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Author Topic: Teleconverter and DOF  (Read 3612 times)

Malte_P

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Teleconverter and DOF
« 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.

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Teleconverter and DOF
« on: March 01, 2013, 06:13:05 AM »

neuroanatomist

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Re: Teleconverter and DOF
« Reply #1 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).
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Malte_P

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Re: Teleconverter and DOF
« Reply #2 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

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.





« Last Edit: March 01, 2013, 07:31:24 AM by Malte_P »

PavelR

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Re: Teleconverter and DOF
« Reply #3 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


Malte_P

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Re: Teleconverter and DOF
« Reply #4 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

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).
« Last Edit: March 01, 2013, 09:23:35 AM by Malte_P »

neuroanatomist

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Re: Teleconverter and DOF
« Reply #5 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

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.
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PavelR

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Re: Teleconverter and DOF
« Reply #6 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

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

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Re: Teleconverter and DOF
« Reply #6 on: March 01, 2013, 09:50:21 AM »

PavelR

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Re: Teleconverter and DOF
« Reply #7 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...

Malte_P

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Re: Teleconverter and DOF
« Reply #8 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

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?


Malte_P

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Re: Teleconverter and DOF
« Reply #9 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'.  


 ???
« Last Edit: March 01, 2013, 10:08:15 AM by Malte_P »

Mt Spokane Photography

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Re: Teleconverter and DOF
« Reply #10 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.
« Last Edit: March 01, 2013, 10:54:16 AM by Mt Spokane Photography »

Malte_P

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Re: Teleconverter and DOF
« Reply #11 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?

marinien

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Re: Teleconverter and DOF
« Reply #12 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?  ;)
« Last Edit: March 01, 2013, 10:42:27 AM by marinien »
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Re: Teleconverter and DOF
« Reply #12 on: March 01, 2013, 10:35:25 AM »

Mt Spokane Photography

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Re: Teleconverter and DOF
« Reply #13 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 :)

Malte_P

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Re: Teleconverter and DOF
« Reply #14 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.
« Last Edit: March 01, 2013, 11:36:01 AM by Malte_P »

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Re: Teleconverter and DOF
« Reply #14 on: March 01, 2013, 10:48:04 AM »