1 or 2 stops of extra exposure.

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jameslongstrong

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When shooting in snow I understand you need to shoot 1 or 2 stops extra exposure. I understand this is so the snow shows as white instead of a dull grey.

I always presumed that this meant shooting the picture over exposed although on closer examination of my camera I get the impression this is not the case.

Can someone explain how the extra stop or 2 of exposure works and why? Also when would you use more or less of it?

Cheers.
 
Way over simplified - what exposure you need depends on light available and how you're metering the scene. If you set the camera to average metering then it may well get thinsg wrong, but you'll get a ballpark figure tostart from, and can apply compensation eitehr way til you get what you want, as you get to know your camera you get to know how it evaluates certain conditions and what sort of compensation if any you need to apply.
 
Camera meters look at a scene and depending on your setting which part, evalauative... whole scene, partila or centre weighted... emphasis on the middle of the frame, spot, just a small part in the centre of the frame. Having chosen which part it uses algorithms to calculate so that it can get to 18% grey, so for example using spot and pointing at a black car will give a different reading to a white car, despite the fact that the actual light level is the same.

So cameras are pretty stupid really, you need to interpret what the meter is telling the camera, so if it sees a lot of snow it thinks, hey it's bright here, so will underexpose, so hence the grey snow. That said they do a pretty good job most of the time, you just need to be aware of how the meter is thinking. For really accurate metering take a reading from a grey card.
 
The cameras meter will set the exposure of a mostly white scene so that the white will come out grey - to combat this you have to add an extra stop or two so that white appears white. The reason that the cameras meter does this is because its set to expose for 'typical' scenes (ie. not all white or all black). Scenes which are either mostly white or mostly black will require some exposure compensation from you to make them appear correctly.
 
This is making more sense now. I think I was confusing myself a little.

I'll let you in to a secret too. I was actually looking at the flash exposure compensation symbol. This is probably why I've got it all wrapped around my head:bang:
 
Simplified & non-technical explanation, this is slightly different to how most people explain it so it may help or it may make it worse :)

The first thing to realise is that the light falling on a scene determines what the exposure should be, the light reflected from a scene varies according to what is doing the reflecting. As camera meters measure reflected light this can cause problems. If you were to measure the light falling on the scene, using an incident light reading, you would not have to worry about adjusting exposure for different scenes which reflect different amounts.

Your camera is taught that all scenes reflect 18% of the light falling on them (Digital cameras may be taught that it is 14% or 12% depending on the camera but I will continue with 18%)

First consider taking a picture of a landscape which, on average, is reflecting 18% of the light falling on it.
When your camera is pointed at the landscape it thinks ' That scene must be reflecting 18% of the light falling on it so I need to set an exposure to suit that light. '
If the light gets brighter the camera thinks ' That scene is still reflecting 18% but it is brighter now so the sun must have come out so I need to reduce the exposure to suit the new brighter light '
If the light gets dimmer the camera thinks ' That scene is still reflecting 18% but it is darker now so the sun must be behind a cloud so I need to increase the exposure to suit the new dimmer light'

So far so good but what about scenes which reflect more than 18% such as snow.
When your camera is pointed at the snow scene it thinks ' That scene must be reflecting 18% of the light falling on it but it is very bright so it must be very sunny so I need to reduce the exposure to suit the very bright light '
This means that, although the camera meter shows the exposure as correct, it has actually reduced the exposure and will cause the white snow to be grey.

A similar thing occurs when you photograph a very dark scene, a black horse for example.
Your camera thinks ' That scene must be reflecting 18% of the light falling on it but it is very dark so it must be very overcast so I need to increase the exposure to suit the very dim light'
This means that, although the camera meter shows the exposure as correct, it has actually increased the exposure and will cause the black horse to be grey.

The histogram will show you what you have recorded and you can can use exposure compensation to get the correct exposure for that particular shot.
 
Cheers Suvv, Now thats the kind of explination I like. v e r y s l o w.

Ha! I'm goin got have a play with different metering modes. I'm not too sure what I am using currently as to be honest, I didn't understand how what would be best in what situation.

This may be a daft question but is there much difference in using different metering modes?
 
jameslongstrong said:
Cheers Suvv, Now thats the kind of explination I like. v e r y s l o w.

Ha! I'm goin got have a play with different metering modes. I'm not too sure what I am using currently as to be honest, I didn't understand how what would be best in what situation.

This may be a daft question but is there much difference in using different metering modes?
Click to expand...

