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I am interested in taking pictures with a very shallow depth of field, and also willing to have the option to have the bokeh effect,

I'm not an expert in photography so I'm really confused about what I'm supposed to look for in a camera,

I did read that one of the things I need to look for is a large sensor, I found a few cameras that have a 22.2 x 14.8 mm sensor, Is that enough for my needs? or do I also need to look for a certain size of lens and stuff like that?

Thanks

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Firstly I'm not sure what you mean by the "bokeh effect". The term bokeh refers to the aesthetic quality of the out of focus area of an image, but it sounds very much like you want to achieve highly blurred backgrounds.

In principal what you want is a camera system that provides a large sensor and the option of large aperture lenses. When I say large aperture I'm taking about the physical opening, not just the f/stop (more on this later).

Ideally then sensor would be 36x24mm, you don't mention a budget but the much more common size of 22.2x14.8mm will provide far better value for money. Focusing close will give you shallow depth of field, but it's not always practical, for example if your subject is bigger than a mouse.

The amount of background blur is also related to the physical size of the aperture. This is given by the focal length (the number in millimetres) divided by the f-number (the number that follows "f/"), so a 50 f/1.8 lens has an aperture that is 50/1.8 = 27.7mm in diameter.

Focal length is important - you have to look past the f/number. Here's an example, the following was shot at f/1.4 at 50mm. Depth of field is shallow, and so the background is blurred:

But now look at this shot, which was taken with a different lens at f/5.6, which is a slow as most lenses go:

The background is not just blurred it's completely obliterated! What's different? The focal length. The first shot was taken with at 50mm f/1.4, so the opening was 36mm across. The second shot was taken at 800mm f/5.6, with an opening a whopping 143mm across!

  • "f/5.6, which is a slow as most lenses go" You mean as fast as most lenses go, surely? And even that's a dubious statement: even among cheap zoom lenses, there are many that will go to f/3.5 or f/4 at the wide end of the zoom range. – David Richerby Jul 5 '14 at 15:50
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"Bokeh" is a feature of the lens used and its adjustment, not of the camera directly.

Bokeh is generally improved by:

  • More rather than less aperture blades in a lens and

  • More rounded aperture blades
    (eg Tamron 18-250 and Sonly SAL18250 are optically identical lenses (both made by Tamron) except Sony chose to round the aperture blades more to improve bokeh.)

Reduced depth of field is with resultant "blurred" background is generally a prerequisite to visible bokeh.

DOF decreases with

  • increasing aperture,

  • increasing focal length and

  • decreasing distance to point of focus.


Special case - the mirror lens.

For a given focal length a mirror lens is low cost, compact and low weight. The disadvantages are fixed aperture, fixed focal length, and an "interesting" bokeh effect.

Mirror lenses are generally considered to have poor bokeh due to out of focus points forming "donut" shapes. This can be controlled but often not fully eliminated. Donut snobs hate them.

500mm mirror lens - donuts are visible but average viewer is unlikely to find them objectionable - a donut snob will hate this effect:

enter image description here

500mm mirror lens forming very bad donuts in background. You don't have to be a donut snob to see these, but the picture is probably still acceptable to many "ordinary people" :-)

enter image description here


Extremely low depth of field example:

Use of a reversed lens on the front of the main lens, to provide a "macro" closeup capability, will result in an extremely shallow depth of field causing the background to lose all features. Whether the resultant bokeh is pleasing in a given case is in the mind/eye/brain of the beholder.

Grass seed head using reversed lens macro. Sensor needs cleaning.

enter image description here

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Firstly, there is nothing like a specific camera for bokeh. Bokeh is basically formed by a light source in the out of focus area of the image. But there are factors in camera and lens kit that affect the bokeh or the DOF.

The main necessity for getting shallow DOF is a lens with large aperture such as f1.8.

As you can understand from the exposure triangle, the swallow depth of field is directly proportional to the larger aperture. ie. larger the aperture shallower the depth-of-field.

A 22.2 x 14.8 mm (APS-C) image sensor would be satisfying for getting a nice bokeh effect in the image. Although a Full frame sensor(36×24 mm) would be much better.

Other than this Focal length of the lens, distance of the subject in focus also affects a fine bokeh.

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The amount of background blur is dependent only on the aperture opening, as some of the answers here already mentioned. From Wikipedia:

b = f*m_s/N * x_d/(s ± x_d) = f*m_s/N / (s/x_d ± 1)

For background bokeh as opposed to foreground bokeh, the ± sign should be + sign. If the background is at infinity, subject distance s is much smaller than subject-to-background distance x_d so the first term vanishes. So,

b = f*m_s/N

where m_s is subject magnification, i.e. x_sensor/x_s (assuming the subject fills the frame, more on that later). So,

b = f*x_sensor/x_s / N
b/x_sensor = f/N / x_s

The figure you're interested in is the ratio of blur disc size b to sensor size x_sensor. It is dependent on the subject size x_s you can't affect and f/N which you CAN affect.

