Light meters?

[Adey Baker]

Originally Posted by Christine

I was only thinking re light meters early on this week,and Adey,I think it was either yourself or Tracker who gave me some advice a couple of years ago,on BF.So does the light meter,tell you exactly which ISO,Aperture and shutter speeds one needs to set on the camera?.If taking a photo of ,say,a Swan,would one point the meter directly at the Swan,how about sunsets etc,I saw a used Grossen advertised in AP this week,and did think re buying.I am totally clueless re speeds etc,but if I have a machine which tells me which settings to put on the cam it would makes things easier,and in time one would know automatically.Would be useful ,I guess taking birds in water in the sunlight,or would it?.Before anyone mentions histograms,no,they mean absolutely nothing except lots of lines and graphs.

Most cameras nowadays will give you a decent exposure in a wide range of situations but the relationship between the different shutter speeds and aperture combinations which will still give a correct exposure is not immediately obvious without scrolling through all the options (and then forgetting where you started!).

This is where a seperate meter of the type illustrated by Don a few posts ago comes in useful. Make sure you get one of a similar design to this rather than a fancy digital model

You have to select the ISO setting, Christine and then take a reading of the light which will give you a numerical value for the light level - usually an 'exposure value' - 'EV.' Then you 'dial-in' the EV number and now you'll have the range of shutter speeds and apertures which will give you a 'correct' exposure - they're on the top half of the dials in Don's shot.

If you point at a swan you'll get an incorrect exposure if you don't adjust the recommended setting because the light meter will be callibrated to give a 'correct' exposure when pointed at a mid-tone.

You can use a meter to check a mid-tone - a special 'grey-card' if you've got one - but grass, brick walls, some road surfaces (those that have been surfaced with chippings rather than tarmac) etc., to get a reading for the prevailing light conditions and then make a note of the EV number - if it is, say, EV12 then by checking your white swan or black crow or whatever in the same light you can see how far away they are from EV12 and then you'll know by how much to adjust the exposure compensation if the subject is filling the frame in your shot

Sounds complicated, I know, but basically the ready reckoner is 'over-compensate a light subject, under-compensate a dark subject' by however much your meter (and the experience you gain!) suggests. For instance, with the above swan, if it gives a reading of EV14 then you know that it is 2 'stops' away from the 'correct' EV12 so you'll need to 'over-expose' 2 stops to bring it back into line. Of course, you'll have to experiment with your camera to see if its meter system is using 'intelligence' to analize your composition and make its own adjustments to compensate - if the swan is against a dark background it may work it out for itself but a white subject against a light background doesn't give it anything to compare with and can lead to under-exposure as it tries to bring all those light tones down to a 'mid-grey.'

Getting a light meter in your hand and pointing it at various subjects in different light will teach you far more easily than I can - oh, and when you've got a really good 'mid-tone' subject, compare the reading from it with a reading from the back of your hand (in the same light direction) and note the difference - if its, say, only half a stop away then you've always got a 'grey-card' on your person that only needs adjusting by half a stop for a 'correct' reading! If you've got 'Mediterannean,' 'Asian' or well-tanned skin colour then it'll probably be spot-on!

[Christine]

Gosg,Adey,thanks,there is a lot of info to digest in your post.It seems more complicated than I thought.I'll have to have a think about this one!!.It is really the white birds in the water who cause most of the exposure problems,esp when it is sunny.It seems to be horses for courses.Make sure one has a bright sunny day,lots of light for digi photography,then the sun light causes havoc with the exposure!!! Very tricky.

[Adey Baker]

Originally Posted by Christine

Gosg,Adey,thanks,there is a lot of info to digest in your post.It seems more complicated than I thought.I'll have to have a think about this one!!.It is really the white birds in the water who cause most of the exposure problems,esp when it is sunny.It seems to be horses for courses.Make sure one has a bright sunny day,lots of light for digi photography,then the sun light causes havoc with the exposure!!! Very tricky.

Unfortunately, this is where your histograms might come in handy! But, yes, white birds on sunny days can be a problem

[John]

Christine,
If the white swan is surrounded by black water (no reflections) this is where the incident light method comes in. You point the meter away from the swan towards the camera. You don't need to leave the camera position, stay where you are turn your back on the subject and take a reading. What you are doing is measuring the light falling on the subject. No matter what the subject brightness distribution may be you will get a correct reading requiring no compensation. To be really precise, you may need to compensate just a little (1/2 a stop) for really bright or really dark subjects. Many consider this to be the most accurate method. Make sure that any meter you buy can use incident light.

[Don Hoey]

Following on from John's comments on incident light metering I thought I would add a comment and explanitory pic or two.

