Thursday, 1 March 2012

Depth Of Field: Everything you need to know

I shall do my best to make this sound as simple as possible. (Text which is in italic is small print and is not important to the general concept)



Depth of Field is considered the distance  (front to back)  (or range within a photo) which is considered to be acceptably sharp.

(Simple statement, but the tricky thing is calculating what is Acceptable and not acceptable (thats where a really silly thing called the Circle of Confusion comes into play but so as not to sound like a smart arse or get things confused with Resolution, print size, subject viewing distance, human visual acuity etc. lets just consider the COC a constant that we don't need to think about (however i shall try and do a bit of explaining of it at the end for those of you who like to be smartarses))
For the purpose of this article i shall refer to a short depth of field as narrow and a long depth of field as Large)

DOF is an OPTICAL phenomenon. 
It is Controlled by 3 things:
  1. Aperture
  2. Actual lens length
  3. and Subject distance
(and if your a smart arse the COC calculation for a given print)

Depth of Field and Aperture:  Aperture is simple.

Give your lens the biggest  Aperture (i.e. f/1.4) and you get the narrowest Depth of Field.
Give your lens the smallest Aperture (i.e. f/22) and you get a larger Depth of Field

i.e. if your subject is 5 meters in front of you, and you are using a big aperture setting, the area of sharpness with be just 10 or so cm in front of your subject and 10 or (more) behind your subject. 
(Generally there is more distance in focus behind the subject than in front, unless you are shooting macro (but that is getting technical so don't worry about it))
If you are using a small aperture setting (i.e. f/22) the area of sharpness will be maybe 2meters in front and 5 meters behind your subject.


Depth of Field and Focal Length:  Focal length is simple

The longer your lens's (actual) focal length, the narrower the depth of field.
The shorter your len's (actual) focal length the larger the depth of field.


i.e. If you are shooting from 5 meters away from your subject and you are shooting with a 200mm lens, only a couple of cm in front and a couple more behind your subject.
If you are in the same position and same distance from your subject and you are using a shorter lens ie a 20mm, the depth of field will be maybe 3 meters in front and 40 meters behind
(However in doing so your FOV (Field of view) will have changed.  The perspective  of your subject and relation to the objects in the background will be the same (just crop in your image and see for yourself) but the over all image will contain far more background and forground (you then also get into the terms of absolute blur vs relative blur and COC when printing / viewing the images but thats just getting confusing!)))


Depth of Field and Subject Distance: Simple but gets confusing

The closer to a subject you are the narrower the depth of field.
The further away from a subject the larger the depth of field.

ie if your subject is 5 feet in front of you compared to 20 feet in front of you, there will be a much narrower depth of field when your subject is closer to you.
(However this becomes void if you get to a lenses hyper focal distance at a given aperture i.e. no matter where the subject is, if they are with in the Hyperfocal range, then everything will be seen as acceptably sharp)



Depth of Field and Sensor Size: Confused

Now this is where a lot of confusion comes in! (for the sake of argument I am going to ignore the Idea of Circle of confusion calculations and even if I did use them They would actually be counter intuitive to the argument i.e. the smaller the sensor the narrower the depth of field is due to the image size having to be enlarged to a greater degree, but that is if you are enlarging you images to a constant size and you also have to take into account pixel density vs print resolution vs perspective to have an equal test which just isn't actually possible but I digress)

The uneducated amongst the imaging community will believe that a larger sensor CREATES shallower depth of field, WRONG! 
Well that is the wrong term.  You have to be pretty careful with what you say on this as some people get up tight.
The best way to say it is "A Larger sensor AFFORDS the Photographer a Shallower Depth of field"

There is a more specific argument that some photographers will say: 

ALL THINGS BEING EQUAL, A larger sensor will give a shallower depth of field.
Now that is kind of right but mostly wrong.

