Posted by Cliff Dodge on January 7, 2013 at 4:09 PM
Why is this? Isnt the sign, even though a reflection shot at 5.6, on the same plane as the road on each side of it? The sign was further down the road, and i was even further the other way to get the sign in the water. I was just goofing off when i took the shot, then when i was looking at it up close, i couldnt understand, and still cant, why the sign in the reflection of the water is in focus and nothing on either side of it isnt, that is, within the focal area shot at 5.6.??
Hi Cliff, ...hmm.... saw this earlier...and surprised no physics-major or eye-docs have jumped on this.
I'll take a stab...and they can laugh at my attempt...altho the image is appearing on the same plane as the roadway, the water is reflecting a sign that is significantly further away. Since the water is flat, it reflects the sign cleanly as if it were actually a distant object... as such it is in focus whilst the road surface is not...or visa versa. That occurs for the same reason that you would hold up a mirror, say, 6 inches from your eye. The image seen on the mirror can be miles away and requires you to focus independent of any smear or item of dirt that is on the glass of the mirror. If you were near-sighted, this would be easily demonstrated because without your eye-glasses you could focus sharply on a dirt spec in the glass mirror ...but, you would be unable to focus as sharply of the image in the mirror... (assumed far distant). You need your prescription eye-glasses to help you focus on that distant object. Your eye can focus on the spec but not the distant image...even though both are located 'on the same image plane'. It all has to do with depth of field and focal lengths. Just like the image above.
I think Ken is saying it correctly. I simple way to think about it is - how far does the light have to travel before it reaches your lens - that then is the effective focal distance - after all the light did not originate on the surface of the of road so focusing there leaves out the distance it had to travel to the surface before being bent toward your lens.
Posted by Cliff Dodge on January 9, 2013 at 5:42 PM
Discussion
3 repliesI'll take a stab...and they can laugh at my attempt...altho the image is appearing on the same plane as the roadway, the water is reflecting a sign that is significantly further away. Since the water is flat, it reflects the sign cleanly as if it were actually a distant object... as such it is in focus whilst the road surface is not...or visa versa. That occurs for the same reason that you would hold up a mirror, say, 6 inches from your eye. The image seen on the mirror can be miles away and requires you to focus independent of any smear or item of dirt that is on the glass of the mirror. If you were near-sighted, this would be easily demonstrated because without your eye-glasses you could focus sharply on a dirt spec in the glass mirror ...but, you would be unable to focus as sharply of the image in the mirror... (assumed far distant). You need your prescription eye-glasses to help you focus on that distant object. Your eye can focus on the spec but not the distant image...even though both are located 'on the same image plane'. It all has to do with depth of field and focal lengths. Just like the image above.
I hope this has not confused you more.