AUGUST 8, 2014

The three legged stool of Correct Exposure...

 

Sometimes all the settings and options on your camera can be completely overwhelming. What is Shutter speed? What's an ISO? Why does changing my aperture matter?

 

Photography, on a technical level, is all about managing and manipulating how much light interacts with the sensor (or film) in your camera. The amount of light that hits your cameras sensor, in all the visible wavelengths, is what is known as the "exposure" (ie exposure of the sensor to the light).

 

Shutter speed, Aperture Value and ISO are the three main tools photographers use to ensure that they achieve the correct exposure for their photographs. Balancing these three values allow you to ensure that your photo is lit the way that you want, stops or blurs the action in the scene in the way that you want, and create the depth of field that you desire.

 

Today, we are just going to talk about how these three values interact to allow you to get a "correct" or balanced exposure. Understanding these tools will allow you to understand how you can leverage them to make really creative images, such as light trails, blurred images and tracking shots, and low DOF shots, but before we go down that road, we need to talk about how each of them work.

 

Shutter Speed

Probably the easiest to understand is Shutter Speed. Shutter speed is literally the amount of time that the cameras sensor is exposed to the light that is emitted by the scene you are photographing. SLR's control this by opening and closing a mirror in a pre-set amount of time to reveal and block the sensor from the light being emitted by the scene you are photographing. The main cause of blurry photos is an insufficiently fast shutter speed for the scene.

 

Having too slow a shutter speed will cause a running child to be a blur in your photo, or a moving car to come out jagged and fuzzy. It can even cause the shaking and vibration of your own hand to cause the image to be blurry. Having too fast a shutter speed can lead to photos that look too "static" - think a flying airplane. The photo will look more interesting with the prop in motion, than frozen still. Same with a moving car - the image looks more dynamic with clear motion from the wheels.

 

Shutter Speed can be controlled primarily by either the Tv, S, Time Value or Shutter Priority mode on your camera (depending on the model - every manufacturer seems to come up with their own name for it.)

 

Aperture Value

Aperture refers to the size of the hole that the sensor gets to look through to see the scene. You can get a feel for this by balling your hand into a fist, and leaving a small hole for your eye to look though. It looks much darker with a small hole, than with a large hole. Aperture values are referred to as "f-numbers", and are represented in the form f/4, f/8, f/22 etc etc

 

Aperture Priority is primarily used to control the DOF (Depth of Field) of an image. You would use this mode when the most important aspect of the photo is how the DOF looks to you. An image with only a small part of the image in focus, and the background having a lovely blurry effect (known as "Bokeh") will use a LARGE aperture. An image where everything needs to be in focus will have a SMALL aperture.

 

Here's where it can be confusing. You would think that a SMALL Aperture would mean a SMALL f number, right? Sadly, it's not quite that easy. You need to think of the f number as 1 OVER the apeture value, ie 1/4 or 1/22.

 

Once we look at it like this, we can clearly see that f/22 is much smaller than f/4.

 

There are downsides to making your aperture as small as possible to keep the DOF long. Overall sharpness of the image starts to degrade as you make the aperture smaller. Most lenses have a maximum sharpness at around f/8 to f/11.

 

Aperture Value can be controlled primarily by either the Av, A or Aperture Priority Modes on your camera - again, each manufacturer has their own way of expressing this mode on their function dials.

 

ISO

ISO refers to how "sensitive" the cameras sensor is to light. The higher the ISO, the more sensitive the sensor. So, by increasing the ISO, we can afford to make the Aperture smaller and the Shutter Speed faster.

 

That's great right? Why not just set it as high as possible?

 

Well, that will allow you to have "faster" photos, and when you are trying to take photos of the kids playing or of people at night, thats a great benefit. But, as with everything in life, nothing is for free, and there is a downside to cranking your ISO way up.

 

Noise.

 

Noise is the little dots and specks of colour that sometimes appear through your image - and it can make your photos look a complete mess. There are software packages, and even camera settings that will reduce the impact of this noise in your image, but it will always be a present problem with higher ISO settings. It is caused by unwanted information hitting your sensor, which is picked up because your camera sensor has been set to be more sensitive to those electrical fluctuations, and even heat from the camera itself.

