Question for an optic exam - All about Dof / Hyperfocal...

Discussion in 'Beginner Questions' started by proy, Oct 30, 2013.

  1. Hi.
    Just started studying photography and I have an exam tomorrow.
    I was given a few demo questions the teacher might ask, and there's one for which I am totally puzzled. Can anyone help out?

    "During the shooting, the focus point is made 0.3m on the main subject. In that scene, objects are still in focus at 20 m, other placed just in front of the main subject are blurred.
    If an object is placed at 60m, will it be in focus? "
    If I'm lucky, I can get a response back from the next few hours..
    Much much thanks for any clarification :))
  2. Presumably since there's an exam there is some sort of course material. Is there a description of hyperfocal distance? Is there an equation for calculating hyperfocal distance?
    I think the question that you quoted is not clearly expressed.
    There is only one subject distance that is theoretically perfectly focused. Everything else is out of focus, but if it's only a little bit out of focus it is considered to be sharp enough (according to some criterion of sharpness, often based on print size). This is the depth of field, and how big it is depends on a number of things (aperture, lens focal length, your fussiness about what is sharp enough)
    Hyperfocal distance is a particular case of normal depth of field considerations.
    When a lens is focused at hyperfocal distance, everything from infinity to half the hyperfocal distance is in focus (and "in focus" according to the allowable fuzziness set in the equation, generally called "circle of confusion")
  3. the focus point is made 0.3m on the main subject​
    Say what? Can someone translate that one into plain English please?
    How can anyone answer that question without knowing at least the focal length and the aperture? As well as the circle of confusion?
  4. Patrick,
    Depth of field is the length of the span before and after the focus point that remains acceptable as to sharpness. This zone straddles the point focused upon but the zone is not split down the middle. The zone of acceptable focus (DOF) extents 2/3 away from the camera and 1/3 back towards the camera as measured from the point focuses upon. The length of this zone is a variable. Short focal length lenses yield lengthened DOF while long focal length lenses yield reduce DOF. If the focus point is near the camera, DOF is shallow. If the focus point is far from the camera, DOF is expanded.
    The wording of the demo question makes for some guesswork. The nebulous part is “focus point is made 0.3m on the main subject”. I think, objects at 60m will not be rendered actable as to sharpness because objects place closer to the camera are blurred. To my way of thinking DOF must be somewhat shallow and unlikely to extend out to 60m.
    To help you a bit: Hyperfocal distance is the lest distance setting on the lens barrel that renders objects at infinity with acceptable focus. Setting the camera to this distance renders objects a infinity ∞ in focus and delivers the greatest DOF possible. As an example suppose the camera is set to 2m and a tiny aperture, say f/22 is used. Under these conditions everything from 1.5m to infinity will be acceptable. The hyperfocal distance setting is pre-set on a simple non-adjustable camera. Such a lash-up shields the novice photographer from the need to ever focus the camera (Brownie box camera use this setting).
  5. Like other have said the question is not stated clearly. It said subjects just in front of the main subject are blurred. How far in front?
    It's not so important to score well in the exam but it's more important to understand what you learned. So I would suggest pose that question to the teacher even if it's not in the exam. After that please share with us the answer. It's just fair that you give us the answer too.
  6. I have an exam tomorrow​
    Good luck on your exam. It's a little late, so you'll need luck.
  7. Gup

    Gup Gup

    Patrick, here's what I get from your question.
    The camera is focusing on an object .3m away from the lens (this is 1ft, if that makes it easier for some). Objects in front of that are out of focus. Objects at 20m (or 65ft) are also in focus. The lens in use must be a short focal length. What length, I'm not sure, but some here might be able to figure it out. The aperture chosen must be fairly small for that kind of depth of field, I'm guessing f16, or so. In this scenario I would say that an object at 60m (195 ft) would be considered as in focus as the object at 20m, for the purposes of this question. So, my answer would be yes.
    I'm not a physicist and am not applying any rules of mathematics, just experience and a guess.
  8. James G. Dainis

