I created a homemade projector for my phone using a shoebox and a magnifying glass lens of 90 mm focal length. The idea was to put my phone between F and 2F to obtain a real, inverted and enlarged image beyond 2F on my wall. However, the image comes out extremely blurry. Nothing I do seems to be helping - I set my phone on the highest level of brightness, I painted the inside of the shoebox black, I adjusted the position of the phone - nothing is working. It is just coming extremely blurry. Can I fix this?

  • $\begingroup$ Hello! Could you give a little more information? How far away from the lens is your phone? How far away is the wall? $\endgroup$
    – Philip
    Jan 4, 2020 at 8:28
  • 2
    $\begingroup$ A single lens will never give a great image due to chromatic and spherical aberration. The lenses in projectors are compound lenses that are built up from a combination of several simple lenses to reduce the aberration. $\endgroup$ Jan 4, 2020 at 8:58
  • $\begingroup$ What lenses can I use to reduce aberration? $\endgroup$
    – Diana
    Jan 4, 2020 at 15:38

1 Answer 1


Changing the brightness and painting the box black will not affect the image sharpness. Simply changing the distance between the phone and lens is not sufficient. The projector must be kept at a certain distance from the wall.

Measure the width of the phone screen and the height of image you desire. $$\text {Magnification}=m=\frac{h_{img}}{h_{obj}}=-\frac{v}{u}$$ Using the lens formula, you can now calculate both the distance of phone from the lens and the distance of projector from the wall for a clear image.


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