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09-09-2010, 10:09 AM   #1
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How does TCs affect aperture?

I understand teleconverters affect effective aperture on lenses. How does that work? Say I have a f4 lens. Would a 2X teleconverter affect effective max aperture? if so, is there is any rule of thumb for that?

Thanks,

09-09-2010, 10:26 AM   #2
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a teleconverter is a magnifier, usually either 1.4x or 2x

since it magnifies the existing image through the lens a 1.4 x magnification in linear dimension is a doubling of "area" or reduction by 50% of the light hitting a particular area of the sensor or film this is 1 stop.

similarly a 2x TC doubles linear image dimension and has a 2 stop reduction in light
09-09-2010, 10:36 AM   #3
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don't forget the effect on DOF as well. this is also important to notify.
09-09-2010, 11:10 AM - 1 Like   #4
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Yes, the rule of thumb is: multiply both focal length and f-stop by the multiplier, which accounts for the DOF also. A 100/2.8 lens with a 1.4x TC becomes a 140/4 lens, with DOF to match. That 100/2.8 on a 2x TC becomes a 200/5.6 lens, with DOF thinned even further.

Why is 1.4 the multiplier in TC's and apertures? Notice: 1.4*1.4=2 and 2*1.4=2.8 and 2.8*1.4=4, et cetera. That's because 1.414141414... is the square root of 2. The aperture diaphragm or iris is (close to) circular. To double its AREA (to double the light passing through), multiply its DIAMETER by the square root of two.

Every telephoto lens has a TC built into it, to make the lens physically shorter than its optical length, but that TC (called the telephoto group) is matched to the optics of the lens. Some higher-end teles have matched add-on TC's available -- matched, to minimize aberrations. Just remember that every added glass surface between subject and sensor reduces contrast somewhat. But that can be fixed in PP, eh?

09-09-2010, 01:06 PM   #5
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Thanks all!
So the 1.4x is effectively decreasing 1 stop and the 2X effectively decreases 2 stops.
For practical purposes I suppose a 1.5X TC can be aprox to 1 stop like the 1.4X But a 1.6X TC would be 1 1/3 stop.
Is the DOF thinner because you are "loosing" light but the diafragm size is fixed? And Does that means that at f2.8 with a 2X TC you'll have the DOF of 2.8 but the light gathering ability of 5.6?

Thanks,
09-09-2010, 01:34 PM   #6
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DoF remains the same because the real aperture remains the same and the TC is simply magnifying. The TC does exactly what it says, it increase the focal length. It does nothing to the real aperture. As a result, the f-stop (what most people refer to when they say aperture) is affected, as it is equal to the real aperture / focal length.
09-09-2010, 01:58 PM   #7
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QuoteOriginally posted by ismaelg Quote
Thanks all!
So the 1.4x is effectively decreasing 1 stop and the 2X effectively decreases 2 stops.
For practical purposes I suppose a 1.5X TC can be aprox to 1 stop like the 1.4X But a 1.6X TC would be 1 1/3 stop.
Is the DOF thinner because you are "loosing" light but the diafragm size is fixed? And Does that means that at f2.8 with a 2X TC you'll have the DOF of 2.8 but the light gathering ability of 5.6?

Thanks,
QuoteOriginally posted by Eruditass Quote
DoF remains the same because the real aperture remains the same and the TC is simply magnifying. The TC does exactly what it says, it increase the focal length. It does nothing to the real aperture. As a result, the f-stop (what most people refer to when they say aperture) is affected, as it is equal to the real aperture / focal length.
The depth of field will be for the same as a real prime lens at that focal length and aperture.

In simple terms it is thinner because you are increasing the focal length, which by it self directly impacts the DOF,

But think of it another way if you consider say a 100mm F2.8 lens, and it's native depth of field, that is based upon a defined image magnification from the image captured on either sensor or film. If you blow it up bigger, than the apparet depth of field is reduced. Depth of field is the area of acceptable focus. A teleconverter, or any telephoto lens, is simply a different way to blow up an image, by enlarging it before the sensor as opposed to after.

the end result is the same as far as depth of field is concerned.
09-09-2010, 04:32 PM   #8
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QuoteOriginally posted by Lowell Goudge Quote
But think of it another way if you consider say a 100mm F2.8 lens, and it's native depth of field, that is based upon a defined image magnification from the image captured on either sensor or film. If you blow it up bigger, than the apparet depth of field is reduced. Depth of field is the area of acceptable focus. A teleconverter, or any telephoto lens, is simply a different way to blow up an image, by enlarging it before the sensor as opposed to after.

the end result is the same as far as depth of field is concerned.
The same as what? If I were the OP I would be thoroughly confused by now. I asked this very same question in an earlier thread (can't remember if it was here or DPR) some time ago - lets verify the answer I got.

