Spot Metering & Histograms

[Peter Ryan]

Personally I am a matrix metering man and the histogram tells me how the tones in the scene are recorded in camera.

Those of you who shoot in Spot metering mode (or Centre Weighted for that matter) cannot look at the histogram in the same way otherwise why not just meter in matrix metering in the first place.

My question then is, how do you read the histogram if you are metering in Spot mode, what does it tell you, not about the scene but how you have metered the spot - can you tell if it was sucessfult or not?

[jiro]

Exactly what Nikon camera are you using, Peter? Your question intrigues me. My old Nikon D70 can only show one histogram without the RGB option. I am not so sure if I can actually isolate the histogram reading on just the area the spot meter reads on mine. I am also more of a matrix metering guy since I practice a lot reading the exposure on my camera and I have a good grasp when to trust the matrix metering and when to apply the spot metering if I am in doubt. I still apply the old system (something similar to the zone system method) - shadow areas with preserved detail on Zone III ( -2 EV from 18% gray) and bright area with texture and detail on Zone VII ( +2 EV).

[Peter Ryan]

I had the D70 (well still have actually) but traded up a little while ago to the D300s, as my publisher wanted more mega pixles for the calendar. Like you I have learned what matrix meter will give me and the histogram shows the tones of the whole scene. This does not change if you change the metering mode; i.e. it does not just show you the tone you metered off if you choose Spot Metering.

So the question is if you use Spot meter what does the histogram tell you about how effective you have been in choosing a mid tone subject or do you just ignore the histogram when you sue spot metering?

[jiro]

I just tried it right now on my D70. Using a gray card with a cross at the center (to aid me on the focusing) there is no difference on the histogram whether I use the spot or the matrix metering mode. That is, if I filled the entire frame with the gray card using the matrix metering mode of the camera.

Here's the exposure result using the matrix meter reading on the whole scene for comparison on the black and white areas.


and here's the exposure result using the spot metered reading on the gray card.


Nikon's matrix metering is pretty reliable almost every time for me unless I am shooting backlit or directly with a strong light in front of a camera to fool it. All shots are SOOC and only converted to jpeg to make the file size small.

[jiro]

I forgot to add, Peter. I rely much on the histogram to alter the exposure so I can "expose to the right" of the histogram. If the matrix metering says OK but still I find the histogram leaning more to the left I override the exposure reading and manually adjust either the shutter speed or the aperture to my liking. Does this makes sense to your question? I think I need more coffee. Hehehe.

[Peter Ryan]

You posted again while I was answering. You might need a strong coffee. I do much as you do with metering and adjsutments. I am just wondering how our spot metering buddies out there read histograms and adjust exposure as we do.

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Sorry Willie, I am not getting point across. I would expect both to be the same if you spot metered or filled the frame with matrix metering on any object.

It is when people use spot metering to say meter off the face of someone and then take a shot. How do you then interpret the histgoram as it does not represent what you metered but the whole scene?

I teach my students a simple shooting method. One dial and two buttons is all you need to do anything in photogrpahy.

If you choose aperture priority and then change the dial (the one dial) to choose the DOF you want you then look at the resultant shutter speed and see if you can hand hold, need a tripod or need to change the ISO (one of the buttons).

The other button you need is the Exposure Compensation button. Shooting in matrix metering we know the story of the black cat and the polar bear. So if there is two stops between black and midtone and two more between midtone and white then how do we know how much compensation we need?

If you look at the histogram (not the tones but the outline of the histogram itself) you have a line at the bottom, one at each each end and interval lines (some at a quarter and some at a fifth, depending on camera make). If you underexpose the histogram so that the brightest part is on the two thirds line then you know you only need to open up 0.7 - 1 on you EV to compensate.

How do you make that kind of an adjustment if using spot metering and studying your histogram.

[jiro]

Maybe, I think I got your point.

