AboutOptics C Expertise I am interested in answering general questions about optical phenomena, devices, experiments, and instrumentation. I have a special interest in the properties of light, interaction of light and matter, and interaction of light and humans (visual optics). Ask me anything to do with light!
Experience I have a classical training in optics, optical phenomenology, optical modeling and laboratory experimentation. I work for a major university research laboratory specializing in infrared optics .
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Hello Curtis,
I couldn't get the thin lens equation to work for me with a double convex lens. I placed an object behind it and in front focused my 35mm camera on the virtual image it produced. I obtained various readings for S1 and S2 where the image of the object came into sharp focus in the camera's viewfinder. A picture of the set up I used is at : -
The results of F I got were variable and didn't match the lens' stated focal length of 7.5cm. The data I got is shown on the webpage.
What have I done wrong?
PS On the webpage above is a picture of a Lissajous figure. It was obtained using an enlarger lens applying the formulae you gave me many weeks ago. The formulae works with the enlarger lens. Thanks to your help I'm making progress with my photo art, which has stalled for years. Concave lens aren't working for me and I'm now turning my attention to convex lens. (My final artwork will employ both an enlarger lens and convex lens working together.)
Best regards,
Usuff
Answer -
The Lissajous figure is very nice. I had never heard the term before.
I tried adding a fixed offset to S1 and S2 to arrive at a consistent value of F. I came up with a value of about 6 cm for S1 and -5 cm for S2, which would mean that your estimate of the location of the lens was wrong by 5.5 cm (the remaining 0.5 cm is consistent with a slight shift of the location of the principal planes due to the thick lens). These values return an average focal length of 7.15 cm, incidentally. However, 5.5 cm seems altogether too large to attribute to measurement error. Any chance?
Here is my revised table:
S1 S2 F
9.5 25.5 6.92
10.5 23.5 7.26
11.5 20.5 7.37
12 19 7.35
12 16 6.86
Best,
Curtis
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Curtis,
Regarding the discrepancy in applying the thin lens equation above, my mistake. I realised this morning on waking up that two lenses are involved, not just the single DCX lens shown in the diagram. There is a 70mm Nikkor macro lens on the camera body. I've amended the picture accordingly at : -
What a silly mistake on my part. I think this accounts for the inconsistency between the observations and the calculations above?
In situations where two lenses are combined, can just one equation be used or does the thin lens equation need to be applied twice? (The results of the first calculation used for the second.)
Glad you like the Lissajous figure. Here is a clearer one shot with a macro lens. With your help on the optics side I can soon expand the variety of designs.
I'll be looking at the psychological aspect of different color combinations. Work ahead for me!
Best,
Usuff
Answer Ok, the addition of the second lens makes a big difference. You can use "gaussian reduction" to combine the two lenses into one effective lens, but I recommend that you simply apply the thin lens equation twice.
You will need to be very careful to use all lenses at or near their design conjugate when you do color.
