Focusing on reflections?

MikePerham · 11-10-2009, 09:01 AM

If you are taking a photograph of a reflection, and the reflective surface is say 5 feet away and the subject being reflected is 15 feet from the reflective surface, is your focusing distance 20 feet, or 5 feet?

Anyone with experience in this with an answer? ...would be appreciated, thanks!

Mike.

**newmikey** · 11-10-2009, 09:10 AM

Originally posted by MikePerham

If you are taking a photograph of a reflection, and the reflective surface is say 5 feet away and the subject being reflected is 15 feet from the reflective surface, is your focusing distance 20 feet, or 5 feet?

Anyone with experience in this with an answer? ...would be appreciated, thanks!

Mike.

Neither. According to the laws of physics, focusing distance is 15 feet - the apparent distance of the image from the mirror.

[EDIT]
Let me give you an example:

1. Focused on the mirror itself (your 5 feet). The reflected image now is out of focus

2. Focused on the image to mirror distance (15 feet) with the reflected image in focus and the mirror itself just out:

aerodave · 11-10-2009, 09:26 AM

Originally posted by newmikey

Neither. According to the laws of physics, focusing distance is 15 feet - the apparent distance of the image from the mirror.

No, the laws of physics would say you focus at 20 feet. That's the distance the light rays have to travel to get from subject to camera, and the fact that they get "folded" by a mirror doesn't affect the focusing distance.

newmikey, maybe in your examples the depth of field is making it work with that error. After all, it's not too hard to focus at 15 feet and get a sharp subject at 20 Feet. For example, at 35mm, f/8 on an APS-C sensor, focusing at 15 ft puts everything between 9.5 ft and 36.8 ft in focus.

**newmikey** · 11-10-2009, 09:36 AM

Originally posted by aerodave

No, the laws of physics would say you focus at 20 feet. That's the distance the light rays have to travel to get from subject to camera, and the fact that they get "folded" by a mirror doesn't affect the focusing distance.

newmikey, maybe in your examples the depth of field is making it work with that error. After all, it's not too hard to focus at 15 feet and get a sharp subject at 20 Feet. For example, at 35mm, f/8 on an APS-C sensor, focusing at 15 ft puts everything between 9.5 ft and 36.8 ft in focus.

On second thought (digging through stacks of stale education) you are absolutely right! The image appears to the camera at the same disctance behind the mirror as it is in front of the mirror and you have to add the camera-to-mirror distance to that.

That's probably what my examples show: 5 feet and 20 feet rather than 5 and 15.

<slap hand to forehead mode>
I should learn to think, then write...
</slap hand to forehead mode>

reeftool · 11-10-2009, 09:56 AM

That's a pretty reasonable answer, aerodave. I don't remember too much physics and have probably lost far too many brain cells through the years of think of things like this. A reflection like the posted shot of the mirror works best when the whole thing is in sharp focus, the mirror, the wall , and the reflected image. I would never rely on a distance scale for that shot other than checking for the max. depth of field. I take a lot of water reflections shots from my kayak and often some of the reflection is only a few feet away while the unreflected shoreline and subjects may be at infinity yet the image looks to be in focus throughout the whole picture. In this photo, the shoreline and reflected image in the water appears in focus but the lilly pads in the foreground are out of focus with the somewhat sharper reflected image under them. I don't remember exactly where my focus point was on this shot but I used a 70-300 Sigma at 133 mm and f/11.

enoeske · 11-10-2009, 10:38 AM

I tried some reflection shots at a wedding I shot recently. I had to switch to manual focus for them because the camera kept focusing on the mirror, not the reflection a farther distance away.

aerodave · 11-11-2009, 07:56 AM

I think in your examples, enoeske, the problem was probably the autofocus sensor catching the stuff on the edge of the mirror...the frame or purse. In both cases, the mirror is a small part of the frame, and one of the edges could have been in the AF sensor's field of view. Or in the first shot it could have been trying to focus on the smudges on the surface of the mirror. But for a perfectly clean, sufficiently large mirror, the camera can't "see" it and won't try to focus on the mirror surface itself.

graphicgr8s · 11-11-2009, 02:53 PM

Originally posted by aerodave

No, the laws of physics would say you focus at 20 feet. That's the distance the light rays have to travel to get from subject to camera, and the fact that they get "folded" by a mirror doesn't affect the focusing distance.

newmikey, maybe in your examples the depth of field is making it work with that error. After all, it's not too hard to focus at 15 feet and get a sharp subject at 20 Feet. For example, at 35mm, f/8 on an APS-C sensor, focusing at 15 ft puts everything between 9.5 ft and 36.8 ft in focus.

