1. Reminder: Please use our affiliate links for holiday shopping!

Question for the Brain Trust regarding DOF with 2x Tele-Extenders

Discussion in 'Open Discussion' started by roundball, Dec 10, 2014.

  1. roundball

    roundball TalkEmount Hall of Famer

    Oct 8, 2013
    USA
    My understanding and first hand experience is that adding a 2x tele-extender to a lens, results in a loss of 2 stops of light.
    For example, I can take a shot with a Canon FDn 200 / 2.8 and note the shutter speed.
    Then add a 2x extender, and in Aperture mode see that the light loss slows down shutter speed a couple steps.
    (The equivalent of an aperture being stopped down 2 stops).

    As we also know, a certain focal length and aperture setting gives a certain amount of DOF at a given distance.
    Using the same 200/2.8 as a reference, at 50 feet it gives a 2 foot DOF. (per an online DOF calculator)

    Now for my question:
    If a 2x extender is added, I know there is the light loss...BUT...does it effectively make the aperture a 5.6 as far as DOF is concerned?
    Would the configuration become a 400/5.6 as far as DOF is concerned...or would it be a 400 with DOF still that of a 2.8?
     
  2. jai

    jai TalkEmount Top Veteran

    589
    Feb 4, 2013
    Don't know from experience, but I am commenting anyway.

    I think the DOF should be similar to the lens without the teleconverter.

    The f number of the lens is determined by the ratio of the physical aperture (which doesn't change) to the focal length of the lens (which does).

    So because your focal length is doubling, your f number will also double. It should act like an F5.6 lens.
     
  3. jai

    jai TalkEmount Top Veteran

    589
    Feb 4, 2013
    Ok so I just went back and looked at a calculator, apparently a 400mm lens at F5.6 has a much shallower DOF than a 200mm F2.8.

    So, if your subject is the same distance, you should get a shallower DOF with the teleconverter. But you are probably using the teleconverter to shoot subjects further away!
     
  4. xXx1

    xXx1 TalkEmount All-Pro

    Jan 15, 2013
    For similar images teleconverter shouldn't affect dof. Teleconverter works by enlarging the image circle so the DOF is same for same enlargement ratio (that is, you shoot two times farther away to get similar image).
     
    • Like Like x 1
  5. OldNoob

    OldNoob TalkEmount Veteran

    223
    Oct 30, 2014
    ( completely ) revising my statement.
    In my experience, adding a 2x teleconverter to a 210mm f5 lens did not PRECIVABLY affect the DOF. For example photographer is at point A with a 200mm lens takes a photo of the subject at point B and the background is off in the distance at point C. the photographer now adds a 2x converter. points A B and C have not changed but ther is now 2x zoom factor on that image. if you were to crop the 200mm image to the same proportions as the 400mm image and then enlarge it to the same size as the 400mm image , the DOF would be very similar, depending on the distance of the background behind the subject.

    But technically it does increase the depth of field along with the f-stop.
    Depending on the lens, the effect can be greater or less noticeable, due to the distance and magnification involved.
    (ive corrected this ten times,, because my percieved experiance is quite different than the technical answer apparently LOL ;) )
     
  6. Kirkp

    Kirkp TalkEmount Regular

    151
    Nov 2, 2014
    This DOF tutorial explains why the DOF is nearly (but not completely) independent of focal length IF the magnification of the subject is held constant.
    http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

    ... This link provides some DOF experiments to illustrate this:
    http://www.luminous-landscape.com/tutorials/dof2.shtml
     
    • Like Like x 1
  7. jai

    jai TalkEmount Top Veteran

    589
    Feb 4, 2013
    The depth of field is only "independent" of focal length only if you hold the magnification constant AND you are comparing lenses with the same F-number.

    But the F-number itself is dependent on focal length!

    So if the fixed quantity is the physical diameter of the aperture, which it is in this case, then yes the DOF actually is dependent on focal length.

    Confusing!

    So! By the logic in your links above, a 200mm F2.8 will have equal depth of field to a 400mm F2.8 with the same magnification. But the teleconverted lens is a 400mm F5.6, so it should actually have a deeper depth of field than the naked lens.

    It's all pretty academic, because either way the DOF is going to be very thin.
     
    • Like Like x 1
  8. roundball

    roundball TalkEmount Hall of Famer

    Oct 8, 2013
    USA
  9. Kirkp

    Kirkp TalkEmount Regular

    151
    Nov 2, 2014
    Good point! I forgot the teleconverter also changed the F number!

    A 200mm F2.8 lens focused 50 feet away will have the same DOF as a 400mm F2.8 lens focused 100 feet away. (Doubling the focus distance keeps magnification the same.) But you are right to point out that a 200mm F2.8 lens with a 2x teleconverter becomes a 400mm F5.6 lens. So with the teleconverter you'll have a much wider DOF.
     
  10. roundball

    roundball TalkEmount Hall of Famer

    Oct 8, 2013
    USA
    I just posted the link to the DOF calculator showing that it cuts the DOF in half...doesn't make it wider.

    200mm/2.8 @ 50' has a DOF of 2.02 feet
    400mm/5.6 @ 50' has a DOF of 1.00 feet.....only half as much
     
  11. Kirkp

    Kirkp TalkEmount Regular

    151
    Nov 2, 2014
    In your example, the 400mm/5.6 lens should be focused at 100' to keep magnification the same. The DOF calculator says 4.0 feet in that case. So it's a factor of 2 wider DOF.

