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)
(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:
- Actual lens length
- 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.
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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