View Full Version : Spot metering

Tri Tran
20-Apr-2009, 10:29
For the most accuracy, what's the maximum distance for 1 degree spot metering? Any ideas? Thanks. TT

Kevin Crisp
20-Apr-2009, 10:39
The usual problem if you are metering a very small area, one that just barely fits in the little circle, is flare and/or lack of total agreement between the shown circle in the finder and what the cell reads.

This is more of a problem when the surrounding area is much brighter than what you are attempting to meter. Even though the circle may be on the less reflective area, the meter picks up some value from the adjacent area. Some meters are better than others for this. It is easy enough to test your meter by metering a darker area next to a very bright area. As you move the finder circle to the edge of the dark area does it start to pick up the adjacent bright area before it is included in the circle in the finder? I have two Pentax digital meters, one with the Zone VI treatment, and both are highly resistant to this kind of reading error.

Generally speaking the distance doesn't affect the reading. In other words, if you meter a bright building at an EV of 14 from two feet away and back up to ten feet, twenty feet, 100 yards, etc. the reading should be the same. If you get far enough away that the finder is reading a very small area next to much different reflectivities then you get into the potential flare/precision problem.

On really long distances atmospheric haze can be a factor, but I assume that is not what you are asking about. I hope this makes sense. I've never noticed that distance to the subject causes surprises on the negative. You must be noticing something?

20-Apr-2009, 13:05
I think that this question deserves a bit more consideration. I don't have an answer, but would like to see someone with knowledge of the interanal workings of one of these meters weigh in.

A 1 deg. spot meter has an optical system that seems to image the subject onto a small photo sensor. Most do not have adjustable focus so they are presumably set for some compromise focusing distance somewhat near infinity. As the subject gets close, the image would be defocused at the sensor and the sensor would be more influenced by adjacent areas of different brightness. In their favor, the optical systems are small and use short focal length lenses, so the defocusing will be relatively small.

I don't have any data, but I sense that I can observe this in my Zone VI modded Pentax digital at distances on the order of 1 ft.

Tri Tran
20-Apr-2009, 15:20
long distances atmospheric haze can be a factor, but I assume that is not what you are asking about. I hope this makes sense.

I have the Minolta and this is what I have the problem with . It looks like the reading is off by surrounding refleted to where I wanted . I usually had perfect reading within 10-20 ft but not around 75 yards or more, so I just have to compensate the exposure according to the amount of lights like flat scene , dark scene , haze from the mountain by experienced otherwise it will give me the wrong data or may be I change the Pentax one. Thanks for your time.

Kevin Crisp
20-Apr-2009, 15:46
Unless you are working in smoke filled auditoriums I wouldn't think there would be enough haze in just 75 yards of distance to make any difference at all. (Actually not even 75 yards if the first reading is from 3 to 5 yards away.)

I haven't personally used the Minolta spot meters but they have a good reputation.

When you back up does the reading of the same thing go up or down?

Jim Noel
20-Apr-2009, 18:55
Since I made a 2" lens hood for my Pentax I have no problem with flare regardless of the light conditions. Of course if there is significant haze the meter reads it. That is OK with me because it will help me place the object in the correct place on the curve. If a dark object appears lighter because of atmospheric haze it just adds depth to the image.

Nathan Potter
20-Apr-2009, 19:23
Don't know. Do a quick test. Set up a light table with diffusing material over it - or easier use a computer screen on bright that is blank. Cover with a piece of black cardboard with a hole in it that will be the size of the spot circle on your meter. You will need to be maybe 3 feet away from the screen and take a meter reading. Then remove the cardboard so that the whole light table or computer screen fills the field of view of the meter and take another reading. Note the EV difference between the two readings and you'll have the maximum inaccuracy due to off axis and scattered light under extreme contrasty conditions. Make sure the room is dark during both tests. There will be an additional small error possible due to the spectral peaks of fluorescent light vs the more blackbody uniformity of daylight (that should be unaltered sunlight) at 6500K or so. And the spectral responsivity of the meter cell will play a part - but I think can be ignored.

