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IBIS missing focal length

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woodrim View Drop Down
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Post Options Post Options   Quote woodrim Quote  Post ReplyReply Direct Link To This Post Topic: IBIS missing focal length
    Posted: 27 March 2019 at 16:52
I have just received a Questar 8/700 and quickly found that my A7 II IBIS offers 600mm and 800mm settings but no 700mm. Has anyone an idea of which would be a better setting; over or under? Or turn off completely? I've tried both settings with some initial tests and think 600mm may be better but still notice some nervousness in the image when viewing.
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woodrim
 



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pegelli View Drop Down
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Post Options Post Options   Quote pegelli Quote  Post ReplyReply Direct Link To This Post Posted: 27 March 2019 at 17:11
My guess would be to use the nearest lower value (at least that's what I do with missing focal length). Basically my logic is that it will then not overshoot with compendation and give a result between no and full IBIS, pretty close to what it should be theoretically.

But I've got no data to support this theory, it's just a hunch.
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Post Options Post Options   Quote woodrim Quote  Post ReplyReply Direct Link To This Post Posted: 27 March 2019 at 17:12
Originally posted by pegelli pegelli wrote:

My guess would be to use the nearest lower value (at least that's what I do with missing focal length). Basically my logic is that it will then not overshoot with compendation and give a result between no and full IBIS, pretty close to what it should be theoretically.

But I've got no data to support this theory, it's just a hunch.


Sounds logical. Thanks.
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woodrim
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Post Options Post Options   Quote addy landzaat Quote  Post ReplyReply Direct Link To This Post Posted: 27 March 2019 at 18:38
Originally posted by woodrim woodrim wrote:

Originally posted by pegelli pegelli wrote:

My guess would be to use the nearest lower value (at least that's what I do with missing focal length). Basically my logic is that it will then not overshoot with compendation and give a result between no and full IBIS, pretty close to what it should be theoretically.

But I've got no data to support this theory, it's just a hunch.


Sounds logical. Thanks.
and it fits your own initial testing
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Post Options Post Options   Quote Basil Quote  Post ReplyReply Direct Link To This Post Posted: 27 March 2019 at 20:29
Congratulations on your new toy. I'm looking forward to seeing some shots taken with it.
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Post Options Post Options   Quote Miranda F Quote  Post ReplyReply Direct Link To This Post Posted: 28 March 2019 at 11:11
If you do the maths, the limited capability of IBIS (3 stops is only 1/8) means that an approximate value for FL is quite adequate. I've used a lot of MF lenses in the 300-350-400-500 region with a '400mm' chipped adapter and IBIS works just fine; it's a statistical thing anyway, so you will always get some blurred shots at low shutter speed.

Edited by Miranda F - 28 March 2019 at 11:15
Miranda F & Sensorex, Sony A58, Nex-6, Dynax 4, 5, 60, 500si/600si/700si/800si, various Sony & Minolta lenses, several Tamrons, lots of MF primes and *far* too many old film cameras . . .
 



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woodrim View Drop Down
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Post Options Post Options   Quote woodrim Quote  Post ReplyReply Direct Link To This Post Posted: 28 March 2019 at 13:52
I have gotten away with using (forgetting to change) wrong setting but I think mostly because I was steady and function not needed. However, when it is too far off the correct number, the viewfinder image gets jittery. I noticed less of that with the 600 setting. Today is the day to test it out formally.

I have been curious about the Questar since reading of its reputation. I am eager to see how it compares with some of my other good mirrors. So far, I am doubting that it will be as good as my two big Russian lenses.   
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woodrim
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Post Options Post Options   Quote darosa Quote  Post ReplyReply Direct Link To This Post Posted: 28 March 2019 at 15:19
Originally posted by pegelli pegelli wrote:

My guess would be to use the nearest lower value (at least that's what I do with missing focal length). Basically my logic is that it will then not overshoot with compendation and give a result between no and full IBIS, pretty close to what it should be theoretically.

But I've got no data to support this theory, it's just a hunch.


+1

I have quite a few lenses with odd focal lengths and I always pick the nearest lower. Better too little than too much imho.
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Post Options Post Options   Quote woodrim Quote  Post ReplyReply Direct Link To This Post Posted: 28 March 2019 at 16:28
Now, thinking for a moment, I realize that I've been setting my 58mm lenses to 60mm IBIS. Can it be better to go 3mm low than 2mm high? And that also brings up the question of how accurate is the 58mm designation? I have read that my 90mm Vivitar Series 1 is actually 87mm. I should start setting it at 85mm? Although I have never noticed any problem at 90mm.
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woodrim
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Post Options Post Options   Quote pegelli Quote  Post ReplyReply Direct Link To This Post Posted: 28 March 2019 at 18:56
Originally posted by woodrim woodrim wrote:

Now, thinking for a moment, I realize that I've been setting my 58mm lenses to 60mm IBIS. Can it be better to go 3mm low than 2mm high? And that also brings up the question of how accurate is the 58mm designation? I have read that my 90mm Vivitar Series 1 is actually 87mm. I should start setting it at 85mm? Although I have never noticed any problem at 90mm.
While my "hunch" is to put it shorter I also think it's not something to be extremely worried about. It's not an exact science, especially if the IBIS system doesn't get distance information. Theoretically you need both to have the best result. Since adapted lenses don't transfer this to the body the IBIS is going to be inaccurate by definition. I also agree with what Miranda F posted above, IBIS will improve your chances to get a sharp shot, but it's no guarantee. So my other "hunch" is that for a 58 mm lens it will really not make a big difference if you set it at 55 or 60 mm. I think the difference between 55 and 58 or 58 and 60 is not really significant vs. the base improvement you're getting with IBIS in the first place. But you would need to take lots of test pics at controlled circumstances and accurate shake measurement (both of the camera as well as of the resulting pictures) to make a better assessment, but who has the equipment, time and energy to do something like that?
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Post Options Post Options   Quote woodrim Quote  Post ReplyReply Direct Link To This Post Posted: 28 March 2019 at 22:12
Not I.
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woodrim
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Post Options Post Options   Quote Miranda F Quote  Post ReplyReply Direct Link To This Post Posted: 29 March 2019 at 06:18
58mm to 60 is ~3% approx. which is very roughly 1/32 which is 5 stops. Therefore using a simplistic analysis, if IBIS was capable of giving you a 6 or more stop benefit it would reduce the effectiveness to about 5 stops.
I don't think IBIS is that good ... and it's probabilistic anyway so you'd have to take a lot of pictures and do some careful calculations to see that small a difference.
Miranda F & Sensorex, Sony A58, Nex-6, Dynax 4, 5, 60, 500si/600si/700si/800si, various Sony & Minolta lenses, several Tamrons, lots of MF primes and *far* too many old film cameras . . .
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