Incident Light Meter Readings

[patrick_j.]

<p>hello --<br>

i am still not sure if i understand this method properly. for example in such a case: i want to take a portrait, and measure exposure using an incident reading. i point the dome of the meter towards the camera, making sure to capture the light that falls on the subject. now: do i simply use that given reading or do i open a stop to make up for skin tone .... or is that for reflective measurements? <br>

the next thing i have always wondered is actually the rgb equivalent of that 18% grey that the meter assumes and how i calculate a given f-stop into rbg values. so in the above case, if i assume that skin tone would be one f-stop lighter than 18% grey, what does that translate to in greyscale rgb?</p>

<p>again, plenty of questions, but still thank you for much in advance for any responses.<br>

patrick.</p>

<p> </p>

[russ_britt3]

<p>If you take an incident reading you dont worry about gray,as you are metering the light falling on the subject, not reflecting off the subject.</p>

[philip_wilson]

<p>As Russ says just hold the lightmeter near the subjects face making sure that the subject or you are not blocking the light getting onto the dome (This is usually obvious outdoors but you can get between a light and the meter in a studio - at least I have done this but I can be a Klutz). The meter records the intesity of light falling on the dome so there is no issue with grey cards. Grey cards are used with meters that measure the refelcted light from a surface - different colours and surfaces will reflect a different proportion of the light - camera meters have traditionally assumed an 18% grey surface when they calculate the light intensity - in essence the incident light is calculated as being 1/18% or 5.56x times the meter reading. This is not completely correct as maost camera meters use abou 12% therse days - this link is very interesting.<br>

http://www.bythom.com/graycards.htm<br>

To your first question just hold the meter by the models face point it towards the camera and take a reading. I usually hold mine by the side on the head and point it at about 30degrees to 45 degrees elevation from the horizontal facing the camera.</p>

[ondebanks]

<p>Grey means colourless, so r=g=b.</p>

<p>A <em>reflective </em> meter is calibrated so that if the greycard itself was in the exposure, it would appear to be the mid-tone; the shade of grey which (to our eyes) lies mid-way between black and white. In actual fact, it is closer to black. Our visual system estimates brightness logarithmically, not linearly. So if black is 0 and white is 100, we do not rate 50 as average grey; we rate 18 as average grey.<br>

See http://www.rogerandfrances.com/photoschool/ps%2018%20per%20cent.html</p>

<p>If a skin tone is 1 stop lighter than the grey card reading, it reflects 2x => 36% of the incident light.<br>

Something 2 stops lighter than the card will reflect 72% of the incident light.<br>

A perfect reflector will reflect 100% of the incident light. But reflecting 100% of the light doesn't necessarily mean that you can't go any higher and you're saturating the highlights. The dynamic range of the film or sensor may extend well beyond this flux level. For this reason, you can't convert 18% grey into an rgb value; that has no meaning. It is not 18% of the dynamic range of the sensor or the film.</p>

 

[frank uhlig]

<p>If you expose at the incidence reading, the picture will be exposed as natural: the face will have the shade it has naturally. If it is a bit darker than lilly-white, it will be rendered so. If you were to open up instead then it would appear lighter than it is.</p>

<p>Your operator choice ... And if you miss, you can always adjust when printing. An overexposed slide will be a bit light with overexposure, though, and cannot be adjusted, except in printing.</p>

[patrick_j.]

<p>hello & thank you all for your very insightful comments. as i mentioned i obviously had confused the reflective reading with the incident reading. so no need to adjust anything, unless i arbitrarily wish to do so. i will of course read the links you posted. a few details that remain unclear to me are these:</p>

 

<ul>

<li>on one article i read that the photographer at times points not at the camera, but rather at a "highlight" for an incident reading. hmmm. does this make any sense?</li>

<li>so if an incident reading measures the light intensity given a specific subject, how would i use it in the field. for example in this photo: http://www.rogerandfrances.com/images/f%20m%20st.%20m%20porch.jpg it seems that the scene has many many different light intensities, depending on where in the shade i would take such a reading. or is this the case in which i would resort to a reflective reading?</li>

</ul>

<p>again, thank you for your patience and thoughtful answers.<br>

patrick.</p>

 

[colin_elliott]

<p>Patrick, This pic. calls for careful selective metering; not one that I would choose to use an Incident meter for.<br>

I would personally take a number of Spot readings (at least 3 and maybe as many as 9), encompassing Light, Dark and Medium tones. Limit the extreme ends of your readings to parts of the picture in which you want to retain detail. A true spotmeter will only read a 1 degree or 1/2 degree field of view.<br>

Having obtained your readings, average them out to provide you with the correct exposure.<br>

For eg. if your High light is <a href="mailto:100th@f16">100th@f16</a> and the Dark is <a href="mailto:100th@f4">100th@f4</a>, then the Average is f8.<br>

You may wish to bracket around this initially, in order to test your exposure technique for such subjects.</p>

[larry_freytag]

<p>I use an incident meter, but point it more at the light than the camera, maybe a nit but it works for me. After all you are measuring the light so point at the light?</p>

[q.g._de_bakker]

<p>Patrick,</p>

<p>You hold the meter such that light is falling on it the same way as it is on the subject. The basic method: you point the meter at the camera.<br>

That's the basic method, your starting point. You have to decide how to deviate from that basic method to suit your demands best. Sometimes (if you want to create a silhouette, or when a subject is lit strongly from the side and you want to expose for the highlights) it is better to point the meter at the light source. <br>

Use your judgement. It's not difficult. Just remember that you are metering the light that is falling onto the subject.</p>

<p>The scene in the picture is easy to handle with an incident light meter. You take two readings: one in the shade near the door, one in the bright sunlight. Then you decide how to balance the two, which one should weigh more when you combine them into the reading you will use to expose your film.</p>

[Kent Shafer]

<p>If just taking a single reading, I point the incident meter at the light source if I'm shooting slides to avoid blowing out highlights. I point it at the camera (and thus give a little more exposure) when shooting negatives to get more shadow detail—negative film's wider dymanic range means the highlights will still have detail too.</p>

<p>On the x-rite color checker chart, square number 22 is a very close match to a Kodak 18% grey card. x-rite specifies the sRGB values for that square as 122, 122, 121.</p>

[robert lee]

<p>Reflective readings can work well too, especially when you want to shoot quickly and move on.</p>

<p>Take a single reading of part of the composition that is to be exposed as mid-gray, say the door panel. Let the rest of the scene fall where they may. Negative film have at least +4 and -3 stops of good detail capture capability. Unless you're doing your own B&W development, there's not you can do about this anyways.</p>

<p>In general, the meter reading gives an idea of how much and how well the scene's brightness range fits into the capture range of the film. If it doesn't fit, then at least the information is there for a judgment call on what to sacrifice. </p>

<p>When I care enough to use an external meter, it's one that reports directly in exposure value, EV. It's a bit easire to work with I think. </p>

<p>Say the door panel measures to EV12, the lighter shadows in the doorway to EV9, but the white wall in direct sun measures to EV17. If the shadow details are important, then a camera exposure setting for EV12 maintains good shadow definition. You also know now that at exposure time that much of the highlight will carry no detail. That can be perfectly fine and work well for the composition.</p>