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Taped Contacts - 1.4 X

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JohnC
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« on: June 19, 2009, 11:21:48 pm »

I think I mentioned about taping the first 3 contacts on the 1.4 X  extender (Canon) to allow auto focus to be retained. I've tried it on my 100-400mm  and it not only retains auto, albeit a little slow to focus  but  it keeps the f5.6 end at 5.6 ant f8 as it normally would. I've now got 896mm f5.6. So,  having seen this  I tried it on the Canon 70 -200 IS  f4 and whilst this lens would retain auto anyway  because it's f4 it would normally lose  a stop to f5.6  but it didn't so the 200mm on the 1.6 X crop camera =320mm  plus the 1.4 X = 448mm at  f4.

Brian..obviously , on your  300mm prime on a 1.6 X crop camera   it  would give you  480mm plus the 1.4 X = 672mm  and yet,  still retain f2.8.

I borrowed my neighbour's 1.4 X and I have to hope that Canon haven't put a stop to this with the latest version 1.4  X  11.
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brianb
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« Reply #1 on: June 19, 2009, 11:44:02 pm »

Ummm, it doesn't make the lens a stop faster, it just means the camera believes the lens to be a stop faster.

Similar things happen when you stack tele-extenders - if I use both 1.4x & 2x extenders on a f/2.8 lens the camera believes the lens to be working at f/5.6 rather than at f/8, the autofocus "functions" but is slow & inaccurate. Switching to manual focus is a big improvement - for me - but then I'm not chasing objects which vary their distance significantly. But the biggest problem with stacking extenders is that the 1.4x has to go next to the camera, put them the other way round and the mount on the 2x hits the glass in the 1.4x and will mark it.

I know of one "pro" who regularly uses two 2x extenders and apparently gets acceptable AF performance, but he's using 1 series bodies which have significantly better AF sensors than the cheaper versions.

You need to be really careful with taping over contacts; if you get the tape too thick the contacts in the body can be displaced so that they don't make proper contact when you would like them to, you will them probably get "error 99" or "error 1" and the camera will lock up when you try to fire the shutter.
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Paul
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« Reply #2 on: June 20, 2009, 10:33:24 am »

This is interesting! As Brian says, youre not really getting more aperture, just fooling the camera. My cameras, being Sonys and Minoltas (same thing - Sony bought Minolta), don't need any such trickery - my 100-200 f4.5 zoom retains autofocus on a 2x convertor where it is an f9, but I'm told it's centre-spot only which is the only mode I ever use anyway. So I don't need to employ such tricks. However, I have a slightly faulty lens - a 24-85mm - where the fault is a poor contact in the encoder that tells the camera what the focal length is, and as the actual FL is adjusted away from the 24 that the sensor defaults to the AF starts hunting a lot, though it get there eventually. This leads me to believe that the servo motor is tuned to the lens and needs to know the FL for best results. My teleconvertors - 1.4x and 2x, both Teleplus, seem to do the conversion on chip which makes matters simple.

Incidentally, I understand the reason Minolta/Sony made the AF work that way was to accomodate the 500mm f8 AF mirror lens, the only one of its type ever sold.
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JohnC
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« Reply #3 on: June 20, 2009, 06:50:07 pm »

 Yes, that's a fair point re. the camera being fooled, so to test  that I  used the 70-200 and took a photo at the F4  it  appeared  to retain and then dropped it to f5.6 and there was a difference in the exposure- the f5.6 photo in the LCD was, of course darker , so it does seem that it actually does give you f4.  BUT lol.. I've just had a thought..I can hear you knowledgeable gentlemen saying "Ah but how do you know that it wasn't taking it at f5.6 (in reality) and the next photo you took was infact  f8 - well, that's a good point I didn't think of that at the time so what I'll do is to to take one photo at f4 with the extender taped and then take the extender off and take one at f5.6 and see if there's  any difference-- is this correct ?

