A really good photograph is one which makes another photographer think “I wish I’d taken that”; a great photograph is one that makes him think that, then buy a print from you to put on his wall.
A good photograph has three elements that harmonise and thus set it apart from the rest of the field. We’ll look at the first element today, Exposure, which is difficult to get right.
There are three elements to be considered when calculating exposure:
Aperture (the tap)
Shutter speed (the sluice-gate)
ISO (the hifi volume control)
I mentioned in a previous post that aperture & shutter speed do “other things” as well as control the volume and duration of the light reaching the sensor, and now is the time for you to become aware of those functions.
Aperture also controls the “depth of field” (DoF) which is the distance range over which objects in the photograph will appear to be “acceptably sharp”. We’ll cover this in more detail in the post on the hyperfocal distance.
A small aperture (large f/No.) such as f/8, f/11, f/22 give a larger depth of field and, conversely, a large aperture (small f/No) f/4, f/2.8, f/1.4 give a smaller depth of field.
It gets more complicated by the fact that the depth of field varies with both the focal length of the lens and the camera to subject distance.
In the days of film, a photographer who needed info on the DoF had to have access to DoF tables – nowadays you can download a free app for your smartphone (mine also calculates the hyperfocal distance).
I spent many years ignoring the hyperfocal distance (as most photographers do) but once you grasp the concept, you can take certain types of photographs all day, without ever focussing your camera!
Shutter speed A slow shutter speed 1/25, 1/8 can result in camera shake or visible subject movement, both of which are a bad thing. A faster shutter speed will freeze movement but at the expense of a higher ISO or wider aperture,
ISO Low ISO (100, 200) gives a cleaner image than a high ISO. A lot of camera sensors will give “noise” in the image (visual equivalent to white noise, or distorted sound if you rack the hifi to 11) from ISO 800. Visible colour noise in an image is also a bad thing. Camera manufacturers often boast of their camera’s high ISO of perhaps 12800, 25600 or higher. Good luck in using it. It is possible, but involves a good bit of post-processing and you really do have to avoid any hint of underexposure. Noise is more noticeable in the shadows so, if you have to add a bit of exposure to the shadows then you will amplify the noise.
If I correctly expose an image (no underexposing) I can use ISO 6400 on my Canon 6D without a problem. Anything much higher than that is reserved for the day that Martians land in my garden at dusk.
I know that I promised you "no charts, tables etc" but this one isn't too complicated and it does have an important messge.
The really important point in this table is that:
For the same scene, ALL of the above combinations of ISO, aperture, and shutter speed give the same exposure.
ISO200 – f/8 – 1/60 puts exactly the same amount of light in the bucket as does
ISO400 – f/11 – 1/60 which is exactly the same as ISO1600 – f/16 – 1/125.
Go and try it out! Set your camera to “M” for manual, choose a nice well-lit subject that isn’t going to move about, and take a few shots at various combinations.
So why chose one setting over another?
We’ve already mentioned that a low ISO gives better image quality and a High ISO allows picture taking in lower light at the expense of colour noise. The considerations for the other two elements are:
A big lens aperture, f/1.4, f/2, f/2.8, means a more expensive & heavier lens; it also means a very shallow DEPTH of FIELD – ie the parts of the photograph that appear acceptably sharp fall in a very narrow band. At f/1.4 focussing on an object 50cm away the depth of field might only be 1cm. Anything closer than 49cm or further than 51cm will not be acceptably sharp.
This can be a good thing – to blur backgrounds & foregrounds – and thus put emphasis on a certain point but you have to be aware of it.
A smaller maximum aperture, f/4, or f/5.6 will generally mean a lighter & cheaper lens but one that will let in less light. This in turn means a Higher ISO (colour noise?) or a slower shutter speed.
A slower shutter speed, say 1/60 at f/4 instead of 1/250 at f/2 might mean camera shake (never acceptable) or some blurring due to subject movement. This can be acceptable, depending on what you want to capture.
Lenses come in a variety of FOCAL LENGTHS from perhaps 17mm (or smaller) which is a WIDE-ANGLE lens to 200mm (or larger) which is a TELEPHOTO lens. The longer the focal length, the harder it is to avoid camera shake (never ever acceptable). As a general rule of thumb, most people can avoid camera shake by using a shutter speed equal to the focal length of the lens ie 1/50 for a 50mm lens, 1/200 for a 200mm lens.
Image stabilisers/optical stabilisers (IS/OS) are a great help but add to the cost & weight of the lens. With a good IS system you might be able to hand-hold a 200mm lens at 1/50.
So you set the ISO, choose a suitable shutter speed/aperture and let the camera’s metering system choose the matching aperture/shutter speed. Er, no. It’s not quite that simple.Yo ur camera has more computing power than was available to the first manned space flights but it has no idea of what you are taking a picture.
Ok, Martians haven't landed but a new Canon 7d Mk2 has and. in the process of putting it & me to the test I tried a couple of hand-held shots at ISO 16,000 and one of them is reproduced below. It's OK-ish if a little plasticy.
Tune in in another couple of weeks to learn about the other two legs that support a great photograph plus: