Focusing on reflections. - DPChallenge Forums

07/13/2006 09:24:39 AM · #1

OK, this may be a stupid question for some of you but I can't figure it out. Why is it that when you focus on the reflection on an object, the object itsel is thrown out of focus and if you change the focus to the object then the reflection is thrown out of focus? It seems to me that since the object and the reflection are the same distance from the camera then they should both be in focus. What am I missing here?

June

hyperfocal

Nikon D7000

Nikon MF 105mm f/2.5 AIS

07/13/2006 09:30:20 AM · #2

Simple. The distance from the lens to the subject is different than than the distance from the lens to the reflection.

Message edited by author 2006-07-13 09:31:52.

Canon EF-S 18-55mm f/3.5-5.6 IS

07/13/2006 09:31:53 AM · #3

Originally posted by hyperfocal:

Simple. The distance from the lens to the subject is different than than the distance from the lens to the reflection.

But that is waht I dont understand, the reflection is ON the subject, even if he object reflected is further away.

June

Konador

Canon EOS-500D Rebel T1i

07/13/2006 09:33:41 AM · #4

It's to do with the distance the light has to travel. Simply put, if it's a reflection, it has to travel both from the object to the mirror (to be shown on it), then from the mirror to the camera, whereas the mirror itself only has to travel from the mirror to the camera.

acrotide

Canon EOS-350D Rebel XT

Canon EF-S 18-55mm f/3.5-5.6

07/13/2006 09:34:17 AM · #5

Originally posted by Chiqui:

Not a science major or anything, and I may be way off base here, but here's what I think (hope this comes out somewhat coherently as I'm not quite sure how to explain it :))...

Eventhough the surface that the reflection is on is the same distance from the camera as the object, the reflection does have the depth of whatever it is reflecting. It's almost like looking out a window. If you focus on the window, what's outside will be out of focus, but if you focus on what's outside, the window will be out of focus. That's the best I can come up with. Hopefully someone else actually knows what they are talking about :)

pix-al

Canon EOS-5D

Tamron AF 28-300mm f/3.5-6.3 XR Di LD Aspherical IF for Canon

07/13/2006 09:46:44 AM · #6

But the object and it's reflection are NOT the same distance from the lens.
If you are crouched in front of a swan on a lake with the camera lens horizontal, if you were to lower the lens slightly and look at the swan's reflection in the water, then the line of sight is diagonal. The horizontal line and the diagonal line will NOT be the same length/ distance, there will be a marginal difference.

Depending on your position in relation to the object and it's reflection, this effect could be exaggerated.

Maybe this could cause what you are finding. Does any of that make sense?!?!?

Canon EF-S 18-55mm f/3.5-5.6 IS

07/13/2006 09:52:20 AM · #7

Nope, makes no sense. I just don't get it. Thankfully I am not this dense for everything! LOL

June

Konador

Canon EOS-500D Rebel T1i

07/13/2006 10:01:13 AM · #8

The camera focus works on the total distance the light has travelled to reach the lens. The object is not actually on the mirror, the mirror just bounces the direction of the light which is still travelling from its original point.

Message edited by author 2006-07-13 10:06:17.

Bugzeye

Fisher Price Kid-Tough Digital

07/13/2006 10:03:15 AM · #9

It is just a little trick that your eyes can do and your camera lens can not.

srdanz

Canon EOS-5D Mark IV

Canon EF-S 10-22mm f/3.5-4.5 USM

07/13/2006 10:08:22 AM · #10

An example of what Konador was explaining...

This was taken with 70-200 that has minimum focal distance of 1.5 meters. I was holding it in a driver's seat, some 30cm away from the rear view mirror - no way it would have been in focus (as is obvious - the blurry effect I've got there is the result of lens not being able to focus that close).

Hope thie helps...

kawesttex

Apple iPhone 11 Pro

Canon EF 17-40mm f/4.0L USM

07/13/2006 10:09:58 AM · #11

Originally posted by Konador:

Good example Konador. My example is more along the lines of building on a lake.

pix-al

Canon EOS-5D

Tamron AF 28-300mm f/3.5-6.3 XR Di LD Aspherical IF for Canon

07/13/2006 10:10:53 AM · #12

Ah - I get what you are trying to do now!

Yes - as per the fantastic diagram by Konador!

Message edited by author 2006-07-13 10:11:20.

darnok

Nikon D850

Nikon AF-S Nikkor 16-35 f/4 ED VR

07/13/2006 10:14:29 AM · #13

Originally posted by Bugzeye:

It is just a little trick that your eyes can do and your camera lens can not.

Your eyes have a very short DOF, but your brain can make many objects appear in focus as the same time.

Canon EF-S 17-55mm f/2.8 IS USM

07/13/2006 10:19:07 AM · #14

Originally posted by Konador:

ROFL

idnic

Canon EOS-60D

07/13/2006 10:21:56 AM · #15

Ben, your camera is falling over! See how those bargain tripods are? lol

Bear_Music

Samsung Galaxy S24 Ultra

Sony 24-70mm F4 Vario-Tessar T* FE OSS

07/13/2006 10:26:14 AM · #16

Look at it this way, June: imagine building a special right-angle shooter attachment to screw on the front of your lens that consisted of a mirror mounted at a 45-degree angle, so you could "shoot around corners" (these actually exist). If your theory were correct, that the reflection were "on" the surface of the mirror, you wouldn't be able to get anything in focus cuz the mirror is actually virtually touching the lens. But in actuality, you'd find focus works exactly as it always does, and so does the DOF, on the "redirected" reflection.

ALL a reflection is doing is "bending" or "redirecting" light rays; focus is still based on the total distance of the object from the camera, measured as distance from camera to mirror + distance of mirror from object. If you set up a camera on a tripod to photograph itself in a mirror, and the camera is 3 feet from the mirror, then the correct focus distance will be 6 feet: the light rays that "are" the camera have to travel 3 feet to the mirror then 3 feet back to the camera. In order to have both mirror frame and reflection in focus on this shot, you'd need 3 feet of DOF.

Does that help?

Robt.