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RichSBV
15-Apr-2005, 11:30
I don't know if anyone else has done this, but curiosity got me to test this...

You can use any spot meter as an incident meter by simply placing the "Lumisphere" dome from a Sekonic L-398M meter in front of the lens of the spot meter!

I have tested this with all kinds of lighting conditions and it works great. Since it's almost impossible to judge reading from different meters, I have compared the results between an L-398M, L-718 and Luna-Pro F meters. The results were all withing a 1/3 stop.

The easy way to mount the dome would be to get a lens cap for the spot meter and make a hole in it to accomodate the dome. Or you could use the cardboard sheet that the dome comes mounted on when new and make a lens cap from more card stock using glue and/or tape.

Once mounted in front of the lens, the meter works as any standard incident meter. And no, you can't see anything through the spot meter this way, except grey... ;-)

Just thought some other people could use this info as I like to get both spot and indident reading and sometimes it's just a pain to use two meters...

One other note. None of the other dome/spheres I have will work. They're all too dark. Only the "Lumisphere" from the Sekonic 398 will work as far as I know...

Al Seyle
15-Apr-2005, 12:24
I find it works with the dome from my Minolta IVF on a Pentax/ZoneVI Dig. Spotmeter. Problem is, I frequently point my incident meter upward and there is no way to lock in a reading on the Pentax. The meter always has to be read "live". It would be pretty difficult to get your eye behind the Pentax to measure subjects below eye level.

george jiri loun
15-Apr-2005, 13:49
You can do this with an SLR camera too - that's the invention of George Wallace (expodisc.com - Wallace Photo Products) . He sells these domes even in a flat shape as he found out how to make the flat pannel sensitive to the direction of light. It's a clever and not so much known invention which didn't find too many followers, unfortunately.

Will Strain
15-Apr-2005, 15:08
the expo disc products are becoming more useful with the advent of digital photography, since you can effectively use them to set a custom white balance.

not that anybody takes the time...

george jiri loun
15-Apr-2005, 16:19
The spotmeter would need to cover with its angle of view the whole diameter of the dome in order to work properly. As this is not the case it only works when the main light shines on it from directly ahead, not from a side.

RichSBV
15-Apr-2005, 19:19
george...

I thought that too when I first tried it. But the nature of the dome diffuser is... diffusion! It works extremely well no matter what the angle of the light is. If it didn't, I wouldn't have bother posting the info...

george jiri loun
16-Apr-2005, 03:06
Rich, the function of the dome is not only to diffuse the light but to capture the appropriate amount of light when it comes from an angle (that's why it must be a dome and not just a flat diffuser !). If the sensor's angle of view doesn't see the whole diameter of the cupola it cannot measure the light comming on the dome from an more extreme angle - it measures just a small part of the dome's surface, the rest is invisible for it. That's why it cannot measure properly. And a spot meter (espescially if it is a 1° spot meter) sees just the top surface of the dome, not the whole dome.

RichSBV
16-Apr-2005, 09:01
george... Have you tried it??? Argue all you want, but I've tested it, it works fine and I've made two dome/lens covers for my spot meters and will happily use them. You can not use this idea. That's fine...

Without doubt there can be certain lighting situations where the spot/dome combo may give a false reading, but that applies to every single design of every light meter made. I have tested this idea with as many lighting conditions as I could think of and it works. The only obvious flaw would be is there was a small spot of bright light falling on the center of the dome where the spot meter reads, and that's a pretty absurd lighting condition and would be considered operator error!

Noone is forcing you to use a dome on your spot. It works so well for me and solved the problem of carrying a seperate incident meter that I thought I'd share it with other in the same situation. For 10 bucks new and a 1.50 used, I can have an incident meter with me without carrying another meter...

What more can be said. It works, I'm _very_ happy with it and all I hope is that someone else finds it useful. If you don't like the idea, don't use it...

george jiri loun
16-Apr-2005, 13:31
"The only obvious flaw would be is there was a small spot of bright light falling on the center of the dome where the spot meter reads" (Rich) - wrong again , it's just the opposite because in this exact condition the meter would read this falling light correctly as it falls on the part of the dome where the sensor sees it. It is when the light falls on the part of the dome where the sensor cannot see it (because of its limited angle of view) that it reads incorrectly. And yes, I constructed a dome for two of my SLR lenses some 15 years ago and use it instead of an incident exposure meter. Of course, there it works perfectly as the lens covers the whole diffusing sphere with its angle of view. That's why it works.

RichSBV
16-Apr-2005, 21:51
george, give it up until _you_ do the testing! I was/am right. Any major light difference between the exact center of the dome, be it a dark spot or a bright spot would produce an error in the reading.

Fine, according to _you_ this whole idea doesn't work. Then DON'T USE IT! and quit at that and leave the rest of us alone!

_I_ tested it for two days under every concievable lighting condition. It works as I stated, within 1/3 stop of three extremely accurate incident meters. So it doesn't work for _you_ even though _you_ never did the testing...