    Doubling the distance to keep magnification constant makes sense if you want to keep the subject the same size in your composition, but are using the teleconverter to allow you to work at a longer distance from the subject. Birders and sports photographers would use it this way.
     
  12. roundball

    roundball TalkEmount Hall of Famer

    Oct 8, 2013
    USA
    I'm sorry but you're making an invalid assumption... :) ..."Birders" don't automatically use them that way.
    I shoot birds almost every day...and in my example, if I wanted to shoot a bird in my back yard out at 50' and think a 200/2.8 will be too small for my tastes, I can add the 2x-B extender to shoot the same bird at 50', with an effective 400/5.6 configuration to get a larger image...just did it today.

    But the DOF will be cut in half according to that DOF calculator I referenced...which is what I started this thread to discuss / verify.
     
  13. jai

    jai TalkEmount Top Veteran

    589
    Feb 4, 2013
    Isn't that weird!

    Shooting at the same distance, the teleconverter is making your DOF shallower than the original lens.

    If you shoot at the same magnification, it's the opposite. The teleconverter has a deeper DOF than the original lens.

    That explains why people get so confused about it, and why the "experience of shooting" feels different to the maths.
     
    • Like Like x 1
  14. xXx1

    xXx1 TalkEmount All-Pro

    Jan 15, 2013
  15. Kirkp

    Kirkp TalkEmount Regular

    151
    Nov 2, 2014
    Fair enough. What I've learned from this thread is that when using a teleconverter I should be aware of how it affects DOF for both situations: fixed distance (where it reduces DOF) and fixed magnification (where it increases DOF).
     
  16. OldNoob

    OldNoob TalkEmount Veteran

    223
    Oct 30, 2014
    And that's the good point that we need to take away from all this. At the OP's stated focal lengths DOF isn't going to matter greatly unless there is an object that is within inches of the subject.

    Honestly i would be more concerned with IQ with the tele-converter attached, if it isn't a OEM converter
     
  17. roundball

    roundball TalkEmount Hall of Famer

    Oct 8, 2013
    USA
    Prior to starting this thread, I had looked at a few articles via Google searches and was struck by how much confusion and various assumptions / understandings / misunderstandings that there seemed to be...not a very simple cut & dried situation due to the variables involved in actual use.

    I've already had the Canon FDn 2x-B Extender for some time...(for use with Canon teles 300mm and greater)...and have used it many times with Canon FDn 300 and 400 lenses...excellent match-up for IQ and everything...including moon shots using the Extender with a Canon FDn 300/4.0-L.
    The extender made it's wide open F4.0 become an F8.0 as a starting point, and then I just set F11 for the moon shots like I always do...and it produced excellent moon shots.

    So I just ordered a Canon FD 2x-A Extender...(for use with Canon lenses shorter than 300mm teles)...and specifically only plan to use it with a Canon FDn 100-300/5.6-L Zoom Macro that I have coming for what I plan (hope) will be my main tool in the tool bag to take moon shots.
    The reason is to get the utility of the zoom letting me frame foliage / moonscape shots I'm confronted with, much easier than starting with a long dedicated tele, then forcing me to make lens changes in poor light, etc.
    I experimented with the 2x-B extender which arrived yesterday on a Canon FDn 80-200/4.0-L zoom and it gave excellent IQ...so I assume the results will be similar when the 100-300/5.6-L gets here.

    Since my typical set up for shooting moon shots is using F11, the wide open setting of 5.6 on the 100-300 will automatically become F11...and like with the extender on the 300 I mentioned above, DOF should not be an issue with the moon 238,000 miles away. Lens should be here next week and I'll soon know.

    Thanks for everyone's input
     
    • Like Like x 1
  18. xXx1

    xXx1 TalkEmount All-Pro

    Jan 15, 2013
    There is huge difference in quality among teleconverters. Good lens and good teleconverter may give good results but average lens and poor teleconverter will produce bad results always . I have Nikon ED 300mm/4.5 and Nikon 1.4 teleconverter and the combination is good enough for me. I try to find the 1.7 teleconverter as the combination is supposed to be quite good too.

    Moon is easy in DOF sense but is moves pretty rapidly. Manual star tracker might be nice toy.

    I might get Samyang 500mm/6.3 as a new toy category lens (it is only 150 € from foto-tip and very light weight) and DOF at 15 meters is only 22 cm (with Nex 7 sensor but that lens resolution is much lower).
     
  19. roundball

    roundball TalkEmount Hall of Famer

    Oct 8, 2013
    USA
    12/12/14 Early Morning Moon Test Shot

    CANON FDn 80-200/4.0-L with CANON FD 2x-B EXTENDER INSTALLED
    Effectively 400mm focal length, ISO200, F11, 1/125
    IQ held up pretty well, and DOF was no issue with the moon 238,000 miles away






     
    • Like Like x 1
  20. roundball

    roundball TalkEmount Hall of Famer

    Oct 8, 2013
    USA
    Couple hours had gone by so I just stepped out, completely set up all over again, and took a couple more shots.
    The moon was so far down near 'moonset' and the rising sun was so intense in the clear blue sky, it has a blue cast.
    But disregarding that, my main interests were:
    DOF
    Sharpness
    And verifying that while the 2x-B Extender doubled the fundamental max ap from 4.0 to 8.0, that all subsequent stop down steps would act normally, ie: 1 click = 1 step.

    Looks like all is well...now just need the FDn 100-300/5.6 to get here.