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of tele converters, DOF and fstops

Discussion in 'Open Gear Talk' started by rbelyell, Aug 3, 2015.

  1. rbelyell

    rbelyell SC Top Veteran

    820
    May 14, 2013
    NY Mtns
    so here is a very basic question/proposition: i put a 2x teleconverter on a 50/2.0 lens. i now have a 100mm lens with DOF of 4.0 and light gathering ability of the original maximum aperture of 2.0. correct, partially correct or entirely incorrect?

    thanks!
     
  2. Luke

    Luke Super Moderator

    Nov 11, 2011
    Milwaukee, WI USA
    Luke
    at least partially incorrect. You lose 2 stops of light gathering ability. Your f2 is now an f4 when shot wide open.

    My guess is your DOF would be like an f4 100mm lens (when shot wide open), but I'm not sure about that.
     
  3. Jock Elliott

    Jock Elliott SC All-Pro

    Jan 3, 2012
    Troy, NY
    As near as I can tell, a 2x teleconverter on your 50mm f/2 would turn it into a 100mm f/4.

    Here's a link to what Ken Rockwell has to say about teleconverters http://www.kenrockwell.com/nikon/tc.htm

    I believe his technical info is essentially correct. I did some research on trying to achieve long reach at reasonable cost with a DSLR or ILC and a teleconverter, and it seemed to me that "you can't get there from here." That's how I became a born-again superzoomer.

    Cheers, Jock
     
  4. Lightmancer

    Lightmancer Super Moderator

    Aug 13, 2011
    Sunny Frimley
    Bill Palmer
    All true. There's no such thing as a free ride, but it is also worth pointing out that a 1.4x converter, whilst giving a tad less reach also loses a tad less light...
     
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  5. rbelyell

    rbelyell SC Top Veteran

    820
    May 14, 2013
    NY Mtns
    first, thank you all. so it seems that while a 'crop'sensor only effects DOF and not native lens light gathering ability, the same is not true with a teleconverter. good to know.
     
  6. bassman

    bassman SC Rookie

    20
    Feb 12, 2014
    New Jersey, USA
    A crop sensor has no effect on DoF. DoF is purely a function of the lens aperture and focal length. The reason many think it does is that the lenses used on a cropped sensor have a shorter FL than the lens one would use to achieve the same field of view as a FF camera. A 25mm lens on an m43 camera at f/2.8 will have exactly the same DoF as it would on an APC sensor, a 1" sensor or a FF sensor. On the other hand, a 25mm lens on m43 has approximately the same field of view as a 35mm lens on APC, or a 50mm lens on FF. But the shorter lens - 25mm vs. 35mm vs. 50mm - will have the deeper DoF.
     
  7. Luke

    Luke Super Moderator

    Nov 11, 2011
    Milwaukee, WI USA
    Luke
    the size of the sensor has everything to do with the DOF. Just plug some numbers into any DOF calculator that you want to use.
     
  8. Tilman Paulin

    Tilman Paulin SC Top Veteran

    681
    Nov 15, 2011
    Dublin, Ireland
    You are both correct - from a theoretical and a practical point of view ;)

    DOF is indeed purely a function of the lens aperture and focal length.
    A 25mm 1.4 lens for example will always project the same image onto a camera's image plane, regardless of the size of the sensor in that image plane.

    What will be different is the composition/field of view of the image.

    A "Full Frame" sensor will record a 'wider' image, whereas a smaller (cropped) sensor will record a cropped version of that same image.
    In order to achieve a similar composition as on "full frame", you'll have to use a wider lens on a cropped sensor camera (which will result in less shallow depth of field).

    This image from wikipedia might help to shed some light on it.
    To get a similar composition, a 70mm lens is used on FullFrame vs a 50mm lens on APS-C.

    https://upload.wikimedia.org/wikipe...s_APS-C.svg/476px-Full-frame_vs_APS-C.svg.png

    Hope this helps to visualize this. I try not to worry about equivalencies too much. Makes my head spin :)
     
  9. Luke

    Luke Super Moderator

    Nov 11, 2011
    Milwaukee, WI USA
    Luke
    Thanks for your explanation TIlman.:bravo-009:

    And I'm glad you corrected me. The size of the sensor doesn't have "everything to do with DOF" as I overstated. :coco:

    But to get the shot one wants (as explained well in that link you provided) a different sized sensor will yield a different DOF. Because DOF is determined by three factors: the focal length of the lens, the f -number of the lens opening (the aperture), and the camera-to-subject distance.

