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07-30-2010, 09:27 AM   #1
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Effects of TC on aperture?

Hi, I keep reading that with a say 1.4x TC a 300/4.0 lens would become 420/5.6 (or something like that). I'd like to learn what that aperture change really means in use. Does it mean with a manual lens, if the lighting meters as f/5.6 I'd have to set lens to f/4.0? What about an "A" lens with an A-capable TC, can I still set camera to shoot f/4.0 and let it work out the correct shutter speed? Thanks.


Last edited by Kguru; 07-30-2010 at 09:33 AM.
07-30-2010, 09:56 AM   #2
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If you are using a separate / external meter, then yes, set your lens one f-stop larger than your meter suggests. But if you're using the meter on the camera, there's no need - the effect of the smaller effective aperture has already been taken into account because the camera is seeing less light than it would without the TC. M42, K-mount, KA-mount, doesn't matter - the camera meter is incapable of *not* seeing the effect of the TC already. Adding a TC is no different than using a filter in that respect, or simply dimming the lights - whatever effect these actions have, the meter sees the effect directly, so there's no need to compensate.
07-30-2010, 10:00 AM   #3
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Teleconverters cut the effective aperture by the same factor that they boost the focal length. If you put a 2x TC on a 300mm F2.8 lens, you'll now effectively have a 600mm F5.6.

If the teleconverter is worth a hoot, it'll compensate for the loss of light through its electrical contacts, and A/F/FA/DA lenses will be usable in P mode. Thus, for example, when the lens is set to F2.8 on its aperture ring, the camera will think that it's actually set to F5.6.

If it isn't, just meter manually. In this case you can completely forget about what the aperture ring is set to, and just focus on your light meter, which measures light through the lens.

As a word of caution, there are some A-capable teleconverters out there that don't apply the loss-of-light factor. If this is the case and you want to meter manually, just make sure you set the EV compensation to your TC's factor. So with my 2x teleconverter, I'd set the EV shift to +2.

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07-30-2010, 10:01 AM   #4
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as far as I know a 2x teleconverter will lose you 2 stops, a 1.4x will lose you 1 and a 1.7 will be somewhere in the middle


as marc said, you don't need to worry about this- your camera will meter what it 'sees'

07-30-2010, 12:27 PM   #5
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QuoteOriginally posted by Kguru Quote
Hi, I keep reading that with a say 1.4x TC a 300/4.0 lens would become 420/5.6 (or something like that). I'd like to learn what that aperture change really means in use. Does it mean with a manual lens, if the lighting meters as f/5.6 I'd have to set lens to f/4.0? What about an "A" lens with an A-capable TC, can I still set camera to shoot f/4.0 and let it work out the correct shutter speed? Thanks.
It all depends on the TC you use

Some TCs simply feed the lens data through others modify the aperture to correct for the change in f- stop so depending on which type of tc you have the answer can change
07-30-2010, 02:00 PM   #6
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QuoteOriginally posted by clark Quote
as far as I know a 2x teleconverter will lose you 2 stops, a 1.4x will lose you 1 and a 1.7 will be somewhere in the middle


as marc said, you don't need to worry about this- your camera will meter what it 'sees'
However, if its an AF lens, it may slam AF or not AF at all depending on the tc and/or lens.
07-30-2010, 08:27 PM   #7
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Thank you all for your explanations, just need to clarify a little bit. Adam & Lowell warn it depends on whether the TC modifies the aperture data before feeding to camera. The other posters say if using the camera's built-in meter only then it's what the camera sees, meaning doesn't matter whether TC can modify aperture data or not. Is that the correct understanding?

PS: Need to double check as I have not seen a TC advert that cares to mention whether it mods, thanks again.
07-30-2010, 11:14 PM   #8
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Whether you need to adjust for the TC when using an external meter would depend on whether the TC compensates or not, I'd agree. But I can't see how it would possibly matter when using the camera meter. The camera meters based on the light received wide open, and regardless of what the lens/TC reports for the aperture wide open, all other apertures will be correct relative to that. Unless I'm missing something, but I don't think I am.

07-31-2010, 04:58 AM   #9
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Although the one would expect the camera to compensate even if the TC does not modify aperture, there are metering issues with most cameras, that are a function of aperture, the K10D being the worst example.

The camera is programmed to correct as a function of these errors, but if it does not know the true aperture (i.e. lens plus TC corrected for the TC) it gets it wrong.


Just take a K10D and a sigma lens with TC out and test it. you get a constant .7 stop error on a sigma APO 70-200 F2,8EX using the sigma 1.4x TC and a constant 1.3 stop error using the 2x TC.

it all comes down to the camera not knowing the correct aperture.

I have posted the following many many times before, and it shows the error in metering using a K50mmF1.4 on several cameras. with the K10D it thinks at F2.8 there is more light than there actually is, and it under exposes, at F8 it think there is less light than there is and over exposes. if the camera is programmed knowing this, it corrects for it, but if the TC does not modify the lens data the camera gets it wrong

the error will be a function of TC camera and lens maximum aperture

07-31-2010, 06:02 AM   #10
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QuoteOriginally posted by Kguru Quote
Thank you all for your explanations, just need to clarify a little bit. Adam & Lowell warn it depends on whether the TC modifies the aperture data before feeding to camera. The other posters say if using the camera's built-in meter only then it's what the camera sees, meaning doesn't matter whether TC can modify aperture data or not. Is that the correct understanding?