There are normally 3 metering modes:

1. Evaluative (Canon) or Matrix (Nikon), these are essentially the same and take readings from the whole of the scene, they also take into account where the camera was focused and compare the readings obtained with information in the camera memory to adjust the suggested exposure if required. For example if you were shooting a snow scene then the situation I explained earlier would apply but the camera would also compare the readings with what it has stored and say ' ah that looks like a snow scene, I will increase the exposure to compensate' As you can imagine this may upset the idea of simply adding 2 stops to every snow scene because the camera may have already tried to compensate.

2. Centre Weighted uses readings from the whole of the scene but gives more attention to an area in the centre of the scene. Cameras vary but it may be in the ratio of 60/40 or 70/30 with the centre area getting the higher weighting. IMO not a very useful method but others may disagree.

3. Spot metering takes the reading from a small area in the centre of the viewfinder, some camera may use an area around the focus point? The idea is that you meter the area that you want to be correctly exposed without worrying about the rest of the scene. Bear in mind however that exposure compensation may be required if the spot metered area is lighter or darker than 18% grey, as mentioned in the earlier posts.
For example if you are taking a portrait you can spot meter off the persons face, if they are Asian then the metered value will be OK, if they are White you need to increase the exposure by 1 stop, if they are Black you need to decrease the exposure by 1 stop. (Purists may say 1 and a third stops but I learnt before you could dial in fractioons of a stop ;) )

One good tip following on from this is to spot meter off the palm of your hand and then compensate depending on your skin colour. As your palm does not tan and you normally have it with you it is useful in all situations.
 
jameslongstrong said:
Cheers Suvv, Now thats the kind of explination I like. v e r y s l o w.

Ha! I'm goin got have a play with different metering modes. I'm not too sure what I am using currently as to be honest, I didn't understand how what would be best in what situation.

This may be a daft question but is there much difference in using different metering modes?
Click to expand...

The simple answer to your opening question is that you are not over exposing the image, you are giving more exposure than the meter 'thinks' you need. Because it's got it wrong, for the reasons explained above.

The various metering modes are basically similar in principle, but for point and shoot ease of use, evaluative will get you closest, most often. But all meters are just guessing at what the exposure might be, based on a set of assumptions that may or may not apply.

The real answer to correct exposure is shown on the LCD, because that is an actual final photo. But since the LCD can be hard to assess, particularly outside in bright light, you should get to know what the histogram is telling you, and also the over exposure warning known as 'blinkies' (in the menu).

Using these very accurate aids is the best way of getting perfect exposure every time.
 
Also, nothing wrong with trying Manual mode in a snowy scene. Once you get the exposure right (histogram & blinkies yes yes), just shoot and don't worry what is in the frame.

I was afraid of manual mode for far too long. What really pushed me over the edge was photographing a stage show outside where the stage was almost white, and all the performers were wearing black tuxedos, running about gathering together and splitting off. Automatic metering was well.. impossible. So I bumped the camera to manual, shot some frames to get the exposure right and then shot about 200 almost perfectly exposed shots :D

Never looked back. Of course if the light changes, you will have to adjust.
 
I remember when I started out in photography and was taught about the difference between reflected and incidence metering methods which relates to this question. The exercise we used was to have a black card, a grey card and a white card and to take a reflected reading from each one, and of course we had three different readings. This was in the days of film, and after developing the film, although we had used three different exposures, we had three negatives all the same , ie grey ones. Then we took an incident light reading from the same squares, and of course the same amount of light was falling on all three so we had just one reading. The resulting negs were obviously clear, grey and black proving the theory.
I should think this experiment would be even easier with digital, the only problem would be being able to take the incident reading without having a separate light meter with an incident attachment, but I feel sure there must be a way around that, perhaps facing the camera towards the light source and covering the lens with a translucent cover?
 
When faced with an exposure dilemma I frame an average scene using evaluative (matrix) and look at what the meter says, then set manually to that as a starting point.
 
Have you tried the AEB automatic exposure bracketing feature?
"An automatic exposure bracketing feature lets you set the total exposure variation (across three shots) at anywhere from +/- one-third or one-half EV, all the way up to +/- 2 EV. The nice part is that the automatic variation is centered around whatever level of manual exposure compensation you have dialed in. Thus, you could manually set a positive exposure compensation of 0.7 EV, and then have the camera give you a variation of +/- 2/3 EV around that point."
Click to expand...

I've always tried to explain it as the camera metering always 'looks' for a mid-tone gray in order to accurately expose a scene. In something extra bright, like snow scenes, it can't 'find' a mid-tone gray so seeks to make what it perceives as too bright into a mid-tone gray, hence the gray looking snow you often see. Same applies in dark scenes, like indoor (poorly lit church, for example) and it tries to make the darker scene lighter to mid-tone gray, so often makes the shutter speed too slow so you get a much brighter image but with blur!

Alternatively, with a pocket compact, you can use the beach/snow setting to get much the same result! Not available on a 50D, though.
 
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