May 23, 2011

Food Photography Basics #5 : Aperture And Depth Of Field (DoF)


In my previous post in this series, I had talked about Exposure and the three factors that affect / play an important part in determining optimum exposure. These three factors (or elements of the Exposure triangle) are aperture, shutter speed and ISO.
While each one of these is as important as the others in its own way, aperture seems to play a larger part in setting the exposure than the other two in food photography. This is because most food photographers use aperture settings to exercise creative control through depth of field (DoF) in their photographs. This is what we shall explore in this post.




Pears and apples will feature largely in the photographs here, as that’s all I had in the fridge. Time to go shopping to stock up on fruit and vegetables!



What is Aperture?


Aperture refers to the opening in the lens, and the size of it, through which the light comes in when you are taking a photograph. Aperture is controlled not by your camera but by the lens you use and so the minimum aperture available to you would depend on the lens you are using. So with the 50mm f/ 1.8 on your camera, your aperture range will start with f/ 1.8.





Aperture is measured in f-stops (or f-numbers) namely f/ 1.4, f/2.0, f/ 2.8, f/ 4, f/ 5.6, f/ 8.0, f/ 11, f/ 16 and f/ 22 and these values are full stops. You might notice that there are other aperture stops, like f/ 3.2 and f/ 3.5 between f/ 2.8 and f/ 4.0. These are 1/3rd f-stop values between full f-stops and allow for finer adjustment of exposure.

There are some basics to understand and remember with aperture. If you’re new to this it could be a bit confusing but will become easier with time and practise. I couldn’t make head or tail of this when I started out with my camera.

The first thing to remember is that the smaller the f-stop or number the larger the size of the opening in your lens and so more light is going to come into your camera and your photograph. So if you take two pictures at the same shutterspeed and ISO, the first at f/ 2.8 and the second at f/ 4, the first picture would be more exposed (have more light in it) than the second one.

You can see how the amount of light coming through the opening in the lens changes with change in aperture in the following set of photographs. Please move clockwise from the upper left hand photograph taken at f/ 2.8.
All were taken at ISO : 100 and shutterspeed : 1/40 with only changes in aperture settings.





I have no idea why whoever it was that came up with this system, decided that if the aperture (or opening) was larger, then the f-stop/ number had to be smaller! Actually there is a scientific explanation for this based on focal length ratios which I don’t really understand myself so I’m not going to try and explain it to you!! (You wouldn’t believe I did study Physics at school and was supposed to be reasonably good at it?)

It can be quite confusing initially and I still remember getting my DSLR and trying to remember whether f/ 5.6 meant a bigger lens opening than f/ 11. Again, this is another thing that becomes second nature with practise. If you look at the diagram that follows you will understand this relationship of larger apertures to smaller f-stops/ numbers and vice versa better.

There are various analogies used to make this easier to understand and remember this relation ship between f-stops/ numbers and size of the opening in the lens.
One is to think of the lens like your eye. If there is too much light then your pupil closes to for a smaller opening to allow less light in (more light = small opening = larger aperture number and vice versa).

Another is to imagine the lens like a window with curtains. When there is too much light coming through your window, you close the curtains to reduce the light (more light = smaller opening through curtains= larger aperture number and vice versa).

Yet another one is water coming out through a hosepipe. When you open the tap all the way through (think big opening, lower aperture number) more water comes out (think more light). Start closing the tap (think closing down aperture to smaller opening) and less water will come through the hose (think less light).

You can use whichever analogy works for you but in the end what it boils down to is “Large f-stop/ number = Small opening = less light” and “Small f-stop/ number = Large opening = more light”.


The other thing to remember with aperture is that with every increase in an f-stop/ number, the amount of light that comes in will decrease by half from the previous stop, and every decrease in an f-stop or number the amount of light coming through the lens opening will be double. This is important to understand as this will help when you want to adjust apertures and corresponding shutterspeed to maintain the same Exposure levels when shooting in manual mode.





If you think about it, it will become easier to understand as you can see in the above set of photographs. Follow the photographs starting from upper left clockwise. I started with the following specifics for exposure - ISO : 100, aperture : f/ 5.6 and shutterspeed : 1/ 50 using the 50mm/ f 1.8 lens.