PS: Need to double check as I have not seen a TC advert that cares to mention whether it mods, thanks again.
I think what you're saying is 'does the TC add the 'converted' data into the EXIF- I think this would be a no, so for example if you shoot using a 135 2.8, the exif reads 135 2.8, if you put a TC on, I think it would still read the same (i.e. it wouldn't say 270mm f5.6)
Although I can't test this because all but 2 of my lenses are completely manual, and so is my teleconverter


I don't get any exposure errors on my camera (ist DL), probably because it is too old and stupid to do any processing so it just does what it's told
07-31-2010, 06:05 AM   #11
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QuoteOriginally posted by Lowell Goudge Quote
the error will be a function of TC camera and lens maximum aperture
Now now ... this is getting to be more complicated than rocket science

Thanks Lowell & Marc for taking time again to explain.
07-31-2010, 06:20 AM   #12
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QuoteOriginally posted by clark Quote
I think what you're saying is 'does the TC add the 'converted' data into the EXIF
Not really, what I asked was if a 2x TC "converts" a f/2.8 lens into f/5.6 what do I need to do when I mount it on camera and shoot in A mode, can I still set aperture to 2.8 or must I limit myself to 5.6 otherwise it would underexpose?

Apparently yes I can set 2.8 with TC, but it'd automatically result in a shutter speed 4 times longer than at 2.8 without the TC. Correct?
07-31-2010, 06:44 AM   #13
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QuoteOriginally posted by Kguru Quote
Not really, what I asked was if a 2x TC "converts" a f/2.8 lens into f/5.6 what do I need to do when I mount it on camera and shoot in A mode, can I still set aperture to 2.8 or must I limit myself to 5.6 otherwise it would underexpose?

Apparently yes I can set 2.8 with TC, but it'd automatically result in a shutter speed 4 times longer than at 2.8 without the TC. Correct?

this will all become super obvious within about 20 second of mounting the lens and taking a few pictures

I think you're making a huge fuss about nothing- the camera will read as much light as the lens and TC feed to it- so if you set the lens to f2.8, the lens feeds that much light to the TC, the TC then loses 2 stops of light in the TC, so the lens feeds the equivalent of f5.6 to the camera, the camera reads the light as f5.6 and sets the appropriate shutter speed- if you want to have a particular depth of field but the shutter speed is still too slow/fast, then tweak the ISO


so to answer your question, yes the shutter speed would be 4 times longer with a 2x teleconverter, so in that respect a 100mm f2.8 with a 2X TC would give you a slower shutter speed than a 200mm f4, so teleconverters aren't the best for speed, but are pretty convenient- and cheaper than a new lens
07-31-2010, 07:11 AM   #14
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QuoteOriginally posted by clark Quote
I think you're making a huge fuss about nothing-
Thanks for the explanation.
Btw I haven't got a TC to know how it behaves with respect to aperture, so before getting one I'd like to learn what the implications are. That's not making a huge fuss is it?
07-31-2010, 09:43 AM   #15
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QuoteOriginally posted by Lowell Goudge Quote
Although the one would expect the camera to compensate even if the TC does not modify aperture, there are metering issues with most cameras, that are a function of aperture, the K10D being the worst example.
True, and I could see how a TC might possibly exacerbate them (although I suppose it is also conceivable that the TC would actually *lessen* the problem).

QuoteQuote:
Just take a K10D and a sigma lens with TC out and test it. you get a constant .7 stop error on a sigma APO 70-200 F2,8EX using the sigma 1.4x TC and a constant 1.3 stop error using the 2x TC.
I don't have that lens, but I certainly don't see anything like that with any other lens I own. Yes, the camera gets the aperture wrong, but it's wrong wide open as well as stopped down, and the *ratio* is still correct. That is, the camera knows that a setting of f/8 is three stops slower than a setting of f/2.8, and therefore chooses a shutter speed 8 times slower. Add a TC, and the camera *still* chooses a shutter speed 8 times slower than it chooses wide open - it just choose a speed only half as fast in the first place because the TC "stole" half the light. You're really metering at f/4 instead of f/2.8, and shooting at f/11 instead of f/8, but either way, it's three stops slower, and 8 times slower shower speed will be the (theoretically) correct setting.

QuoteQuote:
I have posted the following many many times before, and it shows the error in metering using a K50mmF1.4 on several cameras. with the K10D it thinks at F2.8 there is more light than there actually is, and it under exposes, at F8 it think there is less light than there is and over exposes. if the camera is programmed knowing this, it corrects for it, but if the TC does not modify the lens data the camera gets it wrong
But what I don't get here is that the K lens transmits no data whatsoever, so there is nothing for the TC to modify. So aside from the accidental effect it might happen to have on whatever metering issues the lens or camera have in the first place, I can't see how the TC itself would be causing a problem on its own. As far as the camera is concern, all you've really done is turn down the lights - it doesn't know a thing about aperture, but is just measuring the light it sees. I'm not saying that certainly lens / camera combination might have their characteristics change because of the TC, just that the TC itself isn't causing a one-stop error as one might otherwise think it might.
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