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Question regarding softboxes...
Old 01-02-2005, 09:19 PM   #1 (permalink)
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I took some Christmas cash over to Borders and bought this book on model and glamour photography put out by Amherst Publishing.

The author goes into a lot of mathematical derivations for things photographic, but he hit one I had not seen before.

He claims the optimum distance for a softbox (front surface to model) is the same as the diagonal of the box. Thus for a 30 by 40 inch softbox, you'd place it 50 inches from the model. He doesn't go into WHY that's supposed to be optimum, or at least my initial reading didn't register a reason.

He also claims that at this distance the light on the model will "pop".

I presume the geometry of the light source is the basis for the "optimum" distance calculation. I just wonder if anybody knows for sure, and if there is indeed a distance at which something magical happens. I have used a softbox extensively, and I don't recall ever seeing such an effect, but I'm getting older and don't see as well.

And how do you determine if it is indeed "popping" since you are looking at the modeling light, not the strobe output unless you are a lot quicker than I am!






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Re: Question regarding softboxes...
Old 01-02-2005, 11:27 PM   #2 (permalink)
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Part of the reason might be the very purpose a softbox is used, to make the photo softer (the shadows more diffuse). The closer the box, the bigger, the light, the softer the shadows.


Softbox was 57" from model Bridget.

Cheers,
rfs
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Re: Question regarding softboxes...
Old 01-03-2005, 09:31 AM   #3 (permalink)
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With that particular soft box that was his definition of the distance for his images to pop or what he feels his images pop. I disagree with the notion that the light source is not different coming from the panel or diagonal part of the soft box. Light coming from a center of a soft box is more DIRECTIONAL vs scattered light coming from the panel or diagonal side of a soft box.

Now Al, you didn't pay attention in Ohio. I had a brief discussion on the matter.

So let me go into more detail from a scientific perspective.

Distance between the light source and subject will define an image. The penumbra shadow, the part that receives some light, to the umbra, the part that receives no light are all effected by the distance and direction from a light source that will change it's characteristics from different distances and directions from a light source.

Shadows vary tremendously as a function of the lighting sources. They can be hard edged or soft edged and contain both an umbra and a penumbra area. The relative size of the umbra-penumbra is a function of the size and the shape of the light source and its DISTANCE and quality from the object. The umbra is that part of a shadow that is completely cut off from the light source, whereas the penumbra is an area that receives some light from the source. A penumbra surrounds an umbra and there is always a gradual change in intensity from a penumbra to an umbra.

A light source at large distances, in the simplest case, the objects produce umbra with sharp edges.

Distances of the light source that determine the quality of a shadow, enable us to conclude information concerning the nature of the light source. It's very important. Some photographers just take it for granted and say this works and fell into it by nature. Your happy go lucky photographer that hits and misses needs to pay attention to this law and start to understand it and pay attention to DETAIL.

I hope this gives you a better understanding of what he finds is the magical distance from his light source.

J T









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Re: Question regarding softboxes...
Old 01-03-2005, 09:53 AM   #4 (permalink)
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[ QUOTE ]
Shadows vary tremendously as a function of the lighting sources. They can be hard edged or soft edged and contain both an umbra and a penumbra area. The relative size of the umbra-penumbra is a function of the size and the shape of the light source and its DISTANCE and quality from the object. The umbra is that part of a shadow that is completely cut off from the light source, whereas the penumbra is an area that receives some light from the source. A penumbra surrounds an umbra and there is always a gradual change in intensity from a penumbra to an umbra.

[/ QUOTE ]


Hi JT,

I think I'm missing something here. Let's say we are in a dark room and then we turn on the modeling lamps of our strobes. Now are subject is lit but is surrounded by darkness. The dark areas around the otherwise lit subject would be the umbra, is that correct?? And so that would make the penumbra the lit areas of my subject. Correct? In that case, it seems to me that since the darkness is surrounding the otherwise lit subject, it would be the umbra that surrounds the penumbra and not the reverse. I guess I can see both ways working, depending on the angle of incidence and the degree of coarseness to any texture in the image. Perhaps I'm reading too much into this. In any event, your explanation is great and I think I learned more from that quick blurb than I did from my photography course text book and all it's techno-babble. There is something to be said for explaining things in simple ways and you've done just that.

Thanks for sharing and I wish you a very safe, joyful, health, and prosperous new year!

Good day!

Mike
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Re: Question regarding softboxes...
Old 01-03-2005, 10:17 AM   #5 (permalink)
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Think just shadows.......the penumbra is not the specular light, if you will....

Umbra is a shadow that virtually has NO LIGHT

Penumbra is the shadow part that actually would have detail or receive some light.

In a ring light shadow, clearly examined, you would find this.

In my image notice the light on Kelly's cheek gradually fade into a penumbra shadow into an umbra ...... Look at the chair, look at her arms, look at her legs, look everywhere that you see a light fall off that creates the penumbra into the umbra. So in conclusion the penumbra would always surround the umbra if a penumbra existed....in more directional light souces such as parabolox flash reflector heads, umbrellas, grids, etc, you would have more definition of an umbra or sharper edge umbra and less of a penumbra shadow.

Now here is where it gets confusing...The farther your light source is away, you would have sharper more defined umbras.

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Re: Question regarding softboxes...
Old 01-03-2005, 10:22 AM   #6 (permalink)
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Great explanation, JT. I look forward to your instructional sessions at HTL VI.

By the way, did you know that "umbra" is the Italian word for "shadow"? It's also the basis of our word "umbrella" -- a device that casts a shadow. (Okay, we use one mostly to "cast" a dry spot when it's raining, but its original use was to protect the user from the sun.) The prefix "pen" means "next to," so "penumbra" literally means "next to the shadow." I once was given directions in London and told to "get off [the subway] at the penultimate stop." (I think he knew that most "yanks" don't know the true meaning of "penultimate"!) --Randy
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Re: Question regarding softboxes...
Old 01-03-2005, 10:24 AM   #7 (permalink)
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Thanks for, er, shedding some light on the explanation. I understand what you're saying. And this shot of KellyG is spectacular. My goodness, she is so beautiful and you do such an incredible job of highlighting and capturing her beauty.

I think I need to add to my resolutions for this year. I want to attend a Super Shoot at some point if I can see my way clear financially at some point. It would certainly be the highlight of the year.

Thanks again, JT. Always a pleasure talking with you! Have a great week.

Mike
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Re: Question regarding softboxes...
Old 01-03-2005, 10:31 AM   #8 (permalink)
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Hi Randy,

I actually didn't know where "umbra" was derived from but that makes total sense.

Yes, looking forward to HTL VI. Working everyday on it from here on out.....it will be here before you know it.

GOOOOOOOOOOOOOOOOOOOOOOOOOOOO RAMS ;-)

If they would just get out of the UMBRAS of the coaching staff, we might just pull it off.....

J T

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Re: Question regarding softboxes...
Old 01-03-2005, 10:41 AM   #9 (permalink)
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Mike,

Always a pleasure to see you get into it as well. We would love to have you at a Supershoot. I know you would benefit from the educational value and hands on experience of shooting shooting and more shooting.....

Thanks for the kind words on Kelly. Although no one has to tell me she's beautiful...I get that part, obviously....that's why I keep her around.... ;-)

J T

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Re: Question regarding softboxes...
Old 01-03-2005, 11:07 AM   #10 (permalink)
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And to think....I thought you kept her around for her ability to keep you slightly calmer during events and her ability to keep the models on track. I guess it shows that I can be wrong now and then... [img]/ubbthreads/images/graemlins/grin.gif[/img]
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