Aperture, Shutter and ISO
Written by Rob Sloane
This is an edited version of articles that appeared in the club newsletter in May-June 2019.
Members may have problems focusing in dark conditions.
Here are a few tips and explanations.
Most cameras have both auto and manual focus. The switch to change from auto to manual can be on the body, the lens or both.
Auto focus is a function of both the camera and the lens together.
Auto focus relies on the camera sending out a beam of some sort to the subject, bounce back, be measured and adjust the lens.
Auto focus doesn’t always work, for example, if the subject is moving, (especially fast), or the subject is too small, or the subject has no or little detail, or the subject is at a distance and very dark.
Switching to manual focus means you must turn a ring on your lens whilst looking through the viewfinder and adjusting till your eye sees a sharp subject, but you must have enough light on the subject to see it in the first place.
As long as the camera and the subject stay pretty much in the same position, your image should be sharp.
A solution is to temporarily light the subject (with a torch) and focus on it.
You could focus on it, either with auto focus or manual focus, but once focused, switch to manual. This stops the camera from trying to refocus if the subject moves around a little.
Now you are good to go with your exposure.
Note: Most cameras have a number of focusing modes under auto focus as well as being able to change the focus points, accessed via the camera controls or menu.
Above, we had a look at focusing and how to change from Auto focus to manual focus.
Sharp focus means the edges and detail of the image is crisp and “sharp”.
You may sometimes hear a judge comment that the image is a little soft, what she/he means, is the image is not really sharp.
Not all images need to be, or should be sharp. Some are fine to be soft all over, some, as long as the important elements like a persons eyes are sharp, is fine, in the words of the Peter Knight in his Beginners Class, “it all depends”.
Focus controls how sharp your image is, at the spot that you focus on.
Sharp and sharp focus could be slightly different. An image could be in focus, but blurry.
If your subject is not sharp, then there may be more than one reason. You (or your camera) didn’t focus correctly or focused on the wrong thing.
Technically only one distance from the camera can be perfectly in focus, the rest is a compromise and depends a bit on the aperture (Information about Aperture below).
Reasons for blurriness include :
You moved the camera while the shutter was open.
The subject moved while the shutter was open.
You meant it that way.
In the cases where the subject is not sharp because either you or the subject moved while the shutter was open, the result is not really a focus issue, but a “blur” or “motion blur” effect.
Sometimes you may want blur, sometimes you may not:- “It depends”.
This brings us to the shutter.
Lets not worry about how a shutter works, it’s a marvelous piece of technology, it is a relative fragile part of your camera, if it stuffs up it is probably cheaper to buy a new camera.
The shutter won’t effect focus but it will effect any blur, however a MAJOR effect of the shutter is exposure, (how light or dark your image is).
Most (or many) cameras have a shutter speed range from 30 seconds to one 8000th of a second plus a thing called bulb (which some of you may have used for long exposures).
If you look in your view finder (or on the info screen) at the shutter detail, if it shows 4” , that’s 4 with a set of brackets it probably means 4 whole seconds.
If it shows 0”4 that probably means .4 of a second, and if it shows just 4 (no brackets), it probably means ¼ of a second, if it shows 400 it probably means 1/400th of a second, and if it shows 4000, it probably means 1/4000th of a second. (pretty damn fast).
Check that little book or download that came with your camera i.e read the manual!
We will come back to that bulb thingy later.
Remember the shutter has a big effect on your exposure. (along with aperture and ISO and the amount of light you have available).
Let’s pretend for a moment that your camera is on Auto, and you cannot change the aperture (f5.6) or the ISO (400).
If the subject is pretty light, that will force you to use a fast shutter (to get the right exposure),
If the subject is pretty dark, that will force you to use a slow shutter (to get the right exposure).
If the subject is moving fairly fast and you don’t want blur, you will need a fast shutter.
If you are hand holding the camera and have a dose of the shakes, you will need a fast shutter.
So — what can you do if you have low light, a moving subject and you are shaking.
Eg. It is early morning, still a bit dark, you are in a moving vehicle, your subject is lion chasing a gazelle.
Ditch the Auto and go to Manual or Shutter priority.
Choose a suitable shutter speed – the higher the better, a couple of factors can effect this, how fast is the subject moving and what lens you are using, and how bumpy is your vehicle.
Image stabilization will help, the longer the lens, the higher the shutter speed the better. Rule of thumb, if you have a 600mm lens you need at least 1/600th of a second, maybe 1/400th of a second with good image stabilization.
The IS switch is often on your lens, but maybe on your camera.
Choose a suitable aperture, aperture will effect how much is in focus, (Depth of Field, DoF).
If you choose a low number like f2.8 (if your lens goes that low) the lion may be in focus but the gazelle may not be, so maybe its better to choose something like f8.
Finally we still have to get the exposure right, choose a suitable ISO. High ISO will allow you to have a fast enough shutter, and an aperture around f8 to f11 but you may end up with a pretty “grainy” or “noisy” image.
The best solution; stay in bed until the light gets better, and get the lion and gazelle to do their thing, say about 10 am when the light is better.
Nets solution: Get out of the vehicle, (you may end up as lunch), put your camera on a tripod and get the lion and gazelle to finish in a designated spot where you have perfect focus.
