DOF and teleconverters...

Discussion in 'General Technical Discussion' started by manzico, Dec 29, 2005.

  1. This may be a dumb question but here goes. When you attach a teleconverter to a lens you end up with a minimum "effective aperture". Is this called an "effective aperture" because it is caused by the decrease in light caused by the widening of the image circle, or is it, in some way an actual decrease in minimum aperture size? If all the teleconverter does is widen the image circle, I would imagine that, yes, you would get less light on the sensor and so have to take up the slack in your exposure calculations (f-stop is a convenient place to do this), but it won't really effect the DOF of the lens, thus an f/2.8 won't be as "fast" because it requires more light, but it will still have the same DOF. If there is some other limiting factor that prevents the iris from opening as wide, then obviously there is a loss of speed and a loss of light. So which is it?

    thanks,

    Dave
     
  2. Im not the techie here, but my understanding very simply is you are adding an extention of an extra glass element inturn reducing the amount of light into the lens hence loose a stop on a 1.4tc or 2 stops on 2x tc.

    DoF depends only on aperture and magnification. When you put on a teleconverter, the DoF you lose to increase magnification is precisely the same as what's gained by reduced aperture, so there's no net effect. :Smart: I hope that is clear as mud. Where is Bjorn when you need him. LOL
     
  3. nfoto

    nfoto Guest

    Why, I'm right here :biggrin:

    DOF depends on aperture and image magnification, the latter of which also depends on focal length and focused distance.

    Teleconverters make the focal length of the lens longer by "widening" the light cone from the lens. So with a 300/2.8 and a TC 1.4X you get 420 mm (due to the factor 1.4X) and f/4 (because the light projected by the lens is now distributed over a larger area).

    At f/4, DOF is greater than at f/2.8. At a given focused distance, 420 mm gives less DOF than a 300 mm. So, you gain in one aspect and lose in another, and in the end, the changes cancel each out. Thus, DOF remains the same. f/4 @ 420 mm (300+1.4x) gives identical DOF to f/2.8 @ 300 mm.
     
  4. Thank you both...

    That's just what I needed. It's all very interesting stuff. I understand why f/4 has more depth of field than f/2.8. I also understand (I think) why longer focal lengths produce narrower depth of field. I can definitely see that they cancel out. I was seeing as much in the test shots I did with the teleconverter attached to the lens. This leaves me with one more question: why does the teleconverter necessitate the f/4 setting. I guess I'm a little fuzzy on the absolute definition of f-stop. I've always dealt with the number on a lens/lens basis so I've only needed a relative understanding of it. But here I have the same lens with a different minimum f-stop depending on the presense of the teleconverter. Either the teleconverter prevents the iris from opening as widely as it can when the teleconverter isn't present, or f-stop is tied to a relationship between image circle and aperture size. I'm still not clear on what drives the increase in minimum allowable f-stop when the teleconverter is there. What prevents a manufacturer from providing a teleconverter for the affor mentioned 300mm f/2.8 lens that provides a 1.4x focal length increase with the DOF expected with a f/1.4. lens (or would it be 1.8?), albiet with the unavoidable loss of light caused by widening the image circle? Sorry for all of the questions, but, like I said, it's very interesting stuff.

    thanks,

    Dave
     
  5. nfoto

    nfoto Guest

    When you understand the definition of "f-stop" (technically, it's the f-number, not "f-stop"), the mystery is resolved.

    An f-number is the ratio between the focal length of the lens and its entrance pupil. Never heard of the entrance pupil? don't worry, many haven't. Look into your lens from the front side, but at a distance. The "opening" you see is the entrance pupil, it serves to collect light on behalf of the optical system inside the lens barrel. What you really see is the virtual image of the aperture, not the physical aperture as such (most people think of the lens diameter instead so are lead astray).

    When you attach the teleconverter, focal length increases, but the entrance pupil stays put. Hence the f-number changes. If you have a 200/2 lens this means its entrance pupil is 100 mm. Put a 2X converter on the lens and it has a focal length of 400 mm (200 x 2). Now its entrance pupil still is 100 mm, so its f-number is 400/100 = 4. We get a 400mm f/4 lens by putting a 2X converter on a 200/2 lens.
     
