Lighting "texture" of HMI and CDMI

[noah977]

I've been experimenting with an interesting project and would like to get the group's opinion...

A friend of mine is a film maker. He has an entire grip truck of "movie lights" that consists of a ton of toys (stands, lights, scrims, etc.). He has generously offered to let me use them for my photography shoots!!

I've experimented with the HMI, CDMI, and kino-flow lights. The results seem strange to me. (Compared to a "standard" Profoto or Integra strobe.)

The light feels much "flatter". I'm not sure how to describe it, but the model's face looks a bit weird. They are supposed to be "daylight", so I don't think it's the color.

One thought is that the strobes have a mixture of direct light combined with some indirect light at the edges. (Especially when bounced off an umbrella.)

I can't seem to get the same "feel" in a photograph that I'm used to.

I know I'm being vague, but some of you guys must have also experimented with these lights before.

Can anybody offer some opinions or advice?

Thanks!

 

[Mike Wilde]

Some of it is the intended field the film lighting is meant to cover.

People tend to move around in film, and therefiere the lighting gear, in my understanding, is meant to cover whole areas.

Daylight flourescent boxes are lit edge to edge, and are not the diffuser for a point source that photo strobes are, so they should give a more even light.

The other thing is the distance that the lights are meant to be used at. Frequently in film the whole outside of the building is lit up to do interior scenes lit near windows.

 

[noah977]

You may be on to something. Even with barn doors, the lights act like a HUGE light source. Yet, they're still not soft like a big softbox. They are huge and harsh..

 

[Mike Wilde]

I am not familiar with CDMI, but in my experience with theatre lighting, HMI is thought of as a 'european dance look'. It is indeed crisp in it shadows, and thought of as cold relative to tungsten, because of its usual gassing to give daylight colour balance.

The arc of the HMI light while very bright is indeed approaching a point source. Large flash tubes arc over comparatively longer tubes to get the volume of light out, and are much less a point source than an HMI light. HMI lights that I have seen usually have a high pressure 'lozenge' inside a second outer tube. The light in HMI is a tight arc in the centre of the small lozenge portion. Sounds great, except they do shed a lot of heat, and the ballasts for them make any studio power pack, even most old speedotron blacklines look lightweight.

 

[noah977]

Yes, "Cold and Crisp" would be a good way to describe it. Strobes, while supposedly the same color, don't feel as cold or as crisp.

 

[samcomet]

Noah - FWIW I earn my living as a chief lighting technician on films and satisfy my artistic bent as a fine art photographer so perhaps I can add some insight to your questions.....the "light quality" of a source of illumination is mostly based on what happens to it before it hits the subject and not its physical source type. A point source of light such as the sun on a cloudless day is considered hard. Look at your shadow on the ground and see how crisp it is. On a cloudy day the light is diffused as the source illuminating you on the ground becomes the entire cloudy sky - huge relatively to the point source behind it. Consequently ones shadow, if at all discernible, is not at all sharp but rather soft and diffuse. The same analogy can be used with sunlight bouncing off a white wall...the source of light becomes the large white wall instead of the point source of the sun. If the sun illuminates a subject directly PLUS a bounce off the wall, the bounce becomes a soft fill light to bring out the shadow detail. If however a subject is in shadow, the bounce will simply softly light, somewhat shadowlessly, the subject. A caveat here though is the distance from the source the subject is.....the further away the subject and source are, the sharper or harder the light becomes. If we were close enough for the sun to fill the entire sky, it would approximate the soft quality of the cloudy sky from the above example. An "open eye" HMI lamp is an extremely small source of light. It is considered to be a point arc source, as inside the globe are two electrodes separated by a small gap that an electric arc spans to create light. The lamp housing may typically have a clear lens, a convex lens or a fresnel lens to absorb UV radiation and modify the light quality. Typically the clear or convex lenses are used as bounce or used through diffusion frames to reduce harshness. If one uses a diffusion frame, remember the closer the frame is to the subject, the softer the light quality will be almost irrespective of how far the gap is between the actual lamp head and frame is. Also remember that the larger the diffusion is, the softer the light quality and that one should always endeavour to "fill" the entire frame with the light to maximise its usefulness. Fresnel lenses can also be used as a bounce or thru diffusion but it is also useful as a direct source. In this case the concentric rings of the fresnel lens "become" the source like in the diffusion examples above and behave similarly, but a bit harder in quality. The fresnel can also usually be "spotted" or "flooded" to increase or decrease the output illumination. Spotting a lamp as the name implies reduces the size of the beam so the trade off is that the light become harder. Conversely flooding becomes softer and decreases the amount of light. One may also use combinations of say, a bare fresnel as a "key" (principle) source and a bounced light as a soft "fill" for the shadows. Kino's are flouro tubes and as in the above wordy paragraph, the surface area of the tubes become the source and are softer than a point source. Kino's can be diffused but are somewhat impractical to bounce as they do not put out a lot of light relative to HMI's for example. HMI's have a colour temperature of daylight while Kino's can have either daylight or tungsten balanced tubes (or even a mixture of the two). The last thing that you may want to consider is the placement of the sources....that would be a treatise for some other time as my fingers have calluses and I have probably bored you to tears by now, but I hope this helps you understand somewhat the toys your generous friend has let you try out! good luck and cheers,
sam

