# canon rumors FORUM

## Gear Talk => Technical Support => Topic started by: Malte_P on March 01, 2013, 06:13:05 AM

Title: Teleconverter and DOF
Post by: Malte_P on March 01, 2013, 06:13:05 AM
im still kind of a noob.. so is there a website or can someone explain what happens in terms of DOF when i use a teleconverter?

i know i lose 1 or 2 stops of light but what about DOF?

lets say i have a EF 100mm f2 and i attach a kenko 1.4 teleconverter.
Title: Re: Teleconverter and DOF
Post by: neuroanatomist on March 01, 2013, 06:18:31 AM
In your example, you'd effectively have a 140mm f/2.8 lens.  So, you can see the TC effect on DoF by plugging 100mm f/2 vs. 140mm f/2.8 into a DoF calculator (here's one (http://www.dofmaster.com/dofjs.html)).
Title: Re: Teleconverter and DOF
Post by: Malte_P on March 01, 2013, 06:35:47 AM
Quote
In your example, you'd effectively have a 140mm f/2.8 lens.  So, you can see the TC effect on DoF by plugging 100mm f/2 vs. 140mm f/2.8 into a DoF calculator

so this is wrong:

http://photo.stackexchange.com/questions/4928/how-does-a-teleconverter-affect-depth-of-field (http://photo.stackexchange.com/questions/4928/how-does-a-teleconverter-affect-depth-of-field)

Quote
Because the physical aperture size is unchanged, depth of field is unchanged for a given focusing distance.

Quote
Any 600 f/5.6 lens, teleconverted or not, will have the same depth of field as a 300 f/2.8 lens.

when i put the numbers in a DOF calculator they are not the same.

300mm@f2,8 focus distance 30m =  1.68m
600mm@f5.6 focus distance 30m =  0.83m

im confused because i read so many different things.  ???
or maybe i just don´t get it.

so how i understand it.
a teleconverter will not change the physical aperture size but it will change the focal length.
and because it changes the focal length the f-stop of the lens will change.

50mm = 100mm / f2.0
50mm = 100mm x1.4 / f2.8

DOF is not linear to f-stop and focal length.
because a 300mm f2.8 lens has a larger DOF then a 600mm f5.6 lens at a fixed focus distance.

Title: Re: Teleconverter and DOF
Post by: PavelR on March 01, 2013, 08:44:47 AM
DOF will be smaller because of the bigger magnification.
You can see it:
Take two photos - one with TC and one w/o TC.
If you crop the image taken w/o TC to the same framing of the other one, you get two identical results.
DOF is described perfectly at http://toothwalker.org/optics/dof.html (http://toothwalker.org/optics/dof.html)

Title: Re: Teleconverter and DOF
Post by: Malte_P on March 01, 2013, 08:58:41 AM
DOF will be smaller because of the bigger magnification.
You can see it:
Take two photos - one with TC and one w/o TC.
If you crop the image taken w/o TC to the same framing of the other one, you get two identical results.
DOF is described perfectly at http://toothwalker.org/optics/dof.html (http://toothwalker.org/optics/dof.html)

but that includes enlarging.
i guess you mean croping and enlarging to the same size as the other image?
i don´t want to imagine what that does with the COC.... and stuff.

and how can i get the same results when you first say the DOF is different?   ???
because when i enlarge the image the parts that appear sharp will then become unsharp?
because i enlarge the COC?

i want to know if i can calculate the DOF of a lens 100mm f2 + 1.4x TC the same way i would calculate a 140mm f2.8 lens.

if what neuroanatomist wrote is correct, and i can ignore the other stuff i have read (that lenses with the same physical aperture diameter have always the same DOF at the same focus distance).
Title: Re: Teleconverter and DOF
Post by: neuroanatomist on March 01, 2013, 09:22:26 AM
so this is wrong:

http://photo.stackexchange.com/questions/4928/how-does-a-teleconverter-affect-depth-of-field (http://photo.stackexchange.com/questions/4928/how-does-a-teleconverter-affect-depth-of-field)

Quote
Because the physical aperture size is unchanged, depth of field is unchanged for a given focusing distance.

Quote
Any 600 f/5.6 lens, teleconverted or not, will have the same depth of field as a 300 f/2.8 lens.

The statements you quoted are incorrect.

or maybe i just don´t get it.

so how i understand it.
a teleconverter will not change the physical aperture size but it will change the focal length.
and because it changes the focal length the f-stop of the lens will change.

You do get it.  Basically, for a given sensor size there are three factors that determine DoF: aperture, focal length, and subject distance.  If you consider the case of taking 'the same shot' i.e. identical framing, such as filling the frame with a 6' tall person, then focal length and distance cancel each other out (longer focal length means you must be further from the subject), and only aperture determines the DoF.