Like I said, I'll be using it, and quite accurately. You don't use it... We'll all be very happy...

By the way, NO 35mm behind the lens meter will meter totally evenly across the whole film area. So by your own words, your system can't possible work on the SLR either... Ya can't have it both ways... Just give it up and let the rest of us use or not use this as we feel fit...

george jiri loun
17-Apr-2005, 06:49
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.

RichSBV
17-Apr-2005, 07:32
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!

d.s.
17-Apr-2005, 08:06
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

george jiri loun
17-Apr-2005, 08:12
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?

george jiri loun
17-Apr-2005, 08:21
"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...

Oren Grad
17-Apr-2005, 10:26
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.

george jiri loun
17-Apr-2005, 12:44
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...

Oren Grad
17-Apr-2005, 14:48
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.

george jiri loun
17-Apr-2005, 15:04
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.

george jiri loun
17-Apr-2005, 15:22
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.

Oren Grad
17-Apr-2005, 15:23
Well, that's the essence of the matter - whether a incident reading taken using just a few degrees around the vertex of a diffuser dome can ever be of practical use. I don't know, but I can imagine that it might be, and I think that this is a matter to be resolved by testing of the proposed meter setup under the conditions of intended use.

Remember as well that there are a variety of techniques for using an incident meter, not all of which involve taking just a single reading, and not all of which involve pointing the meter at the camera and nowhere else. It is possible that optimal technique for such a "narrow" incident meter will be different from that for a standard incident meter.

You seem to be arguing on theoretical grounds that it could not possibly be of use to anybody under any circumstances. I guess we will just disagree on this.

Cheers...

Oren Grad
17-Apr-2005, 15:25
...an incident reading...

george jiri loun
17-Apr-2005, 15:35
"you seem to be arguing... that it could not possibly be of use to anybody under any circumstances." (Oren) Where did you read this absolute statment in my posts? I cannot find it...

Oren Grad
17-Apr-2005, 16:19
If you think about the geometry, this "narrow area" incident reading off the inside of a diffuser dome should generally give results in between those provided by a full dome reading and those generated with a flat diffuser such as the Sekonic "Lumidisc", but much closer to the result from the flat diffuser. So much for reinventing the wheel...

George - Sorry for the rhetorical overreach. Let's leave it at that.

george jiri loun
17-Apr-2005, 16:32
No hurt feelings, Oren, Cheers!

Oren Grad
17-Apr-2005, 17:29
And good light to all!

Gene Crumpler
2-May-2005, 14:21
My, My such a fuss! How about an 18% gray card?

tim atherton
2-May-2005, 14:47
"My, My such a fuss! How about an 18% gray card?"

Hold it at 18 different angles and get 18 different readigns... :-)

In addition, many light meters aren't calibrated for 18% grey

Ken Lee
8-Feb-2012, 07:33
How big is the Lumisphere (http://www.bhphotovideo.com/c/product/119341-REG/Sekonic_401_821.html) dome ? Its $12 price point is attractive.

Will it fit on a Pentax Digital Spot Meter ?

The smallest ExpoDisc is 52mm, so I guess you could get a step-down ring.

Are there other options ? (This is a 7-year-old thread.)

BetterSense
8-Feb-2012, 07:47
Well, that's the essence of the matter - whether a incident reading taken using just a few degrees around the vertex of a diffuser dome can ever be of practical use.
How would you take such a reading? Not the way you think!


If you think about the geometry, this "narrow area" incident reading off the inside of a diffuser dome

Think about the geometry a bit more. You are having a conceptual failure.

A spot meter is not some type of laser beam. A typical spot meter uses regular thin-lens optics, with a small-area photosensor placed at the focal plane (think of it like a subminiature camera with a sensor instead of film). Focus of the system is set at infinity or hyperfocal. For a given sensor size, there is a fundamental tradeoff between the size of the spot-reading and the intensity at the focal plane (due to numerical aperture), so to maximize both qualities with practical sensor sizes, spot meter designs tend to have fairly large-diameter lenses (low-F-stop). I designed my spot meter with an f/.9 lens.

If you place a dome over the lens, you are not reading "a narrow area" reading off the inside of the dome. Since the dome is so far out-of-focus--it subtends effectively the entire exit pupil of the lens--an area of the dome equal to the exit pupil of the spot meter is registered on the photosensor. So in the special (actually typical) case of thin-lens optics where the dome is the same diameter as the lens, the entire dome is equally weighted by the spot meter's sensor. Assuming a Lambertian dome, there is only cos^4 falloff to account for, which also exists for the conventional incident meter design, and cos^4 falloff is actually worse for the conventional incident meter design due to the near-zero numerical aperture of the typical configuration.