 

[noah977]

Sam,

Thanks for the info. Your explanation of lighting reminds me of one of my favorite texts on the subject, "Light Science and Magic".

I understand all the concepts of small and large lights, hard vs. soft, etc.

The HMI seems to be "hard and large" with a strange color cast. My friend swears it is daylight, yet somehow it looks a bit blue to my eye. The other thing I've noticed is that photos taken with it seem very harsh. EVERY wrinkle, blemish, etc stands out. It looks more like a forensic police photo than a portrait. I've experimented with bouncing off white foamcore and the results are a bit better, but still not great. Perhaps I'll try your suggestion and instead of a bounce, shine the light through a diffuser fairly close to the subject.

Thanks!!!!

-N

The lights are great, I'm just trying to wrap my head around how the perform so that I can use them predictably.

 

[Bruce Watson]

Part of the problem could be that "daylight" can be defined in different ways for different processes. I'm thinking that daylight is often defined to be around 5000K for still photography, but 6500K for cinema. This would make the movie lights look blue in comparison.

If the film you are using is balanced for 5000K, and you show it 6500K, I would expect some of what you are seeing. Think about what it would do to freckles which are generally brownish. The bluer the light, the less yellow in it, which will give the freckles less light to reflect, which will render them darker and make them stand out more.

OTOH, I've never worked with movie lighting and could be completely wrong. I'm sure someone here will correct me if I'm wrong.

 

[noah977]

Bruce,

Excellent point. I'll phone my friend this afternoon to find out exactly what color temperature he means. There are a few HMI, CDMI and 2 Kinos. Its possible that they're all different.

If they aren't 5000K, then I guess I'll have to find the appropriate filter.

Thanks!

 

[samcomet]

Noah & Bruce - Motion picture film that is daylight rated is balanced at 5500 degrees Kelvin. Sunlight at midday is considered to be 5500 but either side of midday can be anything from 3200 ish (tungsten) at sunrise/sunset to 10,000 just prior to sunrise or just post sunset. HMI and Kino's have two designations for "colour". The first is the colour temp which for daylight Kino's and HMI's are 5500 degrees. The second is CRI (colour rendition index). This is a number which suggests how well and object "looks" under the light source - in other words how accurate it appears. The two sources we are talking about can be measured on the "daylight-tungsten" scale (commonly thought of as colour temp) AND the magenta-green scale (commonly thought of as the CRI). Both are actually technically inter-related but for the sake of this argument I won't go into that can of worms. Suffice to say we will think of colour temp and CRI as I broadly (but technically a bit wrong) put forth above. Normal Kino and HMI lamps have a significant amount of mercury and other glowing elements in them to help with ignition. When the amounts of these elements are in the normally accepted ratios the colour temp and CRI are respectively 5500 and 95+ (100 being ideal). As HMI's age they tend to go a bit magenta as these ratios change (although I've seen some go green). This may be the cast that you are seeing in you photos. Slight shifts in either of these measurements are very hard if not impossible to pick, however if one had lamps with different colour temps and CRI's side by side the diff would be more pronounced. In short one can have properly balanced colour temp at 5500 degrees but one can still have the same source as slightly green or magenta. In my humble experience Kino's do not exhibit this CRI shift anywhere near as much as HMI's do. You should check to see if the tubes you are using in the Kino's are all daylight balanced - i.e. they should all have a blue cap at the ends where the pins are....a gold cap will indicate that they are tungsten balanced. There are others but I won't go into that for fear of boring youse guys to tears. Noah the last thing that I think you ought to consider is exposure and lamp placement. A pretty girl slightly over exposed (half a stop to two-thirds) with a fully frontal source will have her wrinkles and blemished seemingly reduced from a source placed off axis to the lens. Again that is an aesthetic choice, but I am just trying to think of some sagely advice for your prob. One last point - it is considered that the softest source is a bounced source, with the next hardest being thru a diffusion frame (trace or otherwise). One can also achieve really really soft light by bouncing thru a frame.....just to give you more headaches!!
anyway hope this helps a bit,
cheers for now,
sam