Is a flash diffuser necessary?
No, because flash diffusers are only going to have an effect under a narrow set of circumstances. Where not used properly, it leads to a waste of flash power and is an unnecessary extra weight to carry. There are better ways to soften light and remove specular highlights that would be more worth the money.
In the following parts of this article, we will look into what diffusers are meant for, and how to use them properly so that they do their job.
Owning a flash diffuser kit from Godox personally and having used them for a few times, I am inclined to say that you don’t need them, and that money can perhaps be better spent on something more essential. You can find out about its current price via the hyperlink.
This is what it looks like fyi - the diffuser is dome-shaped.
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How do you use a speedlight diffuser? What does it do on a flash?
Speedlight diffusers come in various shapes, such as domes, cuboids or cylinders. You mount them on a speedlight flash head usually with screws or magnets. The flash light goes through the diffuser before it reaches your subject, which scatters the light. The light is made weaker overall, and provides shadow fill where effectively bounced.
Light from a speedlight flash is very directional; it comes from the flash head towards the subject. With a diffuser, the light becomes way less directional. The light is scattered across all directions from the flash, now only a fraction of the total flash output hits directly onto the subject.
The effect that a diffuser has is that it decreases the intensity of flash light hitting the subject. The image may appear dimmer with a diffuser on than without.
Where bounce material is available close to the subject, the diffused rays of light from the flash may be effectively bounced back onto shadow areas of the subject to provide shadow fill.
Ideally, the bounce material needs to be held close enough to the flash and the subject for the diffuser to be able to make a difference, because the flash is a relatively weak and specular light source. It needs to operate within a contained space in order to make a visible difference.
Alternatively, it will help if the diffused flash is used in a small room with low ceilings and white walls, which brings the bounced light back onto the subject.
The result is more even illumination, as some light has been redirected from the highlights areas to the shadow areas.
What are the disadvantages of using a diffuser on a flash?
It does not do harm per se, but where not used properly, it cuts down the amount of light hitting your subject and causes a waste of flash power. It is also an excessive weight to carry around. It needs to be used with a reflector for it to do its job. Otherwise, flash diffusers do little to photos.
As explained above, the set of circumstances under which a flash diffuser can have an effect on your photos is rather specific. One, there needs to be a material that is able to bounce that light off, and two, everything needs to be close enough so that the light is not dissipated into the environment.
Where these criteria are not met, the flash diffuser becomes a pointless bit of extra weight on top of your flash head, which can weigh you down where the setup is hung on your shoulders over long hours.
Do you use a flash diffuser outside?
Flash diffusers usually have little use outdoors even with a reflector. The diffused light needs to be bounced back towards the subject for it to make a visible difference. In an outdoor environment, especially an open area without walls, this needs to be done by setting up reflectors. It works best where the flash, bouncer and subject are close to each other.
Technically, the distinction is less so about outdoors vs indoors than the availability of bounce surfaces. I have shot in large rooms where no neutral-coloured wall could act as a bounce, or outdoors where a near-white ball can be used nearby the subject.
The only key to getting a flash diffuser to do what it is supposed to do is to effectively pair it up with a bounce surface. The light that is diffused to other directions need to redirected back to the shadow areas on the subject to provide fill.
If not, you will need to bring reflectors to act as a bounce surface. But as I have explained above, the light from the flash needs to be in close proximity with the reflector and the subject for the flash light to make a conspicuous difference.
What effects do flash diffuser shapes have?
The shape affects the directions in which the flash light travels and their respective proportions. A hemisphere spreads light out equally in all directions, whereas a cuboid mainly redirects the light upwards. The results, though, can be minute depending whether there is an effective bounce material to go with the diffuser.
Flash diffuser vs softbox
The key difference between a diffuser and a softbox is that a softbox enlarges the size of the light source while a diffuser does not. The diffuser spreads the light over a wider area, whereas a softbox produces a directional light. A diffuser can cause a significant loss in light intensity while a softbox mostly retains the flash power.
Difference I - Size of light source
The first difference worth noting is the size of the resulting light source. This is important because it changes the quality of the light in relation to the subject, which alters the tonal contrast in the image.
Diffusers DO NOT alter the size of the light; the flash head remains a point light source. Softboxes DO alter the size of the light; it makes the flash a larger light source (aka usually a square).
As a result, shadows become softer after putting on a softbox and that tones tend to transition smoother from highlight to shadows. Where only a diffuser is used, the hardness of the shadow remains as is.
Difference II - Effective flash intensity
Another difference lies in the effective flash intensity, aka the amount of illumination that reaches your subject, which is what really matters at the end of the day.
A diffuser spreads the light across a larger area, thus resulting in a lower average flash power across the entire frame. A softbox keeps the light largely directional. The subject still gets the majority of the light, but with a softer gradation from highlights to shadows.
The result is that you get a flatter, dimmer image with a flash diffuser, while the softbox retains the contrast levels but softens the harshness of the light.
Flash diffuser vs reflector
The main difference between a diffuser and a reflector is the extent to which they change the direction of light. A reflector mainly redirects light in one general direction, whereas a diffuser redirects light in all directions. The reflector is meant to provide a targeted source of fill light, whereas a diffuser is meant to bounce light all around.
The reflector and diffuser are very different beings, and they work best when used together.
When you only have a diffuser but not a reflector to bounce the light back onto the subject, you lose much of the diffused light to the surroundings. When you have a reflector but not a diffuser, the reflected light remains a relatively concentrated light source that fills a particular area in the shadows, rather than raising the overall values throughout the frame.
Does the wide-angle panel/ push-on diffusers reduce hotspots?
The wide-angle panel on your flash does appear to reduce hotspots and specular highlights, but it also reduces the flash intensity, resulting in a darker photo overall. The panel merely spreads the light to illuminate a wider range without modifying the nature of the light. Turning up the flash power will cause the hotspots to reappear.
The widest coverage for most speedlights is equivalent to the field of vision on a 24mm focal length, which is fine when shooting at a 24mm or longer focal length.
However, in some instances, the photographer might find the need to go wider than a 24mm, for example 16mm. This might be the case of shooting large groups within a tight space.
The flash coverage will need to be widened accordingly, or else the people on the edge of the frame will not be sufficiently lit up, while those closer to the centre of the frame receives most of the illumination.
But a necessary result of spreading the same flash output power across a wider range of subjects is that the flash intensity is lower across the board.
While it might appear that the hotspots have disappeared by adding the wide-angle panel, it is merely because the overall intensity has reduced. Should you need to turn up the flash output, the hotspots will still occur.
Summary
In this article, we talked about:
whether you need a flash diffuser;
how to use a flash diffuser;
the disadvantages of using a flash diffuser;
whether you use flash diffusers outdoors;
the effect of their shapes;
the differences between a flash diffuser vs a soft box vs a reflector; and
the effects of wide-angel panels.
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