To achieve as large background blur as possible, you're looking to maximize f/N. This means:

  1. Prefer telephoto lenses that give larger f
  2. Prefer wide-aperture lenses that give smaller N

Sometimes, these two may be at odds with each other. For example, do you choose 135mm f/2, 200mm f/2.8 or 400mm f/5.6? All of them have about 70mm aperture opening.

Now if the background isn't at infinity preference would be on slightly shorter wide-aperture lenses. For example, if the background is at 30 meters from subject, a telephoto requiring you to take the picture from 10 meters divides the infinity blur by (10/30+1) = 1.3 whereas a shorter lens allowing you to take the picture from 3.5 meters divides the infinity blur by (3.5/30+1) = 1.1167. So, while 400mm f/5.6 is slightly better than 135mm f/2 if the background is at infinity, for background at 30 meters from subject, 135mm f/2 wins. Indoors, I say even 85mm f/1.2 might be better.

Also do keep in mind the depth of field. You may want the entire subject to be in focus. Depth of field is

DoF = 2*u^2*N*c/f^2

where u is the distance to subject, N is the aperture number, c is the circle of confusion (a certain fraction of sensor size), and f is the focal length.

For a particular sensor, you have u/f constant if you use equal framing. Thus, to maximize depth of field, you are looking to increase N from the smallest possible value (but don't increase it so much that background blur vanishes!). Focal length does not matter.

You can play around with the parameters here: https://dofsimulator.net/en/

For example, full frame 200mm f/2.8 american shot of man 2 (1.80 m) has 6.98% background blur and 6.7 cm depth of field in front of the subject and 6.9 cm behind the subject.

In contrast, APS-C 135mm f/2.0 american shot of man 2 (1.80 m) has 6.51% background blur (slightly less due to 135mm/2 being smaller than 200mm/2.8) but now the depth of field is 7.8 cm in front of the subject and 8.0 cm behind. If you had f/1.8 available, the background blur would be 7.24% but depth of field would still be 7.1 cm in front of the subject and 7.2 cm behind the subject.

What if you take 200mm f/2.8 american shot of man 2 (1.80 m) then on APS-C? You need greater distance, but now the depth of field is 11cm in front of the subject and 11.2cm behind. However, 200mm f/2.8 is equivalent to 320mm f/4.48 on full frame so you are gaining the extra depth of field due to longer effective focal length.

So, APS-C can give more depth of field and larger background blur at the same time, if you have the same lens. But the same lens is effectively a longer lens on APS-C, so you could achieve approximately the same by using a longer lens on full frame.

So, all in all:

  • Select full frame vs APS-C based on which has the lens selections you want. Generally, smaller sensors have smaller aperture opening lenses, so something like micro four thirds might not be entirely ideal. I think most who want lots of indoors background blur choose full frame because fast short primes are available only for full frame (example: 50mm f/1.2 is available but APS-C equivalent 31.25mm f/0.84 or anything close to it isn't). For long telephotos primes the situation is better on APS-C, and outdoors telephotos are more convenient, so for outdoors APS-C vs full frame does not matter as much.
  • Use a telephoto prime lens, especially long if the background-to-subject distance is large (although 70-200/2.8 zoom at 200/2.8 or 100-400/4.5-5.6 zoom at 400/5.6 might replace a prime at cost of larger price and heavier weight). Indoors, the lens should not be as long as outdoors.
  • Select the aperture to have the desired depth of field. Ideally with long telephotos, you can achieve both the desired depth of field and the desired background blur at the same time.

It should also be noted that longer lenses result in compressed background. You may or may not want this. Also, the subject should fill the frame for strong bokeh. So, at the selected focal length, you should be as close to the subject as possible. Some of the assumptions I made in the calculations are no longer valid if the subject does not fill the frame.

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Yes, bokeh is actually proportional to the physical width of the lens opening.

Say you focus on a near-field object at a finite distance = Z and have a camera/lens combo that gives you a field of view (FOV) with angular half-width = Q degrees. If you define bokeh as the ratio of the diameter of the blur circle B (blurred image of a background point at infinity) to the width of the image frame W, then

                     bokeh   =   B / W    ~    R / ( Z  * tanQ )

where R is the radius of the lens opening - ie half the diameter (Note: In the above equation, Z should technically be Z - F, where F is the lens focal length, but you can usually ignore the F when looking at a far-away object).

So if you have two cameras, a large DSLR and a small point-and-shoot, both with the same angular FOV (ie, lenses are same 35mm-equivalent), then the camera with the larger diameter lens will give you more bokeh. This is independent of the camera sensor size.

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