Incident light receptor
Seconic 558 (and other) and Gossen Starlight, both has a dual-function retractable lumisphere adjustable for sphere of flat characteristics. When the lumisphere is retracted it resembles a flat receptor for incident light, as e.g. Minolta flashmeter V has.
However when light from side strikes the flat receptor angle of light rays relative to the receptor plane is accounted. When the same side light strikes retracted sphere receptor, things change A LOT: some rays never hit the receptor but the wall around the receptor, and the similar is to the sphere area on opposite side of the light source for it is in the shade. Also some rays hit the receptor with 90 degree to the receptor surfice.
What the hells math is behind it. What actually the retracted sphere is for. IT JUST CANNOT REPLACE FLAT RECEPTOR FOR IT IS NOT FLAT. Does it take “some” average three dimensional photographic condition as sphere does in general, and actually is for nothing for it is not accurate as photog might need.
Did anyone made some tests on this matter.
I dunno! I always have Minolta's which is a bit of a pain to carry and change the receptor. I think it's an acceptable replacement for the flat diffuser.
Acceptible can means many different things. There is no replacement for flat receptor for "accurate" reading. The dome is there to simulate an average three dimensional photographed object, and is far from to be accurate even not retracted. When you shoot head and shoulder with light coming from side and reflected light hits opposisite side of the face, you put a dome in from of the head and they are iluminated in the similar way. You are getting an average of something, not an accurate reading as flat receptor does. For one should pay $600 for an average reading? I can do it with my Gossen sixtino.
Preface: Photographic light meters are practical instruments designed for use by photographers, they are not scientific instruments designed to CIE standards. The ISO Guide (not a standard) 2720:1974 does not define what the angular response of an incident meter must be - it is left to the manufacturer to decide. "Any form which allows satisfactory determination of exposure..." ISO 2720 does give guidelines on the angular response of cosine and cardioid receptors, if they are the chosen forms. The most important thing is to calibrate your photographic system as a system, and then to use consistent technique with adjustments based on experience.
The 'correct' spherical response of a flat receptor is cosine: ie there should be no response at all to light from the side and full response to light from the normal. The shape of the response is like half a figure-of-eight: a sphere sitting on the receptor surface. (The response of a dome is usually described as cardioid - ie it should have a limited backwards response to about 145°, which is often blocked by the meter body.)
When I got my L-508 I wondered about the difference between a flat receptor and a retracted dome (it seemed too easy when compared to swapping receptors), and compared the readings with my Studio Master with a flat receptor, my Spectra meters with flat receptors fitted, and a grey card (which behaves like a flat receptor, not a domed one). The agreement was close enough in terms of angular response, once the systematic differences had been allowed for. If you are really interested I could do a more detailed analysis of the retracted dome against the ISO values for a cosine response.
In practice many domed meters are used with a cupped hand when assessing the contribution of different lights - this is analogous to a retracted dome.
There is a small difference in effective calibration between the dome and retracted dome for the Sekonic meter. I don't know about the Starlite (sic). With the dome out, the incident reading is equivalent to the same meter doing a spot reading from a 12.5% (approx) grey card - though the angular response ('indicatrix') of a grey card is different from that of a dome. With the dome retracted the incident reading is equivalent to a spot reading off a 14% (approx) card. Little difference in practice.
So, it's something to be aware of, but in practice it is unlikely to make any difference. I'm happy to use the retracted dome of my L-508C for repro and for for assessing the contributions of different sources.
Hmm! Like I said I don't really know. My guess that it's must be quite close to the flat diffuser that is why it's an approach that both Gossen and Sekonic take. As for the $600 question? I didn't buy either the Sekonic or Gossen and in fact you do have the Gossen.
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Helen, thank for you time.
"If you are really interested I could do a more detailed analysis of the retracted dome against the ISO values for a cosine response."
Yes I am interested. For the flat receptor I do not not need any explanation. Just I would know how you possition the retracted dome meter in the studio when:
1. The open book is on the desk.
2. We want to reproduce what is on one leaf of the book and the camera is above the leaf.
3. two different [Ws] flashes are used and are on sides of the a leaf of the book
4 Each flash is (for some reason) 80 degree to the normal to the surface of the flat leaf of the book.
5. is the reading of the retracted receptor the same as form flat one.
Yes, the flat and recessed reflectors are used in the same manner.
They are most commonly used to determine if the lighting across the copy is even by measuring center & corner exposure.
Some might say I have a bad attitude! Too bad.
I read Daniel's question to be about whether a recessed dome had the same directional characteristics as a flat disk - ie a true cosine response.
I made a more careful comparison than the one I had made when I originally got my L-508. I fastened it to a board alongside a Sekonic Studio Master with a flat disc, and held them a fixed distance from a clear 100 W lamp with a CCT of 2790 K. The two meters (both Sekonic, both with the same calibration constant) read the same, within the reading accuracy of the Studio Master, at all angles between 0° and 80°. At 80° the L-508 was reading 2.4 stops down from the 0° reading - within the allowable departure, as given by ISO 2720, from the true cosine response.
I admire your diligence! A much better idea to test it and see, than to speculate emptily on the internet.
Helen, thanks a lot for your time. I think your test will be very usefull to me and to many other photogs too. Thanks again.
You are right when beleive in tests. But to fully understand the matter you have to know theory behind it. Just when theory is proved correct, by testing, it is real value and experience. Just to do job leads no where. Again, one have to undestand it especially when the matter is one's life goal..
When light comes from side the iluminated area of sphere is smaller than on flat receptor. But the angle on sphere is lower than on the flat one so that might be the reason. However I will figure it out and post finding.
Again thanks to everyone, and especialy to Helen.