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question: teleconverter effect on DOF
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PostPosted: Wed Oct 10, 2012 6:49 pm    Post subject: question: teleconverter effect on DOF Reply with quote
My understanding is that using a 2x teleconverter doubles the lens's stated apertures setting. so setting a 50/2 lens to f2 whilst on a 2x teleconveter means you are really shooting at an f stop of 4.0.

this being so, what then is the DOF of that same lens attached to a 2x teleconverter at that setting: is it the DOF that corresponds to f2 or f4?
thanks
tony

Last edited by rbelyell on Wed Oct 10, 2012 8:09 pm; edited 1 time in total


PostPosted: Wed Oct 10, 2012 7:07 pm    Post subject: Reply with quote
A 2* teleconverter will give you twice the focal length, but will decrease the light by a factor of 4
as well as decrease the resolution (by a factor of 2).
See http://en.wikipedia.org/wiki/Teleconverter


PostPosted: Thu Oct 11, 2012 1:28 am    Post subject: Reply with quote
f/4

Changing the magnification or the distance changes the dof.


PostPosted: Thu Oct 11, 2012 1:02 pm    Post subject: Reply with quote
The DOF stays theoretically exactly the same.
The teleconverter is only cropping and enlarging a part of the picture the lens normally produces.

Only thing is that with an higher magnification the DOF scale might be tinyer for your eyes
DOF-wise there is no difference between cropping on a PC or using a teleconverter.


PostPosted: Thu Oct 11, 2012 1:17 pm    Post subject: Reply with quote
so FS are you disagreeing with post of Visualopsins? are you saying with teleconverter DOF at any f stop stays normal and does not double?
thanks
tony


PostPosted: Thu Oct 11, 2012 1:27 pm    Post subject: Reply with quote
ForenSeil wrote:
[...]DOF-wise there is no difference between cropping on a PC or using a teleconverter.


So long as the crop is not enlarged this is true. Enlarging (magnifying) the crop changes dof.


PostPosted: Thu Oct 11, 2012 1:59 pm    Post subject: Reply with quote
Yes. That's because the DOF is always subjective. Physically the DOF stays the same when using a teleconverter but due the higher magnifation of the crop it looks more shallow.

If you compare old with modern MF lenses you will see that many modern lenses will have a tighter DOF scale printed on them.
That's because resolution of film/sensor and view size increased during the last decades.

Last edited by ForenSeil on Thu Oct 11, 2012 6:00 pm; edited 1 time in total


PostPosted: Thu Oct 11, 2012 3:54 pm    Post subject: Reply with quote
well, i,m not sure where this is coming out in terms of my original question. my one comment is that DOF imo cannot be subjective. there is either a zone when things are in focus at a given aperture or there is not. since many folks have made careers shooting 'zone focus' , DOF must be objective.
tony


PostPosted: Thu Oct 11, 2012 4:41 pm    Post subject: Reply with quote
rbelyell wrote:
well, i,m not sure where this is coming out in terms of my original question. my one comment is that DOF imo cannot be subjective. there is either a zone when things are in focus at a given aperture or there is not. since many folks have made careers shooting 'zone focus' , DOF must be objective.
tony


Actually not, for example the depth of field depends on the size of the print, together with focal length and size of the "sensor" (or film): so it's a limit of resolution above which we call things on focus, but it's a convention based on perception.

I'm afraid I didn't explain it well, but I don't know how to say some of the things, and it's hard to explain the little I know in english, so sorry if it is not clear.

EDIT
maybe here is better expressed than I will ever be able to:
http://kcbx.net/~mhd/2photo/dof.htm

back on topic, obviously a TC will affect the perceived DOF - I don't think I am able to quantify it.


PostPosted: Thu Oct 11, 2012 5:58 pm    Post subject: Reply with quote
Exactly.

But to finaly quantify it I would say the DOF of a 100/4 will be about the half than from a 50/2 with an identical printsize and identical distance. Wink

So if your DOF was 20cm it will be 10cm if you keep the distance.
If you don't keep distance but the same magnification the DOF would be around 40cm instead of 20cm

Values are only to give you and approximate conception


PostPosted: Thu Oct 11, 2012 6:30 pm    Post subject: Reply with quote
so just so my simple mind understands this correctly, at a given distance a using a teleconverter does NOT effect the original DOF of the lens in use? correct?


PostPosted: Thu Oct 11, 2012 6:55 pm    Post subject: Reply with quote
Short simple answer...

Teleconverters will not affect DOF. Your 200mm F4 (for example) lens is still a 200mm F4 with the same DOF.