So, if you take a head shot with your 100/2, you may be around 2 m from your subject.  If you then put on a 1.4x TC and take the same headshot, you have to back up to around 2.8 m get the subject's whole head in the frame with your 140mm f/2.8 lens.  But you're now shooting with an f/2.8 lens, so the DoF will be deeper with the TC, for 'the same shot'.  If you don't change the subject distance, the 100/2 + 1.4x (aka 140mm f/2.8 lens) will give you a shallower DoF than the 100/2...but your shot will cut off the subject's hair and chin.
Title: Re: Teleconverter and DOF
Post by: PavelR on March 01, 2013, 09:50:21 AM
DOF will be smaller because of the bigger magnification.
You can see it:
Take two photos - one with TC and one w/o TC.
If you crop the image taken w/o TC to the same framing of the other one, you get two identical results.
DOF is described perfectly at http://toothwalker.org/optics/dof.html (http://toothwalker.org/optics/dof.html)

but that includes enlarging.
i guess you mean croping and enlarging to the same size as the other image?
i don´t want to imagine what that does with the COC.... and stuff.

and how can i get the same results when you first say the DOF is different?   ???
because when i enlarge the image the parts that appear sharp will then become unsharp?
because i enlarge the COC?

i want to know if i can calculate the DOF of a lens 100mm f2 + 1.4x TC the same way i would calculate a 140mm f2.8 lens.

if what neuroanatomist wrote is correct, and i can ignore the other stuff i have read (that lenses with the same physical aperture diameter have always the same DOF at the same focus distance).
Yes, I mean cropping and enlarging to the same physical size on the display / paper / ...
Difference can be seen on cropped and uncropped image (TC image is for getting adequate magnification).
I can not help you with math to calculate the DOF, because it depends on the final magnification ~ crop, lens sharpness, viewing distance, and many other factors, thus such number does mean nothing to me... (I need to know my lenses and predict the real result with different apertures; I do not want to convince anybody about the sharpness of the photo because DOF calculator says: it is in the DOF...)
There is another thread discussing DOF and background blur deeply: http://www.canonrumors.com/forum/index.php?topic=11109 (http://www.canonrumors.com/forum/index.php?topic=11109)
Title: Re: Teleconverter and DOF
Post by: PavelR on March 01, 2013, 09:58:05 AM
DOF will be smaller because of the bigger magnification.

If you crop the image taken w/o TC to the same framing of the other one, you get two identical results.

these kind of "explanation" are exactly why 80% don´t get it.   :P  :D
The first sentence is the definition of the change.
The second is the last step to produce the image that can be compared.
Anytime you cut some sentences from the article, you can get words that can not be understood...
Title: Re: Teleconverter and DOF
Post by: Malte_P on March 01, 2013, 10:03:20 AM
Quote
So, if you take a head shot with your 100/2, you may be around 2 m from your subject.  If you then put on a 1.4x TC and take the same headshot, you have to back up to around 2.8 m get the subject's whole head in the frame with your 140mm f/2.8 lens.  But you're now shooting with an f/2.8 lens, so the DoF will be deeper with the TC, for 'the same shot'.  If you don't change the subject distance, the 100/2 + 1.4x (aka 140mm f/2.8 lens) will give you a shallower DoF than the 100/2...but your shot will cut off the subject's hair and chin.

ok thanks so far.
now to the enlarging part.

http://photo.stackexchange.com/questions/4928/how-does-a-teleconverter-affect-depth-of-field (http://photo.stackexchange.com/questions/4928/how-does-a-teleconverter-affect-depth-of-field)

on this site is an example with an 35mm @f1.8 and a 210mm @f11 lens that, after cropping and enlarging, show the same DOF.

when the images are shot from the same position (lets say 3m).
the 35mm @f1.8 would have a DOF of 0,79m.
the 210mm @f11 would have a DOF of 0.13m.

so the DOF is very different.

i crop the 35mm image, so the field of view is the same and i enlarge the crop to the original size. now it looks like both images have the same DOF.

why?
because i enlarge the original COC of the uncropped image?

but that´s not the same as taking the "same picture" from a different distance (as in neuroanatomist example)?

correct?

Title: Re: Teleconverter and DOF
Post by: Malte_P on March 01, 2013, 10:06:20 AM

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.

sorry but isn´t that different to this:

Quote from: neuroanatomist
So, if you take a head shot with your 100/2, you may be around 2 m from your subject.  If you then put on a 1.4x TC and take the same headshot, you have to back up to around 2.8 m get the subject's whole head in the frame with your 140mm f/2.8 lens.  But you're now shooting with an f/2.8 lens, so the DoF will be deeper with the TC, for 'the same shot'.

???
Title: Re: Teleconverter and DOF
Post by: Mt Spokane Photography on March 01, 2013, 10:24:10 AM
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 DECREASE 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 increase.

So, for wildlife where you merely need more focal length and can't get closer, you get more  LESS 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.

