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Thread started 05 Apr 2011 (Tuesday) 13:00
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Can an extension tube push me over the 5.6 AF limit?

 
JamieWilson
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Apr 05, 2011 13:00 |  #1

Hi. Newbie here, and probably a dumb newbie question...
I know that my cheap body (450D) will disable AF for lenses smaller than 5.6 and I also know that if I take a lens over 5.6 via a teleconverter, the same applies, short of tape tricks, but I don't hear about anyone needing tape tricks with extension tubes. Yet, in theory, I know that extension tubes do indeed also increase the effective F number. There will be some light density loss (spread out over a longer internal projection) and no doubt that does AF no favors. My question though is, will the camera see this effective F increase and disable AF the way it would with a teleconverter? (Obviously there'll be no AF if it's a dumb tube, but I'm assuming a smart "auto" tube here.) For instance, if I put a 36 mm smart tube behind a 5.6 lens and the effective maximum aperture becomes say 6.2, will the camera disable AF just as it would if I had put a teleconverter behind a 5.6? Thanks in advance.




  
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DreDaze
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Apr 05, 2011 13:09 |  #2

no, you can still AF fine...in some cases of extreme macro though, MF is easier anyways though


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JamieWilson
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Apr 05, 2011 13:12 |  #3

Just trying to get a tad closer with a long lens actually. Thanks very much DreDaze.




  
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Fiddlefingers
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Apr 05, 2011 13:27 |  #4

Never had a problem with the 400mm f5.6, even with 68mm of extension tubes attached.


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JamieWilson
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Apr 05, 2011 13:56 |  #5

That's exactly what I hoped someone would tell me. Thanks Fiddlefingers.




  
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Alex_Venom
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Apr 05, 2011 14:31 |  #6

400 f/5.6L with Kenko tubes here. Works like a charm.
You lose infinity focus, though...


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phigment
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Apr 05, 2011 14:36 as a reply to  @ Alex_Venom's post |  #7

I know my 'smart' tubes are merely extending the contact pins. No information going to the camera is modified like a TC would (assuming you have a smart TC).

The camera wouldn't know to disable the AF. Focus may not be as accurate though.


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gasrocks
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Apr 05, 2011 16:15 |  #8

Let's see -you have a long lens with a range limit switch. you add some tubes and select 60 feet to infinity. Wonder how AF will do.


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DreDaze
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Apr 05, 2011 16:20 |  #9

gasrocks wrote in post #12164526 (external link)
Let's see -you have a long lens with a range limit switch. you add some tubes and select 60 feet to infinity. Wonder how AF will do.

but you're putting the tubes on to take a picture inside of the MFD...even the 1200mm has a MFD of 45ft...so why are you setting the switch to 60ft to infinity? :)


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big_g
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Apr 05, 2011 16:45 as a reply to  @ DreDaze's post |  #10

Why would the extension tubes change the f number??

Multipliers change the focal length of the lens so they alter the maximum f number as it is a ratio of the aperture to the focal length. An extension tube only moves the lens away from the sensor so it changes the MFD and hence allows larger magnification at the expense of infinity focus


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gasrocks
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Apr 05, 2011 16:49 |  #11

He was probably thinking about how tubes darken the image.

DreDaze - good example. We all shoot with a 1200. (I'm ribbing you.)


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DreDaze
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Apr 05, 2011 16:51 |  #12

gasrocks wrote in post #12164733 (external link)
DreDaze - good example. We all shoot with a 1200. (I'm ribbing you.)

you don't dream when you sleep?


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gasrocks
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Apr 05, 2011 16:52 |  #13

Not for the 1200 - now for the 800/5.6 IS, maybe.


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1Tanker
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Apr 05, 2011 17:01 |  #14

big_g wrote in post #12164700 (external link)
Why would the extension tubes change the f number??

Multipliers change the focal length of the lens so they alter the maximum f number as it is a ratio of the aperture to the focal length. An extension tube only moves the lens away from the sensor so it changes the MFD and hence allows larger magnification at the expense of infinity focus

Yeah.. ETs don't give a longer FL.. only the glass in a TC does. The FL is the distance from front element to farthest rear element.. which changes to the back end of a TC, when used. Tubes do no such thing.
*more technical than that, but the point is valid*


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big_g
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Apr 05, 2011 17:36 |  #15

1Tanker wrote in post #12164800 (external link)
Yeah.. ETs don't give a longer FL.. only the glass in a TC does. The FL is the distance from front element to farthest rear element.. which changes to the back end of a TC, when used. Tubes do no such thing.
*more technical than that, but the point is valid*

The focal length isn't the distance from the front element to the farthest rear element. Sorry to be a pedant but if that were the case all 500mm lenses would be 500mm long. No point in going into too much silly argument on this as my 500 f4 is 387mm long and the 800mm 5.6 (which I wish I had) is 461mm long. The 1.4 converter only adds approx 20mm to the lens length not the 200mm that would be needed to extend the distance from the front lens of a 500mm to the back element to 700mm

Anyway, I still can't see why a tube would change the effective f of a lens. I am happy to be told I am wrong if someone can explain why. Its the only way I can learn :D


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Can an extension tube push me over the 5.6 AF limit?
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