So if I was taking a picture of a reflection in a mirror that is 5 feet away and the reflection is of the moon I should focus at infinity?

aerodave · 11-11-2009, 03:31 PM

Originally posted by graphicgr8s

So if I was taking a picture of a reflection in a mirror that is 5 feet away and the reflection is of the moon I should focus at infinity?

Yup! That's exactly right.

I'd suggest testing it out tonight, but the moon rises pretty late right now (like 2AM). I guess you could try it out in the pre-dawn hours when the moon is fairly low in the eastern sky...or wait a couple weeks for it to be up high during the evening hours.

reeftool · 11-12-2009, 08:22 AM

I never really thought much about this until I saw the thread the other day. I have always focused through the viewfinder and if the AF couldn't get it , I would manually focus. I have had difficulty with water reflections at times, just getting the camera to focus. Shots with the shore line and the reflection always seem to work easily but getting a reflection only shot off the water had to be manually focused as the AF would go nuts. This explains it as the camera can't decide between the reflection, which is distance and at infinity possibly, and the surface of the water which is a few feet. I just assumed it was a weakness in the AF system but that isn't the case. It's because I'm trying to get the AF to focus on infinity and 5 ft. at the same time. I remember a time trying to get a shot of a feather floating by and had to turn the boat around 3 times and finally got it manually focusing. All I had to do was look at some of my own shots and the answer was staring me in the face.Thanks for the explanations.

graphicgr8s · 11-12-2009, 06:13 PM

Originally posted by aerodave

Yup! That's exactly right.

I'd suggest testing it out tonight, but the moon rises pretty late right now (like 2AM). I guess you could try it out in the pre-dawn hours when the moon is fairly low in the eastern sky...or wait a couple weeks for it to be up high during the evening hours.

Oh well. I was actually making a smart alek remark. Of all the wedding shots I've done with mirrors I always manually focused and never paid mind to where it focused. Manual focus because I never owned an AF til recently. Focus was never the reason to toss the picture though. I just found better means to screw it up.

11-10-2009, 09:01 AM	#1
MikePerham Veteran Member Join Date: May 2007 Location: Prince George, BC Canada Photos: Gallery \| Albums Posts: 642	Focusing on reflections? If you are taking a photograph of a reflection, and the reflective surface is say 5 feet away and the subject being reflected is 15 feet from the reflective surface, is your focusing distance 20 feet, or 5 feet? Anyone with experience in this with an answer? ...would be appreciated, thanks! Mike.

11-10-2009, 09:56 AM	#5
reeftool Pentaxian Join Date: Dec 2007 Location: Upstate New York Photos: Gallery \| Albums Posts: 9,555	That's a pretty reasonable answer, aerodave. I don't remember too much physics and have probably lost far too many brain cells through the years of think of things like this. A reflection like the posted shot of the mirror works best when the whole thing is in sharp focus, the mirror, the wall , and the reflected image. I would never rely on a distance scale for that shot other than checking for the max. depth of field. I take a lot of water reflections shots from my kayak and often some of the reflection is only a few feet away while the unreflected shoreline and subjects may be at infinity yet the image looks to be in focus throughout the whole picture. In this photo, the shoreline and reflected image in the water appears in focus but the lilly pads in the foreground are out of focus with the somewhat sharper reflected image under them. I don't remember exactly where my focus point was on this shot but I used a 70-300 Sigma at 133 mm and f/11. Last edited by reeftool; 12-24-2009 at 08:04 AM.

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11-10-2009, 09:10 AM	#2
newmikey Site Supporter Join Date: May 2007 Photos: Gallery Posts: 2,289	Originally posted by MikePerham If you are taking a photograph of a reflection, and the reflective surface is say 5 feet away and the subject being reflected is 15 feet from the reflective surface, is your focusing distance 20 feet, or 5 feet? Anyone with experience in this with an answer? ...would be appreciated, thanks! Mike. Neither. According to the laws of physics, focusing distance is 15 feet - the apparent distance of the image from the mirror. [EDIT] Let me give you an example: 1. Focused on the mirror itself (your 5 feet). The reflected image now is out of focus 2. Focused on the image to mirror distance (15 feet) with the reflected image in focus and the mirror itself just out:

11-10-2009, 09:26 AM	#3
aerodave Veteran Member Join Date: Aug 2007 Location: Dayton, OH Posts: 365	Originally posted by newmikey Neither. According to the laws of physics, focusing distance is 15 feet - the apparent distance of the image from the mirror. No, the laws of physics would say you focus at 20 feet. That's the distance the light rays have to travel to get from subject to camera, and the fact that they get "folded" by a mirror doesn't affect the focusing distance. newmikey, maybe in your examples the depth of field is making it work with that error. After all, it's not too hard to focus at 15 feet and get a sharp subject at 20 Feet. For example, at 35mm, f/8 on an APS-C sensor, focusing at 15 ft puts everything between 9.5 ft and 36.8 ft in focus.

