What about that fresnel lens on your camera flash?

[MarkSturtevant]

I just realized that the plastic cover on camera flashes is actually a fresnel lens, and this of course is to focus the light from the flash. But for close up and macrophotography, a big priority is to diffuse the light from the flash, and so isn't it counter-productive to start out with a focused beam of light? So what about that fresnel lens? Would it be of some benefit to remove it & replace it with something that contributes to diffusion?

[chris_ma]

I doubt that it's worth the trouble.

soft light is a combination of light source size to distance ratio and how much scattering is happening in the light source.

you can increase the scattering by simply putting a diffusion gel on top of the fresnel. if you need more, then simply use two or three layers separated by an air gap. that will have a very similar effect to replacing the fresnel.

if you want a softer light you'll have to increase the light source size to distance ratio, which with macro often means putting a layer of diffusion very close to the object.

[Big Pete]

I took an oval ice cream container and fitted thin white foam plastic instead of the lid. Then I cut out on the rim around the container, a rectangular hole where I insert the flash. In this way, I do not get a direct flash light without it bouncing around in the container and then softened through the white foam. This gives a soft and even light from the flash. You need a diffuser surface that is a couple of times larger than the flash itself and a little distance to the object to be photographed for best results.

[Troels]

To be a true fresnel lens the single grooves must have the shape of cirkular part of a lens surface. This is apparently not the case here. Parts of the groves are just straight lines. I doubt this surface wil be able to focus anything at all. The center square has a completely different structure. I think this pattern is ment to adjust the direction of light to give it more equal distribution and coverage of the field while adding some diffusion at the same time.
The reason for adding bigger diffusors is primarily to make the diffusing surface bigger, to add light from more directions, seen from the object's perspective. Correct me if I am wrong.

[MarkSturtevant]

I did an inquiry about it online, and descriptions I have confirm that the flash covers do feature fresnel lens grooves. The grooves on this one are not complete, but I'd think they would still provide some magnifying or light converging effect albeit not as strong as others, maybe. A lens broken in 1/2 is still a magnifying lens.
In any case the inquiry is moot according to the opinions so far.

[chris_ma]

the flash lens is mainly used to match the light beam to the angle of the lens, so you can usually select something like 24mm to 105mm (FF equivalent).

but as said, if you would take it out and put in a white diffusor instead, it's pretty much the same as if you just leave it in and add a diffusor in front.
might even be that with the zoom angle you manage to get more diffusion with the flash lens still in depending on the angle setting.

[klevin]

To be a true fresnel lens the single grooves must have the shape of cirkular part of a lens surface. This is apparently not the case here. Parts of the groves are just straight lines. I doubt this surface wil be able to focus anything at all. The center square has a completely different structure. I think this pattern is ment to adjust the direction of light to give it more equal distribution and coverage of the field while adding some diffusion at the same time.
The reason for adding bigger diffusors is primarily to make the diffusing surface bigger, to add light from more directions, seen from the object's perspective. Correct me if I am wrong.

OK, I'm game!
Fresnel lenses are now commonly viewed as stepped lenses. Just like you can have lenses that are not circular, such as an anamorphic element, you can have the same thing, except stepped. Think prisms. Remember that a circular lens is just a special case of a prism.
In the case of a flash, the designers are just using a stepped lens to shape the beam into something more rectangular, to both minimize spill light and use the light more efficiently, as you point out!
To comment on the OP's issue, I suspect removing the front stepped lens would actually make the flash more specular. Remember that in a flash, it's not a lens that sharpens or focuses the beam as much as it shapes the beam. As troels points out, it probably diffuses at close range.

[rjlittlefield]

It is an amusing experiment to photograph a flash through a tiny pinhole, so you can actually see the light pattern instead of just seeing a blob of blown-out pixels.

Here is a Yongnuo YN460-II, photographed on axis (and properly fuzzy from being shot at f/400 or so).

As noted by other posters, the job of that fresnel-ish lens is to shape the overall beam of the flash, spreading the light more or less uniformly over the area that will be photographed by the camera under normal shooting conditions, while not sending a lot of light outside that area where it will be wasted. The lenses have to be pretty good at that job, or word would quickly get around and people would buy some other model of flash.

However, we can see in the image above that this particular front-of-flash lens is not a very good diffuser. I've looked at a couple of others using this same method, and they were not good diffusers either. This is not surprising to me, because if the front of the flash did look uniform in this view, then I expect that light would be getting sprayed all over the place and the lens would not be doing its beam-forming function.

Getting back to Mark's questions:

isn't it counter-productive to start out with a focused beam of light? So what about that fresnel lens? Would it be of some benefit to remove it & replace it with something that contributes to diffusion?

We get good diffusion by putting a diffuser close to the subject, so that it spans a wide angle around the subject. Then the light that forms the image is some fraction of whatever light hits the diffuser, and any light that does not hit the diffuser is wasted. So it's not counter-productive in general to start out with a focused beam of light, it's just that we'd like the beam to be focused on the diffuser! Our traditional setups are pretty awful at that last aspect, with most of the light going someplace else, to be wasted at best or cause trouble at worst. A front-of-flash setup that would act more like a spotlight, focusing light on just the diffuser, would be a lot more efficient in terms of wasted light. On the other hand, getting the thing aimed properly could be challenging, and since we're seldom short of light, I'm not sure how the overall cost/benefit ratio would work out.

--Rik

[MarkSturtevant]

Well, at least this turned out to be somewhat interesting! Thank you everyone for your helpful comments.