 

So, what does this mean? Lets take a common example of a problem where people struggle to take a "good photo" due to circumstances beyond their control - a school awards night.

 

Generally, these venues are not well lit, and they ban flash photography, which we would normally use to get some extra light in the scene and take more balanced exposure. Plus, in this situation, you will usually be more than 10m away from your subject, so your flash will not be able to light the entire scene adequately.

 

Usually, the problem we have in these situations is ending up with a blurry photo. This is because the camera is slowing down the shutter speed of the camera to allow more light to hit the sensor to correctly expose the scene. The problem is that your son/daughter is moving and will appear all blurry on the photo because the shutter speed was too long.

 

How do we compensate for this? We can tell the camera to have a fast shutter speed as it's number 1 priority. We can set the shutter speed to something like 1/100 which should be enough to "stop the action" in the scene. But there is a problem - unless we compensate with aperture and ISO, the photo will be dramatically underexposed. The action might be stopped, but it will be too dark to actually see anything.

 

So, lets open our aperture wide open, to the largest figure we can. The lower aperture limits are determined by the lens you are using. If you are going to be shooting in a dark environment, you want the "fastest" lens you can get your hands on (ie an f/1.4 lens will be significantly faster than an f/4 lens, because the widest aperture allows a much faster shutter speed).

 

If we do an exposure calculation now (by pointing your camera at the scene with the shutter button half pressed, the camera will give us a reading of how well exposed the photo will be if we take it now), we find that we are STILL underexposed and the photo will be too dark. Now we can increase the ISO to help. By increasing to a high enough value, we can now take an image that stops the action in the scene AND is exposed well enough to clearly see everything in the photo.

 

This is just one example of how these three things work together to allow you to take a properly exposed photo. There is a lot more to it than this of course, but i hope this encourages you to start experimenting.

 

Remember the stool metaphor? In the prior example, we were forced to have the shutter speed at a certain length in order for the photo come out. We then had to change the "lengths" (ie values) of the other two "legs" (ISO & Av) to have a "balanced stool" (good photo).

 

Lets try a sunset landscape shot. In this case, Aperture Value takes precedence. We want a sharp photo (let's say f/8). But it's the evening and it's getting dark, so we need to have a longer exposure to make an f/8 shot work. We don't want to increase the ISO very much because we don't want extra noise in the image. So, in order to keep the exposure of the scene correct, we need to increase the shutter speed. In this situation, we will probably need a tripod, as the exposure length may be as long as a couple of seconds, and it will be impossible to hold the camera steady enough for the shot to not be blurry.

 

What about one of those images of a river or a waterfall where the water is all blurry and flowing? In that case, we want a longer than normal exposure, maybe as much as half a second. If we were to shoot with a "normal" aperture at half a second, the image would be badly overexposed under normal conditions, even with the ISO at it's lowest value. So we need to make the Aperture smaller to compensate for the longer exposure time (remember - smaller aperture means larger number!). In this case, during a daytime shoot, we may have to make the aperture as small as possible (f/22 or smaller depending on the lens). Shooting with a tripod again (to minimise shaking from your hand), we get a cool effect of the water flowing through the image. There are other ways of achieving this, such as using a ND (Neutral Density) Filter, which has the effect of adding a pair of dark sunglasses to your camera, allowing you to use a wider aperture, and thus achieving a sharper image.

 

As you can see, you can individually change these values to help you achieve the exposure you desire. Don't be afraid to experiment and see what you can achieve.

 

Happy Shooting!

 

AUGUST 8, 2014

The DPI Question...

 

I hear a lot of questions about DPI in my day to day, and i wanted to write a short post about what DPI is, what it means to your camera, and how it affects how you print.

