Bokeh and FX vs DX question

Heitz

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Hi all. Please tell me if this logic is flawed. Bokeh must be produced by the outer (edge) portions of the glass (I realize it is partially the blades, but the light that contributes to bokeh must pass primarily from the outer glass portions). Thats why when you stop down, and you use more of the center portions of the glass, bokeh decreases. If this is true, then consider the following.

1) Suppose you are shooting at f/1.8 on an FX lens. Will bokeh be different on an FX sensor versus a DX sensor? My thought is that it would not, at least for the portion of the image that both sensors have in common. Of course, the FX sensor will have a larger DOF, and that would change, but everything else should be equivalent.

2) Now suppose you hold sensor size constant (lets say DX cropped sensor). Will there be differences in the bokeh for a FX lens and DX lens of equivalent focal length? My thought is that it WOULD differ. Using an FX lens on a DX sensor would force you to use more of the central portions of the glass (decreasing bokeh) whereas the DX lens would be sized such that more of the glass edges do contribute.

Thoughts?
 

kundalini

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KmH

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You are confusing bokeh with depth-of-field (DoF). Bokeh is not adjustable.

Image sensor size does influence DoF, which is why DoF calculators have to be told what size image sensor is being used. www.dofmaster.com
 

zcar21

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It is flawed from the beginning. Bokeh means out of focus, it doesn't come particularly from any part of the lens.
To answer your questions you have the inverse responses.
 
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KmH

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Bokeh is a word made up in 1997 by Mike Johnson, who was then the editor of Photographic Techniques magazine.

Boke is a Japanese word meaning blur or haze, in the sense of a mental haze or senility, and doesn't really relate to photography.

Mike kind of added 2+2, got 7 for an answer, and published his result. The word bokeh is now often used as a way to try and establish some cred, though it is often used in the wrong context and pretty much does just the opposite.

Differences in optical aberrations and lens aperture blade shape and edge finish cause some lens designs to produce a Circle of Confusion (CoC) that is pleasing to the eye (nice bokeh). Other lenses produce a CoC that is unpleasant or distracting (poor bokeh).

There are 2 types of bokeh: Cream Cheese & Hollywood.

Nikon's AF 85 mm f/1.4D lens is nicknamed "The Cream Machine" for the wonderfully smooth, Cream Cheese type CoC it's 9 lens aperture blades and minimal optical aberrations produces, while Canon's inexpensive EF 50 mm f/1/8 II lens and it's 5 poorly shaped lens aperture blades produces "Harsh and distracting bokeh due to pentagonal aperture". Canon EF 50mm F1.8 II Lens Review: 5. Conclusion & samples: Digital Photography Review
Mirror type (catadioptric) telephoto lenses produce a sort of donut shaped CoC that many find very unattractive and distracting.
 

djacobox372

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A larger sensor will produce a thinner plane of focus (blurrier background) at the same aperture due to the need to either get closer or use a longer focal length since the wider sensor produces a wider composition. IE a 50mm f1.8 on full frame creates the same composition and exposure as a 35mm 1.8 on crop sensor, but has noticably narrower dof due to the longer focal length.
 

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