Teleconverter and DOF

[TrumpetPower!]

What makes it confusing is when the discussion compares the same framing versus the same distance to subject.

If you need more reach, and the camera stays in the same place, adding a TC will increase the depth of field.  Only if you move the camera backward so that you have the same image as without the TC, will the depth of field remain the same.

This is exactly right, on both counts, both the source of the confusion and what happens.

So, for wildlife where you merely need more focal length and can't get closer, you get more depth of field with the TC.  For a portrait, if you already have a good composition, its very unlikely that you would add a TC and then move backwards, so that is not a common usage of a TC.

There is actually a very common usage of a(n effective) teleconverter that works exactly that way.

All those variable-aperture zooms out there? They're basically using a variable teleconverter. So, if you're doing portraiture with one, you might well take a waist-up shot wide open and then zoom in (with the lens, not your feet) for a head-and-shoulders shot still wide open, but (obviously) at a smaller aperture. And, even though it's a smaller aperture, it still works out to a smaller depth of field.

It works out to exactly the same smaller depth of field as if you had just taken a single waist-up picture and then cropped out the head-and-shoulders portion and enlarged it to the same size as the original waist-up shot.

If you're using a constant-aperture zoom, the depth of field is even shallower still, because now you not only have the enlargement factor decreasing depth of field, but you've also got the larger physical (or, at least, apparent) aperture at work to boot.

Somebody posted to some other thread an online tool that graphed background blur as a function of focal length, aperture, and distance from subject. Wish I could remember where it was, but the uptake is that a 50mm f/1.0 will have the most blur for things near the subject, and a 400 f/2.8 will have orders of magnitude more blur for things past about ten feet or so. So, if you want the least-recognizable background possible, speed wins for tight spaces but focal length blows it away if you've got the working distance.

It works the other way, too. If you want to maximize depth of field rather than minimize it, you want a short focal length lens stopped down as much as possible with as much distance as possible between you and your subject.

...and then you can use a lens with movements or adjustable aberrations to further complicate matters in either direction....

Cheers,

b&

[Mt Spokane Photography]

Look use this simple example.

100mm f 2 @ 20ft has 1.1ft dof.

Put a 2xTC on above for a 200mm f5.6 @ 20ft and you get 0.54ft dof.

The focal length increased but the actual aperture size (not the f number) stayed the same, so you get less dof.

ok and now go a few steps back and achive the same framing.. what do you have  less or the same DOF?   

some here say the DOF will be the same in that case.
and neuroanatomist says the DOF will be less.

In Neuro's example:
 
The depth of field with a 100mm f/2.8 lens at 2m is 0.05m
 
With a 1.4X TC (140mm)  at f/2.8 and 2.8m the depth of field is the same, 0.05m.
 
It might be confusing as to what he is saying when he says the depth of field is less, but its the same for those two cases where the composition remains the same.
 
Just use the DOF calculator, its difficult to write it down in a manner that is not confusing, and easy (as I've shown) to get it backwards to what you were trying to say.
 
http://www.dofmaster.com/dofjs.html

[Malte_P]

What makes it confusing is when the discussion compares the same framing versus the same distance to subject.

If you need more reach, and the camera stays in the same place, adding a TC will increase the depth of field.  Only if you move the camera backward so that you have the same image as without the TC, will the depth of field remain the same.

This is exactly right, on both counts, both the source of the confusion and what happens]

could you guys please come to a conclusion.

ROTFL.. and i thought i have problems understanding this. 

Adding a TC and moving back so the FoV is identical means deeper DoF

i think you are right.

[Malte_P]

Look use this simple example.

100mm f 2 @ 20ft has 1.1ft dof.

Put a 2xTC on above for a 200mm f5.6 @ 20ft and you get 0.54ft dof.

The focal length increased but the actual aperture size (not the f number) stayed the same, so you get less dof.

ok and now go a few steps back and achive the same framing.. what do you have  less or the same DOF?   

some here say the DOF will be the same in that case.
and neuroanatomist says the DOF will be deeper.

In Neuro's example:
 
The depth of field with a 100mm f/2.8 lens at 2m is 0.05m
 
With a 1.4X TC (140mm)  at f/2.8 and 2.8m the depth of field is the same, 0.05m.
 
It might be confusing as to what he is saying when he says the depth of field is less, but its the same for those two cases where the composition remains the same.
 

the one lens is a 100mm f2 in his example.
the other is a 100mm f2 + 1.4 TC = 140mm f2.8

and maybe im just to dumb to understand him but so far i think that the DOF will be the same it´s NOT what he says:

Adding a TC and moving back so the FoV is identical means deeper DoF

so with a 100mm @f2 + 1.4 TC you have more DOF then with a 100mm @ f2 .. given the same FoV, same framing.