The base line of the histogram (the area of the graph to the right) must hit about 1/2 of the last quarter of the graph (if the histogram is divided into 4 parts) OR must hit 1/3 of the last slice of the graph (if the histogram is divided into 5 parts). Something like this:



This graph was taken from the wonderful book of David Ziser "Captured by the Light". Figure 4 is the adjusted exposure based on the histogram from Figure 5.

[jiro]

Another practical method to get good exposure is to activate the "blinkies" on the camera. If you get blinkies (overblown highlights) on the IMPORTANT BRIGHT AREAS like white wedding dress but not on occasional bright areas in the scene light candles or occasional light sources then lower down the exposure until the blinkies are gone. well, that's how I understood its function.

[Peter Ryan]

Not quiet, I use it to get my students used to knowing how much compensation to make. When they start they do not know how much compensation to adjust for in a scene so your bottom graph is pretty much the starting point in my working model.

This histograpm has marks to divide into fifths so I would say they could safely open up +1.25 EV to get a correct exposure for a normal scene.

So if I swithched my method of teaching and told them to spot meter the scene, what I am trying to find out is what is the purpose of a histogram if you spot meter. I am trying to find out how I would get them to read the histogram and make that type of adjustment as you and I do in matrix metering.

I do not think it possible to spot meter and make a meaningful reading of the histogram to go back and adjust your exposure. It must be done with grey cards or white cards but not using the histogram.

[jiro]

You have a very good and valid point. Though, personally, even I would still be hard pressed to find an exact solution if I will only base my exposure adjustment using the histogram method. The actual scene will still be the key to guide me whether if using the histogram, using the blinkies, or even using the small screen at the back of the camera as an aide for me to nail the exposure right. For me, spot metering is a very powerful tool when you are caught in a very tricky scene. I find it quite helpful when I am trying to shoot a very dark or a very bright scene. If the scene's lighting is evenly distributed then I will trust matrix metering any time of the day. Sometimes, even at difficult shots, matrix metering still gives me favorable exposure that I thought would be a trash image.

I think the best way to comprehend it all is a lot of practice, practice and some more practice.

[Peter Ryan]

I agree that you cannot rely 100% on the histogram but it isa good starting point. We do spend a lot of time talking about the image infront of you and seeing the main shades and tones. Some time back Rob postedsome examples of histograms with an explanation that (with his permission) Iuse to help students understand how to read both the scene in front of them andthe histogram.

But I see so many people wanting to use spot metering that Irealised the histogram does not work for spot metering, you have to use a greycard or white card.

Whatever method you adopt you do need to know its strengths andweaknesses and adjust accordingly. I appreciate your input to this discussion.

[jiro]

...Whatever method you adopt you do need to know its strengths and weaknesses and adjust accordingly.

Couldn't have said it better!

[Peter Ryan]

By the way, I think I prefer the long haired version. I see you have changed your avatar back.

[abhi]

Some time back Rob postedsome examples of histograms with an explanation that (with his permission) Iuse to help students understand how to read both the scene in front of them andthe histogram.

Interesting discussion, gentlemen. Very informational. Peter, are those examples posted here on CiC? If yes, would you happen to have the link to that thread?

[jiro]

By the way, I think I prefer the long haired version. I see you have changed your avatar back.

Thanks! One more year and my hair will be go back that long... and I'll be incognito once again!

[Colin Southern]

Hi Peter,

The histogram is a representation brightness distribution, so (a) it's not affected directly by the metering mode you choose, and (b) needs to be read in conjunction with the scene you're shooting.

Case in point - a closeup of a models face may not have anything in the top quarter, but may still be correctly exposed, because skin isn't a highlight, and you don't want to expose it as a highlight.

Spot metering can tell you where to set your exposure (ball in the middle if you're metering a middle grey, or ball offset as appropriate depending on how far what you spot meter is from a middle grey), but it's actually more useful to see WHERE particular parts of the scene are going to record, for a given pre-exposure. I've done this with collegues to demonstrate: Spot meter the sky and see it's at +3 - spot meter fence see it's at -1 - spot meter ground and see it's at 0 - spot meter a black area and see it's at -3 etc Here comes the cool "party trick" ... take the shot and without looking at the screen - hand over the camera to them - and say "the sky with be slightly blown in places - the black areas will have detail only if you look at the review screen at an angle - ground will be a mid-tone - fence slightly darker.

