One of the things that controls depth of field as we all know is aperture. BUT, is it aperture as expressed by the f/stop or the physical size of the lens opening? To wit, when using a variable aperture zoom, say f/3.5 to 6.3, at the wide end it's f/3.5 and at the tele end it's f/6.3 but the physical size of the opening is the same. It's as large as it can be and the only reason the f/stop number changes is because of the math when the focal length changes. So, then is the depth of field the same at both extremes?
If it's the actual size of the opening that is the control, then the answer would seem to be yes but nobody ever states it that way. I realize this is a very fine distinction and not something practically to normally worry about, but I sure would like to know. Any physics profs out there or optic theory experts? Please enlighten me.
Thanks,
John
Posted by Ramamohan Pai on June 21, 2011 at 11:01 PM
If we take 18-200mm Lens as an example, the size of the subject recorded on the sensor is different when the focal length is 18mm and 200mm - in simple words your magnification changed when going from F/3.5 at 18mm end to F/6.3 at 200mm end. DOF is also a function of Magnification which is not considered in your question above.
Simplistically put (don't want to get beaten by Physicists), if the magnification or reproduction size on the sensor is the same, then DOF is controlled by Aperture.
Posted by Kent Staubus on June 22, 2011 at 12:02 AM
An f-stop is simply a ratio. Roughly, an f-stop is the length of the lens divided by the opening of the aperture. Thus, say you have two lenses. One is 500mm long, the other is 50mm long. The physical measurement of the widest opening in each lens is (say) 30mm. The 500mm is an f16 lens, and the 50mm is an f1.8 lens. (Again, roughly.) DoF will be different on each lens at widest aperture, even thought the "hole" is the same size. Remember that you're dealing with a --ratio--, not an absolute number.
Kent in SD
Posted by Dan Borzynski on June 22, 2011 at 12:07 AM
Ram pretty much sums it up: DOF is a function of magnification and f-stop, nothing else.
But... What causes a lot of confusion is that we all know that wide angle lenses have more depth of field than telephotos. Well, that is true but not because it is a wide angle lens. It is because when you use a wide angle lens, you are almost always using it for the field of view it takes in = smaller subject = less magnification = more DOF at the same f-stop.
If the primary subject is the same size in the frame with a 50 mm lens and in a 500 mm lens, we'll still tend to say that the 50 mm has more DOF. It really doesn't. What is happening is that all of those little things in the background in the 50 mm shot are smaller, so they appear sharper. A tree trunk that is 1 mm wide on the sensor with the 50 mm that appears relatively sharp (but really isn't because DOF doesn't extend that far back) is now 10 mm wide with the 500. Any lack of sharpness in the 50 is now 10 times as obvious with the 500, plus all that other stuff in the background next to that tree trunk is now 10 times bigger, so it all blurs to this nice soft mess. Distinct trees in the 50 mm shot 1 mm apart on the sensor are now blurred together into one in the 500 mm shot.
It is also why cropped sensors have "more" DOF than full frame sensors. To obtain the same composition, you need to magnify the image less for the smaller sensor => less magnification means more DOF at the same f-stop.
Quiz time: for those pocket point and shoot cameras with tiny sensors, why do those seem to have f-stops that don't go beyond f8? If the above made sense, the correct answer is pretty obvious. Run in the opposite direction, is is also why large format cameras are sometimes incorrectly said to have less DOF than 35mm based camers (either film or digital).
Many thanks to all. Here's what started my quest. I read an article online which stated: "The depth of field for a variable aperture zoom lens is the same at both extremes of zoom when wide open because even though the f/stop number changes, the laws of physics do not".
I take it then from all the responses that this statement is false. Correct?
But what are the laws of physics on this? I'm just wondering from a scientific/optics perspective how the size of the opening impacts DOF and realize (since not a scientist) I won't fully understand the answer but would still like to hear it.
There is some mis-information out there, even on reputable sites, that these may help clarify. It persists, because in normal usage, the approximations are good enough. For instance, DOF IS affected by focal length, even for the same magnification on the sensor, and same f-stop. It's not always obvious in practice, but it is. A lot of people will tell you that only magnification and f-stop matter , but not always at the extremes, like the minimum focus distance of your lens.
I agree. There are so many things where something may be "true" in an absolute sense but practically, the difference is so slight as to be safely ignored. To me, if I cannot see whatever the nuance may be, I choose to not worry over it. I do however want to know the facts so I can make the decision to ignore or not in an informed manner. In addition, I do teach photography and feel I need to be armed with factual information and well versed in the absolute answer as well. This one caught me off guard and I'm a bit embarrassed for not being sure of the answer, but hey, that's OK. We all keep learning. Thanks again to all.
Its all based on the laws of diffraction. The two principal criteria are "diameter of Aperture" and "distance to subject". Resolution or sharpness does come into play, if you are computing DOF, a statement of the diameter of the circle of confusion must be considered.
However, all things being approximataely equal, a 50mm lens at f2.0 and a 200mm lens at f8.0 when focused at 10 feet will yield exactly the same DOF (if not tthe same image size), the diameter of aperture was identical as was the distance.
