Mirror Focus?

..."It is all about depth of field. And the shorter the focal length and smaller the aperture ( opening the light passes through in the lens) the greater the distance in focus.

Now the larger the sensor the shorter the focal length you will need to maximize results. However on a full frame camera, a 14mm lens set at f/16 will have in focus everything from around 30cm away to infinity."...

https://colesclassroom.com/depth-of-field-dof-explained-easy/

https://www.quora.com/Can-a-camera-lens-focus-up-close-and-far-away-at-the-same-time

Thanks SE.

That answers my question.An infinity of pictures is worth an infinity of words!

We can always go back and digitally correct the focus with AI, so it's not really set in stone...

The camera that took this picture must have a high f number or the ladies should become noticeably blurry in the distance...

Would they though? All of the ladies images are formed on the same plane in the mirror.

No, the image is behind the mirror, the same distance as the object being reflected.

https://www.inspiritvr.com/plane-mirrors-study-guide/

How about a 2nd mirror reflecting the image in the first mirror,both being a first surface reflector?

The light from the object bounces whenever it hits a flat mirror, angle of incidence = angle of reflection. If it hits a second flat mirror, it bounces again. The ray of light which is exactly in the right direction enters your eye or a camera lens.

The distance that the light has traveled determines how your eye or camera lens has to adjust its focal length for the image on the retina or camera sensor to be in focus.

So why does the camera not treat the reflection the same as a photograph.It is all in focus in the mirror.

Put a picture behind a piece of glass...what is the difference between the reflection and the photo,all conditions being the same.?

As I said,I don't understand all I know about this...it hasn't clicked yet.....

So why does the camera not treat the reflection the same as a photograph.It is all in focus in the mirror.

The depth of field of modern cameras (high f-stop number) makes it hard to detect mild blurring. If you have a camera with a fast lens (say f/1.4)

Put a picture behind a piece of glass...what is the difference between the reflection and the photo,all conditions being the same.?

Parallax. The picture will remain stationary with respect to the mirror as you move left and right. The reflection of a stationary object some distance in front of the mirror will move back and forth showing the image is actually behind the mirror. The light from the object bounces off the mirror, making it appear to originate behind the mirror.

So why does the camera not treat the reflection the same as a photograph?

Because of the distance, the camera will focus on the nearest objects - the photo and the mirror itself, but because the image on the mirror is away, you need to adjust it to be sharp, but then the image of the photo and the mirror itself will be blurry.

what is the difference between the reflection and the photo,all conditions being the same.?

Because of the depth of the image on the mirror. It looks like it is far behind, but if you put it behind glass, it's the same as the photo next to the mirror.

If the mirror is a first surface reflector,like Aluminum or Stainless steel, no glass in front,they are both the same distance from the lens.So what happens then?

The mirror itself and the photo next to it will be in focus.

Ok,another example of my question:If you take a picture in a mirror and it is perfectly focused,then take a picture of the picture from the mirror beside a picture that is not a reflection from a mirror,there should not be any difference, right?

I don't understand your question, but...

If you take a picture of an object reflected in a mirror, light from that object must travel to the mirror, bounce off, and travel back to the camera. This distance is farther than the distance from the mirror to the camera.

But,the image in the mirror is on the first surface of the mirror which is where the camera focus should be,same as a photograph at the same distance.

I understand some loss in brightness,etc,but the focus should be the same,no?

I can understand a picture taking a picture of a distant object being a different focus,but the image is ON the mirror itself.

... but the image is ON the mirror itself.

No, the image is behind the mirror the same distance that the reflected object is in front of the mirror.

One word: Parallax

You can compare the distance of two objects by moving your head left and right. If two objects are the same distance, the parallax will be the same. If they are unequal distance, the nearer will have the greater parallax, that is, it will move right with respect to the farther object when you move your head left and vice versa.

The image you see in the mirror is a virtual image, not a true object ... and a flat mirror has no focal point, so the focal point would be calculated as your distance from the mirror, plus the distance from the mirror to the object...

Exactly. The image behind a plane mirror is a virtual image. The light doesn't go through there, it has been redirected by a mirror or mirrors and comes from that direction, and the location can be determined by parallax.

The other type of image, a real image, the light passes through. Examples are images that fall on camera sensors and the retina of the eye.