100mm f/2 on FF at 20 feet, DoF = 1.54 feet
200mm f/4 on FF at 20 feet, DoF = 0.75 feet

Now, back up so the shot with the 2x TC has the same framing as the shot without the TC:

200mm f/4 on FF at 40 feet, DoF = 3.07 feet

[RLPhoto]

I believed if you stay in the same spot, and zoom in, your perspective and DOF are the same. Only when you move the camera or mess with the aperture is when DOF changes.

[neuroanatomist]

It's a confusing issue, to be sure. Partly, that's because of semantics.  There are lots of statements to the effect that "a TC doesn't affect the DoF."  That's true, if you account for the effects of a TC on both focal length and f/number. In other words, a 200 f/2.8 lens with a 2x TC becomes a 400mm f/5.6 lens.  That 200/2.8+2x combo will have the same framing and DoF as an actual 400mm f/5.6 lens, it will not have the framing of a 400mm lens with the DoF of a an f/2.8 lens (if you want the latter, you've got to pay several thousand $$ more for an actual 400/2.8 lens).

Somebody posted to some other thread an online tool that graphed background blur as a function of focal length, aperture, and distance from subject. Wish I could remember where it was, but the uptake is that a 50mm f/1.0 will have the most blur for things near the subject, and a 400 f/2.8 will have orders of magnitude more blur for things past about ten feet or so. So, if you want the least-recognizable background possible, speed wins for tight spaces but focal length blows it away if you've got the working distance.

True, but as the OP pointed out earlier, DoF isn't an exact conceptual opposite of OOF blur.  DoF defines what is in sharp focus, and that's independent of the distance between the subject and the foreground/background.  OOF blur is critically influenced by distances between the subject and fore-/background.

[Malte_P]

"A TC does not alter the apparent aperture, it does alter the focal length. For the same distance shot using a TC will reduce dof, for the same framed shot the dof is the same."

but neuroanatomist math seems to be right :

100mm f/2 on FF at 20 feet, DoF = 1.54 feet
200mm f/4 on FF at 20 feet, DoF = 0.75 feet

Now, back up so the shot with the 2x TC has the same framing as the shot without the TC:

200mm f/4 on FF at 40 feet, DoF = 3.07 feet

Adding a TC and moving back so the FoV is identical means deeper DoF

how can both be true?

[neuroanatomist]

Remember when I said the only two important things are actual apparent aperture size and subject magnification?
DOF is dependent on two factors alone, subject reproduction ratio and apparent aperture. The bigger either is the less dof you have.

"A TC does not alter the apparent aperture, it does alter the focal length. For the same distance shot using a TC will reduce dof, for the same framed shot the dof is the same."

So, you are saying that if you frame a shot with a 200mm f/2.8 lens, then put your 2x TC behind that lens and double the distance to the subject so the subject magnification (framing) is identical, you will have the same DoF in both shots?  If that's true, the 2x TC turned a 200mm f2.8 lens into 400mm f/2.8 lens.  Unfortunately, my 2xIII doesn't do that.  Where can I get one of your magical teleconverters? 

EDIT:  I was really hoping my 600/4 could be turned into a 1200/4 by adding $300 of glass to the back end.  Damn, foiled again.   

[K-amps]

Let me add to the confusion:

If you want  the same background blur... But

1) Want to stay where you are
2) The subject is also fixed in distance from you, but you want to pull it closer

Then adding a TC will mantain this OOF blur in the background without you needing to move closer to the subject.

However your framing has changed. All the TC buys you is re-positioning the subject in relation to you and the background. The closer the subject is to the sensor, the more oof blur you get for the background. The TC pulls the subject closer however since it reduces the F number, you are back to square 1... the only advantage you get is, you can stay far and shoot and still get some OOF blur in background since the TC has pulled your subject in a bit... confused? 

[Malte_P]

Let me add to the confusion:

If you want  the same background blur... But

1) Want to stay where you are
2) The subject is also fixed in distance from you, but you want to pull it closer

Then adding a TC will mantain this OOF blur in the background without you needing to move closer to the subject.

However your framing has changed. All the TC buys you is re-positioning the subject in relation to you and the background. The closer the subject is to the sensor, the more oof blur you get for the background. The TC pulls the subject closer however since it reduces the F number, you are back to square 1... the only advantage you get is, you can stay far and shoot and still get some OOF blur in background since the TC has pulled your subject in a bit... confused? 

but the object does not get closer. it´s only magnified by the TC.

[K-amps]

Let me add to the confusion:

If you want  the same background blur... But

1) Want to stay where you are
2) The subject is also fixed in distance from you, but you want to pull it closer

Then adding a TC will mantain this OOF blur in the background without you needing to move closer to the subject.