11-10-2009, 09:36 AM	#4
newmikey Site Supporter Join Date: May 2007 Photos: Gallery Posts: 2,289	Originally posted by aerodave No, the laws of physics would say you focus at 20 feet. That's the distance the light rays have to travel to get from subject to camera, and the fact that they get "folded" by a mirror doesn't affect the focusing distance. newmikey, maybe in your examples the depth of field is making it work with that error. After all, it's not too hard to focus at 15 feet and get a sharp subject at 20 Feet. For example, at 35mm, f/8 on an APS-C sensor, focusing at 15 ft puts everything between 9.5 ft and 36.8 ft in focus. On second thought (digging through stacks of stale education) you are absolutely right! The image appears to the camera at the same disctance behind the mirror as it is in front of the mirror and you have to add the camera-to-mirror distance to that. That's probably what my examples show: 5 feet and 20 feet rather than 5 and 15. <slap hand to forehead mode> I should learn to think, then write... </slap hand to forehead mode>

11-10-2009, 10:38 AM	#6
enoeske Veteran Member Join Date: Jul 2009 Location: Surprise, Az Photos: Gallery Posts: 2,136	I tried some reflection shots at a wedding I shot recently. I had to switch to manual focus for them because the camera kept focusing on the mirror, not the reflection a farther distance away.

11-11-2009, 07:56 AM	#7
aerodave Veteran Member Join Date: Aug 2007 Location: Dayton, OH Posts: 365	I think in your examples, enoeske, the problem was probably the autofocus sensor catching the stuff on the edge of the mirror...the frame or purse. In both cases, the mirror is a small part of the frame, and one of the edges could have been in the AF sensor's field of view. Or in the first shot it could have been trying to focus on the smudges on the surface of the mirror. But for a perfectly clean, sufficiently large mirror, the camera can't "see" it and won't try to focus on the mirror surface itself.

11-11-2009, 02:53 PM	#8
graphicgr8s Guest	Originally posted by aerodave No, the laws of physics would say you focus at 20 feet. That's the distance the light rays have to travel to get from subject to camera, and the fact that they get "folded" by a mirror doesn't affect the focusing distance. newmikey, maybe in your examples the depth of field is making it work with that error. After all, it's not too hard to focus at 15 feet and get a sharp subject at 20 Feet. For example, at 35mm, f/8 on an APS-C sensor, focusing at 15 ft puts everything between 9.5 ft and 36.8 ft in focus. So if I was taking a picture of a reflection in a mirror that is 5 feet away and the reflection is of the moon I should focus at infinity?

11-11-2009, 03:31 PM	#9
aerodave Veteran Member Join Date: Aug 2007 Location: Dayton, OH Posts: 365	Originally posted by graphicgr8s So if I was taking a picture of a reflection in a mirror that is 5 feet away and the reflection is of the moon I should focus at infinity? Yup! That's exactly right. I'd suggest testing it out tonight, but the moon rises pretty late right now (like 2AM). I guess you could try it out in the pre-dawn hours when the moon is fairly low in the eastern sky...or wait a couple weeks for it to be up high during the evening hours.

11-12-2009, 08:22 AM	#10
reeftool Pentaxian Join Date: Dec 2007 Location: Upstate New York Photos: Gallery \| Albums Posts: 9,555	I never really thought much about this until I saw the thread the other day. I have always focused through the viewfinder and if the AF couldn't get it , I would manually focus. I have had difficulty with water reflections at times, just getting the camera to focus. Shots with the shore line and the reflection always seem to work easily but getting a reflection only shot off the water had to be manually focused as the AF would go nuts. This explains it as the camera can't decide between the reflection, which is distance and at infinity possibly, and the surface of the water which is a few feet. I just assumed it was a weakness in the AF system but that isn't the case. It's because I'm trying to get the AF to focus on infinity and 5 ft. at the same time. I remember a time trying to get a shot of a feather floating by and had to turn the boat around 3 times and finally got it manually focusing. All I had to do was look at some of my own shots and the answer was staring me in the face.Thanks for the explanations.

11-12-2009, 06:13 PM	#11
graphicgr8s Guest	Originally posted by aerodave Yup! That's exactly right. I'd suggest testing it out tonight, but the moon rises pretty late right now (like 2AM). I guess you could try it out in the pre-dawn hours when the moon is fairly low in the eastern sky...or wait a couple weeks for it to be up high during the evening hours. Oh well. I was actually making a smart alek remark. Of all the wedding shots I've done with mirrors I always manually focused and never paid mind to where it focused. Manual focus because I never owned an AF til recently. Focus was never the reason to toss the picture though. I just found better means to screw it up.