 

DPI stands for "dots per inch" - now, that's fine, but what does that actually mean? Well, it means different things in different "environments" so to speak. You may also hear it refer to as "pixels per inch", "points per inch" or even "dots per cm" - they are similar but not completely interchangable - but for the sake of clarity in this post, we will refer to them as "Dots per Inch" - which is how you will traditionally see your images referred to as as it relates to the printing process.

 

First, lets talk about how your camera interprets photographs...

 

Your Digital Camera/SLR has a sensor inside it, which basically functions as the "eye" of your camera - what it sees is what you get as a photograph. Different cameras have different size sensors and different "pixel density". The size of the sensor and the pixel density combine to give you the Megapixel (MP) rating of your camera. (Megapixel is literally shorthand for "million pixels")

 

Your standard low-mid range SLR or point and shoot camera will normally have a MP value in the region of 12MP (ie 12 Million Pixels). Normally this means that the sensor will have an overall size (on an APS-C size sensor) of 3000x4000 pixels. 3000 multipled by 4000 equals 12 million - so we can see how the number comes about. Cameras that have higher megapixel figures generally have the same size sensor as their brethren, but they have a higher pixel density, meaning that in the same space they have more pixels, because the individual pixels are physically smaller (the current industry leader in DSLR cameras is the Nikon D810 with 36.3MP - although there are medium format cameras that have up to 80MP!).

 

So, how does this relate to DPI for your photos?

 

Lets say you want to print a photo at 300 dpi with your 12MP Camera. How big will this photo be?

 

To find out, we need to work with the native resolution of the camera. Basically, it is a very simply equation.

 

Our original image is 4000x3000 pixels in dimension as we have discussed.

 

If the photo we want is to be 300dpi, we simply divide those dimensions by 300 to discover how many inches in size the photo is!

 

4000/300 = 13.3in

3000/300 = 10in

 

So a photo taken on our 12MP camera will be 13.3x10in @ 300dpi.

 

This also works in reverse. If i have a photo and i want to blow it up to - let's say - 40in (approx 1 meter), what will the native resolution of the image be? So, we divide the pixel width of the image by our desired physical size, and we get...

 

4000/40 = 100dpi

 

Now, is that resolution suitable for printing? Well, that will be the subject of another post, but the short answer is - it depends.

 

So, how do people end up with large photos at really high resolutions? Well, there are two ways to achieve this. Either use a camera with a bigger/more dense sensor (remember - more pixels equals more dots per inch at a given size) OR they use post-processing software such as Adobe Photoshop or GIMP to do something known as "re-sampling" - in which the photo is resized larger, but the physical pixels are either kept the same size, or reduced in size and extra pixels are introduced to "fill in the gaps". This has both benefits and drawbacks depending on the circumstance.

 

From the point of view of someone who prints images every day - DPI isn't everything. It is important, but it is important to keep in mind just how you will be using your photo. A big enlargement that will be framed or canvas printed and placed on your wall can afford to be at a much lower resolution that an image you will be reading from a book because you will be physically further away from it - so from your point of view the physical "pixel size" will look smaller. Re-sampling an image in this situation can often lead to the image losing sharpness because the added pixels may serve to "smooth off" the sharp edges in your photo.

 

I've also had questions about why the image on their computer only seems to be at 72dpi. There are a lot of reasons for this happening, but it is mainly due to the software that your computer is using. 72dpi is traditionally seen as the desired resolution for viewing an image on a computer screen, as the physical resolution of your computer screen is traditionally no higher than that. It would be a waste of resolution as your screen would not be able to display the image at that resolution anyway. Does this mean that your image is "low resolution"? Well, yes and no. 72dpi is generally seen as quite a low resolution for PRINTING, but it common for computer screens. But, as we learnt before, because the resolution is low the physical image size will be large. How large? Well, on our 12MP example...

 

4000/72 = 55.5in

3000/72 = 41.6in

 

So, 55.5in x 41.6in (140.97cm x 105.66cm) is the size of our image at 72dpi - which is pretty massive i'm sure you will agree! As we make the physical image smaller, the pixel density will increase, increasing our resolution.

 

I hope this helps clarify things for you in how your cameras resolution works. Happy shooting!