    Last edited by a moderator: Dec 29, 2005
  6. F15Todd

    F15Todd

    Feb 1, 2005
    Tennessee
    Got to love those techno geeks, especially the ones that can explain things for the mass audience. Just giving you a hard time Bjorn. Thanks for being an active member of the Café, your info really does help people to learn.
     
  7. Gale

    Gale

    978
    Jan 26, 2005
    Viera Fl
    Hello Bjorn,

    Thank you so much for being an active member of the forum
    We are very fortunate.
    I love your way of explaining things for us.

    You say it like it is!!!!!!!! I appreciate that.

    Thank You

    Happy New Year.
    Best Regards
    Gale
     
  8. Thanks Bjorn, I needed that!
     
  9. Thanks Bjorn

    That is just the explanation I needed. Everything becomes clear when you get that last bit of information.

    thanks again,

    Dave
     
  10. Flew

    Flew

    994
    Jan 25, 2005
    Alabama
    Thanks Bjorn. The best, most straightforward explanation that I've seen.:smile:
     
  11. gvk

    gvk

    388
    Jun 17, 2005
    Mystic, CT

    I have to disagree with Bjorn's DOF calculation above. I totally agree with his explanation of how a TC affects focal length and aperture. However, for subject distances much longer than the focal length (i.e. outside the macro range; at magnifications larger than about 1:10 you have to account for effective aperture which depends on the ratio of image to pupil magnification) and much less than the hyperfocal distance, the DOF is directly proportional to f-number and inversely proportional to the magnification squared. Since magnification varies directly with focal length and inversely with distance, in this range the DOF is inversely proportional to focal length squared. Thus the aperture change with TC does not completely compensate for the increased focal length. In the 1.4X example given by Bjorn, the DOF at the same subject distance is actually 1.4X smaller when using the TC.
     
  12. nfoto

    nfoto Guest

    There is no such thing as a free lunch. On the other hand what you see is what you get. A teleconverter costs light, gives more reach, and gains nothing as far as DOF is concerned.

    May I suggest you back up your assertion by showing results from a well-documented, well designed experiment?
     
  13. gvk

    gvk

    388
    Jun 17, 2005
    Mystic, CT
    Free lunch? Of course not. I never suggested there was. I only stated that depth of field is not the same with and without TC, or in general for two lenses of different focal length at the same subject distance and with aperture adjusted in proportion to the ratio of focal length. I did not intend to start an argument, particularly with such an accomplished photograper and well known lens tester.

    This is hardly a radical assertion, more an established fact within the stated distance limits, and the usual approximation of Gaussian geometric optics (i.e. neglecting aberrations, diffraction, etc.). You can find these relationships in any lens reference that discusses DOF, such as section 22.2 of Applied Photograpic Optics by S.F. Ray.

    However, I was intrigued by your suggestion of an experimental demonstration. I have been busy the last couple of days shooting some portraits and an indoor track meet, so I did not have time to devise a test until this morning.

    I don't have a 1.4X TC anymore, so I used my TC-17EII with a 70-200 VR f/2.8. I shot both tests with zoom at the 105 mm setting. The nominal subject distance was around 2.2 m resulting in a magnification of about 1:20 @ 105 mm. With the TC-17EII mounted the equivalent focal length is about 180 mm and the maximum aperture f/4.8. The greater focal length with TC increased the magnification to around 1:11.7.

    The following photos were taken with a D2X mounted on a Gitzo 1325 tripod. Several shots were taken with the camera moved a few mm (with the aid of a leveled RRS MPR-192 rail) each side of the manual focus point to avoid focusing error. The sharpest results at the desired focus point were chosen for the comparison. Raw NEF files were processed to tiffs using Nikon Capture 4.4 with normal contrast and low sharpening settings. These tiffs were cropped and combined using Photoshop, resized by a linear factor of 0.4X and converted to jpeg. No other post processing or sharpening was applied.

    The meter stick was leaning against a wall at a 30 degree angle and the point of focus was at the top. The photos thus illustrate front depth of field with each cm mark on the scale corresponding to a half cm distance toward the camera. As expected, the DOF is somewhere around 1.7X greater without the TC. You have to allow for the difference in magnifications and the slight degradation of resolution with the TC, but the DOF difference is clearly visible.