My concern, though,  is that I can hear the lens hunting and wonder if it may damage the contacts somehow. When it hunts and assuming there is time, I can can go to manual get it focused and switch to auto and take the photo,  as and when, there may be a minimum of adjustment but no hunting. What I also found was that the hunting problem is mainly with hand held ,when it was tripod mounted the auto focus was fast, hardly, if any difference. However, the focusing without the extender was instant .
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JohnC
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« Reply #4 on: June 22, 2009, 10:55:16 pm »

Brian & Paul.

To test what you've suggested might be the case I tried this. I hope it was the correct way to determine whether the camera is only fooled into thinking that the lens is at f5.6 with the extender when it should be at f8 (at 400mm in the case of my lens)
I found that to do this  without introducing other factors I had to have the priority in Av thus controlling the shutter speed.

Photo 1. With the taped  extender. f5.6   gave 1/80sec.

          2. f.5.6 without the extender.................1/160 sec.

          3. Without the extender at f8 (what you think it might actually be)..1/80 sec.

So the shutter speed for f5.6  (with the extender) and f8 (the real f8 without the extender) is the same which suggests to me that the f5.6 in the first photo with the extender is infact f8 even though it's reading f5.6.           

What do you think ? Have I got this test right ? I mean have I carried it  out correctly to get the result I was wanting- ie. to find out if f5.6 with the ext. really is f5.6. ?

It now appears that you are right.
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brianb
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« Reply #5 on: June 22, 2009, 11:39:22 pm »

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So the shutter speed for f5.6  (with the extender) and f8 (the real f8 without the extender) is the same which suggests to me that the f5.6 in the first photo with the extender is infact f8 even though it's reading f5.6.           
Yes.

But this is absolutely straight forward - with the lens fully open, the linear aperture is obviously unchanged, so the extender must change the f ratio by the same amount as it changes the focal length (since the f ratio is simply the focal length divided by the linear aperture).

What the electronics are fooled into saying is irrelevant - so long as you're using TTL metering. If you're using external metering, you'd better know what the actual f-number is ... this is where one of those old fashioned aperture setting rings comes in handy, even though you do have to do a bit of simple mental arithmetic, there's never any doubt as to what size hole you have set ... and personally I find it a lot less confusing than using those multi mode dials.
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JohnC
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« Reply #6 on: June 22, 2009, 11:50:33 pm »

 So, where I am on this is that  I would still   retain auto  (which I didn't before I put the tape on)  but lose one stop ?

The tape I used was electricians tape.
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brianb
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« Reply #7 on: June 23, 2009, 12:17:51 am »

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So, where I am on this is that  I would still   retain auto  (which I didn't before I put the tape on)  but lose one stop ?
Yes but don't be surprised if the auto is unreliable.  The AF sensor needs a certain "steepness" in order to detect phase lock correctly. I've noticed with recommended Canon lens/extender combinations that AF is not quite as accurate, for astro subjects I definitely prefer to focus manually using the type S precision focusing screen and a magnifying angle finder.

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JohnC
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« Reply #8 on: June 23, 2009, 02:46:05 pm »

Mmm. Well, it does take a few secs. to get in focus compared with no taped extender but if you're saying that it may not be as accurate ie. sharp even when it locks on then  maybe I should abandon this strategy. Is that what you're saying, Brian ,or are you just saying it may take a little longer to get a sharp focus  but it will get there as sharp as with no taped extender ? It's just that I don't have the belief that my manual focusing is as sharp as auto - my neighbour doesn't like manual focus either for the same reason.
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brianb
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« Reply #9 on: June 23, 2009, 04:08:24 pm »

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Is that what you're saying, Brian ,or are you just saying it may take a little longer to get a sharp focus  but it will get there as sharp as with no taped extender ?
You'll have to suck & see. I wasn't happy.
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JohnC
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« Reply #10 on: June 23, 2009, 04:16:38 pm »

Ok..I'll do a test photo.  Mind you, if you're not happy with your results I'm almost tempted to just leave it at that and tbh. with one or to shots I've taken I had a suspicion  just looking through the viewfinder that it wasn't quite right but others were ok so I dismissed it , anyway... thanks for all the info. too,  all very interesting.
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