DOF and teleconverters

[Bogdan Coltea]

This is a thought that's been bugging me for some time. I've researched it on the internet but there are a lot of contradictions.
I can't imagine a better place to pop the question.

So here goes: what is the DOF of a lens with TC mounted on it?

For the sake of argument: we throw in a 300/2.8 with TC2x and obtain a 600mm with f/5.6 effective aperture.
What is the DOF for this combo when used wide open?
That of F/2.8 of the original lens, or F/5.6 of the effective aperture?

PS: Just as a side note - something similar should apply to macro lenses, as the effective aperture decreases with magnification.

[Robert O'Toole]

This is a thought that's been bugging me for some time. I've researched it on the internet but there are a lot of contradictions.
I can't imagine a better place to pop the question.

So here goes: what is the DOF of a lens with TC mounted on it?

For the sake of argument: we throw in a 300/2.8 with TC2x and obtain a 600mm with f/5.6 effective aperture.
What is the DOF for this combo when used wide open?
That of F/2.8 of the original lens, or F/5.6 of the effective aperture?

PS: Just as a side note - something similar should apply to macro lenses, as the effective aperture decreases with magnification.

Nothing will change assuming you do not change position. The TC just "crops" the image.

Macro is the same. A lens that can give you a 1:1 ratio will still only give you a 1:1 ratio on the sensor with a TC, the image will be only cropped. Try it.

Robert

[Bogdan Coltea]

Thank you Robert for the quick reply.
The starter for this puzzle was a short report written by Moose Peterson complaining about the reduced DOF when using TCs, especially with his 300/2.8.
That led me to believe that DOF remains the same as the bare lens, as you said, without changing position. Just increasing magnification.

So basically shooting with a 600 f/4 closed down to 5.6 gives greater DOF than a 300/2.8 + TC 2x wide open? (again assuming the same position is used)

For the macro lenses I was referring at something else: when focusing towards 1:1 macro lenses loose light, attaining an effective aperture lower than the original one (at infinity or normal distances).
I believe it's something like f/5.6 at 1:1 focus, against f/2.8 at normal distance.
Again the same question: when shot wide open does this lens have a DOF of f/2.8 or f/5.6 ?

PS: I would have loved to conduct some tests of my own, but unfortunately I don't own a teleconverter anymore.

[Roger Clark]

Nothing will change assuming you do not change position. The TC just "crops" the image.

Robert,
I disagree. The TC magnifies. Changing magnification changes depth of field (magnification is in the full depth of field equations).

Macro is the same. A lens that can give you a 1:1 ratio will still only give you a 1:1 ratio on the sensor with a TC, the image will be only cropped. Try it.

Robert

I do this. It is not a crop. The TC magnifies, that is how the field of view gets magnified. For example, in my lab at work I use a 65 mm 5:1 macro lens on a 22.2 x 14.8 mm sensor. With no TC, I image 4.4 x 2.96 mm area at 5:1 magnification. But I normally use a 2x TC, giving 10:1 magnification and and a 2.2 x 1.48 mm field.

Roger

[Roger Clark]

Thank you Robert for the quick reply.
The starter for this puzzle was a short report written by Moose Peterson complaining about the reduced DOF when using TCs, especially with his 300/2.8.
That led me to believe that DOF remains the same as the bare lens, as you said, without changing position. Just increasing magnification.

Bogdan,
But magnification is in the equations for depth of field. When you change the focal length with a TC, the depth of field calculation should be done with the focal length and aperture of the combined system. So a 300 f/2.8 with a 2x TC, giving 600 mm f/5.6 gves the exact same depth of field as a 600 mm f/5.6 lens on the same camera.

So basically shooting with a 600 f/4 closed down to 5.6 gives greater DOF than a 300/2.8 + TC 2x wide open? (again assuming the same position is used)

No, it is the same.

For the macro lenses I was referring at something else: when focusing towards 1:1 macro lenses loose light, attaining an effective aperture lower than the original one (at infinity or normal distances).
I believe it's something like f/5.6 at 1:1 focus, against f/2.8 at normal distance.
Again the same question: when shot wide open does this lens have a DOF of f/2.8 or f/5.6 ?

Depth of field in macro is also a function of magnification, as magnification increases, depth of field decreases.

Depth of field, T, is given by T = 2*C*N*(m+1)/m²

where C is the size of the circle of confusion you wish to achieve, m is magnification, and N is the f-number.

Roger

[Robert O'Toole]

Robert,
I disagree. The TC magnifies. Changing magnification changes depth of field (magnification is in the full depth of field equations).

yes I do agree with your statement, magnification is correct of course. The effect on DOF is still the same. The TC magnifies with a effective change in aperture so the DOF you lose is gained back by the change in effective aperture so there is no change.

I do this. It is not a crop. The TC magnifies, that is how the field of view gets magnified. For example, in my lab at work I use a 65 mm 5:1 macro lens on a 22.2 x 14.8 mm sensor. With no TC, I image 4.4 x 2.96 mm area at 5:1 magnification. But I normally use a 2x TC, giving 10:1 magnification and and a 2.2 x 1.48 mm field.
Roger

You are correct again Roger, for some reason I was thinking about Sensor size change from 1x to 1.3 X or 1.5x and the effect on magnification ratio, which is 0.

[Roger Clark]

[quote=Robert O'Toole;181047]yes I do agree with your statement, magnification is correct of course. The effect on DOF is still the same. The TC magnifies with a effective change in aperture so the DOF you lose is gained back by the change in effective aperture so there is no change.
/quote]

Robert,
Depth of field is certainly a confusing subject, and usually not intuitive. I think part if this is because as photographers were are trained to think in f/stops. But for depth of field we should be thinking in aperture (the diameter of the lens, or more accurately, the entrance pupil) and magnification or focal length. For some more head spinning, see my article on the "Depth of Field Myth and Digital cameras:"

http://www.clarkvision.com/photoinfo/dof_myth

Roger

[Bogdan Coltea]

Thank you for the answer and link Clark.
Very helpful and informative.

[c.w. moynihan]

http://dofmaster.com/dofjs.html

with the same body, a 600mm f/4 lense @ f/5.6 will give you more DOF than
a 600mm f/4 + 1.4TC ~ 840mm @ f/5.6.

[Ed Erkes]

Moose Peterson has, for many years, continually repeated this inaccurate idea about DOF and teleconconverters, despite discussions by George Lepp, John Shaw and many others that it is not true. A 400mm f2.8 lens with 2X converter will give the same DOF as a 800 f5.6 lens. One way to look at is to remember that a 2X converter is simply additional glass elements that can be added and removed from the lens. If you were to encase the 2X converter elements into a 400mm f2.8 lens body and market it as a single lens, then you've got an 800mm f5.6 lens.