Getting off Auto 2 - Exposure

Last time I let my inner geek get carried away it was about the DSLR camera modes.  It struck me, after another hide based camera chat (well, you have to do something whilst waiting for something to turn up!) that perhaps I should talk a bit more about exposure.  Same caveats as before about this being in Canon DSLR-speak, but it applies to all.

I’m sure everyone will agree that getting the correct exposure is always desirable, but why?  You can open up a picture in Photoshop and adjust the exposure, so why worry too much?  There are a number of reasons, #1 of which is probably laziness.  Why make life hard for yourself when you don’t need to?  More to the point, there are limits to how much you can tweak a picture and there are often penalties in doing that as well.  If a picture is too dark (under-exposed), you can lift it a bit, but if it is TOO under-exposed, there’ll be no detail to lift, and it will be useless.  You’ll also find you end up with the parts of the picture that were in shadow can be rather noisy & unpleasant when you brighten them.

Similarly, if you over-expose too much, you’ll end up blowing the highlights (in other words the bright white bits).  In this situation, you’ve burned the detail out of the image and won’t be able to recover it.  It can be useful to over-expose slightly – you’ll hear it referred to as “expose to the right”, but don’t get too carried away.  Why expose to the right?  If you lift an under-exposed shot, you bring up noise in the shadows.  If you drop an over-exposed shot, you lessen any noise and that’s always desirable. 

Left, Right, eh what? 

That’s all talking about the histogram.  That’s the little graph you see on the back of the camera, or in software on the PC, like these three screengrabs.  One under, one over, one correctly exposed. 


See how the histogram shows more bulk at the lower/left hand end on the under-exposed shot, more up the right with the over-exposed one, nicely spread when it's correctly exposed?  If you get the histogram rammed up the right hand end, like this one, you know you’ve got blown highlights and there’s not much you can do….


Here’s a top tip though.  Look through your camera menu for something called “Highlight Alert” or similar.  Enable that (if you have it) and when you look at the image on the camera’s rear screen, blown highlights will flash.  It’s a handy quick check.  Don’t worry about the odd glint on a chrome bumper, or on a bit of the image you know you’ll crop anyway, but if you have large blinking blown highlights, you might want to fiddle with the exposure compensation a bit.

Why does the camera get exposure wrong in the first place?  Cameras work on the assumption that every picture is just a little bit average.  18% grey most of the time (think grey as in brightness on a black & white image, not grey on a colour picture).  If your subject is far off that, the camera has trouble working out what it should do, but you can help by choosing the exposure metering mode wisely.  There are often a number of these, but they typically include something quite simple (average across the whole picture), an evaluative one that takes note of the whole frame but places greater emphasis on the middle bit and spot metering, which only worries about the brightness of the centre spot.  Whilst spot metering is often recommended for bird photography, I find it only really works well when the bird fills the frame nicely and is NOT white, black or a mixture (swans, blackbirds, magpies as examples).  If you spot meter on white, you tend to get under-exposed shots.  On black it tends to over-expose.  On a Magpie, exposure ends up all over the place as you move between black and white feathers (incidentally, it’s why good wedding photography is hard – all those white dresses and dark suits in the same pictures).  What you may find is the centre weighted evaluative metering mode is a better general purpose mode, then learn to recognise situations where a stop of over or under exposure compensation will help.  Dark bird against light sky?  Bird on reflective water?  Over-expose by a stop.  You may blow some highlights, but the bird will be closer to correct exposure and that’s (probably) what you’re after.  Otherwise you’ll get a lovely silhouette against a perfect sky, but no detail.  Too many blinking highlights on your rear display?  Try a stop of under-exposure and see if that’s better.

I just used another techie term.  Stops.  Hmm, ok, let me take a breather and expand on that in a bit….

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  • So what is all this “Stop” stuff about?  Before we get to that, we need to talk about the “Exposure Triangle”.

    To get a picture properly exposed, you need to get three things lined up.  Shutter speed, aperture and ISO are the three legs of the triangle.  Shutter speed is the amount of time the shutter is open and allowing light to reach the sensor, aperture is the size of the hole the light comes through and ISO is the sensitivity of the sensor to light.  Back in film days we used film of 100ASA, 200ASA etc, now we have ISO100, ISO200 and the like.  Same basic principle.  Higher the ISO, less light you need to get a picture, so ISO100 would be used in sunshine, ISO3200 is for indoors or lousy weather.  The problem with high ISOs are noise, reduced dynamic range and colours – but the benefits of being able to take pictures in low light are huge.