    So because that third factor (the camera-to-subject) distance will change when using a different sensor, the DOF will change.

    So it's not the size of the sensor (in and of itself), but one can't remove the size of the sensor from the calculation.
     
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  10. rbelyell

    rbelyell SC Top Veteran

    820
    May 14, 2013
    NY Mtns
    luke i dont know if its a different DOF. my 50/2 will yield a DOF similar to the 100/4 it becomes on a 2x m4/3 sensor, and as well with a 2x teleconverter. in other words, shooting the 50 FF at f2 at ten feet will have the same area in focus as it will shooting wide open on a 2x crop sensor or with a teleconverter (though the FOV will be different). if the FF situation above yields say one foot in focus, so will the 2x crop sensor/teleconverter situations.

    what seems to be different back to the second part of OT is the light gathering ability in those two situations, where, if i understand it, the 2x m4/3 sensor preserves the 2.0 and the teleconverter does not. this was what i was trying to figure out. i still do not get why that is the case, though i do not doubt that it is in fact the case.
     
  11. Luke

    Luke Super Moderator

    Nov 11, 2011
    Milwaukee, WI USA
    Luke
    I wish I could explain why the light-gathering ability is affected with a teleconverter, but I know for sure that cropping doesn't. And I know from experience that using a teleconverter does.

    All that other mumbo-jumbo from the previous few posts has nothing to do with your original question and I'm sorry your thread is nearly getting de-railed.
     
  12. rbelyell

    rbelyell SC Top Veteran

    820
    May 14, 2013
    NY Mtns
    oh no, not at all. getting all kinds of information is only a good thing. and nothing was OT that i saw, but even if it was, the more info the better!

    actually, i am very interested in your point about the size of the sensor effecting DOF. my understanding, kind of consistant with my above thoughts, is that the DOF--or more specifically the measurable in focus area--is actually the same, though the FOV changes between FF and crop sensors shot at the same distance at the same aperture. the problem for crop sensor shooters shooting a 50/2 is they want their cropped 100mm fov to have a native max aperture of 2.0 for DOF purposes, vs what they actually get, which is a max aperture of 4.0 for DOF purposes. this cuts down on the oof areas they are looking for, but i think what they fail to understand is if theyre happy with the amount of oof area shooting a 50/2 wide open, they will get the same shooting that lens on crop.
     
    Last edited: Aug 4, 2015
  13. Luke

    Luke Super Moderator

    Nov 11, 2011
    Milwaukee, WI USA
    Luke
    I think what often confuses people trying to understand it is the importance of the distance to subject. To get the same framing for both shots, the subject will be a different distance from the sensor which affects the DOF. Here's a couple shots to show you the difference in the DOF with the same lens on 2 different sized sensors..... same lens, same aperture, same subject distance.
    20286883965_04c4460178_b. IMGP0137 by Luke, on Flickr
    19664240804_16b5f5b0e9_b. IMGP0135 by Luke, on Flickr

    Of course, if you shoot using the same lens on 2 different sized sensors with your subject at the same distance, you will have two totally different photos.
     
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  14. rbelyell

    rbelyell SC Top Veteran

    820
    May 14, 2013
    NY Mtns
    yes, distance is critical. i think we are absolutely saying the same thing, just from a different place! to your last sentence i would add 'but you would have the same measurable area in focus'. i believe we are in total agreement.
     
    Last edited: Aug 4, 2015
  15. Luke

    Luke Super Moderator

    Nov 11, 2011
    Milwaukee, WI USA
    Luke
  16. rbelyell

    rbelyell SC Top Veteran

    820
    May 14, 2013
    NY Mtns
    well luke that article doesnt do much for my lifelong preference of reducing complex issues to a series of simple statements!
     
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  17. Luke

    Luke Super Moderator

    Nov 11, 2011
    Milwaukee, WI USA
    Luke
    LOL.

    Maybe this is as close as one can come to simplifying it all down to one simple statement.