The reason why it is mostly wrong is because of the fact that all things can't be equal when we talk about a final image and comparing it with another one when changing the 3 OPTICAL variables (Aperture, subject distance and Focal length) Because you have to take into account FOV (or to put it another way, you have to consider the amount the subject is filling the image)

All THINGS BEING EQUAL: SAME FOV (Different set up)

  • All things being equal with regards to having the same looking image, you have to change your Subject distance or your Focal length: This is known as the Same Image Comparison.With a full frame camera if you are using a 75mm lens, on a cropped sensor camera you will use a 50mm lens. The 50mm lens on a cropped sensor camera will have a cropped FOV as the 75mm lens on the full frame and thus give the same looking image but with a shallower depth of field, however this is in effect not a property of the sensor but, as you have guessed it, the effect of changing the Lens Length. 
  • Additionally the other procedure is to change the Subject distance however this although possible to give the same FOV will give a different ANGLE of View which means there will be a different interaction between the subject and the background.

  • And thirdly this interaction is only valid when comparing subject and background when there is adequate distance to negate the changes of reaching a shorter focal lengths hyper focal distance at a given aperture (OH man that sounds complicated, and it is, and that is why so many get confused)

ALL THINGS BEING EQUAL PART 2: (DIFFERENT FOV, SAME SET UP)

  • The next argument is the idea of all things being equal in terms of settings, and distances. With a larger Sensor you get a greater FOV which in effect means that with the same lens you get a different image and different subject size in relation to the image compared to a smaller sensor.  However the actual distance within the frame i.e. front to back, will not change. Simply not possible as the optical properties have not changed.  

  • Another example of this is the 50mm lensThe human eye is close to the same optical perspective and characteristics as a 50mm lens.If you put your camera up to your eye with a 50mm you can happily walk around using the eye through your camera and not give yourself too much of a headache. If you go crazy with the focal length ie put it up to 100mm or have a 14mm lens on the perspective is too different and your eyeball may explode.Now put that 50mm lens on a Full frame camera or even better put it on a medium format camera.  Nothing apart from the FOV changes.  a 50mm is a 50 mm no matter what format you put it on. The optical properties do not change, only the FOV

So the term a larger sensor affords the photographer shallower depth of field can be true because to get the same image of a subject, i.e. how much they fill the frame, the photographer will either have to move closer (change subject distance) or use a longer focal length which in effect gives narrower DOF.

Simple

Kind of.

















OK so here is a bit more for you who want to know what the COC is:

For the purpose of Calculating DOF camera companies have to consider the COC (circle of confusion)
The COC is the when a human can distinguish when a spot of light stops being a spot and becomes a blur on a print.  The larger you print the more you are able to assertain that a spot of light is less sharp.  Therfore the larger the print the shallower the depth of field.
But you also have to factor in the distance that the viewer will be scrutinising the image. If you are standing with your nose almost pressed up against the image you will see the spots of light look blurry,  if you are looking at an image from the other side of a room blurry shots will not be noticeable.
The closer a viewer is to a print the shallower the depth of field

Some DOF calculators are using old calculations made for 35mm film and medium format to calculate the COC this does not take into account the resolution of the sensors.
A camera with a high resolution will have more pixels in a smaller area, thus more likelyhood of spot of light spreading to more pixels and thus looking blurry. (if printed to a specific print size)
High Pixel Density = Shallower Depth Of field 

COC calculators are using 2 different methods, A) is multiplying the film size by a constant (ie sensor size x 5) or B) multiplying a resolution to a fixed size. (ie only 12inch prints) And that is what you have to figure out for yourself.

If you are shooting high MP and printing large prints and your viewers are going to be up close and personal with your shots your COC will be a far tighter consideration than if you are shooting HD video Resolution.

Easy example is take a photo at a big aperture, look at it on the back of the screen, try and guess which bits are out of focus?  Then put the image on your computer and bingo you really see which bits are out of focus,  The COC has suddenly got much bigger when your viewing you images on a 27 inch screen compared to a 3 inch screen.  

