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DPChallenge Forums >> Tips, Tricks, and Q&A >> DOF in reflections - same as straight or not?
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 07/14/2006 03:01:28 PM · #1
Looking at this:


I'm amazed at how the reflection is sharp while the DOF is obviously very very shallow.

Is that normal? Do reflections normally carry infinte DOF? Don't really understand why they would. Or is it a function of it being a ball instead of a flat mirror?

Sometimes I wish I remembered more from my physics B.S. ;)

Edited a typo.

Message edited by author 2006-07-14 15:19:29.
 07/14/2006 03:08:34 PM · #2
A very cool photo!

My answer is yes, yes, no. :-)
 07/14/2006 03:09:15 PM · #3
Interesting question. I don't think reflections carry infinite DOF, that just doesn't seem right, or even possible. But I don't have the answer, just a hunch.

Added: Obviously I have no idea what I'm talking about.

Message edited by author 2006-07-14 15:10:50.
 07/14/2006 03:10:11 PM · #4
Originally posted by Strikeslip:
A very cool photo!

My answer is yes, yes, no. :-)


They do? Because the reflection becomes a much smaller area in the ball? That is so cool.
 07/14/2006 03:13:26 PM · #5
The reflection is all at the same distance (the mirror surface) -- DOF only comes into play if the surface of the mirror is angled (or curved as in the gazing ball) -- but then it's just the normal DOF of the surface, not what is being reflected.

David
 07/14/2006 03:17:38 PM · #6
The reason you're confused is because you're thinking that the camera is focusing on the reflected object that is far away, when in fact, it is focused on the actual plane of the reflection.

And at that point, the reflection is a two-dimensional image. It is sharp and in focus.

07/14/2006 03:21:21 PM · #7
.

Message edited by author 2006-07-14 15:25:23.
 07/14/2006 03:22:40 PM · #8
Originally posted by Tygerr:
The reason you're confused is because you're thinking that the camera is focusing on the reflected object that is far away, when in fact, it is focused on the actual plane of the reflection.

And at that point, the reflection is a two-dimensional image. It is sharp and in focus.


Okay, that makes sense.

The camera will autofocus on the mirror's plane, then? I don't need to manually focus? Makes sense, since it's looking for the sharpest edges, which will happen when it focuses on the mirror.

Thanks!
07/14/2006 03:24:17 PM · #9
Originally posted by David.C:
The reflection is all at the same distance (the mirror surface) -- DOF only comes into play if the surface of the mirror is angled (or curved as in the gazing ball) -- but then it's just the normal DOF of the surface, not what is being reflected.

David

That's the long version of my answer. :-) Well put.
 07/14/2006 03:25:50 PM · #10
A good question though. Sort of like the question of travelling back in time to visit yourself. :-)
 07/14/2006 03:33:34 PM · #11
Hmm. I liked the answer until I just tested it. I don't have a cable with me to upload, but here's what I did and the results, using a flat mirror on the wall of the bathroom.

All shots were at F/2.8.

1) Focused on the wall in macro mode, maybe 10 cm from the wall, and locked in the focus. Then slid the camera sideways and took a picture of the camera in the mirror. The camera's in sharp focus, but the background's blurry.

2) Pointed the camera at itself and let it autofocus. Same result: sharp camera, blurry me.

What's different between this and the original shot I posted? Put another way, how do you focus to get infinite reflected DOF while minimal straight-up DOF?

Edited to add the aperture.

Message edited by author 2006-07-14 15:37:46.
 07/14/2006 04:50:49 PM · #12
Originally posted by David.C:
The reflection is all at the same distance (the mirror surface) -- DOF only comes into play if the surface of the mirror is angled (or curved as in the gazing ball) -- but then it's just the normal DOF of the surface, not what is being reflected.

David


Nope!
There was a thread a couple days ago that delved into this for the special case of a flat mirror. To summarize, the focus distance for an object viewed in a flat mirror is the sum of the distances from the object to the mirror and the distance from the camera to the mirror. In other words, it's the distance the light rays travel.
For a non-flat mirror, it is not simple at all! The mirror must be considered a component of the optical system. A strongly convex mirror like the one in the OP's post has a very short, negative focal length (the "focal point" is imaginary and is located behind the mirror surface). The practical implication of this is that the mirror acts as a very short focal length "wide angle adapter" and the DoF is therefore very great.
 07/14/2006 04:54:54 PM · #13
Originally posted by levyj413:
What's different between this and the original shot I posted? Put another way, how do you focus to get infinite reflected DOF while minimal straight-up DOF?


You focus like you would if you were shooting the reflected scene directly. In other words, use an approximation of a hyperfocal technique, focusing about 1/3 of the way into the reflected image. You can let the camera auto-focus if you use a single AF point and put it on the area of interest.
 07/14/2006 05:54:16 PM · #14
Originally posted by kirbic:

Nope!
There was a thread a couple days ago that delved into this for the special case of a flat mirror. To summarize, the focus distance for an object viewed in a flat mirror is the sum of the distances from the object to the mirror and the distance from the camera to the mirror. In other words, it's the distance the light rays travel.
For a non-flat mirror, it is not simple at all! The mirror must be considered a component of the optical system. A strongly convex mirror like the one in the OP's post has a very short, negative focal length (the "focal point" is imaginary and is located behind the mirror surface). The practical implication of this is that the mirror acts as a very short focal length "wide angle adapter" and the DoF is therefore very great.


Ah-HAH! THAT jibes with what I remember from optics. I just couldn't figure out how that ball worked.

Thanks a million! Now ... how can I fit those test shots into abstract food ... ;)
 07/15/2006 05:40:37 AM · #15
Originally posted by kirbic:
Originally posted by David.C:
The reflection is all at the same distance (the mirror surface) -- DOF only comes into play if the surface of the mirror is angled (or curved as in the gazing ball) -- but then it's just the normal DOF of the surface, not what is being reflected.

David


Nope!
There was a thread a couple days ago that delved into this for the special case of a flat mirror. To summarize, the focus distance for an object viewed in a flat mirror is the sum of the distances from the object to the mirror and the distance from the camera to the mirror. In other words, it's the distance the light rays travel.
For a non-flat mirror, it is not simple at all! The mirror must be considered a component of the optical system. A strongly convex mirror like the one in the OP's post has a very short, negative focal length (the "focal point" is imaginary and is located behind the mirror surface). The practical implication of this is that the mirror acts as a very short focal length "wide angle adapter" and the DoF is therefore very great.

Got it -- thanks.

But it doesn't help to know my misconception was so common. :( I wonder where I picked it up from? :/

David
 07/15/2006 06:12:50 AM · #16
Thank goodness for kirbic!

The only thing that I would say about food pics is obviously, you would use DOF calculators to get the optimal DOF for the whole set of food...

If using a mirror behind the subject, you would measure the distance from the front of the food to the reflective surface of the mirror.

Then you would need to adjust your DOF (either by moving the camera farther back or by changing the aperture) to include that distance added on BEHIND the mirror.

Just like what it looks like.

As mentioned before, this is obviously much simpler if you are dealing with a mirror which is in plane with the camera's sensor plane.

If you wanted to deal with a curved surface or a non-aligned mirror, things get a bit more complicated, but experimentation should work if you just take measurements off the basic primary line of focus and increase the DOF a bit.

Message edited by author 2006-07-15 06:13:24.
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