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07-20-2009, 11:34 AM   #1
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TC and Aperture

I'm still trying to learn to work with a TC.
I know a TC reduces the aperture by its multiplication factor (1.4, 1.5, 1.7 or 2) but how does this work in practice?
If I mount an f2.5 or an f4 lens to the TC, I can still dial in the original aperture.
How does the metering work then?
I mean, when in Av and set to f4, how does the camera 'know' to expect less light by the multiplication factor of the TC? It still works in 'A' mode, doesn't it?

Thanks for any info.

07-20-2009, 12:43 PM   #2
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In general, a TC is just a magnifier. It takes a part of the image produced by the main lens, magnifies it to the size of the sensor/film. This is why you "lose" light (light of a small area now spreads out to a larger area).

The camera doesn't know that the TC is there.

Assuming the TC has electrical linkages (essentially just a bunch of direct connects) from the camera to the main lens, the camera will read whatever signals produced by the main lens.

Let's say you have a 2X TC. When the camera shows f/2.8, the main lens is at f/2.8, but the main lens + TC combo is f/5.6.

The camera body does not 'know' to expect less light. There is actually less light hitting the camera's light sensors.

Last edited by SOldBear; 07-20-2009 at 02:10 PM.
07-20-2009, 12:50 PM   #3
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QuoteOriginally posted by SOldBear Quote
In general, a TC is just a magnifier. It takes a part of the image produced by the main lens, magnifies it to the size of the sensor/film. This is why you "lose" light (light of a small area now spreads out to a larger area).

The camera doesn't know that the TC is there.

Assuming the TC has electrical linkages (essentially just a bunch of direct connect) from the camara to the main lens, the camera will read whatever signals produced by the main lens.

Let's say you have a 2X TC. When the camera shows f/2.8, the main lens is at f/2.8, but the main lens + TC combo is f/5.6.

The camera body does not 'know' to expect less light. There is actually less light hitting the camera's light sensors.
I will be the first to admit I don't know exactly how the TC passes information, if any, but if it worked like SOldbear describes, the camera could never meter correctly without green button/stop-down metering. In other words all calculations are out the window. Since I use a TC and it appears to meter OK, I have to wonder if above is accurate. Did I miss something in the thought process here?
07-20-2009, 01:05 PM   #4
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Well, it was my thinking as well that the metering should be fairly off but that is not what I observe. I mean if a f5.6 gets used but the cameras 'sees' or 'expects' a f2.8, there should be an exposure error, shouldn't there?
So, I still don't get it - sorry.
There must be missing something...

07-20-2009, 01:33 PM   #5
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just to set everybody straight.

As noted in other posts, the loss of F stops is due to spreading the image out over a bigger area, 2 or 4 times (for 1.4x or 2x TSs) the metering sees this simply as reduced light. On old film cameras it did not matter because the metering was able to make the necessary adjustments. BUT as things have advanced, some corrections are made in softweare today to correct for errors. some cameras seem to meter differently at different F stops (K10D and K20D specifically) and these cameras need to know the real aperture (including the TC induced change) to get exposure correct. My K10D for example meters correctly with my Sigma 70-200 F2.8, but if I add a sigma TC then i need to make exposure corrections od 1 and 1.3 stops respectively for 1.4x and 2x TCs due to metering issues that have been tracked down to the focusing screen reflectivity.

some TCs do modify aperture information fed to the camera, to correct for the change due to enlargement of the image. The pentax SMC-F 1.7x AF TC is one such unit. Sigma TCs for example do not make this correction. I believe Kenko does make the correction, but that is only based upon some postings I have seen.

as a result things are a little confusing because
- some TCs correct the problem, some don't.
- Some cameras are susceptible to errors if aperture is not known others are not.


The bottom line is test your lenses and TCs in combo on your camera and learn all the quirks before you go out and have to shoot the shot of your life.