For digital sensors, the CoC cannot be smaller than the physical size of two pixels (image elements).   Obviously nothing smaller can be resolved









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28 comments:

  1. Dom,

    this is really good stuff mate! That was not an easy thing to clearly explain and you succeeded. As you said a lot of folks get confused between full frame sensor and cropped and FOV and the famous BOKEH.
    Nice job.

    ReplyDelete
  2. in your youtube video when you say size of sensor, is it like full, medium or croped frame you're talking about ?

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  3. doesnt bloody matter!

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  4. I think the easiest way to understand DoF and sensor size is to take a photo of a ruler at an angle giving it some sort of DoF and out of focus bits, then imagine smaller sized sensor are cropping into the image. The actual DoF does not change, only the relative DoF.

    ReplyDelete
  5. 3 questions:

    1) DoF calculators, such as the one available in the below link, allows to select cameras with different sized sensors, and hence yield a different result.

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

    For instance when I select the "Nikon D7000, D5100, D5000, D3200, D3100, D3000" option and focal length as 50mm, f-stop as f/4 and subject distance as 10m, the total DoF is 7.09 meters. However when I select "D800, D800E, D700", the total DoF is 12.4 meters.

    Does this substantiate that sensor size affect DoF?

    2) If the myth says "larger sensor creates shallower depth of field", then how come the total DoF of the DX sensor is shallower than the FF sensor in the above example?

    3) Considering tiny size sensors of cell phones never ever yield photos like DSLRs in terms of shallowness of DoF, isn't there any truth at all in the statement "larger sensor creates shallower depth of field"? (Perhaps it is not applicable or discernable beyond a certain sized sensor, that is to say, one would not discern or barely discern a difference in terms of (shallowness of) DoF between photos taken with a DX and FF (perhaps even medium and large format) sensor, but any photo taken with a sensor that is less than, say, an inch(?) would be discernable in terms of (shallowness of) DoF when compared to sensors larger than DX)

    ReplyDelete
    Replies
    1. 1) DOF master uses COC calculations that dont take into account pixel density or final print size, read my tips on COC, and if you look closely you will see that it shows that the smaller the sensor the shallower the depth of field which is against the arguement.

      2) exactly, bur read my bit on COC, it is explained there.

      3) camera phones etc the sensors are tiny. but so is the focal length of their lenses, maybe only 1mm or less. try getting a 1mm lens on a dslr!

      Delete
  6. Considering you had previously refuted the so-called dynamic range advantage of FF sensors and now that you are saying DoF is not shallower on FF sensor, would you say the only advantage of FF is that it produces cleaner images at equivalent ISO values?

    Quite frankly I cannot think of any other advantage of FF other than ISO and usability of ultra-ultra-wide angle lenses (the latter being further negated by the availability of such lenses by Tokina and Nikon).

    If you agree with me, would you mind telling us why you are using a FF camera? For ISO advantage?

    ReplyDelete
    Replies
    1. [would you mind telling us why you are using a FF camera?]
      Possible reasons for using a FF camera:
      1) It´s expensive, so you can impress your friends and your clients that don´t have one.
      2) Usually you can pair them with big lenses. The total equipment would be huge in size. Not only clients and friends would be impressed, but girls would think that that´s a sign that your penis is huge too.
      3) Maybe this post about FF and DOF have the purpose of creating noise and audience only, and have some hidden assumptions that would make you think that no practical difference in the behavior of DOF will occur when changing sensor size. Such misleading idea can be refuted empirically without invoking one single rule of optics or any fancy theory about what DOF is and how it works.
      4) Maybe all the above.

      Delete
    2. I asked to Dom why he uses FF for his work, and not you. Even though the reasons you mentioned might be true for most people out there and thus beneficial for such people, based on Dom's videos and blog he seems to be a person using FF for reasons other then yours. Hence my question to him and why I value his answer.