    From wikipedia....... "DOF is determined by subject magnification at the film / sensor plane and the selected lens aperture or f-number. For a given f-number, increasing the magnification, either by moving closer to the subject or using a lens of greater focal length, decreases the DOF; decreasing magnification increases DOF. For a given subject magnification, increasing the f-number (decreasing the aperture diameter) increases the DOF; decreasing f-number decreases DOF."
     
  18. bartjeej

    bartjeej SC Hall of Famer

    Nov 12, 2010
    bart
    The reason why teleconverters affect the effective aperture is that they are mounted behind the lens; they basically take all the light that enters the lens and spread it further outwards, so that the central portion now covers the entire sensor. The amount of light that is now aimed away from the sensor is "wasted".

    Similarly, a wide angle converter takes the light that enters the lens, and points it inwards, so that part of the light that would normally end up besides the sensor, now lands up inside the sensor. This means there is more total light that ends up on the sensor, which means a faster effective aperture. Ofcourse if the lens is tailored for the sensor size, it won't have much light falling besides the sensor and a wide angle converter only creates vignetting, which is why the Speedbooster converters don't work on full frame cameras (unless they were to make a version that lets you use medium format lenses on full frame bodies).

    Converters that are mounted in front of the lens (as opposed to between the lens and the body) usually don't change the effective aperture, since what matters to determine the f-stop is the size of the aperture as seen from the front of the lens (the direction from which the light is coming), and with a front-mounted converter, the aperture as "seen" by the incoming light changes with the same ratio as the angle of view.


    As for DOF: that follows the light gathering ability every time; there's no separating light gathering ability and DOF where f-stops are concerned. That's because shallow DOF is caused by off-axis light (that doesn't travel in a straight line between whatever it's bouncing off of and the center of the lens) still gets captured by the outer regions of the lens, and then ends up somewhere near the straight-line light, but not at exactly the same place; in other words, the light from 1 point ends up in different places on the sensor, rendering that point blurry. Increasing the aperture size doesn't increase the amount of straight-line light that gets captured, but it does increase the amount of off-axis light.

    If that's confusing, think of it as aiming pingpong balls at a small circle inside a bucket; increasing the bucket size doesn't increase the number of times you hit that small circle, but it does increase the number of pingpong balls that you manage to get in the bucket. The ones that would've missed the smaller bucket completely, now land inside the bucket, but somewhere besides the small circle. Those balls represent the result of a larger aperture; increasing the total light gathered, and blurring the image that's not on the focal plane.

    Regarding the question of what delivers shallower DOF: it depends completely, based on which parameters you keep the same. A smaller sensor with the same absolute focal length and f-stop ( = same physical aperture size) will have shallower perceived depth of field, even though the absolute depth of field stays the same. The reason for this is that, assuming you view / print the images at the same size, the smaller sensor's image is magnified more than the bigger sensor's image. The result of that is that any blur that might be invisible due to the low magnification of the bigger sensor's image, does becomes visible in the smaller image. For instance, if the lens renders a point at 1m behind the subject as a 0.02mm blotch on the sensor plane, that would be 0.055% of the width of a full frame sensor, but 0.11% of a 4/3 sensor. If you print both images at the same size, the point will be spread out over a larger portion of the image with the smaller sensor, so the fact that it's rendered as a blotch instead of a point will become obvious sooner.

    In other words, areas that might appear as sharply rendered points in a full frame sensor, can turn out to be blurry blotches in the 4/3 image, even though you used the same lens, f-stop and subject distance. That effect is expressed by the "circle of confusion" in the DOF calculator.

    As for cropping affecting light-gathering ability: yes and no. The density of the light falling on the sensor doesn't change, but the total amount of light that you capture (or use) does change. If you then print / view the image at the same size as you would the uncropped image, you'll see more noise - since the noise that was there in the central portion of the image gets magnified - which makes the result the same as if you'd used a longer lens with slower f-stop and higher ISO on the uncropped image.
     
    Last edited: Aug 4, 2015
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  19. rbelyell

    rbelyell SC Top Veteran

    820
    May 14, 2013
    NY Mtns
    bart you are the einstein of photolounge! great explanations. i think where my analysis went wrong was i did not know of nor understand the concept of 'perceived DOF' vs 'absolute DOF', assuming incorrectly that 'absolute' meant 'absolute' absolutely. ):
     
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