A 100mm lens at f/2.8 + 2X teleconverter becomes equivalent to a 200mm lens at f/5.6 in terms of exposure. That much is clear. The answer I got earlier was that it is equivalent to a 200mm lens at f/5.6 also in terms of DOF. Are you agreeing with this?

09-09-2010, 08:32 PM   #9
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QuoteOriginally posted by Lowell Goudge Quote
The depth of field will be for the same as a real prime lens at that focal length and aperture.
QuoteOriginally posted by timo Quote
A 100mm lens at f/2.8 + 2X teleconverter becomes equivalent to a 200mm lens at f/5.6 in terms of exposure. That much is clear. The answer I got earlier was that it is equivalent to a 200mm lens at f/5.6 also in terms of DOF. Are you agreeing with this?
That's what he said. That's what I say. DOF doesn't depend on how you get there; it's only determined in-camera by the effective focal length and aperture of the lens system, and the frame (film/sensor) size. All the other factors affecting DOF relate to position, distance, and aspects of presentation (including lighting, enlargement, and the viewer's eyeballs).

Consider this: Longer focal-length lenses may be either long tubes, or telephotos. What makes a lens a tele isn't its length, but whether it contains optical elements called a telephoto group, which I mentioned above. The telephoto group is an internal teleconverter, which keeps long lenses from being even longer. So my cheap Hanimar 135/3.5 long-tube is over 110mm from base to front of objective, while my somewhat pricier Enna 35/3.5 tele is only 70mm. If I had a Mukitar 90/2.33 lens with a 1.5x TC, the resulting lens system would be 135/3.5. The Mukitar+TC, Enna, and Hanimar would all have the same DOF if used the same. (Don't bother googling for Mukitar, eh?)
09-10-2010, 05:25 AM   #10
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QuoteOriginally posted by timo Quote
The same as what? If I were the OP I would be thoroughly confused by now. I asked this very same question in an earlier thread (can't remember if it was here or DPR) some time ago - lets verify the answer I got.

A 100mm lens at f/2.8 + 2X teleconverter becomes equivalent to a 200mm lens at f/5.6 in terms of exposure. That much is clear. The answer I got earlier was that it is equivalent to a 200mm lens at f/5.6 also in terms of DOF. Are you agreeing with this?
Yes I am agreeing with it.

ismaelg and Eruditass made an error by saying the DOF of a 100mm F2.8 and a 200mm F5.6 would be the same. I disagreed with that he was considering that the impact of the teleconverter was only on light, and not on the depth of field, which is a function of overall image magnification, he somehow thought/implied in his response the DOF would be unchanged by adding the teleconverter.

The real issue is that depth of field is a function of the lens, aperture, subject distance, and enlargement size of the final image. as you scale thie final image up relitive to the origonal subject size, the depth of field reduces. How you achieve the overall magnification ultimately falls out of the equation, with the exception that the smaller the aperture, the larger the depth of field, or apparent range of acceptable focus.

Last edited by Lowell Goudge; 09-10-2010 at 05:33 AM.
09-10-2010, 07:01 AM   #11
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QuoteOriginally posted by RioRico Quote
Yes, the rule of thumb is: multiply both focal length and f-stop by the multiplier, which accounts for the DOF also. A 100/2.8 lens with a 1.4x TC becomes a 140/4 lens, with DOF to match. That 100/2.8 on a 2x TC becomes a 200/5.6 lens, with DOF thinned even further.

Why is 1.4 the multiplier in TC's and apertures? Notice: 1.4*1.4=2 and 2*1.4=2.8 and 2.8*1.4=4, et cetera. That's because 1.414141414... is the square root of 2. The aperture diaphragm or iris is (close to) circular. To double its AREA (to double the light passing through), multiply its DIAMETER by the square root of two.

