That's REALLY good that you noticed the frame!
You know then that it probably isn't astigmatism (which is pretty common -- my eyes would focus a vertical line at a different focus than a horizontal line, because the curvature of my eye is different for those two directions
). But unless the len is really sloppy in your frame, you looked at the tree in the same orientation as the candle... excellent!
I personally also lean towards counting out chromatic aberation... you didn't notice the image of the tree LOOKING some funny color (like a bit too blue or yellow), did you? I think based on this, that there were probably just errors in measurements that you need to think about. So we'll talk about "error" more...
To get a good idea of how much your measurements have error, go back and do the two experiments again with the same lens and (even hopefully the same meter stick) -- but doing ALL the measurements of length, height, etc. without looking at all at your previous work. Then when you find the new results -- What is the average of the two tree experiments compared to the average of the two candle experiments? Are the averages closer than the initial two measurements? What if after this you go back and do it a third time... are the averages getting even closer? This type of error from measurement to measurement is probably best classified as "statistical error"... so doing the experiment a few times and averaging the results is better than just doing it once.
However, there is another type of error probably best known as "procedural error." If your averages aren't getting closer ("converging") then this is probably the case. This would happen, say if someone cut a centimeter off your meter stick, you didn't notice, and this changed the results of the two methods differently (this would maybe be called "machine error" because the measurement machine isn't working right, so you need to fix it by relabeling the numbers on the stick or getting a new stick. Other procedural errors would include human errors like analyzing the data wrong in some way. Did you somehow read the meter stick wrong, maybe using inches for some measurements and cm for others if both are marked on the stick? Or -- Is your lens really thick? That would mean that the common "thin lens" equations you are probably using wouldn't really describe your experimental data. You might need to look up "thick lens" equations.
a little help please? T.T
