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#151
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film vs CMOS
"nospam" wrote in message
... That goes against everything I've ever learned about photography, it's a common myth. and the fringe benefit of using larger film (the main one being finer level of detail for the same type of film). in other words, different image quality. I hadn't appreciated that image quality affected DOF. So same lens, same camera, same aperture (but different shutter speed) on very slow fine-grained film and very fast coarse-grained film will produce different DOF? Well, well. this explains it exceptionally well: http://www.clarkvision.com/articles/dof_myth/ A commonly cited advantage of smaller digital cameras is their greater depth-of-field. This is incorrect. Interesting article. The reason for the myth is that we are comparing the wrong things and keeping the wrong things constant in the comparison. The longer lens needed to produce the same field of view on the camera with the larger sensor needs to be stopped down further make the *absolute* aperture the same as for the camera with a smaller sensor and correspondingly smaller focal length to give same field of view. Comparing f5.6 on the longer lens with f5.6 on the shorter lens is wrong in terms of DOF. Understood! Of course, in practical photographic terms, we tends to constraint aperture to *roughly* the same range of f numbers for any lens. A lens that is 10x as long doesn't have apertures which are roughly 1/10 (in f number terms) - on all lenses, they will always be around f2 - f16 give or take a couple of f numbers either way. That's so the light-gathering abilities of the lenses are comparable. So a camera with a small sensor will have a lens that has usable apertures in terms of light-gathering capabilities which equate to a camera with a larger sensor and hence a longer lens that has much smaller apertures and therefore needs much longer exposures if film speed / CMOS sensitivity is the same. So a smaller camera doesn't inherently produce a greater depth of field, but when it used in the same conditions of image brightness and film speed, and the need to avoid diffraction due to excessively small apertures, the smaller camera's range of *available* apertures produce a greater DOF than the *available* range of apertures on a longer lens on a larger camera. In other words, it's the age-old difference between theory and practical usability. |
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#152
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film vs CMOS
"NY" wrote in message
o.uk... Interesting article. The reason for the myth is that we are comparing the wrong things and keeping the wrong things constant in the comparison. The longer lens needed to produce the same field of view on the camera with the larger sensor needs to be stopped down further make the *absolute* aperture the same as for the camera with a smaller sensor and correspondingly smaller focal length to give same field of view. Comparing f5.6 on the longer lens with f5.6 on the shorter lens is wrong in terms of DOF. Understood! The only thing that I'll need to read a few more times is the issue of number of photons. I can see that this affects how much light the film/sensor receives and therefore the brightness of the image. But I can't see how it has any effect on the DOF of that image which is a purely optical, lens issue: a dim image and a bright image will still have the same objects in sharp focus and the same objects blurred by some amount. So if in some way you halve the number of photos reaching the sensor (by making the aperture smaller, by using a neutral density or by dimming the illumination of the subject) then as long as you make the sensor twice as sensitive or double the exposure time then the recorded image will be the same. Of these, only altering the aperture alters the DOF. As long as the aperture doesn't change, you can change all the other parameters and as long as you do so proportionally, the image will be identical, both in brightness and in DOF. To think of it another way, suppose you have two sensors which have pixels which are same size. In one case, the pixels border each other, without any space between them; in the other case the pixels are the same size but spaced more widely. Sensor 1 x x x x x x x x x versus Sensor 2 x x x x x x x x x The pixel size (the x) is the same size. So the size of the buckets is the same. But if the pixels are spaced more widely, the sensor will be physically larger and will need a longer lens to capture the same field of view (and so will need a smaller f number to give the same absolute size of aperture and hence DOF). But if we replace Sensor 2 by the more normal situation where the pixels are bigger although their spacing hasn't altered (Sensor 3), the same lens is needed and for the same aperture the DOF will be the same. I presume... Sensor 3 X X X X X X X X X The spacing is the same but the pixel size has increased from x to X which means the larger buckets can gather more pixels. I'm still struggling to see how altering the "size of the buckets" has affected the DOF, if the pitch of the buckets is the same. My brain hurts. I'm probably over-thinking all this :-) |
#153
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film vs CMOS