Nice "party trick" but in essence what spot-metering has given you is total CONTROL over your exposure.

With reflective scenes, this isn't a big deal because there's only roughly a 4 stop range anyway, so on a histogram - with matrix / evaluative metering it'll stop about 1 stop short of the right hand side (assuming something white in the scene, with an average reflectivity of 18%) - you can expose to the right by upping the exposure by a stop - but then you still have to drag it down by a stop in post processing (usually) and you really don't gain anything. The game changer is when photographing scenes with back lighting - then the dynamic range required can jump by 3 stops or more in the blink of an eye - and it's here that exposure becomes more critical as 4 stops of reflectivity - plus 3 stops of backlighting - plus a couple of stops of shadow detail take you to 9 stops and most cameras will only get to around 11 or 12 - so you're getting pretty close to the noise floor and risk shadow noise when you raise the shadow levels in PP, if the shot is under-exposed to start with.

So to (finally) answer the question (sorta!) - if shooting a scene with back lighting (or any scene with a higher dnamic range that a purely reflective scene) then use the blinkies to detect any areas of over-exposure, and the histogram to detect overall under-exposure (or put another way, over-exposure to the point channels are clipped is usually a bad thing with digital photopraphy if detail in those blown areas are important - BUT - the higher the dynamic range of the scene, the more important it is to get the exposure closer to the "max" - BUT - the higher the dynamic range of the scene, the more likely the wide-scope metering modes like matrix / evaluative will be "sub-optimal".

Spot metering gives total control when one knows how to apply what one is seeing - histograms give feedback as to the results of the spot-metering choices

PS: ALL of the above assumes manual exposure selction with spot metering - with auto-exposure it's different.

[Dave Humphries]

Hi Peter,

I often spot meter these days and while that often* completely changes the way I apply EC when in Av (aperture priority) mode (compared to centre weighted or matrix), it has NO effect on my interpretation of the histogram.

Why would I interpret it differently? As you (and others) say, it doesn''t change when you change meter modes.
The histogram performs the same function; an overall exposure judgement aid (post capture), along with blinkies.

* this is dependent upon image content, if the scene is very flat (low contrast) and mono-tone, it'll have little effect, but if comprised of whites and blacks, it can have a massive effect; often reversing how you need to think about, and apply, EC.

HTH,

[Wirefox]

I think put simply the histogram evaluates the brightness distribution captured by the sensor NOT the area of the image covered by the metering. I think Peter's post implies that the histogram only represents the area metered....I could be wrong though The spot metering is one means to evaluate the exposure you want across the image (along with any compensation). The histogram shows the result of that evaluation...so it must have a use.

Why am I getting that sinking feeling that I may have missed the point somewhere along the line

[Peter Ryan]

I think Peter's post implies that the histogram only represents the area metered....I could be wrong though Why am I getting that sinking feeling that I may have missed the point somewhere along the line

Hi Steve,

Thanks for joining in. No, I was saying the histogram measures the brightness of the whole scene but the spot meter measures only part of the scene so how do those who actively use spot metering use the histogram to make exposure fine tune adjustments with the EV compensation when you are only measuring off a small part of the scene and you don't know if that part is actually mid tone. Yes using a grey card or white card and adjusting accordingly works but I hear many say meaure off some object withing the scene itself and I pretty sure most cannot pick a mid tone to measure off. So if they picked the wrong tone then looking at the histogram will not hel them understand they got it wrong.

[Peter Ryan]

Hi dave,

Thanks for contributing. I understand the histogram tells you the same information but as in the above post, the histogram does not tell you if you chose an incorrect tone to measure off whereas with matrix metering you can make EV adjustments based on the blinkies and the histogram to adjust your exposure.