Every telephoto lens has a TC built into it, to make the lens physically shorter than its optical length, but that TC (called the telephoto group) is matched to the optics of the lens. Some higher-end teles have matched add-on TC's available -- matched, to minimize aberrations. Just remember that every added glass surface between subject and sensor reduces contrast somewhat. But that can be fixed in PP, eh?
Very good explanation. There is so much info to learn from people in this forum.
09-10-2010, 07:18 AM   #12
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QuoteOriginally posted by jjdgti Quote
Very good explanation. There is so much info to learn from people in this forum.
You're right, I tried to oversimplify, but was trying to make it easier to understand, and I believe it's true from an only optical standpoint.

An increase in FL, TC, cropping and sensor-size all could basically be grouped into enlargement or circle of confusion (albeit with different amounts of quality degredation. So the only variables would be real aperture, subject distance, and enlargement, correct?

Last edited by Eruditass; 09-10-2010 at 08:00 AM.
09-10-2010, 07:42 AM   #13
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QuoteOriginally posted by RioRico Quote
That's because 1.414141414... is the square root of 2.
With all the references to math lately, I can't let this pass without picking a small nit:

The square root of two is not a repeating decimal like that. It does start out 1.414, but becomes effectively random after that, like any other irrational number.

As for the subject of the thread, I used find this confusing until I realized something pretty simple. f-stop is usually expressed as a fraction or ration: f/2.8 or 1:2.8. That's because it literally is the focal length of the lens divided by the diameter of the physical aperture. OK, sometimes - especially with constant aperture zooms - there is optical trickery involved such that the "effective" aperture differs from the actual diameter, but I'm going to ignore this, because I don't understand it, and don't think it's relevant.

A TC increases focal length but doesn't change the diameter of the physical aperture. Therefore, the f-stop changes. If you had a 100mm lens with a maxium f-stop of 4, that means the diameter of the physical aperture was 25mm, since 100/25 = 4.. If you put a 2X TC on it, you now have a 200mm lens, but the diameter of the aperture hasn't changed - it's still 25mm. So the new maximum f-stop is f/8, since 200/25 = 8. And there is no way that is going to somehow mean one thing when it comes to exposure but another when it comes to DOF. All 200mm lenses with a physical aperture diameter of 25mm are going to behave the same way in both respects.

Last edited by Marc Sabatella; 09-10-2010 at 08:30 AM.
09-10-2010, 07:57 AM   #14
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Thank you all for taking the time to explain this.
I wish I had access to a TC to test it but in the meantime, let me take some baby steps here to ensure I'm on the right path.

- TC increases focal lenght. Clear
- A 1.4X TC will affect exposure by 1 stop and a 2X by 2 stops. Math can be done for all other values. Clear


DOF: I get the point that DOF depends not only on aperture but also focal lenght, distance, sensor size, mother in law's mood etc.
So changing the FL by itself is alredy affecting DOF. What I'm trying to deeply understand is how. (Can't help it. I'm an engineer and suffer from "overcomplicaditis")

I took these 2 images to illustrate DOF on a seminar a few months ago.
Same spot, same lens f2 vs f22
Let's use them as reference.






DOF is affected by a TC.
In the first picture, the DOF is small due to the larger aperture.
If I put a TC from the same spot, same lens and adjust shutter speed for proper exposure, but leave the lens at f2, I would obviouly see less of the image, just like cropping.
Now, would the DOF look the same as if I had crop this original picture to an equivalent size? or will I see more of the fence tubes in focus (bigger DOF) or less (smaller DOF) than in the first pic?

I think some of my confussion comes from the fact that if I'm effectively reducing the apertue (i.e f2.8 to f5.6) I expect a larger DOF but that doesn't seem the case.
I understand I'm trying to compare lemons to oranges but that's part of the fun.

Thanks,
09-10-2010, 08:37 AM   #15
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Reducing aperture only increases DOF if everything else stays the same. Similarly, increasing focal length decreases DOF, if everything else stays the same. If you both reduce aperture *and* increase focal length, then these factors may cancel each other out.

There are online DOF calucaltors you can use to try out different values. I find that calculations are bit misleading, though, because they tell you something what appears in focus, but don't tell you anything about how the out of focus areas change. And that's what I really notice as I shoot longer focal lengths. Even in cases where I'm getting roughly the same DOF with a longer lens and the a shorter lens (because I've adjust distance and/or aperture), the out of focus areas generally look *more* out of focus with the longer lens. It also depends on the distance to subject and the distance to background, so it's hard to pin down.
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