Forgot Password
Pentax Camera Forums Home
 

Reply
Show Printable Version Search this Thread
01-17-2014, 06:18 PM - 1 Like   #1
Pentaxian
ToyTank's Avatar

Join Date: Mar 2013
Photos: Albums
Posts: 675
Question about DOF on reflected surface

Hi I've been been looking for an answer to this question, but I can't find a satisfactory answer. I've noticed when a photograph on a reflective surface(metallic or non metallic) The DOF in the reflection seems infinite, but the answers I'm finding indicate the DOF distance should be the sum distance from film plane to mirror to he subject. Can someone provide a link, or an information to help me understand this? A couple samples to illustrate my question

In the reflection everything from my eyelashs to the mountains are in focus, but my actual eyelashes are both in and out of focus.




Here it's the TINY head of a ball point pen and the reflection of me and my kitchen. Taken on Q w/ tamy 90 and bellows. Not a great shot but just playing around and really got me wondering about reflections




Thanks for any help!


Last edited by ToyTank; 01-17-2014 at 09:04 PM. Reason: Grammar-It's not just cool, it's the law!
01-17-2014, 07:03 PM - 1 Like   #2
dms
Site Supporter




Join Date: Aug 2011
Location: New York, NY
Photos: Gallery
Posts: 1,623
The problem is you are photographing a curved surface which is a lens (a mirrror lens) and with the camera at the focal point sees the distance, while outside the curved surface (area) the close up material is in focus. If the mirror was a flat plane it would be the way you thought.
01-17-2014, 07:08 PM - 1 Like   #3
Pentaxian
halfspin's Avatar

Join Date: Aug 2012
Photos: Gallery
Posts: 254
The reflection has no DOF, it is like a 2d image - if you shot a photo of a printed photo that has a large DOF you could use a very narrow DOF and still see everything in focus.
01-17-2014, 07:32 PM   #4
Pentaxian
ToyTank's Avatar

Join Date: Mar 2013
Photos: Albums
Posts: 675
Original Poster
Thanks for the answers guys, not sure the reflection acts like a 2d object, because of the effect perspective has so light scatters you'd still need another lens to "see" it right? just as any object without a pinhole or lens, though I'm t just trying to understand, and I've seen the same effect photographing small flat 1" polished brass surfaces reflecting the sky with details like clouds and fine details of the object.

If it's a len effect(don't know anything about mirror lenses) I'd say it's collecting alot of light - sunset reflecting off a lake the reflection is almost as bright as the sky. I'd always thought it was an illusion but the camera sees them as almost equally bright. at the right angle


Last edited by ToyTank; 01-17-2014 at 09:19 PM.
01-17-2014, 07:33 PM - 1 Like   #5
Site Supporter




Join Date: Mar 2012
Location: Perrineville, NJ
Posts: 1,365
Never really thought about this, but the responses above are correct. A curved reflective surface is indeed a lens, but a reflective rather than refractive type. It has it's own optical properties that can be measured and characterized. (This is the same principle behind reflector lenses and reflector telescopes.) You are simply putting another lens in the optical path to your subject, albeit a small portion of the overall optical path.

If you were to use a mildly concave mirror (rather than the convex mirrors in your examples), your DOF should be shallower than the rest of the surrounding image. Although it might take a bit of work to set up a good practical demonstration.

Thanks for bringing this up, takes me back to some old physics classes.
01-17-2014, 07:53 PM - 1 Like   #6
Senior Member




Join Date: Sep 2011
Location: Victoria, Australia
Photos: Gallery
Posts: 252
You can think of the scene you see in a reflection as being for all optical purposes like a 3D object located behind the mirror: the image. This is because rays of light emanating from the real object, no matter what direction they are going in, reflect off the mirror at such an angle that they appear to have come from the direction and distance of the object's image and therefore behave in exactly the same way as if they really did come from an object located at the image.

(A not all that helpful explanation and diagram is at Virtual image - Wikipedia, the free encyclopedia )

For a plane mirror, distance from the image to the mirror equals the distance from the object to the mirror. That's when the rule you quoted about the sum of the distances would apply.

For a convex mirror, the image is closer (but smaller than the object - see pic on wiki page) so the focal distance will need to be shorter than the sum and the DOF will be that you expect of a (smaller) object at that (lesser) distance.

For concave, it's potentially vice versa (but it gets more complicated here with inverted images etc - see Curved mirror - Wikipedia, the free encyclopedia ).

If you really wanted to do the calculations to work out your image distance and DOF (I wouldn't... talk about spoiling the fun of photography ), it can be done pretty easily. Let f = focal distance = half of the radius of curvature of a spherical mirror. Then the lens equation, 1/f = 1/u + 1/v will allow you to take the distance (u) from any point on your object and find out the distance (v) between the mirror and the corresponding point of the image, or vice versa. (v will be negative if the image is behind the mirror)
01-17-2014, 07:54 PM   #7
Pentaxian
ToyTank's Avatar

Join Date: Mar 2013
Photos: Albums
Posts: 675
Original Poster
Well I'm going to have some more fun with this and I'll take your answers on faith, I think i'd have to enroll in some night classes to really understand it. Just thinking if it is a lens the optical properties are interesting just for the sake it's DOF is 1:1 ~ infinity and did not loose a ton of light.
01-17-2014, 08:05 PM   #8
Pentaxian
ToyTank's Avatar

Join Date: Mar 2013
Photos: Albums
Posts: 675
Original Poster
Thank you, Russell the diagrams and information you've linked are just what I was looking for and would have never found on my own with the search terms I was using. Thank you so much.

