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Thread started 14 Mar 2011 (Monday) 19:27
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400L and 1.4x Extender on 7D?

 
noahcomet
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Mar 14, 2011 19:27 |  #1

I've read conflicting reports and am hoping to get some clarification---on a 7D, my 400 5.6L with the Canon 1.4x will not allow me to use autofocus, correct?


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scubthebub
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Mar 14, 2011 19:30 |  #2

noahcomet wrote in post #12020579 (external link)
I've read conflicting reports and am hoping to get some clarification---on a 7D, my 400 5.6L with the Canon 1.4x will not allow me to use autofocus, correct?

Correct, the non 1 series bodies can only auto focus upt o f/5.6 and the 1 series can only use center point focus at f/8.

With a 1.4x you lose 1 stop and you get a 560 f/8 manual lens on your 7D


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noahcomet
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Mar 14, 2011 19:33 |  #3

Thank you Matt!


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nikmar08
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Mar 14, 2011 19:42 |  #4

Second Matt on that.

Additional info for workaround involving taping the pins using either clear scotch tape or the black electrical:
http://www.michaelfurt​man.com/taping_the_pin​s.htm (external link)

Not everything remains hunky dory though with such workarounds, so suggest you go through this thread before taking a call:
https://photography-on-the.net …p=11838240&post​count=2707


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Mk1Racer
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Mar 14, 2011 20:00 |  #5

Read something in another thread where a 7D will AF in Live View w/ an f/8 min lens. So a 400 f/5.6 + 1.4x TC should AF on a 7D, but only in live view.


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scubthebub
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Mar 14, 2011 20:10 |  #6

Mk1Racer wrote in post #12020830 (external link)
Read something in another thread where a 7D will AF in Live View w/ an f/8 min lens. So a 400 f/5.6 + 1.4x TC should AF on a 7D, but only in live view.

Hmm.. it looks like that might work as long as you have time to use live view. From a different forum online:

"Its the difference between Contrast detection (live view) and phase detection (normal viewfinder AF). Apparently in phase detection the type of sensor matters whereas it does not in contrast detection. The downside to contrast detection is that is slower."

Contrast detection is what is used in point and shoot cameras. It will tend to focus hunt more in low contrast situation. This makes sense because the mirror and focusing screens flip up to expose the sensor for live view.

So I guess the correct answer is no if you want to look through the viewfinder, but yes if you have the time to use live view. Good to know.


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scubthebub
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Mar 14, 2011 20:14 |  #7

Also from Wikipedia:
http://en.wikipedia.or​g/wiki/Autofocus (external link)

Contrast measurement

"Contrast measurement is achieved by measuring contrast within a sensor field, through the lens. The intensity difference between adjacent pixels of the sensor naturally increases with correct image focus. The optical system can thereby be adjusted until the maximum contrast is detected. In this method, AF does not involve actual distance measurement at all and is generally slower than phase detection systems, especially when operating under dim light. Furthermore, as the autofocus system does not calculate whether the subject is in front focus or back focus, focus tracking is not feasible. As it does not use a separate sensor, however, contrast-detect autofocus can be more flexible (as it is implemented in software) and potentially more accurate. This is a common method in video cameras and consumer-level digital cameras that lack shutters and reflex mirrors. Most DSLRs use this method (or a hybrid of both contrast and phase detection autofocus) when focusing in their live-view modes. Mirrorless interchangeable-lens cameras, including Micro Four Thirds, exclusively use contrast measurement autofocus, and their manufacturers claim performance comparable to phase detection systems."

But remember as the effective f-stop increases the image gets darker and the image will lose contrast. So you may be able to get it to focus at f/8 in daylight, but not right after sunset.


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nikmar08
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Mar 14, 2011 20:27 |  #8

scubthebub wrote in post #12020912 (external link)
...Furthermore, as the autofocus system does not calculate whether the subject is in front focus or back focus, focus tracking is not feasible....

Will that mean AI Servo option goes out of the window - which pretty much throws a lot of wildlife/birding etc out of the window as well?