Also buy a camera that goes to ISO 500,000 with absolutely no noise/grain (Canon may make one one day, but probably an impossibility for Fuji, Nikon etc)**
None of this is the shutters fault, it had plenty of range left, it is just the aperture and ISO that’s the problem.
So with the above situation, I would go for:- make sure the IS (image stabilization is on), try and hold the camera as steady as possible, maybe use a bean bag, 1/400th of a second, f8 aperture and you may get away with ISO 2000.
For focus, maybe use the predictive focus option if you have confidence in it.
Also set your camera to take a series of shots when you push the shutter button (drive). Cameras today can take 5, 8, and some up to 50+ frames (shutter operations) in 1 second. There are still no guarantees you are going to get the shot.
Lets finish with that other shutter option Bulb, an old photography term.
Mostly the slowest shutter speed our cameras can select is around 30 secs. However sometimes you need longer, like for star trails, painting with light etc.
Bulb is the solution. Set to bulb, your camera will open the shutter when you press the button, and close the shutter when you let the button go, (or when the battery goes flat, whichever comes first).
Always try to keep in mind:-
ISO effects exposure and noise/grain.
Aperture effects exposure and depth of field, (how much of the image looks sharp)
Shutter effects exposure and motion blur, (blurriness caused by camera shake or the subject moving).
Choosing the shutter speed gives you the opportunity to be creative, maybe having your subject sharp or blurred or streaky, as well as how bright or dark your image is.
In some cameras the shutter (shutter priority) is designated by S, in others by Tv (time value) but it basically means the same, you set the shutter speed and the camera tries to choose a suitable aperture.
You get to choose the ISO unless you have ISO set to Auto.
In manual you get to choose everything.
In Full Auto you get to choose nothing.
Above, we have discussed both focus and shutter effects.
Occasionally we need to switch our camera from auto focus to manual focus.
It is important you know how to do this. If you don’t – read the manual.
We sometimes need to set the shutter speed ourselves, (not let the camera do it automatically).
If you have the camera in auto mode including program mode it will adjust the shutter to what it thinks is best.
In some circumstances, this is not what you want. If in Aperture Priority the camera will choose the shutter speed that may be an issue for example when using “studio flash”.
If it is important for you to set the shutter speed, then you need Shutter Priority or Manual mode. You will probably need to set the shutter speed when “painting with light”, “studio flash”, “creating blur eg. waterfalls”, “freezing motion”, “star trails”, “light trails”, “avoiding shake”, etc.
Shutter speed has a big effect on Exposure, so does ISO, we will come back to that.
Aperture is the next major contributor to Exposure.
Aperture is the size of the hole in the lens that lets the light in, it can be varied (in most lenses), and is measured in f values. Eg, f2.8, f5.4, f8, f22 etc.
They are odd ball numbers because they are a function of the area of a circle.
Things to remember with aperture.
Small numbers like f2.8, f4 = big hole, lots of light, shallow depth of field. (eg. nose may be sharp, but ears could be out of focus).
Larger numbers like f11, f22 = little hole, not much light, large depth of field, (nose, ears and everything in the background will also be pretty sharp.
Most lenses that we buy go from about f4 to f22 or thereabouts.
You can buy lens that go to f1, f1.4, f2 (they are called fast lenses), but are often expensive.
And lastly for getting our “exposure” right, ISO.
Most cameras you can choose ISO from about 200 to about 1600.
Cameras may start at ISO 50, some cameras may go to ISO 20,000, 50,000 and beyond.
You can usually choose what ISO you want, or on many cameras you can choose A for auto ISO.
The higher the ISO, you (or the camera chooses), the more likely you will have “noise” in your image. That is, your image has all these tiny blobs of colours, which is not good.
If motion (by you or the subject) is important, eg, you are using a long lens, you are say on a rocking boat, the subject is moving wildlife, or a performing athlete, or a moving car, or motorcycle then try and choose a FAST SHUTTER SPEED.
If you want things to blur like a waterfall, or star trails choose a SLOW SHUTTER SPEED.
If depth of field is important, then if you want shallow DoF with background blurred then choose small f number.
If you want lots in focus like for a “Landscape” try to choose a large f value.
Unless you deliberately want a “noisy” pic try to choose the lowest ISO you can get away with.
Unfortunately: all these things, shutter speed, aperture, and ISO all work in together to control the exposure.
If conditions are dark then it will force you to compromise with “slower shutter”, “bigger hole aperture” and “higher ISO”.
If you let the camera do it all, you may be lucky, you may not.
If you are using studio flash you must set the shutter speed within the sync range of your camera, say 1/60th Sec – 1/200th Sec, then depending on the ISO chosen, you must set the f value to correctly expose the way you want or adjust the light output of your lights.
Know how to change the auto focus modes on your camera, and how to change to manual focus.
Know how to change your camera to different exposure modes eg. Shutter Priority (S, Tv), Aperture Priority (A, Av) and how to switch to MANUAL mode.
Know how to change ISO, in any mode, how to change the shutter in shutter priority, how to change the aperture in aperture priority, and MOST of all, how to change ISO, Shutter, and Aperture in MANUAL mode.
You may need to read the manual (damn, damn, damn).
** Yeah, right (Editor).