Keeping my exposure compensation at “0”, I varied the aperture settings from f/ 2.8 through every full stop till f/ 16, and allowed my camera to set the appropriate shutterspeed.
When I changed aperture from f/ 5.6 to f/ 4.0 while maintaining the same exposure, the camera set the shutterspeed 1/25 from 1/50.
Following this logic you will see that with every photograph, everytime the aperture increases by one f-stop/ number the shutterspeed comes down by half from the previous one. You can also see how the a shallow depth of field (blurred background) with the large aperture (small f-stop/ number) slowly gives way to a deeper depth of field (sharper background) with smaller apertures (larger f-stop/ numbers).

Moving down by 1 full-stop by changing my aperture from f/ 5.6 (smaller opening) to f/ 4.0 (bigger opening) I doubled the amount of light coming through my camera. To maintain the same exposure, my camera halved the shutter speed from 1/50 to 1/25.

Similarly, when I halved the amount of light by going from f/ 5.6 to f/ 8.0 (at same ISO), my camera compensated by doubling the shutterspeed from 1/ 50 to 1/100 to maintain the same level of exposure. This holds provided the source of light is constant.

One other important aspect of aperture is that it controls depth of field (DoF) another name for that interesting blurriness in the background we tend to see in a lot of photographs, especially of food.



Depth Of Field (DoF):


Depth of field refers to the distance between the nearest and farthest points of the sharpest area in your photograph or how much of your photograph will be in focus and sharp. Aperture controls depth of field to a large extant and this gives you creative control over your photography.

Typically you can adjust aperture settings to give you a shallow or narrow/ smaller depth of field where a large part of your photograph will be out of focus or “blurry”, or a deep or wide/ larger depth of field where most of your photograph will be in focus and sharper.
Smaller aperture settings (big opening, smaller f-stop/ numbers) will give you a shallower depth of field (more blurriness) whereas larger aperture settings (small opening, bigger f-stop/ numbers) will give you deeper depth of field (sharper images).

A shallow depth of field is of more interest in areas of close-up photography like food photography. Using smaller apertures (larger openings, smaller f-stop/ numbers) creates a shallow depth of field, creating a sharp focus on that particular part that you want to bring out while the rest of the photograph is out of focus/ blurry.
You can see this in the photographs below. Again, please proceed clockwise from the upper left.




At an aperture setting of f/ 2.8 you can see the paper and the phone in the background are quite blurred (shallow DoF), less blured at f/ 5.6 and so sharp at f/ 16 that you can see the text on the newspaper very clearly.
Of course, a very shallow depth of field does not work for every food photograph and should depend on the subject and composition of the photograph.

Depth of field is controlled not only by aperture setting, but also by the distance of your camera from the subject. So if you have a particular aperture setting, say f/ 1.8, if you take your photograph from close to your subject you will have a more shallow depth of field (blurry effect) than if you shoot from a further distance from your subject.




The photographs above illustrate this. I took the one on the left very close to the subject and the one on the right over double the distance away (I forgot to measure the distances!). You can see that the pear in the background is sharper in the second photograph (check the stalk) when compared to the first. Both photographs were shot with a 50mm f/ 1.8 lens at ISO : 100, apertur e: f/ 1.8, shutterspeed : 1/250. Of course, since I shot the one on the right at a distance I had to crop it for comparison.



Some other stuff to remember about aperture/ depth of field.


When one talks of depth of field, it is commonly stated that approximately 1/3 of the DoF is in front of the subject and approximately 2/3 is beyond it. This depends on focussing distance of the camera from the subject. Generally, the closer the subject to the camera, the DoF is more evenly distributed in front of and behind the subject. As distance of focus increases, the DOF will usually be more behind than in front of the subject/ focused area.

When shooting at lower apertures (larger opening, smaller f-stop/ numbers), especially with lenses like the f/ 1.2 or f/ 1.4 lenses it can be difficult to get your desired area of subject into focus as a large part of your composition would be out of focus or blurry because of the shallow DoF. This can be overcome with practise.

When shooting with higher apertures (smaller opening, bigger f-stop/ number), a correct exposure could require slower shutterspeed resulting in the camera registering movement (camera shake). At this point you would need to “stop down” to the next f-stop to increase your shutterspeed, but if you want to shoot at the same aperture then you would need to use a tripod (assuming you were shooting with your camera hand held.



Shooting on Aperture Priority (Av) mode:


A lot of photographers who shoot food tend to use the Aperture Priority or Av mode. In this semi-automatic mode, you set the required aperture and ISO and the camera sets the shutterspeed for optimum exposure. In food photography, aperture plays a more decisive role than shutterspeed as DoF plays an important role.

