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Let's take, for example, the Moment Tele Lens. This lens is pitched as a 60mm lens that allows you to get 2x "closer" to the subject.

The iPhone 6s uses a f/2.2 lens.

Does the mounting of the tele lens onto the iPhone change any characteristic that would affect the ability to produce bokeh? If so, how?

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Yes and No.

The FL of a lens has a significant impact on the DOF, what is not in focus, and how it is rendered.

This is a 2x adaptor which makes the FL twice as long and the FOV 1/2 as wide. For any subject distance and aperture setting a lens that is twice the focal length will have approximately one quarter the DOF of the shorter lens. There will also be a loss of exposure due to spreading the light over a larger area (magnification) equivalent to doubling the aperture to f/4.4. However, I'm not sure if this type of adaptor will cause that to also affect the DOF the way a 2x teleconverter does for a DSLR lens... if it does, the net reduction in DOF is approximately 1/4 (2x focal length and aperture #) for any given subject distance.

Additionally, the narrower FOV and greater magnification will impact the size at which BG elements are rendered (amount of blur/bokeh characteristics). This can range from negligible if the BG is very close behind the subject, to very significant if there is a lot of BG separation (it can result in an entirely different image overall).

The "no" part is due to the fact that it is for use on a very tiny cell phone sensor. So even though it will have less DOF, it will still probably be quite a lot... the difference may not be evident in many/most situations (most evident for very near subjects).

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  • Would it be safe to say that putting the 2x element in front of the native lens (iPhone attachment) vs behind (teleconverter) has the same optical effects on DOF?
    – OnBreak.
    Jan 19 '18 at 16:06
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    No; the front attachment will maintain f/# while the rear attachment will maintain entrance pupil diameter. If you put a 2x front attachment on a 4.2mm f/2.2, you get an 8.4mm f/2.2. If you put a 2x rear attachment, you get an 8.4mm f/4.4. Jan 19 '18 at 20:25
  • The f#/f-ratio is an "effective ratio" dependent on the size of the aperture opening as seen by the objective lens (the effective size). If you put a diopter (magnifying lens) prior to the aperture it increases the magnification and focal length, but it equally magnifies the apparent size of the aperture from the objective end perspective. Therefore, the f-ratio remains the same. This is how constant aperture zoom lenses work, they change the magnification of the aperture opening as they zoom while variable aperture zoom lenses do not (or they do it less). Mar 21 '18 at 20:02
  • The reduction of light with a TC is also an "effective aperture," but for a different reason. The issue is that you are taking the light from a smaller portion of the image circle and enlarging it (spreading the light thinner/farther)... this is referred to as "bellows factor" in other applications. Mar 21 '18 at 20:02

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