"Optical image, the apparent reproduction of an object, formed by a lens or mirror system from reflected, refracted, or diffracted light waves. There are two kinds of images, real and virtual. In a real image the light rays actually are brought to a focus at the image position, and the real image may be made visible on a screen—e.g., a sheet of paper—whereas a virtual image cannot. Examples of real images are those made by a camera lens on film or a projection lens on a motion-picture screen. Virtual images are made by rays that do not actually come from where the image seems to be; e.g., the virtual image in a plane mirror is at some distance behind the mirror."

https://www.britannica.com/technology/optical-image

Consider this scenario:

A photograph is placed beside,in the same plane, a mirror whose glass front is exactly the thickness of the photo.

Or print an image directly on the mirror surface. This would put both images at the same focal distance. Would they both be in focus?

Don't get hung up on the thickness of the photograph or even the thickness of the glass on the mirror. It's insignificant. Assume the photograph is zero thickness and the mirror is a front surface mirror. That's not the issue.

Would they both be in focus?

"both" meaning printed image and reflected image?

No, an object in front of a mirror has a reflection the same distance behind the mirror, not in the plane of the mirror. See the video in #26.

The reflection is a virtual image. The light seen coming from the reflection is actually coming from the object, bouncing off the mirror.

Here's a more physical explanation. Light consists of vibrating electric and magnetic fields. When it hits a flat metal surface at some angle, the free electrons on the surface move to cancel out the electric field parallel to the surface. They generate an equal and opposite field parallel to the surface which creates the reflected light. The light from a reflected object is recreated, and travels in the opposite direction. It appears to be arriving from the virtual image behind the mirror.

Well first of all a mirror doesn't reflect 100% of the light, there is some loss of brightness, so some loss of detail and color intensity...

..."Mirrors reflect between 85% and 99.9% of the light which is incident on them. Aluminum and silver mirrors reflect about 90 and 95 percent of incident respectively. These are the most common types of mirrors used in day to day life. 99.9% reflection is only found in perfect mirrors such as dielectric mirrors."...

...and then there's the ghosts...

Click on this: How far does the light travel from the object to the camera? For the frame of the mirror, it is from the frame to the camera. For the image in the mirror, it is from the object producing the image to the mirror plus the distance from the mirror to the camera. for Rixter's example 10' and 10' + 5'.

A related question:

I have a photo from the early 1920's or forties.It is a sepia tone.It shows my uncle sitting in front of a window and he is in perfect focus.Out the window is a building with "Atlanta Electric..." the building is also in perfect focus.

So how the perfect focus for near and far images?

A further exploring of my question found this,which explains a lot of my unanswered questions.Very informative and enlightening.

https://artlist.io/blog/types-of-focus/

This answer was in the previous comments. The camera has a range over which it is in focus. If this range includes both distances, both would be in focus.

As Rixter says, the image is formed on the reflective, back, surface of the mirror. There will be a small change in focal length due to the image passing through the glass which has a greater refractive index than the air. This error will be small approx. 1/3 of the thickness of the glass so probably negligible.

There will be no difference in focal length if the mirror is a first surface reflector like metal for instance.

So the question remains:Why is the reflection not created as an image at the same distance,like a photograph.As SE showed,it is likely focused to infinity.

I think SE is right.

I put a piece of tape on the bathroom mirror. I took two pictures, one focused on the tape, and the other focused on the reflection of the far wall. As you can see, the reflection is not in the plane of the mirror and both are not in focus at the same time.

OK.Two pictures worth two thousand words.I got it now.

Thanks everyone!

Let me try to explain it better with an example. You have a camera with a calibrated focus lens. Ten feet in front of the camera is a photograph and a mirror. In the mirror is a reflection of an object that is five feet in front of the mirror.

If you focus the camera at 10 feet, the photograph will be in focus but the object reflected in the mirror will not be in focus.

If you focus the camera at 15 feet (10 feet to the mirror + 5 feet back to the object), the reflected object will be in focus but the photograph will not be in focus.

There is no way for the photograph and the reflected object to both be in focus.

That was the point in my comment #6.

Correct; the focus depends on the light path length. In this case 10 or 10 + 5.

The image of the mirror as a mirror and the other photo next to the mirror will be in focus. But the image of the object on the mirror may be out of focus, it depends on how far the object is from the mirror. I'm almost 100% sure that the mirror image will be out of focus.