For this reason, there is incredible silliness involved in suggesting that wide angle lenses have more DOF. If in fact a 16mm lens and a 500mm lens have exactly the same diameter of aperture (NOT F STOP) then when focused at the same distance, the DOF will be the same..
Technologically, if you wanted to waste a great deal of time, study Dawes and Dawes Limits.
...a 50mm lens at f2.0 and a 200mm lens at f8.0 when focused at 10 feet will yield exactly the same DOF...
That's not correct Lynn. DOF is not entirely dependent on the diffraction due to the size of the aperture. A couple of examples using DOF Master's Online Calculator set to a 5D MkII, and keeping the default CoC throughout :
Focal Length : 50mm
F-number : 2
Focus Distance : 10 ft
DOF : 1.45 ft
Focal Length : 200mm
F-number : 8
Focus Distance : 10 ft
DOF : 0.34 ft
----------------------------------------------
Focal Length : 50mm
F-number : 2
Focus Distance : 50 ft
DOF : 42 ft
Focal Length : 200mm
F-number : 8
Focus Distance : 50 ft
DOF : 9.1 ft
Discussion
9 repliesIf it's the actual size of the opening that is the control, then the answer would seem to be yes but nobody ever states it that way. I realize this is a very fine distinction and not something practically to normally worry about, but I sure would like to know. Any physics profs out there or optic theory experts? Please enlighten me.
Thanks,
John
Simplistically put (don't want to get beaten by Physicists), if the magnification or reproduction size on the sensor is the same, then DOF is controlled by Aperture.
Kent in SD
But... What causes a lot of confusion is that we all know that wide angle lenses have more depth of field than telephotos. Well, that is true but not because it is a wide angle lens. It is because when you use a wide angle lens, you are almost always using it for the field of view it takes in = smaller subject = less magnification = more DOF at the same f-stop.
If the primary subject is the same size in the frame with a 50 mm lens and in a 500 mm lens, we'll still tend to say that the 50 mm has more DOF. It really doesn't. What is happening is that all of those little things in the background in the 50 mm shot are smaller, so they appear sharper. A tree trunk that is 1 mm wide on the sensor with the 50 mm that appears relatively sharp (but really isn't because DOF doesn't extend that far back) is now 10 mm wide with the 500. Any lack of sharpness in the 50 is now 10 times as obvious with the 500, plus all that other stuff in the background next to that tree trunk is now 10 times bigger, so it all blurs to this nice soft mess. Distinct trees in the 50 mm shot 1 mm apart on the sensor are now blurred together into one in the 500 mm shot.
It is also why cropped sensors have "more" DOF than full frame sensors. To obtain the same composition, you need to magnify the image less for the smaller sensor => less magnification means more DOF at the same f-stop.
Quiz time: for those pocket point and shoot cameras with tiny sensors, why do those seem to have f-stops that don't go beyond f8? If the above made sense, the correct answer is pretty obvious. Run in the opposite direction, is is also why large format cameras are sometimes incorrectly said to have less DOF than 35mm based camers (either film or digital).
I take it then from all the responses that this statement is false. Correct?
But what are the laws of physics on this? I'm just wondering from a scientific/optics perspective how the size of the opening impacts DOF and realize (since not a scientist) I won't fully understand the answer but would still like to hear it.
Thanks,
JR
Depth of Field Hell
Depth of Field Hell - The Sequel
Depth of Field Hell - The Coda
This is more in depth, and has links to the actual equations determining DOF. Still readable for everyone though.
Depth of Field
There is some mis-information out there, even on reputable sites, that these may help clarify. It persists, because in normal usage, the approximations are good enough. For instance, DOF IS affected by focal length, even for the same magnification on the sensor, and same f-stop. It's not always obvious in practice, but it is. A lot of people will tell you that only magnification and f-stop matter
However, all things being approximataely equal, a 50mm lens at f2.0 and a 200mm lens at f8.0 when focused at 10 feet will yield exactly the same DOF (if not tthe same image size), the diameter of aperture was identical as was the distance.
For this reason, there is incredible silliness involved in suggesting that wide angle lenses have more DOF. If in fact a 16mm lens and a 500mm lens have exactly the same diameter of aperture (NOT F STOP) then when focused at the same distance, the DOF will be the same..
Technologically, if you wanted to waste a great deal of time, study Dawes and Dawes Limits.
Lynn.
That's not correct Lynn. DOF is not entirely dependent on the diffraction due to the size of the aperture. A couple of examples using DOF Master's Online Calculator set to a 5D MkII, and keeping the default CoC throughout :
Focal Length : 50mm
F-number : 2
Focus Distance : 10 ft
DOF : 1.45 ft
Focal Length : 200mm
F-number : 8
Focus Distance : 10 ft
DOF : 0.34 ft
----------------------------------------------
Focal Length : 50mm
F-number : 2
Focus Distance : 50 ft
DOF : 42 ft
Focal Length : 200mm
F-number : 8
Focus Distance : 50 ft
DOF : 9.1 ft