Do clip-on telephoto lenses for phones change depth of field and bokeh?
Asked 1/18/2018
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If I add a clip-on telephoto lens such as a 2x phone attachment lens to a smartphone camera (for example, an iPhone with an f/2.2 built-in lens), will it change depth of field or background blur? How does the longer effective focal length affect bokeh, field of view, and exposure compared with using the phone lens alone?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
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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).
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
8y ago
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Yes—adding a 2x tele attachment can increase background blur, but with some caveats.
A tele attachment effectively makes the phone’s focal length longer and narrows the field of view. For the same subject distance and aperture setting, a longer focal length gives shallower depth of field, so out-of-focus backgrounds can appear more blurred.
A 2x tele adapter also magnifies the image, which can reduce the amount of light reaching the sensor per area, roughly like losing about 2 stops of light (similar to going from f/2.2 to about f/4.4 in exposure terms). That means you may need more light or a slower shutter speed.
The exact depth-of-field effect depends on how the attachment is designed. If it behaves like a teleconverter, the effective aperture changes along with focal length, which offsets some of the DOF gain. Even so, the narrower angle of view and higher magnification still tend to make the background look more compressed and blurrier than with the bare phone lens.
So: a clip-on tele lens can help produce more bokeh than the phone alone, but not to the same degree as a much larger camera sensor and fast portrait lens.
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