BTW I've always meant to do this myself but have never gotten around to it. Maybe someone else on the Forum has some data.

Nate Potter, Austin TX.

Alan Davenport
20-Apr-2009, 20:51
The most accurate distance for spot metering is the exact distance the camera is setup. That's where the film is, so that's where you need to set the exposure.

Tri Tran
20-Apr-2009, 23:02
The most accurate distance for spot metering is the exact distance the camera is setup. That's where the film is, so that's where you need to set the exposure.

That's brilliant! I will try this to see if I get the consistence result and ofcourse each meter acted differently under certain lighting condition. Thank you all.

21-Apr-2009, 07:24
I am always amazed when the Art of photography becomes bogged down by the Science of photography. The science of photography is simple cause and effect. You do this and you get that. When it comes to metering with a spot meter, it is normal to select the areas of importance, measure their luminance values and place them where your artistic vision wants them to be. The science of this process is unforgiving but the flaw in the science is the way the mind works coupled with the mechanics of shutter function and the development process.

Think about it. I start the process of metering by making judgments about where I want selected areas of a scene to fall. I select a spot and measure its luminance value and proceed to make a negative and develop it based on all the testing of equipment and process that is the science of photography. No matter how accurate and repeatable my shutter is (most are not extremely accurate or repeatable) and no matter how precisely I process the film (there are always slight variations in temperature or chemical concentration or PH value etc.) there is going to be a slight variation in the process.

The good news is; it doesn’t matter. The biggest variable in the process is that first one, the selection of a spot to meter and the judgment call as to what value should be assigned to it. If the result of that is the creation of image data in the area in question, there is so much latitude in the printing process that there is simply no reason to get bent out of shape about how exact the science is.

As for Tri’s original question, “For the most accuracy, what's the maximum distance for 1 degree spot metering? Any ideas?” I would say there is none. I always meter from the camera position. As Alan said “That’s where the film is.” If there is a shadow that is so small that the 1 degree circle is not filled by it, it probably doesn’t matter anyway. If it is important to me for some reason I can move closer or, if I can’t get close enough, find a shadow close to me that is exposed to the same lighting.


21-Apr-2009, 10:27
OK since nobody would do the work for me I tried some experiments similar to what Nate suggested, using adjacent light and dark areas on a computer screen and other targets.

The difference between the two areas was about three stops. I found that whether from 27 ft or from 2 feet, the meter would change by its one third stop resolution just as soon as the lighter area crossed into the about 1/2 degree clear ring surrounding the 1 degree circle in my Pentax digital spot. The viewing system seems to have a fresnel and a non-focusing screen. The fresnel seems to starts at the outside of that 1/2 degree clear ring.

However, the defocusing of the spotmeter's image is visible as well (If the viewing system really is aerial, perhaps this reflects only my inability to accommodate the different image distance) and at 2 feet the defocusing of the bright edge seems to be about 1/2 degree wide. The incursion of this defocused bright edge into the 1/2 degree wide annular boundary will also will begin to affect the measurement of the darker spot by that 1/3 stop metering resolution.

21-Apr-2009, 14:41
I agree with most of what has been written on this blog. The problem deals with ambient light from distant objects. Best solution is trust the meter and shoot a back up negative then develop and print the first negative. If you like what you see develop the back up negative and adjust development time to increase or decrease development time!

21-Apr-2009, 16:47
Its easy enough to test in the field. The problem is when the darkest areas are surrounded by very bright areas. In my unmodified spotmeter, it might read 1/2 stop higher under those conditions. Not a big deal, just something to bear in mind.

I heartily agree with the prior poster's comments about the art of photography vs. the science. I think the best work comes from those who know what they're doing, but can go with the flow and work with what's in front of them.

Spend a shoot or 2 getting to know you're meter, taking notes, and then let it go, so to speak. Just my opinion...