    James G. Dainis Moderator

    I think you are paraphrasing the question and don't have it quite right. If a lens is focused at 0.3m only objects at 0.3m will be in focus all others will be out of focus. The degree that they are out of focus but still okay is known as acceptable focus. I don't see that "acceptable" used there anywhere. The DOF and acceptable focus would depend on the lens focal length, format and the circle of confusion. Just how far in front of the main subject are the other objects? 1/16th of an inch? And they are blurred? I sort of doubt that. To me it sounds like he is saying the near limit of acceptable focus is 0.3m not that the lens is focused at 0.3m but I don't know if that is what is meant for the question.
  9. "Just in front....are blurred"
    That should have been more specific. Apparently 0.3m is not the hyperfocal distance. If it was, everything from 0.15m to infinity would be in focus. As others have said, the lens in use would be a wide angle, but whether there would be acceptable focus at 60m would be difficult to know given the small amount of information available.
  10. James G. Dainis

    James G. Dainis Moderator

    By definition when talking about hyperfocal distance the far limit of acceptable focus is infinity. If the hyperfocal distance is 0.6m everything will be in acceptable focus from 0.3m to infinity.

    When setting the hyperfocal distance for whatever focal length and aperture combination you’re using, the depth of field (all things in acceptable focus) will stretch from roughly half the hyperfocal distance to infinity.
  11. Well, as I thought, the answer has nothing to do with hyperfocal. Indeed we do not have either the focal length or the f-stop that is used. And yes, the question IS complete. Howerver...
    There was a typo in the initial question given to me... it marked 30 cm (which I converted to 0.3m). It really should of read 30 METERS. Secondly, I wrongly translated to post on this forum "...other placed just in front of the main subject are blurred". That should of simply read "... others in front are blurred"

    So the recomposed question:
    "During the shooting, the focus point is made 30 METERS on the main subject. In that scene, objects are still in focus at 20 m, others in front (meaning in front of the 20m) are blurred.

    If an object is placed at 60m, will it be in focus? "

    It seems like Alan Marcus is right on the spot. It's all about the 1/3 in front, 2/3 in the back of the focus point!
    In this case, the focus extends from 20m ... 30m (focus is here)... all the way to 50m. Answer is therefor NO objects would not be in focus at 60m. Theoretically speaking at least, since we can't really calculate Hyperfocal in this example!