As John describes taking an incident light reading is dead easy. You are measuring the light falling on the subject so the meter needs to be pointed from the subject towards the camera. If you are doing it from the camera position remember that it will only be accurate if the lighting condition is similar as that at the subject position. It would not be accurate if you were in the shade of a tree and the subject in bright light or vice a versa for example. Only a reflected light reading would work here. The incident method of light reading is totally accurate where the light reading is taken from the subject position and the meter pointing towards the camera.

In response to Christine's concerns about using histograms to determine the accuracy of exposure, I will now probably commit heresy now by saying I have NEVER used that method ..... there you go, I've said it.

The light was getting a bit low when I tried the set up from which I made a composite, ( incident reading 1/15 sec @ f5.6 ). I lost the exif info when I rotated the images so unfortunately cannot give that info. The thinking was that it would show in line with Duncans comments in his exposure threads that a light meter will always give a reading to correctly expose to 18% grey. Left to their own devices they will give exposures that will turn a dominant white/light scene to grey and a dominant black/dark scene to grey. ( Under exposure and over exposure ) The images have only been cropped to form a composite and a touch of sharpness added, otherwise they are straight out of the camera.

There are 2 light meter images.

One is to show the diffuser that allows incident light reading. This has to be slid over the meter eye for such a reading. So any meter without that cannot be used to take incident light readings.

The second image shows how easy it is to see the effect of an increase in ISO on shutter speed and aperture. When looking at the meter only bother to look at the ISO setting, time and aperture. This is a pro meter, so is capable of more complex measurements to which the other numbers refer.

Before someone mentions spot metering, I am quite happy to acknowledge that as long as you understand what to meter, that is an excellent method, and is well suited to a wide range of subjects.

Don

[Adey Baker]

You could write a whole book on light metering, but those three flower shots and the meter with diffuser pointing straight at the camera sum up incident light metering at a stroke - there's an O.B.E. in the post, Don!

[Don Hoey]

Originally Posted by Adey Baker

You could write a whole book on light metering, but those three flower shots and the meter with diffuser pointing straight at the camera sum up incident light metering at a stroke - there's an O.B.E. in the post, Don!

Thanks Adey,

Shame about the lack of light. I will try again when its a bit brighter and note the exif info before I loose it. Some would be amazed at the exposure range between the two different backgrounds. Incident reading a ' snap '.

Don

[John]

I have noticed that those who have used auto exposure modes exclusively sometimes misuderstand the ISO setting. They think it alters exposure. In fact it does not, unless you are using 'manual mode' It simply alters the shutter speed/Aperture combination. The ISO number is a measure of the sensors sensitivity. Raising the ISO permits faster shutter speeds at a given aperture size (f stop eg f2.8) whilst maintaining correct exposure. Or smaller apertures at a given shutter speed. The converses are true when the ISO setting is reduced. High ISO numbers tend to increase noise, therefore, it is good practice to use the smallest ISO setting that will give a faster enough shutter speed. With the Canon 20D noise control is so good that I don't even think about it up to IS0 400 and I would not hesitate to use ISO 800 if I needed too.

[Nogbad]

Hi, sorry to appear quite dim but I am now more confussed than ever. Are you saying I need to use a light meter rather than the cameras own system?

From the photos I took on Saturday I found the camera had hiked the ISO upto 1600 because of the dim light. I would have preffered to have used a lower ISO setting and have less noise. Would it have been better to have used a light meter under these conditions?

While I found the articles by Don & Adey helpful to some extent, i still found it confussing.

Looking at the pictures of the light meters i immediately thought it all looks rather complicated!

This learning curve seems to get steeper all the time.

Nogbad

[Adey Baker]

Hi Nogbad

To a certain extent I was replying to Christine's mention of an earlier post I'd made on Birdforum. I was trying to explain how you could use a meter to 'explore' different lighting conditions as much as finding a correct exposure for a particular shot.

In the 'good old days' before many cameras had built-in meters, many photographers would routinely check their light meters against anything which might perhaps make a good shot and in so doing they gained a great deal of experience in judging a situation even before lifting the camera to the eye.

Providing you can recognise a 'difficult' situation and know how to over-ride a camera's auto settings to deal with it then there's no real reason to use a seperate meter, especially with digital where you can take a lot of shots at different settings and then judge them closely afterwards.

Light meters are not at all complicated when you get them in your hand - by first choosing your ISO, then setting the EV value from your subject you get the shutter speeds on one dial coinciding with apertures on the other. Any of the combinations will give you the correct exposure - you choose whichever shutter-speed or aperture you require and set the other accordingly. If you see, say, 1/125th sec opposite F8 then the next one along will have 1/250th sec opposite F5.6 and so on - if the precise combination you require doesn't appear then you can adjust the ISO until you get what you want (or wait for the sun to shine!)