    Shutter speeds are measured in seconds – or more likely fractions of a second.  A shutter speed range between 30sec and 1/8000sec is not uncommon, but you’ll normally use something in the middle.  You’ll need a tripod and very low light for 30secs – and it wouldn’t be much good for moving targets.  In general bird photography, you’re going to be looking for something pretty fast – maybe 1/1000sec or more for birds in flight (see part 1 of this series for the rule of thumb), less if the bird is stationary or you have a shorter focal length.

    Aperture is a bit more complex.  It’s actually a ratio of the focal length and the lens at the front the light comes through, which is why it’s written in the form of f/4 or f/5.6, but for a given lens, keep it simple and just work on the basis of it being the size of the hole in front of the sensor.  Just like a fraction, the smaller the number underneath, the larger the number/hole – in other words, f/4 is a bigger hole than f/8.

    If you keep one leg of our exposure triangle stable, you continue to get a properly exposed picture by moving the other two settings in opposite directions.  So, if you fix ISO and get a picture with f/4 and 1/1000sec, you’ll also get a proper exposure with f/5.6 and 1/500sec.  The latter has a smaller hole, so less light gets through, which means you need to leave the shutter open for longer to get the same amount of light to the sensor overall.  Think of it like filling a bucket of water.  You can have slow running tap (small hole) for a long while or a fast running tap (big hole) for a short time. 

    So what is a stop?

    Every time you double or halve one of the settings, it’s known as a stop.  So ISO 100 to 200 is a stop, 1/500 to 1/1000 is a stop, f/4 to f/5.6 is a stop.  Er, ok, that last is a bit confusing.  Whole aperture stops are f/2, f/2.8, f/4, f/5.6, f/8 and so on.  (Geek alert!  Trick to remember these, is to think about how you work out the area of a circle (the amount of light you get through an aperture, is, after all, proportional to the area of the circle).  That’s πr2 in case you’ve forgotten :-)  so, 22 is 4, 2.82 is 8, 42 is 16, 5.62 is 32… well, you get the point I’m sure – 1 stop = double the area for light to get through).

    Despite all the above, talking in stops does make it a bit easier when messing around with settings.  We can talk about over-exposing by one stop and people know they can achieve that by doubling ISO (makes the sensor twice as sensitive), halving the shutter speed (light reaching the sensor for twice the time) or opening up the lens a little wider to get double the light through.

    Other uses of the term?  Taking a 300mm f/4 lens and putting a 1.4x converter on it gives an effective 420mm f/5.6 lens – the converter gives you more focal length at the expense of losing one stop.  A 2x converter loses 2 stops – makes it a 600mm f/8 lens.

    Hope that makes sense :-)

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    Find me on Flickr / All about your camera - The Getting off Auto Index

  • In reply to Whistling Joe:

    One further thought on the subject of stops.  A stop is quite a large change, so typically you have the ability to alter settings by 1/2 or 1/3 of a stop.  So a lens may have settings of f/6.3 and f/7.1 between the whole values of f/5.6 and f/8.  Same with ISO and shutter speed, you'll have intermediate settings to give you more granular control.  If you're using exposure compensation to lighten or darken an image, you'll find each click of the wheel alters the setting in 1/3 stop increments (typically).  There's normally a display in the viewfinder/on the screen that shows you how much you've altered it

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    Find me on Flickr / All about your camera - The Getting off Auto Index

  • In reply to Whistling Joe:

    Lots of great info here, Joe, thanks for taking the time and trouble to lay it all out.

    Paul

    My bird photos HERE

  • In reply to Paul A:

    Really appreciate all this photography help WJ, I'm printing it all off for a good read.  I think one of my problems is using too large an aperture at times, which I do to ensure a faster shutter speed then find that not enough of the little birdie is in focus.  I only have a 300 lens ... perhaps I need the birder's minimum 400 ... lol or the Panasonic Lumix that seymouraves talks about ... but I don't understand focal lengths anyway ... (sad face).  Thanks again for taking all this time to help us.  Jill

    Jill

  • In reply to birdiebeginner:

    The focal length of a lens is the distance in mm between the sensor and the OPTICAL centre of the lens.  As a lens has a lot of, er, lenses in it, that doesn't necessarily equate to the actual distance, but tbh, the physics don't really matter too much.  A 50mm lens (on a full frame sensor) gives roughly the same field of view as the human eye - it's why 50mm is a common size and popular.  Anything longer (ie more mm) than 50mm is called telephoto (and gives a narrower field of view, like a telescope does), anything shorter is wide-angle (or UWA Ultra wide-angle when you're getting extreme).  