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scubthebub
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Mar 14, 2011 21:28 |  #9

nikmar08 wrote in post #12021005 (external link)
Will that mean AI Servo option goes out of the window - which pretty much throws a lot of wildlife/birding etc out of the window as well?

Correct, the contrast method changes focus back and forth until an area that that has light/dark next to each other have the maximum contrast (in focus and not blurry). Unless this was super quick (which it's not) there would be no chance for it to keep up with a moving object. Another online explination:

"It typically starts with the lens at the infinity position and moves step by step to the close end of the focusing range, gauging the contrast to see if it increases or decreases. As the contrast increases, the system knows it’s getting closer to an accurate focusing point. Once it passes this point and the contrast begins to decrease, the system has identified the peak of contrast and locks down focus at the peak. "

So yes the lens would focus in live view, but it has a small practical application unless you prefocus on a branch and a bird happens to land there. This is why we basically say it cannot autofocus at f/8. Most the time it is wanting to get more out of a telephoto lens for wildlife that will not have the patients for the live view routine.


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|| 5Dc+Grip | 20D || 24-105
L | 50 f/1.8 II | Tamron 28-300 f/3.5-6.3 <-Crap on FF |
| 550EX | Yongnuo RF 602 triggers | Aperture 3 | Lowepro Pro Trekker 300 AW |

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scubthebub
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Mar 14, 2011 21:34 |  #10

Some more in depth explanations of of AF works
http://www.cambridgein​colour.com …ials/camera-autofocus.htm (external link)
http://www.lensrentals​.com …how-autofocus-often-works (external link)


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L | 50 f/1.8 II | Tamron 28-300 f/3.5-6.3 <-Crap on FF |
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artyman
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Mar 15, 2011 03:38 |  #11

Manual focusing isn't so hard, OK you may not be quick enough for birds in flight, but should be OK for stationery ones.


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big_g
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Mar 15, 2011 03:46 as a reply to  @ artyman's post |  #12

Also your shutter speed will be compromised due to f8 so it will be even more difficult to get a good shot. The viewfinder will also be pretty gloomy. Set you aperture on any lens to f8 then use the DOF preview button to see what I mean. Finally, manual focussing is very difficult with the focus screen on the 7D and you cannot change it.


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weeatmice
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Mar 15, 2011 05:23 |  #13

You can tape the pins on the extender, then the 7D will try to AF. When I tried this though it never locked focus on anything - I have spotted a focus speed setting however in the 7D menu maybe that could help. Otherwise your best bet is MF.


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Mar 15, 2011 05:41 |  #14

weeatmice wrote in post #12023001 (external link)
You can tape the pins on the extender, then the 7D will try to AF. When I tried this though it never locked focus on anything - I have spotted a focus speed setting however in the 7D menu maybe that could help

Nope. C.Fn III-1 won't affect this at all. It's used to change how quickly the focus jumps from one thing to another when using AI Servo.

As for taping pins - it makes the AF slow (if it works at all). It's faster, more accurate and more reliable to use the contrast-based AF in Live-View than to use taped pins.


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phreeky
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Mar 15, 2011 05:47 |  #15

hollis_f wrote in post #12023040 (external link)
As for taping pins - it makes the AF slow (if it works at all). It's faster, more accurate and more reliable to use the contrast-based AF in Live-View than to use taped pins.

In my experience taping the pins (or using a non-reporting TC) actually DOESN'T make it slow, which is one of the problems. The lens doesn't know the TC is in use, so the AF motor rolls along at full speed like there has been no change. Where as if you use a reporting TC on a lens of the appropriate aperture the lens will typically slow the AF down to improve accuracy.

I use my 400 with a 1.4x TC on a 7D. I find it more useful for BIF than static shots, actually. Probably because I'm normally doing BIF with a sky in the background, so there is very significant contrast. AI Servo seems to work alright. I've normally used a Kenko 1.4x Pro300 DG, just received my DGX model and had a brief play - don't have to tape the pins with that, AF performance seemed the same, and it means I get correct data in the EXIF which is handy indeed.




  
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400L and 1.4x Extender on 7D?
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