Understanding "Understanding Exposure"

[psybear]

Okay so I am an amateur, but keen to learn and improve. Already have a basic understanding of relationaship between ISO, f-stops and shutter speeds, or so I thought. Hence I followed many recommendations here and elsewhere and bought Understanding Exposure by Bryan Peterson. I've barely started the book however and he has lost me already.

In explaining how shutter speed works he makes reference to the "camera's light meter". What light meter???

He says for example (camera on Manual) "set your lens to f/5.6, place subject up against house... look through viewfinder and focus on subject. Adjust your shutter speed until the camera's light meter indicates a "correct" exposure in your viewfinder and take the photograph".

I've tried this with my D90 and I'm getting a focus lock and a green light at various shutters speeds. Okay, sometimes I get a flash indicator icon, sometimes not - is this what he means by the light meter?

It's the same when he explains ISO - he talks you through an exercise using different ISO speeds of 200/400/800. Supposedly your 'light meter' will indicate a different shutter speed for each setting - I get the flash indicator going off at 1/250 but it doesn't come on at any shutter speed when I move to 400...

I feel more of a noob now than I did before I started this book! :bonk:

 

[susie]

If you put the camera on manual, look through the viewfinder and 1/2 press the shutter there will be a
-2________0________+2
type thing with a rectangle in it. The in camera meter will move the rectangle from 0 (camera thinks you have a correct exposure) along the scale if it thinks you are under or over exposed.

 

[EdBray]

Not trying to be flippant, but have you read your cameras manual? I know it's not a 'man thing' to do but it may give you a bit more of an insight.

Your camera will have a light meter, it will also allow you to set an ISO manually, I suspect you have yours set to some auto setting.

There is a relationship between ISO, Shutter Speed and aperture. making a change on one may well affect the others depending on how your camera is set up.

 

[Alzibiff]

In explaining how shutter speed works he makes reference to the "camera's light meter". What light meter???

Your camera has an "reflective light meter" built in which enables it to set the shutter speed and aperture value when in any of its various shooting modes. For example, if you have a fully automatic mode, it will set both of them, if you have a sports mode, it will most likely set a low aperture value and high shutter speed to enable a decent photograph to be taken.

The meter is "reflective" as it measures light coming off your subject - light entering the lens. To gauge which values to set, the camera will make a number of assumptions about the amount of light and dark in the scene - all down to the microprocessor embedded in the camera.

For answers to your other questions, I will leave the Nikon boys and girls to comment.

Alan

 

[psybear]

Wow, quick responses, thanks.

Susie, that's perfect - got it now, thank you very much.

Ed - Have read it several times - it's a question of understanding it! As I said, my camera is on full Manual setting, but I didn't now what to look for.

Alan - thansk, that helps my understanding.

 

[Cold Rival]

Does this help?

I just made it, and was somewhat rushed, so excuse the quality.

 

[EdBray]

Does this help?

I just made it, and was somewhat rushed, so excuse the quality.

Very good, although not strictly true as it depends on what you are metering to start with, for an average scene it will be fine, for something out of the ordinary, not so.

 

[psybear]

Tom, thanks for that, it certainly does help, got it now.

 

[tdodd]

He says for example (camera on Manual) "set your lens to f/5.6, place subject up against house... look through viewfinder and focus on subject. Adjust your shutter speed until the camera's light meter indicates a "correct" exposure in your viewfinder and take the photograph".

This is typical of the book's vague and unhelpful style. Personally I dislike the book intensely, but for some reason, many people seem to love it. Anyway, the important thing to understand is that your camera's built in meter (already explained by others) is assuming that the scene/subject that you are metering should average out to a a "middle" grey level of brightness. For an "average" scene it may be true that "middle" grey is a good balance to aim for.

However, every scene does not always average out to middle grey. Think of a snow scene, or a night scene, or something in a dark forest, etc.. Also, you have the option of different metering modes, which meter more or less of the subject/scene (spot, partial, centre weighted average and evaluative in Canon speak - Nikon will have similar options but perhaps with different terminology) so you need to decide what metering mode is best and also how to interpret the brightness of the part of the subject/scene that you are metering. To keep things simple for now, let's stick with evaluative (I think it's called matrix metering in the Nikon system).....

Following Peterson's example above, of a person against the side of a house, what colour/tone/shade is the house - is it red brick, painted white, perhaps dark stained wood, or maybe grey concrete? The camera certainly won't have a clue what colour/tone it should be. All it can use as a guideline for interpretation is that overall the scene should average out to middle grey - the safe bet. That might work fairly well if the house os finished in brick or concrete, but what if it is painted white? The expanse of white will fool the camera (and its meter) which will try to lower the exposure to turn the white into middle grey. If you are shooting with manual exposure and use the meter needle to guide you then, if you centre the needle, you will be accomplishing the same thing that the camera would on auto by aiming to get the scene to be middle grey.