PostPosted: Thu Oct 11, 2012 7:21 pm    Post subject: Reply with quote
thanks to everyone, much appreciated!
tony


PostPosted: Sun Oct 14, 2012 2:23 am    Post subject: Reply with quote
Dof scale on lens is valid with or without TC.

Without TC, lens set at f/2, use distances between f/2 marks for dof.

With 2X TC, lens set at f/2, 2X TC loses 2 stops, so use distances between f/4 marks for dof.


PostPosted: Sun Oct 14, 2012 5:04 am    Post subject: Reply with quote
martinsmith99 wrote:
Short simple answer...

Teleconverters will not affect DOF. Your 200mm F4 (for example) lens is still a 200mm F4 with the same DOF.


visualopsins wrote:

Dof scale on lens is valid with or without TC.

Without TC, lens set at f/2, use distances between f/2 marks for dof.

With 2X TC, lens set at f/2, 2X TC loses 2 stops, so use distances between f/4 marks for dof.


Folks, this makes no sense! Suppose you focus at 2m with the lens @f2. As you yourself just explained, 2x TC just crops the picture and enlarges the crop to the original size. Well, do this with any picture, it will look blurrier! That means that DOF has effectively shrunk compared to the shot taken with naked lens @f2. The DOF scale for F4 shows even greater DOF than DOF for F2 so it can not be accurate either. So to answer OP, DOF scale is inaccurate with TC.

What is true, is that if you maintain the same magnification then DOF will remain the same. However, in order to do so with TC, you'll have to step back. In other words, you'll have to focus at different (larger) distance, say 4m. In this scenario (same magnification), you can read DOF from F4 DOF scale for 2m. However, this is not how TC is typically used.


PostPosted: Mon Oct 15, 2012 4:00 am    Post subject: Reply with quote
The things are getting mixed up a bit

DOF Calculator:
http://www.dofmaster.com/dofjs.html

I hope this solves the mystery
Laughing

Last edited by ForenSeil on Mon Oct 15, 2012 6:27 pm; edited 1 time in total


PostPosted: Mon Oct 15, 2012 12:10 pm    Post subject: Reply with quote
i'm not sure how that chart helps, sorry. because the point of the question is that both the FL and the aperture of a given lens changes with the use of a TC. thus the initial question still remains re the chart: which numbers to plug into the equation, the original, the modified, or some combination of the two?
tony


PostPosted: Mon Oct 15, 2012 12:23 pm    Post subject: Reply with quote
You plug in modified numbers. 50mm lens @ f2 with 2xTC becomes 100mm lens @f4. So you plug in 100 for FL and f4 for aperture.


PostPosted: Mon Oct 15, 2012 12:41 pm    Post subject: Reply with quote
According to dof calculator, F/2 50mm @3m gives dof = 0.284, and F/4 100mm @6m gives dof = 0.568. Note 0.568 is exactly twice 0.284.


PostPosted: Mon Oct 15, 2012 1:03 pm    Post subject: Reply with quote
but a given object 3meters away is STILL 3 meters away when using a TC, right? using a TC does NOT change how far away an object is, does it? it just changes how far it appears.


PostPosted: Mon Oct 15, 2012 1:18 pm    Post subject: Reply with quote
Yep, the distance you plug in is the real object distance.


PostPosted: Mon Oct 15, 2012 1:25 pm    Post subject: Reply with quote
According to dof calculator, F/2 50mm @3m gives dof = 0.284m, and F/4 100mm @3m gives dof = 0.139m, about 1/2 of 0.284m...


PostPosted: Mon Oct 15, 2012 3:03 pm    Post subject: Reply with quote
so what youre saying is according to DOF calculator, the DOF of a specific lens shooting the same object at the same distance at a given aperture setting will be half as much with a teleconverter as without it? in other words everything being equal a teleconverter halves a given lens's DOF???


PostPosted: Thu Oct 18, 2012 6:45 am    Post subject: Reply with quote
rbelyell wrote:
so what youre saying is according to DOF calculator, the DOF of a specific lens shooting the same object at the same distance at a given aperture setting will be half as much with a teleconverter as without it? in other words everything being equal a teleconverter halves a given lens's DOF???


Confirmed by dof calculator.

A 50/2 lens with 2X TC is essentially equivalent to a 100/4 lens. Put 2X TC on 50/2 lens, then use the dof tables for 100/4 lens.

Note what I wrote earlier about the lens dof scale remaining accurate with TC is nonsense. Sorry for that confusion.


PostPosted: Thu Oct 18, 2012 12:03 pm    Post subject: Reply with quote
okey dokey! thanks everyone! hope this helps others as well. i think DOF is often misunderstood, and frankly more often overlooked as one of the most important considerations in all of photography.
tony
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