However your framing has changed. All the TC buys you is re-positioning the subject in relation to you and the background. The closer the subject is to the sensor, the more oof blur you get for the background. The TC pulls the subject closer however since it reduces the F number, you are back to square 1... the only advantage you get is, you can stay far and shoot and still get some OOF blur in background since the TC has pulled your subject in a bit... confused? 

but the object does not get closer. it´s only magnified by the TC.

Correct, it gets closer to the sensor and appears larger (framing change)

[Malte_P]

Obviously we agreed on the same framed shot scenario, I just misspoke re the same place scenario.

actually your not.

but the other way around.

you said:

For the same distance shot using a TC will reduce dof,
for the same framed shot the dof is the same."

neuroanatomist said:

Adding a TC and shooting at the same distance means shallower DoF
Adding a TC and moving back so the FoV is identical means deeper DoF

[TrumpetPower!]

It's a confusing issue, to be sure. Partly, that's because of semantics.  There are lots of statements to the effect that "a TC doesn't affect the DoF."  That's true, if you account for the effects of a TC on both focal length and f/number. In other words, a 200 f/2.8 lens with a 2x TC becomes a 400mm f/5.6 lens.  That 200/2.8+2x combo will have the same framing and DoF as an actual 400mm f/5.6 lens, it will not have the framing of a 400mm lens with the DoF of a an f/2.8 lens (if you want the latter, you've got to pay several thousand $$ more for an actual 400/2.8 lens).

Somebody posted to some other thread an online tool that graphed background blur as a function of focal length, aperture, and distance from subject. Wish I could remember where it was, but the uptake is that a 50mm f/1.0 will have the most blur for things near the subject, and a 400 f/2.8 will have orders of magnitude more blur for things past about ten feet or so. So, if you want the least-recognizable background possible, speed wins for tight spaces but focal length blows it away if you've got the working distance.

True, but as the OP pointed out earlier, DoF isn't an exact conceptual opposite of OOF blur.  DoF defines what is in sharp focus, and that's independent of the distance between the subject and the foreground/background.  OOF blur is critically influenced by distances between the subject and fore-/background.

Agreed, of course, but with a caveat.

Most people looking for a shallow depth of field are actually looking to maximize background blur.

I don't think very many people actually want to take portraits where the pupil is in focus but the iris isn't, but many want to take portraits with the level of background blur that comes with using a 50 f/1.0 or 85 f/1.2 wide open at those distances.

I would suggest that the ideal portrait lens for that style is one with at least a couple inches of depth of field but which blurs the background beyond recognition.

That lens would just happen to be the 400 f/2.8...but only if you have an assistant and a walkie-talkie set....

Cheers,

b&

[K-amps]

That lens would just happen to be the 400 f/2.8...but only if you have an assistant and a walkie-talkie set....

Cheers,

b&

or the 200mm F/2.0

[dougkerr]

When comparing DoF issues, we must be very careful about what the conditions of the two situations are. We may say, "all other things being equal", but what does that mean? Same focal length? Same field of view?

For now. let's compare a situation of the same lens with and without a 2x focal length converter. Let's assume that focus is at the same distance, the focal length setting of the base lens is the same (and thus the focal length of the combination is twice that), and the f-number of the base lens is the same (so the f-number of the combination is twice that). And we will assume a consistent circle of confusion diameter limit (COCDL)

Then, in the configuration with the focal length converter in place, the depth of field will be substantially less than for the configuration with no focal length convertor.

If the setting of the base lens is f =  100 mm and f/3.5, with focus at a distance of 10 m, then, using a COCDL of 0.031 mm, the full depth of field is:

No focal length converter:  10.62 m

2x focal length converter:  4.50 m

But perhaps for our purposes it would be best to think in terms of a constant field of view (a constant focal length). Thus, with the focal length converter in place, we would set the focal length of the basic lens to (for our example) 25 mm. Then the results would be:

No focal length converter:  10.62 m

2x focal length converter:  67.98 m

Woof!

If we look at out-of-focus blur performance (which is what we are sometimes more interested in than depth of field), we also see interesting results.

Suppose that in both cases we assume focus at 10 m and are interested in the blur on a background object at 50 m. We will always use the base lens at its maximum aperture, f/2.0.

First suppose that with no focal length converter aboard, we set the focal length of the base lens to 50 mm, and we leave it set thus when we add the focal length converter. Then the results are (in terms of the diameter of the circle of confusion (blur figure) produces for a point on the background object:

No focal length converter:  0.10 mm (3.24 times our reference COCDL)

2x focal length converter:  0.20 mm  (6.52 times our reference COCDL)

This time, when we mount the focal length convertor, we reset the focal length of the base lens to 25 mm (to hold the actual focal length and thus the field of view). Now the results are:

No focal length converter:  0.10 mm (3.24 times our reference COCDL)

2x focal length converter:  0.05 mm  (1.62 times our reference COCDL)

Best regards,

Doug