Diffusing and using Gels with a 600EX?

[Jopa]

Jopa, nothing will at this size unless you shoot macro at 1' to the subject

I follow the following light modifier size rule:

optimal camera to subject distance equals or less than sum of linear dimensions of the light modifier
i.e. for 80 cm x 80 cm square softbox optimal distance to subject equals or less than 1.6m
for 1.2m octagonal , deca hexagonal or round softbox optimal distance to subject equals or less than 2.4m

therefore for the 40cm large round softbox optimal distance to subject is 80cm only. 1.5m is not ideal and really pushing it but I would say somewhat acceptable in run and gun situations.

at US$7.37 price level you might as well give it a go

http://www.ebay.com/itm/Neewer-16-40cm-Softbox-with-Grey-Balance-Card-for-Canon-Nikon-Neewer-Speedlight-/381453820183

A while ago I tried the original modifier from fstoppers https://www.amazon.com/Fstoppers-FlashDisc-Portable-Speedlight-Softbox/dp/B00KHBZ85E but for some reason it didn't give me much satisfaction with results. I just checked the specs - it's 12". Maybe those extra 4" of the Neewer modifier will make a difference. IMHO they should have made a 7' disc - no compromises LOL!

I never thought about the actual numbers, but I think yours make sense.

At $7.37 the 16" Neewer is definitely worth trying!

 

[hne]

Jopa, nothing will at this size unless you shoot macro at 1' to the subject

I follow the following light modifier size rule:

optimal camera to subject distance equals or less than sum of linear dimensions of the light modifier
i.e. for 80 cm x 80 cm square softbox optimal distance to subject equals or less than 1.6m
for 1.2m octagonal , deca hexagonal or round softbox optimal distance to subject equals or less than 2.4m

therefore for the 40cm round softbox optimal distance to subject is 80cm only. 1.5m is not ideal and really pushing it but I would say somewhat acceptable in run and gun situations.

at US$7.37 price level you might as well give it a go

http://www.ebay.com/itm/Neewer-16-40cm-Softbox-with-Grey-Balance-Card-for-Canon-Nikon-Neewer-Speedlight-/381453820183

A while ago I tried the original modifier from fstoppers https://www.amazon.com/Fstoppers-FlashDisc-Portable-Speedlight-Softbox/dp/B00KHBZ85E but for some reason it didn't give me much satisfaction with results. I just checked the specs - it's 12". Maybe those extra 4" of the Neewer modifier will make a difference. IMHO they should have made a 7' disc - no compromises LOL!

Interesting idea, to use twice the average width of the light source as a rule of thumb for reach as a soft source. It translates to the light source being 28° wide as seen from the subject. This could indeed be a reasonable rule of thumb for soft light as it is quite easy to calculate unaided.

However, the amount of light wrapping around the subject is also dependent on the relative sizes of the subject and the light source: a 1"x1" softbox 2" from the eye will not light the entire face even though it is 28° wide as seen from the eye. A wall-sized softbox at twice the wall length distance will create a fantastic light on someones face.
To light half a sphere, you need a light source at infinite distance or the same diameter as the sphere.

To light a person with soft light, I would argue that you need to have a light source roughly as wide as a person. To light a face only, you can get away with something smaller.

 

[SecureGSM]

yup, due to fast light fall off. Inverse-Square rule of light in action. the further away light source from the subject, the lesser light fall off affects the outer areas of the frame. how much of this affects the image also depends on the lens focal length and softbox beam angle. incidentally, FOV of 85mm lens on FF body is also 28 degree.(diagonally)

"... use twice the average width of the light source..." - correct for round / octa softboxes.
for rectangular softboxes it works out as 1/2 x perimeter
i.e. 80cm x 120 cm soft box - 2m distance as a very rough estimation. vertically or horizontally - is another story

Interesting idea, to use twice the average width of the light source as a rule of thumb for reach as a soft source. It translates to the light source being 28° wide as seen from the subject. This could indeed be a reasonable rule of thumb for soft light as it is quite easy to calculate unaided.

However, the amount of light wrapping around the subject is also dependent on the relative sizes of the subject and the light source: a 1"x1" softbox 2" from the eye will not light the entire face even though it is 28° wide as seen from the eye. A wall-sized softbox at twice the wall length distance will create a fantastic light on someones face.
To light half a sphere, you need a light source at infinite distance or the same diameter as the sphere.

To light a person with soft light, I would argue that you need to have a light source roughly as wide as a person. To light a face only, you can get away with something smaller.

 

[LDS]

yup, due to fast light fall off. Inverse-Square rule of light in action. the further away light source from the subject, the lesser light fall off affects the outer areas of the frame. how much of this affects the image also depends on the lens focal length and softbox beam angle. incidentally, FOV of 85mm lens on FF body is also 28 degree.(diagonally)

Are your rules based on a specific kind of subjects - i.e. portraits? Because IMHO in general, without taking into account size, shape and surface type of the subject, they are going to fail in several situations. Also perimeter puts narrows stripboxes together square softboxes.

 

[SecureGSM]

Correct, for portraits, single or small group of people, with a single softbox of round, Octa, square or rectangular (typicaly 2:3 sides ratio) shape.. there is no universal rule to cover all scenarios and specific cases:
Multiple light sources, stripboxes, types of light modifiers, required quality of light, shadows and directionality of light.

Are your rules based on a specific kind of subjects - i.e. portraits? Because IMHO in general, without taking into account size, shape and surface type of the subject, they are going to fail in several situations. Also perimeter puts narrows stripboxes together square softboxes.