Rich, for a long time that manufacturers of exposure meters know that you can change a reflective meter into an incident meter when you add a diffusor dome on the reflective meter's lens. So they produce exposure meteres in this way. Gossen Luna Pro is just one example of the technique. It works. How come they didn't put the same diffusing dome on a spot meter? Well, they have a good reason not to do so. Read my posts in this tread again and maybe, maybe you will understand the reasons.
george, just drop it unless you are willing to test it and prove it works or doesn't. Period!
Just because you _THINK_ it won't work doesn't mean it doesn't. And what the hell is your problem in the first place? WHy do you care so much that this idea won't work??? Does it somehow bruise your ego? Are you upset that someone besides you came up with the idea?? Does is maybe conflict with some idea you had that you had hopes would make you money?? Give it up already. The Sun rises in the morning, sets in the evening, and a Lumisphere in front of a spot meter works; and NOTHIN _YOU_ say will prevent it from being true!
I don't have to care why a manufacturer doesn't make a dome for a spot meter. I don't have to care that some idiot thinks the idea doesn't work. All I have to do is put my Lumishpere onto one of my spot meters and happily and accurately measure incident light.
Absolutely NOTHING _YOU_ say or do can ever prevent this from working! You can get a similar Minolta Spot meter, buy a 398M Lumisphere, do all the testing you want, come back here and "prove" the idea doesn't work; and it STILL will NOT make any difference to me! Why? Because in the real world, I still put my Lumisphere onto my spot meter and get accurate readings in any light. I could care less what may happen with any equipment you may have. Maybe I'm lucky with what I have, maybe you're not lucky with what you have. I don't care. It works for me and that's all that really matters!
Why you have some personal problem with this whole idea is beyond me. I'm sorry if it offends you or ruins some commercial project you were working on. But facts are facts and no amount of ranting by some lunatic will change those facts.
Go have a happy life george. I'm sorry I may have ruined part of it with my announcement of this incident/spot meter. I'm sorry if this has caused you problems in a mental or financial way. But if it wasn't me, someone would have eventually put a 398M Lumisphere onto a spot meter and come up with the same resulots and been nice enough to share it with the rest of us. I was amazed myself that noone had already and I did quite an extensive search just for that reason.
I can understand why most people really wouldn't care about this. I canb understand why some people would think it would not work, as I did myself _before_ testing it. I can understand why someone would go and do some testing themselves just to see if it did work even though thay had no real interest in it as some poeple do have a nasty streak. But I really never expected some lunatic to go ranting along about how it is impossible for it to work so vehemently without any proof or willingness to test it himself!
So let it be known that I am real happy about trying to contribute to this forum and photography in general! I would have been happy if anyone had said 'thanks' for the idea. I would have expected someone to try and prove it wrong or right. I would have been slightly upset if I found out that several other's meters didn't work this way and I somehow managed to get the only three in the world that would work as I described, but I would have understood. I would have been suprised and not too upset if someone had mentioned that everyone already knew about this and what was the big deal. I could have accepted easily many responses and have seen them here many times over the years. But I really didn't expect some nut job to go crazy and jump on my case for trying to share what I thought weas useful and unique information to the rest of the photo-world.
Nope, changed my mind (not that anyone here would know). I'm not going to let some a##hole ruin my day. george, take a hike and stuff it where the Sum don't shine. Period! Until YOU test it and PROVE me wrong, shut the hell up!!!! And that's that. Ya got somethin else to say to me, come right HERE and say. I'm just fed up with some idiot trying to ruin another peoson's day with their rantings, and today I'm not going to take it. Sticvk it george and shut up or show up. That's the end of that!
A dome is a dome. No matter what the size, if the shape is a dome, and you read it with a averaging meter, it should give you an incident reading if the meter is centered behind the dome.
So a 35mm camera with an averaging meter would be an incident meter when a dome is placed over the lens. However , lens focus could effect the reading. Maybe setting the lens at infinity or at the closest focus would be necessary.
A spot meter is an averaging meter that sees a narrower view. 1 degree, 5 degree, 8 degree ect. what ever their angle of view, they are still an averageing meter, just seeing a narrower view. BUT, if a dome is centered over the meter then the spot will read the center of the dome, which is still a dome in shape and that section of the dome is the incident reading part of the dome. A Weston Master V meter sees an angle of 70 degrees (flap open) , and 50 degrees (flap closed). It is an averaging reflected light meter. "very large spot." The incident cone can be used in either configureation (spelling?), Thus converting it to an incident meter.
A center weighted light meter, or other hybrid meter types found on 35mm cameras won't work because their reading is weighted more in some areas, less in others. The dome would have to be a special shape (no longer a dome), to work.
Having said all this, a dome is half of a sphere. And ususlly a incident dome is properly sized to the meter so as to place the meter equally distanced from all the inside surface area of the dome averaging the reading and is likely to be more accurate than aiming a spot meter into the center. The meters lens probably helps in this.
