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Author Topic: Understanding Light  (Read 5107 times)

Stephenaweiss

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Understanding Light
« on: May 19, 2007, 12:30:58 PM »

I would like help finding references on the quality of light as it reaches the lens, and how it is affected by surface texture, color, reflectance. What is it about portraits of some photographs using slow exposure times that causes an almost textural or glowing quality to the image.

I don't think I am asking about filters.

I know a lot of this has to do with the temperature of light

This has a lot to do with skin tones and the sharpness of portraits in black and white.


thanks
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DiaAzul

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Understanding Light
« Reply #1 on: May 19, 2007, 12:54:39 PM »

Quote
I would like help finding references on the quality of light as it reaches the lens, and how it is affected by surface texture, color, reflectance. What is it about portraits of some photographs using slow exposure times that causes an almost textural or glowing quality to the image.

I don't think I am asking about filters.

I know a lot of this has to do with the temperature of light

This has a lot to do with skin tones and the sharpness of portraits in black and white.
thanks
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Using slow shutter speeds in portrait photography is a way of slightly blurring the facial features in order to hide blemishes and such like. It has a similar effect to the gaussian blur filter in photoshop and was/is used to improve the texture of skin in portrait photography. Where you have highlights then you will get a glow around the edge of the face as it moves around the centered position. Colour temperature of the light shouldn't affect it too much (as far as I know).
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David Plummer    http://photo.tanzo.org/

Stephenaweiss

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Understanding Light
« Reply #2 on: May 20, 2007, 11:19:37 PM »

thank you
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howiesmith

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Understanding Light
« Reply #3 on: May 21, 2007, 02:23:48 PM »

Using a slow shutter speed to control portrait light quality may not be such a great idea.  It will not work with strobe as the "shutter speed" with strobe is the flash duration.  Flash duration (a few thousandths of a second perhaps) is very short, so the subject isn't likely to move much at all while the flash is on.

It usually cannot be counted on for the subject to move about the "zero" point.  The light will cast a longer, deeper shadow as the light moves to a shallower angle. and vice versa.

It is much more reliable and repeatable to move the light or make it relatively larger.

Light quantity (exposure) is controlled by shutter speed, f/stop and ISO.  These are all set in the camera and have no effect on what hits the lens.

Light quality ranges from diffused to specular, largely controlled by the size of the light related to the subject.  It is also affected by the subject - diffused or specular.  This can be changed by powder on the subject (more diffused) or oil (more specular).

A simple exercise you can do yourself is to photograph a diffuse and specular subject with the same light setup.  I used a racket ball for diffused and a glass Christmas tree orniment for specular.  Both blue.  Use a bare bulb on each and see what happens as you change the light to subject distance.  Use a soft box and change the light to subject distance.

You may see that a bare light very close to a small subject will look diffused while a small soft box far away will look specular.  The Sun on Mercury is much more diffused than it is on Pluto.  An oily model may look specular while the same model with a dab of powder on those shiney high lights will look more difused.

I don't think the answer is just one thing.  Like many other things, lighting can be complicated.
« Last Edit: May 21, 2007, 05:30:40 PM by howiesmith »
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Stephenaweiss

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« Reply #4 on: May 23, 2007, 01:17:30 AM »

Howie, thank you for the time you took to compose this answer. James Joyce always thought it important to use words that required the reader to learn something. You sent me to Wikipedia for "specular", between that and your answer I hope I am a bit smarter, will try the experiments you suggest.
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howiesmith

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Understanding Light
« Reply #5 on: May 23, 2007, 12:17:54 PM »

You may also see specular lighting referred to as hard, and diffused as soft.  The real problem is in the real world of photography, there is no specular or diffused lights, but a mix.  The sun on earth is usually considered specular, but the sky makes it more diffused.  Add clouds, and the shift is even more noticible.
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James Godman

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« Reply #6 on: May 23, 2007, 05:10:08 PM »

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You may also see specular lighting referred to as hard, and diffused as soft.  The real problem is in the real world of photography, there is no specular or diffused lights, but a mix.  The sun on earth is usually considered specular, but the sky makes it more diffused.  Add clouds, and the shift is even more noticible.
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Howie is right on this one, but I would also like to bring up another point.  Specular and diffused in practical photography should be thought of relative to the subject being photographed.  For example, if you have a room with one window and a northern exposure and place a person rather close to it, you have a diffused light falling on the subject.  But as that subject moves away from the window the lighting pattern becomes more specular.

Hope that is helpful.

howiesmith

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Understanding Light
« Reply #7 on: May 23, 2007, 05:26:52 PM »

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You may see that a bare light very close to a small subject will look diffused while a small soft box far away will look specular.  The Sun on Mercury is much more diffused than it is on Pluto.

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James is correct.  I thought I was saying something similar when I wrote the above.  Specular and diffused depend on the relative size of the light source with respect to the subject.

A window up close (large compared to the subject) will become more specular as the subject is moved away (made smaller with respect to the very same, unchanged window).

Sorry that I was confusing and/or didn't explain myself well.
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James Godman

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Understanding Light
« Reply #8 on: May 24, 2007, 12:25:51 AM »

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I would like help finding references on the quality of light as it reaches the lens, and how it is affected by surface texture, color, reflectance. What is it about portraits of some photographs using slow exposure times that causes an almost textural or glowing quality to the image.

I don't think I am asking about filters.

I know a lot of this has to do with the temperature of light

This has a lot to do with skin tones and the sharpness of portraits in black and white.
thanks
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I think you also may be referring to the bokeh produced by a given lens.  Out of focus highlights are rendered differently by different lenses (bokeh).

Ray

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Understanding Light
« Reply #9 on: May 24, 2007, 11:01:18 PM »

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For example, if you have a room with one window and a northern exposure and place a person rather close to it, you have a diffused light falling on the subject.  But as that subject moves away from the window the lighting pattern becomes more specular.
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This might not be the case in the Southern Hemisphere, though.
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