So why not shoot in Aperture Priority instead of using a full Manual mode where one would spend time on the whole exposure conundrum?

A lot of photographers will tell you “real” photographers shoot only in Manual mode! Not true. The choice is entirely up to the photographer. No one way is right or wrong as long as you get the “perfectly” exposed photograph. In fact, using this mode is great way to understand the DoF you can achieve with each aperture setting, without having to worry about the shutterspeed settings.

However, if you really want to understand and master exposure using the Manual mode is the way to go. So I would recommend shooting on Manual if only to learn photography. Shooting in Manual mode does have consistency and saves time in post processing.

Let me explain. Last month, I was able to catch the weekend photography workshop by Zach Arias at CreativeLIVE. As a professional, he was saying how photography eats into time he could be spending with his family and friends.
He mentioned shooting in Aperture Priority only when his light source was not constant and he didn’t want to waste time metering for light and so let the camera set the shutterspeed. Given that he usually shoots in situations where his light source is constant, shooting Aperture Priority is rare. Even when light could vary, he prefers to meter the light and shoot in Manual mode.

Sometimes the camera meter may not read the available light correctly (if there is too much contrast), and you can ignore the camera meter while in Manual mode and set your exposure as you want it. In such a situation, the Aperture Priority mode will use the camera meter to set what it feels is the best shutterspeed for best exposure and it might not be what you are looking for.

In situations where one typically takes multiple images during a shoot (as one would even for a food blog) and then edits them to pick out one or more “hero” shots shooting in Manual mode gives you consistency. Since you set the aperture and shutterspeed after metering, you get consistent exposure in every photograph you take by just making proportional adjustments of aperture and shutterspeed. This means once your photographs are on the computer, you can make necessary adjustments to one image in post processing and then batch process the entire lot.

In comparison, visualise the scenario where using Aperture Priority mode means that even small changes in light will cause the camera to keep metering and adjusting for exposure. This means you might have a batch of photographs which are well exposed but the exposure (variable shutterspeed in this case) is not constant across the images. This means more time would be needed to edit individual images during post processing. This is more of a problem if your camera is on Auto white balance (we’ll do this later).

Since food is usually shot in constant light situations and will not move, the argument would be that there is no need to use Aperture Priority. Light can be metered and then food can be shot in Manual mode making exposure changes if necessary. But at the end of the day, what really matters is what you are comfortable with and prefer to use, so long as you get the exposure you want in your photographs!



And finally, about some confusing words/ phrases used in photography.


Photographers have their own jargon when it comes to words and phrases describing their gear or photography. With a little practice these will lose their mystery and might just become a part of your own vocabulary too.

So you might hear someone who “stopped down” their aperture. This just means they changed their aperture to a smaller opening (larger f-stop/ number) as in from f/ 5.6 to f/ 8.0 or f/ 11. When they “opened up” the aperture, they mean the let in more light by changing to a larger aperture (larger opening, smaller aperture or f-stop/ number).

Some people talk about fast lenses and slow lenses. A “fast” lens is a lens that has a very large minimum aperture (large opening, smaller f-stop/ number) and allows more light into the camera. So a 50mm f/ 1.4 lens is a faster lens than a 50mm f/ 1.8 lens or a 100mm f/ 2.8 lens.
Faster lenses are therefore very good to take photographs in low light situations without using additional light from a flash or other source. They’re also usually more expensive and sometimes heavier than slower lenses. Those lenses which let in comparatively less light are referred to as “slow” lenses.

So that’s it for this post. In the next post in this series, we’ll look at Shutter speed and ISO. If you have any questions please mail me or leave a comment at this post and I’ll do my best to answer it.



Useful Links:


Using Your Aperture

A Tedious Explanation Of The f-Stop

To Blur Or Not To Blur

How To Get A Shallow Depth Of Field In Your Digital Photos

Shooting In Aperture Priority Mode



So Far In This Series:


Food Photography Basics #1 : Do I Need A DSLR To Get Good Photographs?

Food Photography Basics #2 : Which Camera? What Lenses?

Food Photography Basics #3 : Getting Started

Food Photography Basics #4: It’s All About Light – Exposure

1 comments:

Kris Ngoei said...

Very useful post for all of us who are interested in food photography... Thanks for taking the time to share with us :-)