On 2018-08-14 09:08, nospam wrote:
In article , NY wrote: One other factor to bear in mind: the depth of field varies with lens focal length, not field of view of the subject. actually, it's aperture. actually, it's the ratio of focal length to aperture. |
#154
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film vs CMOS
In article , +++ATH0
wrote: One other factor to bear in mind: the depth of field varies with lens focal length, not field of view of the subject. actually, it's aperture. actually, it's the ratio of focal length to aperture. nope. depth of field is a function of physical aperture. what you describe is f/stop, which is used for exposure purposes, and in some cases (usually movies), t/stops are used, which is actual light transmission through the lens, not a simple ratio. |
#155
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film vs CMOS
On 2018-08-25 15:06, nospam wrote:
In article , +++ATH0 wrote: One other factor to bear in mind: the depth of field varies with lens focal length, not field of view of the subject. actually, it's aperture. actually, it's the ratio of focal length to aperture. nope. depth of field is a function of physical aperture. what you describe is f/stop, which is used for exposure purposes, and in some cases (usually movies), t/stops are used, which is actual light transmission through the lens, not a simple ratio. Are you claiming that focal length has no bearing on depth of field? That's an interesting viewpoint. |
#156
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film vs CMOS
In article , +++ATH0
wrote: One other factor to bear in mind: the depth of field varies with lens focal length, not field of view of the subject. actually, it's aperture. actually, it's the ratio of focal length to aperture. nope. depth of field is a function of physical aperture. what you describe is f/stop, which is used for exposure purposes, and in some cases (usually movies), t/stops are used, which is actual light transmission through the lens, not a simple ratio. Are you claiming that focal length has no bearing on depth of field? for the same subject size and same image quality (coc), no. That's an interesting viewpoint. not really. it's just math. |
#157
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film vs CMOS
+++ATH0 wrote in
: On 2018-08-14 09:08, nospam wrote: In article , NY wrote: One other factor to bear in mind: the depth of field varies with lens focal length, not field of view of the subject. actually, it's aperture. actually, it's the ratio of focal length to aperture. That is correct. For any given focal length, the smaller the aperature, the greater the depth of field. That is why pinhole cameras focus from closeup to infinity without a lens. |
#158
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film vs CMOS
nospam wrote in
: In article , +++ATH0 wrote: One other factor to bear in mind: the depth of field varies with lens focal length, not field of view of the subject. actually, it's aperture. actually, it's the ratio of focal length to aperture. nope. depth of field is a function of physical aperture. what you describe is f/stop, which is used for exposure purposes, and in some cases (usually movies), t/stops are used, which is actual light transmission through the lens, not a simple ratio. Are you claiming that focal length has no bearing on depth of field? for the same subject size and same image quality (coc), no. That's an interesting viewpoint. not really. it's just math. No matter what the focal length of the lense is, the further away the focal plane is, the greater the depth of field will be for any aperature. As an example, if one is taking a head and shoulders portrait with a large aperature, it is quite likely that part of the subject will be out of focus slighty. Moving back a few feet with the same lense and aperature will result in a deeper depth of field, so that all of the subject should be in focus. The drawback is that the image size will be smaller, and thus require more enlargement to obtain the same size image, with the resulting loss of resolution with the enlarged image. |
#159
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film vs CMOS
On 08/25/2018 06:06 PM, nospam wrote:
In article , +++ATH0 wrote: One other factor to bear in mind: the depth of field varies with lens focal length, not field of view of the subject. actually, it's aperture. actually, it's the ratio of focal length to aperture. nope. depth of field is a function of physical aperture. what you describe is f/stop, which is used for exposure purposes, and in some cases (usually movies), t/stops are used, which is actual light transmission through the lens, not a simple ratio. For an ideal optical system, the depth of _focus_ (on the image side) is a function only of wavelength and numerical aperture, i.e. the sine of the half-angle of the cone defined by the rim rays (i.e. the illuminated cone). That's where wave properties come in. On the object side, the depth of _field_ equals the depth of focus scaled by the square of the magnification. Magnification is of course the ratio of the object distance to the image distance. Aberrated optical systems degrade a bit more slowly because they're not as good to begin with. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com |
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