Thanks everyone for your help, having a elementary understanding of this effect should help me be a better photographer.

01-17-2014, 08:12 PM   #9
Pentaxian
ToyTank's Avatar

Join Date: Mar 2013
Photos: Albums
Posts: 675
Original Poster
Trying to think of reflective concave surfaces in my house to play with(flashlight reflector) and realized I wouldn't be able to photograph much in the reflection but my lens or maybe something placed down in the center and get a 360 degree view of it?

if I can figure lighting out Oh taking photos is fun! I could just be watching TV now
01-17-2014, 08:28 PM   #10
dms
Site Supporter




Join Date: Aug 2011
Location: New York, NY
Photos: Gallery
Posts: 1,623
In your last post what you mention what is (was?) in fact a commercial attachment to do 360 degrees photos. A fisheye except it was a curved mirror held some distance in front of the lens. Never saw one but heard of it.
01-17-2014, 08:56 PM   #11
Pentaxian
ToyTank's Avatar

Join Date: Mar 2013
Photos: Albums
Posts: 675
Original Poster
I just realized in that first photo, my eye is two lenses at the same time

Would be interesting to see the commercial 360 degree rig, mine came on a maglite for $10

PS: Reading the OP I'm a very sloppy typer. The fact you all decrypted my cipher and did not give up half way through my post is a amazing. Thanks again

Last edited by ToyTank; 01-17-2014 at 09:07 PM.
01-17-2014, 09:03 PM   #12
Loyal Site Supporter
THoog's Avatar

Join Date: Aug 2012
Location: North Carolina
Photos: Gallery | Albums
Posts: 3,486
QuoteOriginally posted by dms Quote
In your last post what you mention what is (was?) in fact a commercial attachment to do 360 degrees photos. A fisheye except it was a curved mirror held some distance in front of the lens. Never saw one but heard of it.
Do you mean this:

Spira.com -- The History of Photography As Seen Through the Spira Collection - Images
01-17-2014, 09:10 PM   #13
Pentaxian
ToyTank's Avatar

Join Date: Mar 2013
Photos: Albums
Posts: 675
Original Poster
Neat! Thanks for the link

Edit Lol it looks like one of those convex trucker mirrors in a plexi tube! I got a weekend project now lol
01-17-2014, 10:02 PM   #14
Senior Member




Join Date: Sep 2011
Location: Victoria, Australia
Photos: Gallery
Posts: 252
The other thing I forgot to mention when it comes to convex mirrors like your ballpoint pen.

All images will be at most one focal length behind the mirror (which is one quarter of the diameter of curvature).

So in your example, let's say the ball had a diameter of 1 mm. So long as you had 0.25 mm of DOF and you focussed correctly, you could have the entire reflected scene in focus from zero to infinity.

Pretty tricky. Hadn't really occurred to me before but pretty easy to show with ray diagrams or the lens equation.

Edit to add: mind you, ray tracing and the lens equation are both generalisations for a "thin" lens/mirror so it will start to break down towards the edges of that very curved surface.

Last edited by russell2pi; 01-17-2014 at 10:09 PM.
01-17-2014, 11:55 PM   #15
Veteran Member
maltfalc's Avatar

Join Date: May 2010
Location: Winnipeg
Photos: Albums
Posts: 394
QuoteOriginally posted by halfspin Quote
The reflection has no DOF, it is like a 2d image - if you shot a photo of a printed photo that has a large DOF you could use a very narrow DOF and still see everything in focus.
that's completely ridiculous nonsense. i seriously don't understand how anyone who's ever looked in a mirror could fail to notice the reflection has depth. are you blind in one eye? if this were true, people would never shoot at f22, they'd just shoot through a mirror to get infinite dof at f1.4.
Reply

Bookmarks
  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
answer, camera, dof, flickr, focus, help, mirror, pentax help, photography, question, question about dof, reflection
Thread Tools Search this Thread
Search this Thread:

Advanced Search


Similar Threads
Thread Thread Starter Forum Replies Last Post
Rumors about a new Ricoh GXR camera surface online Cynog Ap Brychan Pentax News and Rumors 70 08-01-2013 06:57 AM
Question about buy k5 on the internet foxrock619 Troubleshooting and Beginner Help 6 11-18-2012 10:26 AM
Dumb question about resolution on Optio RS1500 Alliecat Troubleshooting and Beginner Help 6 11-09-2012 07:20 AM
Question about DOF Alberts Troubleshooting and Beginner Help 17 11-03-2011 04:14 AM
Question about DOF aliquis Photographic Technique 12 04-01-2011 07:24 AM



All times are GMT -7. The time now is 04:04 PM. | See also: NikonForums.com, CanonForums.com part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
register
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]
Top