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# Thread: Spot meter & ZS vs. BTZS & incident light

1. SO you have one hell of a difference there. I hope someone chimes in and interprets this. I have never seen a difference between incident and sopt that was that large. I usually get a difference of maybe two stops.

2. Originally Posted by Helen B
OK, the sun came out, but there were no aluminium-painted roofs handy, so I had to make do with wet blacktop.

Here are the readings to interpret as you wish:
Incident meter directed straight at the sun: f/11½ (no surprise!)
Spot meter reading off wet blacktop: f/64½

No matter which way I pointed the incident meter, the highest reading I could get was f/11½.

Meter set at EI 100, 1/125 second. Clear sky, slight haze. 2 pm, New York.

Best,
Helen
Joining the thread late so apologies if I missed something above that makes my answer not make sense...

The above results are what I would expect, the spot meter reading of the highlights is about 2.5 stops over the incident reading. The incident meter is giving you the middle of the assumed 5 stop range (so, zone 5-ish) the spot meter of the wet pavement is giving you a highlight reading (zone 7-8 or so) about 2.5 stops above the average.

The spot meter is trying to tell you how bright the thing you're pointing it at is. The incident is trying to tell you how strong the lighting its in is. Similar ideas but different enough for 2.5 stops.

Dean

3. Dean,

Er... 2½ stops from f/11½ to f/64½?

4. Helen, I think he's trying to say there is a difference on the high end of 2 1/2 stops. I counted this one on my fingers (my calculator, the original "Plam") and I still get a 5 stop difference between the readings.

This is why I prefer to use a spot meter in these situations. That, and I don't own an incident meter. tim

P.S. Thanks for taking these readings for us.

5. Tim

Both of helen's readings were the high values. Not the low and the high. She has no shadow values, if I read the post right.

I am going to assume she measured with the incident meter right next to the road facing the sun and spotted the same area.

6. Mark,

You are correct. The idea was to show readings taken from a real-world object that one would wish to record detail in, using normal photographic metering techniques. All metering caveats apply. This was not a scientific test, nor is it relevant to anything else apart from that one particular situation. It's just an illustration of the kind of subject that Tim asked about, as far as I understand.

If you did only allow 2½ stops over the fully-illuminated incident reading for the brightest highlight, the actual brightness would be about 2½ stops over your allowance, in this instance.

Best,
Helen

7. that is what I figured.

8. Forgive my confusion, I have not delved into the BTZS (nor new math) but when I was trained you took an incident reading from the subject facing the camera, and a reflective reading facing the subject from the camera. I don't know if Helen was pointing both the spot and incident meters from the view point of the camera toward the subject. The spot meter reading seems underexposed to render black top 18% gray.

Confused in Phoenix

9. I think I understand...

If Hellen had taken an actual picture of a gray card with the incident meter reading and the sun behind her the gray card would appear on zone v. Bright spots and shadows fall where they may.

If Hellen had taken the picture of the grey card with the spot meter reading, Wouldn't she have had to open up 3-5 stops to get the bright spot white and the grey card on zone V? Or do you use the long exposure and reduce development dramatically to get it all on a wide lattitude film. I don't see this working at all for color slides.

Thus there is really 2 or less real stops to create the same middle grey exposure. It might have been useful to take a reflected spot reading of a grey card from the camera position to determine the variances between the reflected sunlight on the grey card and the bright wet pavement.

10. Originally Posted by esearing
I think I understand...

If Hellen had taken an actual picture of a gray card with the incident meter reading and the sun behind her the gray card would appear on zone v. Bright spots and shadows fall where they may.
Agreed. An incident reading ignores the reflectivity of the objects in the scene, thus rendering them in their "true tonality" - assuming the film or media has sufficient range.

Originally Posted by esearing
If Hellen had taken the picture of the grey card with the spot meter reading, Wouldn't she have had to open up 3-5 stops to get the bright spot white and the grey card on zone V? Or do you use the long exposure and reduce development dramatically to get it all on a wide lattitude film. I don't see this working at all for color slides.

Thus there is really 2 or less real stops to create the same middle grey exposure. It might have been useful to take a reflected spot reading of a grey card from the camera position to determine the variances between the reflected sunlight on the grey card and the bright wet pavement.
I don't know that we can actually determine this. I'm not clear whether Helen's spot reading was of a maximum-black area of the wet pavement, or of a reflective highlight area. That makes a huge difference in how one would interpret the reading. If the spot reading was of a highlight in which detail was to be retained, one might open up 2-3 stops, for example. If of a black area, the adjustment would be in the opposite direction - closing down 3-4 stops, perhaps.

The key point to remember is that a reflective reading gives the exposure needed to render the area being read as a middle gray (e.g. Zone V). Assuming you know accurately what area is being measured (as you would with a spot meter), you can then adjust the suggested exposure to "place" that value where it should be (or, where you want it), and everything else falls wherever it will accordingly. With B&W films (some responding better than others), you can adjust development to either expand or contract the contrast range for the desired effect. Naturally, that's not really possible with color film. The BTZS and Zone System methods are simply two different approaches to metering and development to achieve the desired result.

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