mostly, due to the extraordinary behaviour of our native birds.
 

by Doug on Wed Nov 15, 2017 1:39 am
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kiwijohn wrote:
Chris Ross is right, photographing the same branch with and without the 36mm extension tube reveals a slight magnification (measured on the computer screen) of 110.5% with the extension tube as might be expected. (So a 600mm lens becomes 662mm)


Technically the 600mm lens doesn't become a 662mm lens when using an extension tube. The difference in magnification is real but it's because close focus using internal focus changes the optical configuration to something less than 600mm; the closer the lens is focused the shorter the effective focal length becomes.  In other words it's not the extension tube making the focal length longer as much as it is the IF without the extension tube making the focal length shorter.
Doug Herr
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http://www.wildlightphoto.com
 

by kiwijohn on Fri Nov 17, 2017 2:47 pm
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Doug wrote:
kiwijohn wrote:
Chris Ross is right, photographing the same branch with and without the 36mm extension tube reveals a slight magnification (measured on the computer screen) of 110.5% with the extension tube as might be expected. (So a 600mm lens becomes 662mm)


Technically the 600mm lens doesn't become a 662mm lens when using an extension tube. The difference in magnification is real but it's because close focus using internal focus changes the optical configuration to something less than 600mm; the closer the lens is focused the shorter the effective focal length becomes.  In other words it's not the extension tube making the focal length longer as much as it is the IF without the extension tube making the focal length shorter.


Thanks Doug,

Thats a good point - I hadn't anicipated a focus breathing issue with the 600mm lens on close focus settings.

John S
 

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