      Dom: I will appreciate to hear the reasons why you are using FF. Thanks.

      Delete
    3. when i got the nikon D700 it was far superior in terms of iso and noise to any aps-c sized camera on the market, however today there is bugger all difference mostly it is the body shape/size (not weight) that is better for me ergonomically but what ever sensor they put in it nowadays i dont really care.

      Delete
  7. Your explanation seems rock solid, and I even agree that the larger sensor can´t "create" any DOF at all. With identical lenses, you will be forced to frame the image differently to produce the same picture, so it´s not that DOF would be created, but simply that the larger sensor will ask for a different subjects distance, and the the subject´s distance WILL affect DOF. Image framing and lens being equal, DOF will change with sensor size. It´s impossible not to happen unless DOF is not affected by subject´s distance at all, something we all agree here, is not the case.

    It´s an interesting post, and knowledge about anything is always desirable. But in real life, such technical details are somewhat irrelevant. One can surely follow his life believing that larger sensors produces more narrow DOF than smaller sensors. In practice that´s how things would look like, although this is just an illusion created by the different subject´s distance needed to compose the same picture frame.

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  8. So basically, when you have a Full Frame camera and you take a headshot at f/2.8 and 200mm you have shallower depth of field than a APS-C camera because when you recompose without moving from the same spot you need a shorter focal lenght like 130mm or so because of the 1.5x crop.
    Finally you get a shot with more depth of field in the smaller sensor camera because you dont need so much focal lenght and in case you use that 200mm like in the FF, for getting the same shot you would need to step back also losing that shallower depth of field. Sounds correct for me and great explanation Dom. I'm sorry if my writing is not perfect because I'm Spanish and English its not my native lenguage.

    ReplyDelete
  9. Please dont get confused by DOF master, its calculations on DOF take into account Circuls of confusion which in its calculations do not factor in for pixel density nor final print size, and if you look closely the smaller the sensor the shallower the depth of field. so that is counter to the arguement even further anyway!

    ReplyDelete
  10. http://www.youtube.com/watch?v=vBEpmKqBhjE

    ReplyDelete
    Replies
    1. Nice vid. Start shooting with an 85mm on a crop sensor and an 85 on full frame. In small rooms its hard to get enough room sometimes for portraits. Lets say you are doing a head an shoulders. An 85mm at f1.8 on full frame lets you get closer to the subject to just get a head and shoulders shot. Therefore you get narrower depth of field. On a crop sensor same settings to just get the head and shoulders you have to stand further back. There will be more depth of field. Its simple.

      Delete
  11. People often confuse the DOF on the objekt side with the DOF on the picture side, wich leads to seamingly contradicting propositions. When I first saw your Youtube video I too thought you were wrong, but now I see what you mean.

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  12. BUT!

    In the real world, people aren't using two cameras together and comparing, trying to obtain an identical image. In practical terms, the larger the image-capturing device (whether it be a CCD or a piece of film), the harder it is to get an image sharp at the same aperture. Ever tried a portrait at f5.6 on a 5x4 camera? While what you're saying is basically correct, it's kinda arguing against itself.

    http://photo.net/learn/optics/dofdigital/

    ReplyDelete
    Replies
    1. i guess practical terms and factual terms dont mix well.
      however i have done a couple of portraits with a large format digital hasselblad with a 85mm lens. found it to be pretty good.

      Delete
  13. Hello Dom, I never wondered about the if there is a difference between FX and DX dof, as I thought there the only optic difference is the focal length however I as I am using often hyper distances I checked on a simulation app the difference between the 2 sensors.
    dofmaster.com/dofjs
    The simulation I made was on 5d vs 7d with a focal length of 55mm, f 1.4 subject distance 5 meters and this tool shows that the dof on 5d is of 0.7m while on the 7d is 0.44m.

