Got a question? - DPChallenge Forums

08/28/2006 10:20:11 PM · #51

Originally posted by Spazmo99:

On the other hand, moving the camera closer or further away does nothing to affect the exposure.

There is something else happening to the subject from the camera's point of view, what is it?

Hmmm... I'm sticking to my story. The light from the subject to the camera does not disperse, it travels in a straight line from subject to lens.

The only reason light diminishes with the square of the distance is that the area covered by the light increases with the square of the distance. (i.e. if you light an area one foot square at one foot distance, and then double the distance, you are now lighting an area that is four square feet in size and so the amount of light landing on the original one foot square is only 1/4th the amount of light).

08/28/2006 10:20:40 PM · #52

Originally posted by routerguy666:

Whatever amount of light hitting the subject, it is a constant value as far as your camera is concerned when viewing it.

Doesn't the light from the subject diffuse as well? It obeys the same laws of physics as the light from the source. Why doesn't that diffusion affect the exposure?

Message edited by author 2006-08-28 22:32:26.

08/28/2006 10:22:14 PM · #53

Originally posted by Spazmo99:

Doesn't the light from the camera diffuse as well? It obeys the same laws of physics as the light from the source. Why doesn't that diffusion affect the exposure?

It would if your lens got bigger with the square of the distance. But it stays the same size.

08/28/2006 10:23:27 PM · #54

That was a stupid answer I just gave. If your lens got bigger, it would gather MORE light, not less. (hmmm... head scratching time)

08/28/2006 10:26:58 PM · #55

Originally posted by dwterry:

Originally posted by Spazmo99:

On the other hand, moving the camera closer or further away does nothing to affect the exposure.

There is something else happening to the subject from the camera's point of view, what is it?

Hmmm... I'm sticking to my story. The light from the subject to the camera does not disperse, it travels in a straight line from subject to lens.

The only reason light diminishes with the square of the distance is that the area covered by the light increases with the square of the distance. (i.e. if you light an area one foot square at one foot distance, and then double the distance, you are now lighting an area that is four square feet in size and so the amount of light landing on the original one foot square is only 1/4th the amount of light).

The second part is correct, that is the inverse square law, but just because the light is reflected from the subject, that law isn't invalidated, the reflected light still spreads out.

You reasoning is correct, but there is something else that obeys the inverse square law. Think about an isolated subject against a plain background. What happens to the image of the subject at the sensor/film plane when you move the camera closer or father away?

Message edited by author 2006-08-28 22:27:49.

Canon EF 100-400mm f/4.5-5.6L IS

08/28/2006 10:31:33 PM · #56

doh!

Message edited by author 2006-08-28 22:31:58.

Jacbowron

Canon EOS-30D

08/28/2006 10:33:11 PM · #57

Originally posted by pearlseyes:

I can't seem to achieve accurate focus when I shoot people. The focus typically is somewhere than the intended place. I have a shot of a red head on my page that the focus seems to be on her shoulder strap yet I thought I locked focus on her eyes. This seems to be the case with most of my shots when people are involved since I bought my DSLR. Maybe I need a lesson on how to achieve proper focus. When I press the shutter button halfway then recompose my shot does it throw off focus if I remove my finger before pressing the shutter button completely? Maybe it has more to do with the aperture I'm selecting and the distance from the subject. I don't know. Any thoughts? I'm still trying to figure all this out. Thank you.

I can only assume the 20D is similar to the 30D in terms of focusing. So if I am wrong please disregard what you are about to read.

If you press the button that is on the farthest top right back of the camera and look through the viewfinder you should see all of the focus points lit up. If you dont then that may be your problem. Use the joystick on the back of the camera to cycle through the focus points until they are all lit up, or in certain situations, until the focus point is where you want it to be for your photo. If they are all lit up then read on. I remember when I first got my 30D in the mail I hit this said button and messed up the focus without knowing it. I was greatly frustrated for about 45 minutes while all of my shots were focusing in the wrong spot.

The answer to the question: is the focus thrown off after you have focused, recomposed and remove your finger from the shutter button and then take the picture. Is yes, it will be. Each time you press the shutter button while the camera is set on auto focus, the camera will refocus the lens.

Generally speaking if you want to avoid this you must keep the shutter button held half way down the whole time. Also you can try using the selective focus feature on the 20D (mentioned above) so you can compose the shot the way you want it and not worry about the focus being off.

If none of this helped please let me know.

08/28/2006 11:00:08 PM · #58

Originally posted by Spazmo99:

The second part is correct, that is the inverse square law, but just because the light is reflected from the subject, that law isn't invalidated, the reflected light still spreads out.

You are confusing reflected light and light illuminating an object. The amount of light an object reflects is constant based on the light that is hitting it. However, what amount is hitting it varies based on the strength of the source and its distance from the object. With a few exceptions, you use a camera to capture reflected light.

You should google on the Law of Reflection. This is why the light reflecting off an object is not scattering the same as the light coming out of a bulb. What you see are the rays of light reflecting off the object directly into your lense. The strength of that reflection has been determined by the strength of the light source (and its distance from the object as already mentioned). It is a constant value. If you can see the object it is because your eye or camera is catching the reflected light from that object. Again, that reflected light's strength is based entirely on the source. The only part of the equation that affects how brightly the object is lit is the source, not your eye nor where you are standing when you look at it.