I have tested all of my lenses with my K10D and *istD, but sadly am a little behind with checking things again on my K10D since adding the split image, and on my K7, where I have just begun to play
07-20-2009, 01:40 PM   #6
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QuoteOriginally posted by imtheguy Quote
I will be the first to admit I don't know exactly how the TC passes information, if any, but if it worked like SOldbear describes, the camera could never meter correctly without green button/stop-down metering. In other words all calculations are out the window. Since I use a TC and it appears to meter OK, I have to wonder if above is accurate. Did I miss something in the thought process here?
Yes, I think so. Lowell has put essentially the same thing a bit differently. Here's my take:

Metering with a TC works for the exact same reason that metering with an ND filter on works. The meter sees the *actual* amount of light entering the camera, and it's simply less light than it would be without the TC or filter. So the camera automatically chooses a slower shutter speed than it would without the TC or filter.
07-20-2009, 01:55 PM   #7
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QuoteOriginally posted by Lowell Goudge Quote
My K10D for example meters correctly with my Sigma 70-200 F2.8, but if I add a sigma TC then i need to make exposure corrections od 1 and 1.3 stops respectively for 1.4x and 2x TCs due to metering issues that have been tracked down to the focusing screen reflectivity.
The nasty focusing screen again!

I don't use TC a lot, but I've never had problem with metering when I do.

I think the reason my K10D bodies never had the metering problem is that they both the have stock focusing screen replaced with an LL-60.
07-20-2009, 02:06 PM   #8
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QuoteOriginally posted by SOldBear Quote
The nasty focusing screen again!

I don't use TC a lot, but I've never had problem with metering when I do.

I think the reason my K10D bodies never had the metering problem is that they both the have stock focusing screen replaced with an LL-60.
that'll do it! But that does not mean you don't have problems, you have just eliminated the problem

07-21-2009, 01:55 AM   #9
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QuoteOriginally posted by Marc Sabatella Quote
Yes, I think so. Lowell has put essentially the same thing a bit differently. Here's my take:

Metering with a TC works for the exact same reason that metering with an ND filter on works. The meter sees the *actual* amount of light entering the camera, and it's simply less light than it would be without the TC or filter. So the camera automatically chooses a slower shutter speed than it would without the TC or filter.
Ok, I believe this I understand.
I don't use a TC a lot either (yet?) but when I do, I don't seem to notice any problems with metering. And I still use the stock screen. Hmm. Should I worry I don't notice a problem?

By the way, using a K10D and a Pz-AF 1.7x Teleplus SHQ TC (Kenko, I believe).

And thanks for the help everyone!
07-21-2009, 12:12 PM   #10
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QuoteOriginally posted by Bart Quote
I'm still trying to learn to work with a TC.
I know a TC reduces the aperture by its multiplication factor (1.4, 1.5, 1.7 or 2) but how does this work in practice?
If I mount an f2.5 or an f4 lens to the TC, I can still dial in the original aperture.
How does the metering work then?
I mean, when in Av and set to f4, how does the camera 'know' to expect less light by the multiplication factor of the TC? It still works in 'A' mode, doesn't it?

Thanks for any info.
Actually, a lens aperture is usually specified as an f-number, the ratio of focal length to effective aperture diameter. With a TC you do not change the outer aperture diameter, you change the focal length by extending the lens (moving it away from the sensor) and correcting the optical focus display.

Let's do some math:
- Lens: f 100mm
- F-number: 4
- Lens diameter: 100mm/4 = 25mm

Using a 2xTC:
- Lens: f 200mm
- Lens diameter: 25mm
- New f-number: 200mm/25mm = f8
Disregarding any loss off light by TC optical material.

And Marc is right, the light metering is done after the lens, so that should be no issue.
Also, a TC holds electric circuits to change the lens info for the camera (focus length etc)

- Bert
07-22-2009, 11:47 AM   #11
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QuoteOriginally posted by bymy141 Quote
...Also, a TC holds electric circuits to change the lens info for the camera (focus length etc)...
The part about the modified focal length being reported to the camera, I can't confirm with my TC.
I used it with A lenses for the most part where you enter the focal length yourself. No problem there. But when I tried it out with some FA lenses, the reported focal length remains the original one (200mm lens + TC was still recorded as 200mm).
I realize this is not directly linked to my original question but do you know of a TC where the 'actual' focal length of the lens + TC might be reported to the camera?

I would find this an interesting feature for sure.
(perhaps even enough to trade in my TC for another type )
07-22-2009, 02:44 PM   #12
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QuoteOriginally posted by Bart Quote
The part about the modified focal length being reported to the camera, I can't confirm with my TC.
I used it with A lenses for the most part where you enter the focal length yourself. No problem there. But when I tried it out with some FA lenses, the reported focal length remains the original one (200mm lens + TC was still recorded as 200mm).
I realize this is not directly linked to my original question but do you know of a TC where the 'actual' focal length of the lens + TC might be reported to the camera?

I would find this an interesting feature for sure.
(perhaps even enough to trade in my TC for another type )
Hum, I have a Kenko 1.5x TC at home, I'll have to check the EXIF info to be 100% sure....
Maybe this weekend, I've got little time right now.

- Bert
07-22-2009, 03:33 PM   #13
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As long as you show me yours I'll show you mine right now. Mine is a Tammy 1.4x TC, the SDM compatible version and the EXIF does not change regardless of whether I have the TC mounted or not. I used PhotoME and looked everywhere that made sense.

Reading through this interesting stuff got me thinking that we are possibly looking at it too complex when there is a simple answer. Consider the case of a 2x TC used in Ap Priority mode. It passes only 1/4 the light of the lens its attached to. Since the camera meter is on the other side of the TC it is aware only of the reduced light and can go blissfully about its business of calculating the shutter speed requirement for the focal length the lens is set for and the ISO the camera is set for.

So my question now is does the camera actually USE more than the ABSOLUTE amount of light to calculate the metering in this case? If the lens is a constant f/4 and we want to shoot at f/8, it will calculate without knowing what is lost in the middle. I am suggesting that it washes in the calculation such that the TC does not transmit any information about itself to alter the results.

Fire away boys....
07-22-2009, 06:25 PM   #14
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I think that the aperture thing is just a guide so that we would expect that the light available for the exposure would be similar to the 1.4X or doubled if you have TC's on.
It's for calculation purposes on our part so that we know how many stops the exposure has been devalued.
The camera just actually meters the available light when the TC is coupled.
It might not know that it has to double the aperture value.
07-23-2009, 12:22 AM   #15
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QuoteOriginally posted by imtheguy Quote
...Reading through this interesting stuff got me thinking that we are possibly looking at it too complex when there is a simple answer. Consider the case of a 2x TC used in Ap Priority mode. It passes only 1/4 the light of the lens its attached to. Since the camera meter is on the other side of the TC it is aware only of the reduced light and can go blissfully about its business of calculating the shutter speed requirement for the focal length the lens is set for and the ISO the camera is set for.

So my question now is does the camera actually USE more than the ABSOLUTE amount of light to calculate the metering in this case? If the lens is a constant f/4 and we want to shoot at f/8, it will calculate without knowing what is lost in the middle. I am suggesting that it washes in the calculation such that the TC does not transmit any information about itself to alter the results...
QuoteOriginally posted by GerryL Quote
I think that the aperture thing is just a guide so that we would expect that the light available for the exposure would be similar to the 1.4X or doubled if you have TC's on.
It's for calculation purposes on our part so that we know how many stops the exposure has been devalued.
The camera just actually meters the available light when the TC is coupled.
It might not know that it has to double the aperture value.
QuoteOriginally posted by Marc Sabatella Quote
...Metering with a TC works for the exact same reason that metering with an ND filter on works. The meter sees the *actual* amount of light entering the camera, and it's simply less light than it would be without the TC or filter. So the camera automatically chooses a slower shutter speed than it would without the TC or filter.
Well, this is also what I understood from Marks explanation. The available light is reduced "as if" the aperture of the lens would be reduced by the factor of the TC.
Camera calculations then start from the aperture of the lens itself, the available (reduced) light and the aperture dialed in for the shot. Should (and does seem to) work.
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