If the house was very dark then the same thing would happen in reverse. Left on auto the camera would increase the exposure to brighten it up to middle grey. If you shoot in manual and aim for a centred needle then you too will be aiming for an image that looks middle brightness, rather than capturing the true darkness of the house.

The problem is compounded if you adjust your composition, by zooming with the lens or with your feet to include more or less of the house in the picture. Suppose your subject is a groom, in a black tuxedo, against a dark building. The only paler elements of the scene will be his skin and his shirt and tie. If you follow the instructions to adjust the exposure until the meter indicates a "correct" exposure, where should the meter needle be? I can tell you now that it will not be in the middle! If you do set the needle in the middle then the scene will be overexposed and your groom's face will be bleached to pure white. If you were to add the bride to the scene, and still keep a lot of the building as part of the image then her dress would also be blown out and lose all detail and shape.

One big weakness of the book is that he uses these glib, casual phrases, like "correct exposure", but does not give you any clue to understand what that really means in terms of positioning the meter needle or dialing in some Exposure Compensation when shooting in one of the autoexposure modes. He also completely fails to advise use of the histogram on the camera to check that your exposure is indeed correct. It is not sufficient to look at the image and make conclusions about the accuracy of your exposure from that. The histogram is what you need to check.

Bottom line - camera meters are really stupid. They do not know what you are pointing them at or how bright that thing should be. You have to guide the camera, or interpret the meter and set the exposure accordingly. Darker scenes should show an exposure below the zero mark while brighter scenes should show an exposure above the zero mark. Just how much depends on the exact makeup of the scene and which metering technique you choose to use. With skill and experience those judgements become easier to make, but with digital photography and the ready availability of the histogram it is easy enough to check your results and adjust and try again if you need to.

Sorry to waffle on.

Here's some more reading....

http://www.luminous-landscape.com/tutorials/understanding-series/understandexposure.shtml
http://www.apogeephoto.com/march2005/jaltengarten32005.shtml
http://www.cambridgeincolour.com/tutorials/camera-metering.htm
http://www.cambridgeincolour.com/tutorials/histograms1.htm
http://daystarvisions.com/Docs/Tuts/Meter/pg1.html
http://www.photozone.de/Technique
http://ronbigelow.com/articles/histograms-1/histograms-1.htm
http://www.michaelthementor.com/lessons.cfm?lessonID=28

 

[psybear]

Tim that's really useful, thanks for taking the time to write all that. I can see what you mean by the book having weaknesses - I'm going to go through it all anyway, it will force me to do further research at times to aid my understanding, and I'm glad there are people like you on this forum to keep me right!

Going to work through those links now.

 

[HoppyUK]

I think there are two separate things being talked about together here, which is maybe confusing.

The first is exposure metering, which is the calculation the camera makes to determine the correct exposure value.

Once that value has been calculated, the correct exposure setting can be made, using various combinations of shutter speed, aperture and ISO.

These are two separate operations.

 

[HoppyUK]

I've lifted this from a previous thread which some people were kind enough to say was helpful:

"Compare exposure setting to filling a glass with water.

You need a certain amount of water to fill the glass. Same as you need a certain amount of light for the correct exposure.

The tap is your lens aperture, and you can either turn the water on hard (wide aperture, low f/number) or just a slow trickle (small aperture, high f/number).

And you can leave the tap on for a short time (high shutter speed) or for as long as you like (long shutter speed).

You can fill the glass either by turning the tap on hard for a short time, or slowly for a long time. The glass still gets filled the same.

Changing the ISO changes the size of the glass. A low ISO number is a big glass needing a lot of water, and a high ISO is a small glass, needing less.

At all times, the camera knows the size of the glass, it knows how much water comes out of the tap as you turn it, and it knows how long you can turn it on for.

The only variable the camera doesn't know is how bright the sun is shining - that's the water pressure in this analogy. Lots of light is high water pressure which will gush out of the tap fast, even if it's only turned on a little, but if it's darker then the water pressure is low and it will only trickle out even when the tap is full on.

In exposure metering, the camera measures the brighness of the light, the water pressure, and because it knows exactly what all the other factors are, it then presents you with a combination of camera settings - how hard to turn on the tap, and for how long - to fill the glass exactly, and give you correct exposure.

I just thought that up. Does it make things any clearer? Sure exposure settings can get confusing, but the concept is actually very simple. Like filling a glass.

The important thing to understand about camera settings is the halving and doubling relationship between all the exposure controls. 1/125sec is obviously twice as long as 1/250sec, and ISO 100 is half as sensitive to light as ISO 200. Less obvious is that f/4 passes twice as much light as f/5.6, which passes twice as much light as f/8.

That is what exposure setting is about. Exposure metering (measuring how bright the sun is, the water pressure in the analogy above) is a different thing. That is when spot, evaluative, and centre-weighted etc methods come in as all scenes are a bit different and some ways work better than others."