    So, thank you all for your help in me understanding this!! :))
  12. Actually, I thought the Q was tricky on purpose....including a *split diopter*....allowing you to focus close (0.3m) and also further up at 20m, etc. Thanks for the follow up.
  13. Well I've learned something in this thread...the meaning of "Teach to the test".
    I've got quite a few landscapes I've been working on that no where near fits that 1/3 in front, 2/3 in back sharpness appearance hyperfocal zone parameter. I've got a subject fishing at a lake I focused on 30 feet away but have a ton of sharp detail over 100 yards away from the subject captured with an APS-C sensor.
    (How come I'm getting a misspelling alert in my browser on "Hyperfocal"? Will somebody commit on what's right in the education industry?! This is giving me "Academia Doubt", an affliction that causes me to question my abilities as a reasonably learned 54 year old that shouldn't be the case because I feel I'm too stupid to have lived this long!)
    Case in point, it's the first I've ever heard of such a zone mentioned in the OP's test question.
  14. Hi Patrick - Very interesting demo question. I am not aware of what all or how the optics information was taught. Other posters are correct from the standpoint of it appearing there is insufficient information.
    What I believe the teacher is/was asking with such a question is ones deeper understanding of DOF.
    With the focus point is at .3 meters and we know that it was given it is "in" focus at 20 meters, it can be determined from an understanding of the equations that that amount of extra blur between 20M and infinity at maximum 1.5% more. Another way to say this is that lets say the DOF equations showed that 20M was the furthest limit of being in focus. The amount of acceptable focus blur to be "in focus" (when viewing an 8x10 print from 10 inches) is the same as the definition for Circle of Confusion. If by example this was .02 mm which is often used for a Nikon DX sensor then just changing the circle of confusion to be 1.5% larger (.0203) then the focus changes from 20M to infinity. That change is imperceptible to the eye so from any practical standpoint it is in focus at infinity as much as it is at 20M. You eye would know "no difference".
    That 1.5% difference for this problem is independent of focal length, Aperture, or initial Circle of Confusion used BTW. All that matter was the focus distance and the distance to an object "deemed" in focus at a much greater relative distance.
    Bravo to the teacher for asking a question that is not a strictly "solve" the equation question for an exact answer yet going beyond looking for a deeper more practical understanding.
    Sorry I did not see your question until it was too late for use by you though.
  15. Depth of field and hyperfocal distance are complex calculation. The arithmetic must intertwine umpteen variables.
    1. Observer’s perception of sharpness (subjective).
    2. Size of the circle of confusion.
    3. Focal length of lens uses.
    4. Aperture setting of lens.
    5. Distance focused upon.
    6. Degree of enlargement to render displayed image.
    7. Distance observer to displayed image.
    8. Some I don't know
    This chaos has as its roots the fact that a disk viewed from a distance 3000 times its diameter is seen by the observer as a dimensionless point. This works out to a 1 meter circular object viewed from 3000 meters. Now the photographic image due to flare, aberrations and contrast limitations is less strict. For imaging, this works out to a disk viewed with an angle of 3.4 minutes of arc. This would be a circle 1/1000 of the viewing distance equivalent to 1/100 of an inch viewed from 10 inches (0.25mm viewed from 255mm). If the viewing distance of the final image is greater, the circle size is relaxed.
    Now to make an 8x10 from a full frame, the magnification required is 10X. Thus the permisable circle size is 0.25 ÷ 10 = 0.025mm. Because the industry relates focal length to format size, 1/1000 of the focal length is generally used to calculate tables and charts. For a 50mm lens this works out to 50÷1000=0.050mm.
    The hyperfocal distance calculation is (Focal length X 1000 X 0.0033)=hyperfocal distance in feet. The 0.0033 convers to feet. For the 50mm lens at f/22, this works out to:
    Hyper focal distance = (50 X 1000 X 0.0033) ÷ 22
    Hyper focal distance = 165 ÷ 22 = 7.5 feet (2.29m).
    The zone of acceptable sharpness is from ½ the distance focused upon to infinity (∞). This works out to 3 ¾ feet to infinity (1.14m to ∞).
    Looking at 50mm focused at 15 feet set to f/5.6 – Near focus is 9 feet 11 inches 24% in front to 30 feet 8 inches 76% behind.
    I am often output gobbledygook so you can take what I write as lore not data. At 75 1/3 I am still learning. I read the Little Golden Book of Photography over and over and each time I learn something new.
  16. Apologies and ignore my last response. Apparently not getting all the email notifications and missed Patricks update on
    incorrectly stated problem.
    However, with 30 M focus and 20 M in focus. 60 M will also be in focus. Easy to check on
    Set focus to 30M. And adjust values until closer point is at 20M. You will see far point is at 60M
  17. In this case, the focus extends from 20m ... 30m (focus is here)... all the way to 50m. Answer is therefor NO objects would not be in focus at 60m.​
    There is only one plane of focus, and it's at 30 meters. Unless you are using tilt or swing movements, the photo is out of focus at 20 and 50 meters. However, if it is only faintly out of focus and you don't enlarge the photo too much, your eyes won't be able to tell the difference.

    If you enlarge the photo enough or examine it too closely, you'll discover that details at 20 and 50 meters are indeed out of focus.

    The problem as stated has flaws. The f-stop and focal length are not mentioned. Are you shooting at 400 mm or 14 mm? Are you shooting at f/2 or f/32? These parameters will impact the amount that the 20 mm and 50 mm objects are out of focus.
  18. I'm getting the impression that the folks that are teaching the OP are proposing a test question meant more as a leading question to get the OP to say what Alan Marcus originally indicated about 1/3 in front-2/3 behind the subject that is the area of focus.
    It is the typical "keep it simple" so the one being tested doesn't have to know all the required technicalities that make this kind of knowledge functional as a photographer out in the field. It really should've been proposed as an essay type question.
    It's a simplified "rote memory" sourced question so folks can pass a test. But of course it is to the benefit of the OP he ask the question here and get a better understanding on the subject from PN contributors so the OP can use the info in order to be functional out in the field rather than use it to just pass a test.
    Oh, I just remembered to ask...does anyone know how to spot an object that's at 60m without a telescope or IR feedback indicator? A camera was never designed as a surveyor's tool. Think about it.

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