    In the birding world, as you say, 400mm tends to be what people aim for as a minimum as that's the sort of length you need in order to get an acceptably sized image of a small bird at the sort of distances birds normally let you get (ie not that close).

    A lens is described as a Prime lens if it is one focal length only (eg 300mm, 400mm).  A zoom lens is one that can alter its focal length (eg 100mm to 400mm).  Primes are typically sharper than zooms, but less flexible (masters of one thing vs jack of all trades).  The wider a zoom range, the harder it is to get good performance across that range, so a 100-400mm lens (a 4x zoom) isn't too bad, but a 18-200mm lens (11x zoom) will struggle to be sharp across the whole range of focal lengths.

    What's important to realise is that the focal length of a lens is a constant amount - it doesn't change depending on what camera you put it on.  So a 400mm lens is ALWAYS a 400mm lens, whether on a full frame or crop sensor body.  What alters is the effective field of view - the crop sensor, because it cannot capture the entire image circle thrown by the lens, has a narrower field of view when viewed full-screen on your PC.  This is useful for the wildlife photographer, because a bird that touches the edges of the frame on the crop sensor camera would have space all around it on the full frame one.  To get the same field of view (ie bird touching the edges) on full frame would require a longer lens.  So we end up describing the field of view difference as the crop factor - on Canon kit it's 1.6x (as mentioned before), so our 400mm lens, when used on a crop camera, gives the same field of view as a 640mm lens would on a full frame one.

    Compact & Bridge cameras use even smaller sensors to give very narrow effective fields of view but they often quote the 35mm equivalent to help people compare.  Smaller sensors mean the lens can be smaller & lighter (and cheaper) which is why they remain so portable whilst having a long reach.

    In the DSLR world however, people only really talk about the ACTUAL focal length, even when using a crop camera and that can lead to some confusion when a Bridge user compares their "600mm" picture with that from a 400mm lens on a crop DSLR and can't work out why the DSLR picture looks bigger (it's because the Bridge user is already talking about the effective "length" whilst the DSLR user is talking actual).

    I might have to do some pictures to demonstrate this better...

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    Find me on Flickr / All about your camera - The Getting off Auto Index

  • In reply to Whistling Joe:

    WJ,  Nice article.    I like the 3 legs of a triangle scenario and am glad you talk about fixing one of these.  I do see some people make the mistake of trying to adjust all three as they go along.  It makes so much sense to fix the ISO unless the light changes dramatically.

    The Cotswold Water park sightings website

    My Flicker page

  • In reply to Bob Philpott:

    The danger of fixing the ISO is that the light CAN change quite dramatically when you're strolling around a nature reserve - one minute you can be out in the mid-day sun, one minute under the shade of the trees, so most of the time I leave it on Auto-ISO.  Modern cameras are pretty good with high-ISO and the settings-choice algorithms seem better (they do try and keep ISO low).  Where I fix ISO is, as you say, where the light is far more consistent and predictable or I'm in full control of the situation.  If I'm shooting people at a party with flash for example, or taking my time with a landscape picture.  Wildlife is somewhat unpredictable, so I try and keep the camera settings as simple as possible :-)

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    Find me on Flickr / All about your camera - The Getting off Auto Index

  • In reply to Whistling Joe:

    Many thanks for giving us so much valuable information.  My problem is that I now have the camera set to auto-iso, with the dial on the other side set to TV.  This has improved my in-flight shots - but everything is still coming out much too light.  I took my camera to Felixstowe Ferry on Sunday - it was a very bright, sunny morning - I didn't have a single good snap among the nearly 150 snaps I took.  I would try to manually adjust the F setting but with the camera set to TV I can't do that.  Any suggestions would be gratefully received!!

    Our herring gulls are red listed birds.  Think about that the next time you hear some flaming idiot calling for a cull of them.

  • In reply to Clare:

    this is a great thread WJ, most helpful and easier to understand than the manuals  lol    thanks :)

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    Regards, Hazel 

    "Each kindness shown to birds or men is sure to flutter back again" 

  • In reply to Clare:

    Clare,  The problem you have unwittingly created is that you are only controlling the speed whilst the ISO and aperture are both left on auto.  This means that with a shot with a darker background the camera will try and correct this apparent problem and lighten the image by over exposing.   If this is a recurrent problem then you can override this with the +/- button and force the camera to underexpose.  I tend to make a lot of use of that mode sometimes even going 2 stops under.  I tend to fix the ISO (which you can change on an occasional basis during the day) and use AV fixing the aperture as well (changing the +/- as needed).  Although I do accept the reasons for leaving ISO on Auto, my camera is too old to be allowed high ISO levels.

    The Cotswold Water park sightings website

    My Flicker page