    ReplyDelete
    Replies
    1. I already covered DOFmaster in the clever bit about circle of confusion

      Delete
  14. "High Pixel Density = Shallower Depth Of field"
    So why on earth one can never achieve any shallow DoF on a point and shoot cameras or cell phone cameras with 8Mpix?

    ReplyDelete
    Replies
    1. because their sensors are tiny and their focal lengths equally small. so small that almost everything is within the hyperfocal range. surely you understood that.

      Delete
  15. While the article did go into detail to prove a point, it's only partially correct.

    The sensor side does not change the DoF directly. If I have a 50mm lens at f/2.8 and I'm 5 feet from my subject, it doesn't matter what the size of the sensor is. I will have the same DoF. The apparent focal length will be different, making your framing different.

    To get the same frame, the smaller the sensor, the further back you have to go. The composition of a photograph is a huge part of what makes an image pleasing. Getting the same composition to capture what the photographer wants to see in the frame means that they MUST back up when using a smaller sensor vs. when using a larger one to capture the same intended image. Backing up to achieve this makes the DoF larger.

    The statement that this is due to a perceived focal length change is not valid, and rather counter-intuitive. A longer focal length gives you a shallower DoF, which is not the case when using a smaller sensor with the same composition. You have to move back, but the physical properties of the lens did not change, the only thing that actually changed is your distance from your subject, which we all know gives a larger DoF.

    As far as the point and shoot cameras with the really small sensors, they have very wide angle lenses on them, which are required to get a usable apparent field of view for composition. Having to use such wide angle lens makes the DoF much larger. And the reason they have the wide angle lenses is due to the size of the sensor.

    So while the size of the sensor doesn't directly effect the DoF, it does effect your choice of lens to get the composition you want. And the choice of a lens effects your DoF, therefor the sensor size does play a role in the DoF of the final image you want to produce.

    ReplyDelete
    Replies
    1. You are correct but using the lens wrong.

      When on a cropped sensor use a 35mm lens instead and you will get the same composition & DOF and so on. And then everything is the same as on the FF in theory. (You usually get what you pay for)

      Delete
  16. You are correct but using the lens wrong.

    When on a cropped sensor use a 35mm lens instead and you will get the same composition & DOF and so on. And then everything is the same as on the FF in theory. (You usually get what you pay for)


    ReplyDelete
    Replies
    1. It has to be the same lens - the point was to change only the sensor size to see the effect it has on the DoF an image with a desired composition.

      Using a 35mm on an APS-C sensor from 5 feet away gives a different DoF then using a 50mm on an Full Frame - the composition will be the same, but since the focal length of the lens is physically different and the subject distance is the same, the DoF will be different. Prove it out with a DoF calculator.

      Nikon FX f2.8 50mm @ 5 feet DoF = 6.019 inches.
      Nikon DX f2.8 35mm @ 5 feet DoF = 8.290 inches.

      While two inches isn't a large amount, when you get into larger focal lengths the different is much bigger.

      Nikon FX f2.8 300mm @ 100 feet DoF = 68.34 inches.
      Nikon DX f2.8 200mm @ 100 feet DoF = 103.0 inches.

      In every case when considering the composition of your frame, a smaller sensor will always give you a larger DoF.

      I think I'm done with this argument. Both sides understand it completely but they look at it differently.

      Some people consider the composition and will step back to maintain it which will end up with a larger DoF.

      Other people don't consider the composition and will not move back (taking the picture of the nose instead of the entire head) and they will have the same DoF.

      Delete
    2. re-read the part about COC, that is one of the parts of the formula that DOFmaster uses,

      Delete
    3. I'm familiar with the CoC. The calculator I used takes them into account. If it didn't, then the difference would be even greater.

      No CoC calculation change (assuming Full Frame for both):

      f2.8 300mm @ 100 feet DoF = 68.34 inches.
      f2.8 200mm @ 100 feet DoF = 154.8 inches.

      Delete