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Go Back   Olympus UK E-System User Group > Cameras, lenses and system accessories > Lens focus > Converters, adapters and extension tubes

Converters, adapters and extension tubes All those lens accessories that get in between the lens and the camera.

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Old 14th May 2010
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Genuine tech question about EC14

I regularly use my EC14 and regard it as quite remarkable. It barely changes the lens characteristics and you lose ony one stop (equally the magnification is only 1.4x)

OK so far. What I want to know is: What effect does adding a teleconverter to a lens have on DoF?? I have looked on several shouty forums and folks get all shouty about this one. There seems to be two answers.

1: It makes no difference, idiot. The focal length got longer but the aperture got smaller.

2: It will be different, just put the appropriate numbers into a DoF calculator and you will see...

Any comments from the our non-shouty foum??

Pete
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Last edited by snaarman; 14th May 2010 at 01:57 PM.
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Old 14th May 2010
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Re: Genuine tech question about EC14

Well we had this little debate a couple of years ago on 4/3 user.

http://www.fourthirds-user.com/forum...read.php?t=865

Still don't know the answer.
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Old 14th May 2010
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Re: Genuine tech question about EC14

Logic suggest that you calculate using the new focal length and the new (adjusted) aperture. That's just my thoughts which may/will be total rubbish, of course.
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Old 14th May 2010
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Re: Genuine tech question about EC14

Thinking about it logically:

The lens is in front of the tele-converter. Light passed into the lens. The aperture, focal length adjust the amount of light with the magnification/angle-of-view as it passes through the lens. Then the light from the lens is passed into the front of the tele-convertor. The images is then magnified and in the process you lose 1 or 2F (EC14/EC20) stops of light. The light then is passed into the camera.


Of couse I could be talking out of my ass
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Old 14th May 2010
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Thumbs up Re: Genuine tech question about EC14

Quote:
Originally Posted by snaarman View Post
I regularly use my EC14 and regard it as quite remarkable. It barely changes the lens characteristics and you lose ony one stop (equally the magnification is only 1.4x)

OK so far. What I want to know is: What effect does adding a teleconverter to a lens have on DoF?? I have looked on several shouty forums and folks get all shouty about this one. There seems to be two answers.

1: It makes no difference, idiot. The focal length got longer but the aperture got smaller.

2: It will be different, just put the appropriate numbers into a DoF calculator and you will see...

Any comments from the our-shouty foum??

Pete
I too think the EC14 is brilliant and with my new 14-35 gives me a perfect working range for portraits.

There are two types of answer to questions such as this:

1: The technical, that will present all the graphs for DoF and Hyperfocal Distance, for all the lenses from 7mm to 300mm at every aperture from f/2.0 to f/64 for each focal length and focus setting.

or

2: The "Pragmatic" realist, that uses skills developed from many years of experience, who knows where the sweet spot is for the lens in use and will adjust accordingly (with the use of the preview button) to produce the desired results regardless of what the theory may say.

While I have an interest in the "techy", I tend to "use the force" and enjoy the surprize of a well produced shot

Most important of all - Have Fun...

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Old 14th May 2010
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Re: Genuine tech question about EC14

Quote:
Originally Posted by StephenL View Post
Logic suggest that you calculate using the new focal length and the new (adjusted) aperture. That's just my thoughts which may/will be total rubbish, of course.
Yes, I did that using dofmaster and it says the dof gets shallower when you put the EC14 on. Here's a real world set of calculations...

54mm f3.6 5m gives dof of 0.91m - this is the long end of the 14-54 as near as I can enter the numbers.

76mm f4.8 and 5m gives dof of 0.61m - this is the same setup but with the EC14 added. This tells us the dof is quite a bit shallower.

However - you now have a smaller field of view, 'cos you are still 5m from the target but you now have the EC14 fitted... so lets step backwards to 7m (5m * 1.4) from the target to get the original field of view...

76mm f4.8 and 7m gives dof of 1.21m

So neither case gives the same value as the original setting. Shouty forum was right, I am an idiot :-)

Pete

PS.. The EC14 is not quite vice free IMHO.. It changes the quality of the bokeh (at least on the 14-54) to give a slightly harder edge to it, and I bet that has a sublte effect on the perceived dof :-)
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Old 14th May 2010
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Re: Genuine tech question about EC14

Personally, I wouldn't think that calculating the DoF for any lens with a x1.4 or x2 on is as simple as using a DoF calculation.