Message edited by author 2006-08-28 23:05:37.

08/28/2006 11:02:01 PM · #59

Originally posted by routerguy666:

Originally posted by Spazmo99:

The second part is correct, that is the inverse square law, but just because the light is reflected from the subject, that law isn't invalidated, the reflected light still spreads out.

You are confusing reflected light and light illuminating an object. The amount of light an object reflects is constant based on the light that is hitting it. However, what amount is hitting it varies based on the strength of the source and its distance from the object. With a few exceptions, you use a camera to capture reflected light.

No, light moving through space obeys the inverse square law, reflected or incident. Even a tightly focused beam like a laser, still spreads out.

Message edited by author 2006-08-28 23:05:55.

08/28/2006 11:05:28 PM · #60

Originally posted by Spazmo99:

You reasoning is correct, but there is something else that obeys the inverse square law. Think about an isolated subject against a plain background. What happens to the image of the subject at the sensor/film plane when you move the camera closer or father away?

Ah... the light from the subject is reducing with the square of the distance, BUT the image on the sensor is also reducing with the square of the distance making the amount of light appear to be equal.

Right?

TooCool

Canon EOS-300D Rebel

Canon EF 50mm f/1.8 II

08/28/2006 11:05:45 PM · #61

Hey, I asked a question too... :-(

08/28/2006 11:06:22 PM · #62

Originally posted by dwterry:

Originally posted by Spazmo99:
You reasoning is correct, but there is something else that obeys the inverse square law. Think about an isolated subject against a plain background. What happens to the image of the subject at the sensor/film plane when you move the camera closer or father away?

Ah... the light from the subject is reducing with the square of the distance, BUT the image on the sensor is also reducing with the square of the distance making the amount of light appear to be equal.

Right?

We have a winner!!!!

08/28/2006 11:07:26 PM · #63

Originally posted by Spazmo99:

Originally posted by routerguy666:

Originally posted by Spazmo99:

The second part is correct, that is the inverse square law, but just because the light is reflected from the subject, that law isn't invalidated, the reflected light still spreads out.

You are confusing reflected light and light illuminating an object. The amount of light an object reflects is constant based on the light that is hitting it. However, what amount is hitting it varies based on the strength of the source and its distance from the object. With a few exceptions, you use a camera to capture reflected light.

No, light moving through space obeys the inverse square law, reflected or incident. Even a tightly focused beam like a laser, still spreads out.

I'm sorry, but you obviously do not comprehend the law you are hung up on. The Inverse Square Law for Light deals SOLELY with light emitting from a point source.

08/28/2006 11:08:17 PM · #64

Originally posted by TooCool:

It means you're normal.

08/28/2006 11:08:53 PM · #65

Originally posted by routerguy666:

Originally posted by Spazmo99:

Originally posted by routerguy666:

Originally posted by Spazmo99:

The second part is correct, that is the inverse square law, but just because the light is reflected from the subject, that law isn't invalidated, the reflected light still spreads out.

You are confusing reflected light and light illuminating an object. The amount of light an object reflects is constant based on the light that is hitting it. However, what amount is hitting it varies based on the strength of the source and its distance from the object. With a few exceptions, you use a camera to capture reflected light.

No, light moving through space obeys the inverse square law, reflected or incident. Even a tightly focused beam like a laser, still spreads out.

I'm sorry, but you obviously do not comprehend the law you are hung up on. The Inverse Square Law for Light deals SOLELY with light emitting from a point source.

Wrong.

At a distance ANY source of reflected light effectively becomes a point source, yet that will not change the exposure for that reflective surface.

Also, the law can be applied to reflected sources if you consider tham as groups of point sources.

Message edited by author 2006-08-28 23:13:49.

08/28/2006 11:09:18 PM · #66

For those who care, here is the law being referenced:

Any point source which spreads its influence equally in all directions without a limit to its range will obey the inverse square law.

TooCool

Canon EOS-300D Rebel

Canon EF 50mm f/1.8 II

08/28/2006 11:10:19 PM · #67

Originally posted by Spazmo99:

08/28/2006 11:10:23 PM · #68

Originally posted by Spazmo99:

Wrong.

Ahh, a stunning rebuttal. I'll head off to bed now in a fit of intellectual despair.

Canon EF 100-400mm f/4.5-5.6L IS

08/28/2006 11:14:49 PM · #69

Originally posted by routerguy666:

Originally posted by Spazmo99:

Wrong.

Ahh, a stunning rebuttal. I'll head off to bed now in a fit of intellectual despair.

Please do. Maybe you'll wake up illuminated, but somehow I doubt it.

Jacbowron

Canon EOS-30D

08/29/2006 05:40:55 PM · #70

Well, there was a thread killer.... Arent there more questions?

chrissycampbell

Canon EOS-20D

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

08/29/2006 07:53:04 PM · #71

I am fairly new to photography and very new to this site, I have entered a couple of challenges where people have commented that something is wrong with my IMO. Please explain..