    [​IMG]
     
  14. nfoto

    nfoto Guest

    What have you proved? That DOF is dependent on magnification, not on focal length when we move into the close distance range? That was known in advance. Also we know DOF isn't an intrinsic property of the lens, it is a perceived quantity of the end image including its post-recording magnification and viewing distance. Plus the usual approximations which necessitates standardised conditions in which to evaluate DOF.

    Could you repeat with a faster lens and the subject towards infinity, please? I'm not trying to disregard your experiment, I just need more data on which to ponder the issue. After New Year's Eve, my head is a little aching :biggrin: And for the record, I don't mind being proven wrong at all. This has known to happen before and certainly will happen again but hopefully, from my perspective, not too frequently :wink:
     
  15. gvk

    gvk

    388
    Jun 17, 2005
    Mystic, CT
    Yes, I think I demonstrated that DOF is dependent on magnification in a specific way, i.e. inversely proportional to M squared. Since M is directly proportional to focal length, DOF is also inversely proportional to f squared. That is all I stated in my original post. These relations apply only in the range where M << 1 and the subject distance is much less than the hyperfocal distance for the lens and aperture combination. These limits, however, cover most non-macro use of telephoto lenses. For example, with the 105 mm f/2.8 lens settings in my test, I calculate a hyperfocal distance of about 242 m (assuming a circle of confusion of 16 microns that is often considered appropriate for the DX sensor size). When subject distance is a significant fraction of the hyperfocal distance back DOF begins to increase hyperbolicly, so these approximations no longer apply.

    Your comment about post-recording magnification is very relevant to this discussion. Assuming you want to reproduce the same field in the final print by cropping instead of using the TC, you would need to use a smaller circle of confusion to compensate for the increased enlargement factor in order to see equivalent out of focus blur and apparent DOF. Since this increase in enlargement factor is identical to the TC focal length factor, and since DOF depends directly on COC as well as f-number, you now have two factors to cancel with the M (or f) squared in the denominator, and perceived DOF is theoretically identical. In other words, perceived DOF is dependent on the total "magnification" including image magnification and later enlargement factor (as well as viewing distance). I did this often in the mid-1960's when my only lenses for a Konica SLR were a 52 mm and 135 mm; I did my "zooming" with the enlarger, sacrificing increased grain and some sharpness degradation to get the framing I wanted without the ideal focal length lens.

    I chose my test conditions above to illustrate DOF dependence in a small image, suitable for posting. I also took a couple of shots at twice the distance, but would have needed a much larger image to illustrate them due to the greater DOF, and the results were similar. Any larger subject distances would not fit in my basement, and it was snowing here yesterday. :wink:

    My TC-17EII does not work with my other lenses (I once had a TC-14A, but maybe I lent it to a friend, or lost it. While my lens collection is tiny compared to yours, I suspect this may be a case similar to your 45 mm f/2.8 experience!). A similar test could be done using two lenses of different focal length. Maybe in the Spring, when it gets warm enough here to set up a controlled test outside, I can continue.

    I also rarely get upset about making mistakes, as long as I can eventually correct them; they are an essential part of the learning process, and we are all subject to human frailties. As a scientist (in real life) and as a photographer I often depend on intuition, and sometimes it fails me. Thankfully, I have caught most of my professional blunders long before I published them. Having been involved in many animated technical discussions with colleagues, I understand well the difference between arguing about ideas, versus the personality conflicts that seem to occur regularly in this type of discussion on other boards. Often differences in opinion are due to unstated assumptions that affect the comparison. That is why I have tried to be as explicit as possible in explaining my reasoning, and I fear that I ended up being long-winded.

    Although I spent a quiet New Years Eve at home with my wife and son, I did have a few glasses of wine, and stayed up much later than usual--my head is not aching, but it does seem a little harder to get the old body started this morning. :smile:

    By the way, I always enjoy your artistic contributions to the Cafe, as well as, your equipment discussions and more technical explanations, even when I have a small disagreement with them. :smile:
     
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