Placing a dome over the lens of a spot meter makes a very suitable incident light meter, which is no worse and possibly better than a conventional incident light meter in omnidirectionality! The only disadvantage compared to a conventional incident light meter design is less sensitivity due to the imperfect transmittance of the lens(es) and the larger numeric aperture of the system. In fact, if you wanted to make an incident meter with improved omnidirectionality compared to the usual incident meter design (something not necessary in practice), and you were willing to trade off size and/or sensitivity, you would do it in exactly this way.

Taking a small-area spot reading off the inside of a diffusing dome--a situation which you keep bringing up--would require a spot meter that could adjust the focus down to the mm range...I have never seen such a device before.

Bill Burk
8-Feb-2012, 11:54
Roughly 1 inch white dome diameter, 1 1/4 inch including the black mount.

Thebes
8-Feb-2012, 17:43
I use a spot meter for incident metering all the time. I don't need no silly luminidiscs either, just my balled fist. My skin is one stop lighter than middle-grey. Compare lighting ratios in a second with the new lumni-fist, not available at finer photo-stores anywhere.

Gem Singer
8-Feb-2012, 18:54
Thebes,

Reading the light intensity off of your fist with a spot meter is a reflected light reading, not an incident light reading.

cowanw
9-Feb-2012, 06:11
True, but IF your reflective surface is calibrated and constant then your adjusted reading is equivalent to an incident reading.

Gem Singer
9-Feb-2012, 07:14
It doesn't matter whether you are taking a spot meter reading off of an 18% grey card or your hand.

Although the two types of metering might give the equivalent exposure, you are still reading reflected light, not incident light.

(see: A.Adams," The Negative").

Ken Lee
9-Feb-2012, 07:21
The hand might make a better target than a grey card: as Tim pointed out, a card held at 18 different angles, may give 18 different readings. A hand, IF less shiny and not subject to changes in pigmentation, is more diffuse - will give something closer to a genuine incident reading.

That being said, if we want a diffused omni-directional reading, that problem has already been solved with the invention of a translucent dome. We can mimic it if we try, but mimicry is just... mimicry ;)

Helen Bach
9-Feb-2012, 08:17
It doesn't matter whether you are taking a spot meter reading off of an 18% grey card or your hand.

Although the two types of metering might give the equivalent exposure, you are still reading reflected light, not incident light.

(see: A.Adams," The Negative").

Well if you are going to be really technical about it, most incident exposure meters don't read incident light either. Light falls on the dome or disc, then the meter cell reads the light coming from the dome - they don't read the incident light itself. That's not much different from allowing the light to fall on a standard reflector and reading the light coming off the reflector, is it?

Both methods are basing their measurement on the amount of light incident on the subject, not on the light reflected from the subject.


Best,
Helen

Ken Lee
9-Feb-2012, 08:24
Good point !

At the risk of circular reasoning: if there were no difference - or no appreciable difference - then why would the translucent dome be adopted universally: convenience ? consistency ?

Perhaps it's best to defer to the physicists and Optics gurus on this one.

Helen Bach
9-Feb-2012, 08:29
I think that it is a combination of convenience and, perhaps more importantly, controlled directional response.

Best,
Helen

rdenney
9-Feb-2012, 08:34
It seems to me that the disk will remove some of the light reaching the sensor, and thus should be accounted for in some way. My L-718 has a sight hole for reflected average metering, which reduces (using an aperture) some of the light reaching the sensor. It also has a flat translucent diffuser and a dome diffuser that replaces that aperture.

Assuming a diffuser reflects as much light as it transmits, I would expect it to knock off a stop when used on a meter that is not calibrated for use with a diffuser.

Rick "noting that most of the participants have probably long forgotten their participation in this historical thread" Denney

Sevo
9-Feb-2012, 08:45
It seems to me that the disk will remove some of the light reaching the sensor, and thus should be accounted for in some way.

It better should remove some light, or the meter would be way off. Meters generally are calibrated for reflective reading off a about 15-20% reflective surface. So all that is needed is a corresponding transmission value for the dome or disk - which is trivial to achieve. All meters I am aware of need no separate adjustment for incident.

Bill Burk
10-Feb-2012, 14:30
Here's my opinion, and I'm starting to figure it out... It is a rough and casual opinion, not intended to be scientifically accurate or unimpeachable...

Incident reading can be done off the dome in the meter.

Aiming a reflected light meter at the palm or ball of hand... or gray card or any other "known" reflectance and then "placing" that reading "where it belongs" accomplishes the same thing.

You are evaluating the amount of light falling upon the scene and calculating exposure based on that.

But...

Aiming an averaging reflected light meter at a scene, or using a center-weighted meter inside a camera. This depends on the distribution of light in the scene and works on the law of averages to give you exposures that, on average, should be right. Black cat in coal mine or Adobe on white sands are going to be thrown off.

... And... Spotmetering and placing readings on Zones is yet another beast altogether.

Stephen Benskin
11-Feb-2012, 23:34
One other note. None of the other dome/spheres I have will work. They're all too dark. Only the "Lumisphere" from the Sekonic 398 will work as far as I know...

Do you happen to have the values for C for those meters?