I think Rich's solution is a good work around for one who dosen't have a dedicated incident meter, and as he states, he tested it and found it to be accurate for him.
just me rambling on on a sundy morning.
dee
Rich, why not to tell those big guys that make exposure meters that it's enough to put a diffusing dome in front of their reflective spot meters and they will get a working incident meter? Big bucks $$$$$$ waiting... forget the incident meters now. No, - just kidding. Of course it wouldn't work. Why not? It's simple - the spot meter doesn't see the whole sphere gathering the light you want to measure. You know that by now, don't you?
"A center weighted light meter... won't work because their reading is weighted more in some areas, less in others." (dee) - you're absolutely right, dee. A spot meter is even more "weighted in some areas" than in others - it only measures the front vertex of the dome. Thus all the light that falls on the side of this dome is not seen at all hence not measured at all by the sensor. Well, the rest has been said...
I dredged up my Pentax Digital Spot and played with it a bit. The depth of field of the optical system in that model means that if you were to mount an incident diffuser dome on the front, the part of the dome seen by the meter cell would be well out of focus. It has also been widely documented that there's a fair amount of flare in the optical system of the Pentax, so the reading is affected by bright light well beyond the nominal reading area. The dome itself also contributes to this effect, smearing out across a broader surface the light incident on it from any particular direction.
The bottom line is that a reading taken in this way could actually be integrating light from a much broader angle of incidence than the "1 degree" specification of the spot meter would imply, and under many circumstances might be accurate enough to be useful. The only way to know would be to test a particular meter+dome configuration under the lighting conditions one wants to use it in, which is exactly what Rich has done. I'm not sure one can conclude from his results that any spot meter would work well if used in this way, but it's probably equally invalid to conclude on purely theoretical grounds that it could never work well enough to be useful.
Oren, it's all simpler than that. Just reading the post from Rich you can answer your doubts. Doesn't he say that only 1 kind of a lumisphere works on his spot meter? While he thinks it's because the others "are all too dark" the problem is elsewhere. The shape of the dome is chosen so that it corresponds to the angle of view of the meter itself. The meter must see the whole diameter of the cupola beacuse the light you measure is projected on the whole half dome - only in this way you can measure light that is directional. If for ex. 75% of the light is comming on the dome from an extreme angle it projects a light spot on the side of the dome. Regardless of the fact that your spot meter reads 1 or 2 or 3 ° (put there all the flare you like if you think it's going to help) the cupola is not visible all and the light gathering there is of no importance to the meter. Yet the same light is of importance on the scene. Any combination could give an impression of being working if the light around the cupola is uniform - as soon as the light is comming from an angle where it is not seen on the cupola the reading is off. The dome is not there just to diffuse the light but to make light comming from all angles visible to the sensor looking just ahead.
After all, when you played with it, did it work? Even for a light source shining from 60° off the axes? This is not a rocket science just a question of geometry. As I said - the manufactures of our light meters were capable of putting a dome on an reflective meter to get an incident meter. Do you really think they couldn't do so with a spot meter? They would grab the solution too - they just knew why it wouldn't work. It's incredibly amazing that some people cannot understand it...
George -
I don't have a diffuser dome that's the right size to allow for a proper test of this with the Pentax meter. However, I do have a Sekonic L-318. This meter has a single photocell, but allows the user to make incident, reflected or 5 degree spot readings using different head attachments. I just made some observations with the 5 degree head mounted and the incident diffuser dome placed in front of that. With this arrangement, the meter registers a substantial response to light sources well off-axis - yes, 60 degrees or more. Inspection of the diffuser dome alone shows the incident light to be distributed across much of the interior surface of the dome, including the vertex. This is not surprising - except under extremely artificial conditions, the incoming light is not projected onto the dome as a sharply-focused spot, but rather as a section of a diverging cone which would cover an entire half of a sphere placed in its path.
I don't know whether readings made in this way with this meter would be consistent enough for practical use in any real-world situations. But it's at least conceivable.
No doubt, Oren, that the light is "distributed across much of the interior surface of the dome, including the vertex". That's normal, every dome is constructed for it. And no wonder the meter "registers a substantial respons to light sources well off-axis" because the vertex catches some light in all cases. The point is that the measurement corresponds just for the vertex part and not the whole dome - thus the sensor registers "correctly" the light changes but not the amount of light the dome gets on all of its surface. Hence the difference in a correct and a non correct reading - the bigger is the off axis area lit by the light the bigger is the difference. In some cases it can be small, in other cases just a nonsensic reading.
The only way how you could make the spot meter to measure the dome illuminance correctly is to put the dome at such a distance from the spot meter that the meter would see its whole diameter in its angle of view. You would need a long tube at which end you put the dome. No care would need to be taken for proper focusing (Wallace diffusers work properly on SLR lenses) but some correction (+ -) would still be needed for other reasons.
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