Let's use say the 70-300mm lens as an example. I may be wrong of course, and probably am wrong... but the path of light entering the lens ên route to the sensor isn't the same for the 70-300mm lens at 100mm with EC14 on it (effective fl = 140mm) as it would be for the same lens to be set at 140mm without the EC14.

With the EC14, and lens set at 100mm, the light comes into the 70-300, through those optics and then somewhere before it would have hit the sensor had the EC14 not been on, it hits the elements in the Ec14. This simply spreads the image via a factor of 1.4x and thus less of the overall image is projected onto the sensor thus giving the effect of magnification.

Without the EC14, there is no spreading once the light passes through the rear element of the 70-300mm. It is then sent directly to the sensor.

In my humble opinion - and though I have no idea how to accurately calculate the DoF in this example - I feel that it simply cannot be the same as calculating the DoF for the same FL and aperture as you'd get without it.

Make sense...?

No... I thought so... hehe
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Old 14th May 2010
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Re: Genuine tech question about EC14

(whispering mode - or as far away from shouting as is possible......)

consider the TC as additional lens elements - it's just that you can add them as and when required

dof is direct function of magnification (focal length and distance) and aperture after all

so just use your working aperture, focal length and distance in the normal way

I think what people mean when they say 'it stays the same' is that for any given working distance, and assuming you leave the lens aperture fixed, adding the TC will have no effect on the DOF because the increased magnification will be cancelled out by the increased aperture - which is fair enough as far as it goes.

I agree with the view that the bokeh changes, but then again this happens within a single zoom lens at different focal lengths, or the same fl at different working distances as well. No real surprise that something that impacts on fl, working distance and aperture effects it.
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Old 14th May 2010
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Re: Genuine tech question about EC14

Trying to make sense of all this, at the same physical distance, if you put a teleconverter on you will get less depth of field, but the view is magnified. So if you want to maintain the framing or field of view, you have to move back and increase the distance to the subject.

Let's take a 150mm f/2 at 20 feet from the subject. The angle of view is 8.2 degrees. The depth of field is one third of a foot, or slightly smaller than 4 inches. With a 2x converter you have a 300mm f/4, and at 20 feet the depth of field is just under two inches.

If my maths is correct, the field of view (or distance of the subject from corner to corner as seen in the frame) is 2.9 feet at 20 feet (tangent of half the angle of view (8.2 degrees) times the distance times 2).

But if you step back to see the same field of view as the 150mm lens, the lens to subject distance is doubled. So at 40 feet, the depth of field of a 300mm f/4 is just over 4 inches - very close to the dof of the 150mm f/2.

So to summarise, if you use a teleconverter you can get less dof at the same distance but there is negligible difference if you maintain the same field of view by stepping back after fitting the teleconverter (without altering the aperture setting).

Ian
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Re: Genuine tech question about EC14

I think we're all very happy with the Ec14.

So go out and use it.

Don't worry be happy.
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Old 15th May 2010
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Re: Genuine tech question about EC14

Quote:
Originally Posted by andym View Post
I think we're all very happy with the Ec14.

So go out and use it.

Don't worry be happy.
I think this deserves a prize for the best e-group answer in the forums history.



Nick
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Old 15th May 2010
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Re: Genuine tech question about EC14

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Originally Posted by Nick Temple-Fry View Post
I think this deserves a prize for the best e-group answer in the forums history.



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Old 15th May 2010
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Re: Genuine tech question about EC14

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Originally Posted by andym View Post
I think we're all very happy with the Ec14.

So go out and use it.

Don't worry be happy.
Could not agree more. I have just been trying to get my head around scheimplung points, & after about an hour of total confusion have just said B***** to it. I have not needed to understand them so far in my photography so I can live without understanding them now & just enjoy my hobby
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Old 15th May 2010
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Re: Genuine tech question about EC14

Hiya,

You're giving an example DoF or 0.9m with the basic lens. And then adding the EC14 adds a lille bit of telephoto, so this WILL reduce the DoF. And if we divide your 0.9m by 1.4, we get around 0.6m! so roughly-speaking it reduces the numerical DoF by 1.4 for an EC14 - simple!

Adding an EC to ANY lens makes it more of a telephoto - that's why you're doing it - and a telephoto has less DoF than a normal lens. QED.

Hope this stirs the mud even more<g>
RAC
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