How do I know I have the correct exposure when shooting in manual mode?

Question

I have a Canon XTI which I've been using for a few years now. Most of the time I'm been shooting in one of the "semi-manual" modes like aperture priority or shutter priority, but now I'd like to try using full manual mode. I should have a new EF-S 17-55 f/2.8 delivered when I get home today. Before I was using a variable aperture Sigma 17-70 which made manual mode more difficult with the aperture changing arbitrarily.

I understand the effect of changing aperture and shutter speed, but right now I'm accustomed to only controlling one at a time. The camera is usually setting the other to get the right exposure automatically for me. So if I'm controlling both how do I know I have the correct exposure? I understand I need to use the metering in my camera but I'm not sure exactly how do do that.

First when should I be using each of the metering modes (Evaluative, Partial, Center weighted, or Spot)? These just change the area of the scene which is metered correct?

How do I know what the meter should read off a specific object? Obviously if I just happen to be shooting a 18% gray wall then it should be at zero, but that can't apply to everything? If I'm shooting a black cat and take a spot meter reading off of the cat then the meter should read less then zero right? But how do I know how much less then zero? Are there rules of thumb I can follow? I've ordered a couple gray cards with my new lens. So experimenting with those should help.

How often should I be changing these settings? Say I'm at a party indoors which is consistently lit. I should be able to set the aperture, shutter speed, and ISO and just forget about it unless the lighting changes correct? Does the same apply to outdoor scenes? I've been hesitant to use manual mode in the past because I felt like I was spending a lot of times messing with settings before each shot. But not I'm realizing that this is really only the case when the lighting changes, or I want a different style of photo such as a different depth of field.

Finally, does exposure change with distance to the subject? For example say I want to take a picture of a statue with correct exposure using a gray card. I'd place the card on the statue, and while being careful not the block the light on the card find the correct exposure by spot metering the card. If I were then to step back 20' to get the whole statue in frame would I still have the correct exposure?

Answer 1

When in manual mode, the camera actually still meters using the set metering mode (multi-segment, center-weighed, average, spot, etc). This is shown in the exposure-meter which usually displays EC. In manual mode though, the camera shows the difference between the metered exposure and the set exposure.

Now, if you change your exposure until the indicated difference is 0, you've missed the point of using manual exposure. Well, in some cases, such as for stitching panoramas, this is actually useful, but generally manual mode is to decide your own exposure.

The way you set it depends on the image you want to create. Aperture controls depth-of-field, shutter-speed controls the rendering of motion, so set them as you need to realize the image you want.

Once you set the parameters needed (A or T or both), you need to figure out how to set the remaining parameters to create an image of acceptable brightness. On some Pentax camera there is a semi-automatic called TAv which automatically sets ISO according to a set aperture and shutter-speed.

To set the remaining parameters, look at your subject and decide where to meter. The most pleasing way of doing this is to meter on the brightest part of your subject and add +3 (or so, depending on the dynamic-range of your camera) to make that part bright and everything else should fall in place. On Olympus DSLRs there is a metering mode called Spot Highlight to do this automatically. If the results are too dark, then you've exceeded the DR of your camera and are going to have to blow some unimportant highlights (if there are any).

For the action of metering you may be able to do it in manual mode using the AE-L button or the Green button (on Pentax). Otherwise you can use an automatic mode (P, A, S) and then dial in the same values in manual mode as a starting point.

Answer 2

In sunlight, there is a very handy rule of thumb called "sunny sixteen". It simply states that for photographing sun-lit objects, you should let the shutter speed equal the ISO you are shooting at, and the aperture should be f/16. This will give a correct exposure. In Northern Europe, read "sunny eleven", in the Arizona desert read "sunny twenty-two", but the principle remains the same. For shadow subject, open up two stops or so - "shady eight".

Your light-meter gives a reading based on the assumption that whatever you are metering is "average" in brightness; 18 percent gray or something like that. Most photographs are "average", a good portion are not. It can be useful to know that green grass is "average", point the light-meter at grass and you get a decent reading. Mediterranean skin is average also. Point the meter at something that is not averagely bright and you must start to think about what you are doing and compensate accordingly. The blue sky will often be a stop or so above average; snow is about two stops above. The same goes for sun-lit sand on the beach. Note that the camera has the same problem in the automatic modes, it is not unique to manual mode! This is why 99% of snow photos come out too dark, people trust the light-meter to get it right and by definition it will not - snow is not averagely gray but the light-meter assumes it must be.

Anyway, once you have set your exposure, you don't have to fiddle any more with it until the light changes. Check the histogram from time to time and you are golden.

The metering modes vary in how much of the entire scene they measure. A center-weighted mode will look mostly at the center part of the photo, disregarding the edges. A spot meter will only look at the very small bit dead center in the viewfinder, disregarding everything else. Evaluative tries to look at the entire scene and be clever about it, comparing it to a stored database of photos and trying to find a match there... it can be a bit unpredictable. When it works it should be great, when it fails it isn't easy to understand why.

Answer 3

I'm going to answer about exposing in manual mode, for metering, see this question.

First of all, in tV mode, the camera uses a specific exposure value, a shutter speed, and sets the aperture. In aV, it takes the exposure reading and aperture mode, and sets the shutter speed. Manual lets you set shutter and aV, showing the resulting exposure.

In the bottom of your viewfinder, you will see the exposure setting. If the image is properly exposed, the arrow will be pointing at the zero. The correct exposure for the scene may be higher or lower than this, creative judgment will be required to know what that should be.

An alternative to this involves using the histogram mode of your camera. Take one picture close to the correct exposure. Try to increase or decrease the exposure such that the histogram is shifted to the right as much as possible without actually touching the wall.

The exposure does not change with distance to the subject, it only changes by the amount of light reaching the sensor. Basically the amount of light per given area remains the same.

And lastly, just to give a few ideas of when manual exposure is really helpful. First of all, if you have ETTL flash, the flash will automatically be set to match with the settings you have, ie, it'll be more powerful if you need it to be. Secondly, it is almost required for panoramic stitching, it's really difficult to match the exposure otherwise. Lastly, if you have a subject which is against a background of a very different color, bring your camera close so that only the subject is exposed, set your camera to the correct exposure, and then return to the original position. Take the picture, and you'll expose the subject perfectly, although the background might be bleached out. If that's okay, then you're set.

Answer 4

A warning about the grey card. An 18% gray card may not fall dead center of your camera's meter, and it will be highly dependent on the angle of the card and how it reflects light at your meter. It's not a bad start, if you want to get an initial exposure, but do check your histogram. If you don't have a grey card you can meter off green grass, blue sky or your subject's skin, which will be close enough to neutral for a starting point.

The way I usually shoot in manual mode is to start in Aperture priority, set my aperture, and use spot or center weighted metering on my subject and note the shutter speed. Then set the exposure mode to manual and check that my aperture and shutter speed are set to those values. Shoot a test shot and check the histogram. I find that way easier than starting in manual mode and having to figure out which way to turn shutter speed and aperture dials to move the meter the right direction! You could also start in shutter priority mode if shutter speed is the more important setting for your shot.

There is a rule of thumb when metering a black cat, or white snow, and that is to adjust the exposure by about 2 stops.