Here is a image taken at a close distance with my 70-200mm at f/4 and a 1.4X TC.  The brown wall in the background was about a foot away and very much out of focus.  The bird was about 10 ft away and is cropped.
You can do this by adding a TC and getting closer to the subject, which decreases the depth of field.
With my 1D MK III, the depth of field with TC at 10 ft is 0.09 ft, If I did not move, just removed the TC, it would be increased to 0.13 ft.
(http://www.mount-spokane-photography.com/Nature/Birds/i-9FrwL8T/0/L/Finchesextender%20test%201.4XEMW_815320100320-L.jpg)
Title: Re: Teleconverter and DOF
Post by: Malte_P on March 01, 2013, 10:26:39 AM

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.

please look at the post above yours.
im to stupid? or isn´t that different to what neuroanatomist wrote?
Title: Re: Teleconverter and DOF
Post by: marinien on March 01, 2013, 10:35:25 AM
Yes it is different, I think Neuro has made a small error.

The subject magnification stayed the same (the framing) the apparent aperture is the same (the hole in the lens stayed the same physical size) so the dof is the same.

Sorry PBD, but Neuro made no mistake. If you want the DoF on the 140mm stay the same, it needs to be at f/2 (supposing that the 100mm is at f/2). However, the 140mm is at f/2.8, so the DoF is deeper.

Let's exaggerate a little bit: do you think that for the same framing, the 50mm @f/1.0 will have the same DoF as the 500mm @f/10?  ;)
Title: Re: Teleconverter and DOF
Post by: Mt Spokane Photography on March 01, 2013, 10:37:23 AM

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.

please look at the post above yours.
im to stupid? or isn´t that different to what neuroanatomist wrote?

OOPS, I confused my self :)
Title: Re: Teleconverter and DOF
Post by: Malte_P on March 01, 2013, 10:48:04 AM
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.
Title: Re: Teleconverter and DOF
Post by: neuroanatomist on March 01, 2013, 10:55:21 AM
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.

Sorry, but a 100mm f/2 lens with a 2x TC would be a 200mm f/4 lens, not f/5.6.

So, let's repeat your example with correct math (and a specified sensor size, since that does matter):

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

Plug those numbers (100/2, 200/4, 20 ft vs. 40 ft) into your DoF calculator of choice...you may get slightly different absolute values (different calculators make different assumptions about CoC, print size and viewing distance, which affects the calculated DoF values), but the relative differences will be the same.

Compared to a bare lens:
• Adding a TC and shooting at the same distance means shallower DoF (and a smaller FoV
• Adding a TC and moving back so the FoV is identical means deeper DoF
Title: Re: Teleconverter and DOF
Post by: TrumpetPower! on March 01, 2013, 11:02:43 AM
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.

Quote

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&
Title: Re: Teleconverter and DOF
Post by: Mt Spokane Photography on March 01, 2013, 11:08:33 AM
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 (http://www.dofmaster.com/dofjs.html)
Title: Re: Teleconverter and DOF
Post by: Malte_P on March 01, 2013, 11:19:00 AM
neuroanatomist must be jobless or retired without a life.

no sane guy spends op many hours on a internet forum.....  ::)
Title: Re: Teleconverter and DOF
Post by: Malte_P on March 01, 2013, 11:23:50 AM
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:

Quote
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.

Quote from: neuroanatomist
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
Title: Re: Teleconverter and DOF
Post by: RLPhoto on March 01, 2013, 11:29:12 AM
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.
Title: Re: Teleconverter and DOF
Post by: neuroanatomist on March 01, 2013, 11:36:10 AM
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.
Title: Re: Teleconverter and DOF
Post by: Malte_P on March 01, 2013, 11:40:49 AM
Quote
"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 :

Quote
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?

Title: Re: Teleconverter and DOF
Post by: neuroanatomist on March 01, 2013, 11:45:29 AM
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.

Quote
"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?  :P

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.   ;)
Title: Re: Teleconverter and DOF
Post by: K-amps on March 01, 2013, 11:48:15 AM
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?  :D
Title: Re: Teleconverter and DOF
Post by: Malte_P on March 01, 2013, 11:52:03 AM
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?  :D

but the object does not get closer. it´s only magnified by the TC.
Title: Re: Teleconverter and DOF
Post by: K-amps on March 01, 2013, 11:53:45 AM
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?  :D

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)
Title: Re: Teleconverter and DOF
Post by: Malte_P on March 01, 2013, 11:56:31 AM
Obviously we agreed on the same framed shot scenario, I just misspoke re the same place scenario.

but the other way around.

you said:

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

neuroanatomist said:

Quote
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
Title: Re: Teleconverter and DOF
Post by: TrumpetPower! on March 01, 2013, 12:10:56 PM
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&
Title: Re: Teleconverter and DOF
Post by: K-amps on March 01, 2013, 12:56:27 PM

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
Title: Re: Teleconverter and DOF
Post by: dougkerr on March 01, 2013, 01:08:21 PM
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