Tag Archives: focal length

Understanding Your Lens (Part 3)

In this third instalment of understanding your lens series, we will be concentrating on f-stops.

First, we need to clarify the word f-stop which a lot of photographers (including myself) refer to as aperture. Although f-stop and aperture are related, they are different. Aperture refers to the actual opening of the lens which is usually measured by the diameter of the opening. F-stop, on the other hand, is the ratio of the focal length and the diameter of the aperture of the lens. This is a very important distinction between the two and we will look into that in detail later on.

Every photographer should be familiar with the common f-stop numbers. They are as follows:

f22, f16, f11, f8, f5.6, f4, f2.8, f2, f1.4

Memorize those numbers because they are very important especially when you start using manual exposure mode. You should also know by now that the smaller the f-stop number, the bigger the lens opening. That is, f16 is a smaller opening compared to f8 on the same lens. The photo below shows the huge difference between f1.4 (left) and f16 (right).

Image

(Photo taken from Wikipedia)

Notice that I emphasized on the same lens. Different lenses have different opening sizes. On a telephoto lens, f8 will have a larger opening compared to a normal lens at f2.8. Let’s see why …

Consider a 300mm telephoto lens and a 50mm normal lens. Recall that f-stop is the ratio of focal length and aperture diameter or mathematically:

f-stop = ( focal length / aperture diameter)

or

aperture diameter = (focal length / f-stop)

So for the 300mm lens at f8 the opening is:

300mm / 8 = 37.50mm

and for the 50mm at f2.8:

50mm / 2.8 = 17.85mm

As you can see in the above example, there is a huge difference in the diameter of the lens openings. You can actually see this for yourself if you have a zoom lens. Look through the front element of your zoom lens and then zoom in and out without changing the aperture. You will notice how the opening increases in size as you zoom to a longer focal length.

Understanding this very basic concept is important because f-stops control exposure. F-stops regulate the amount of light that hits the camera’s sensor. It is quite obvious that the larger the opening the more light comes in. The problem is that it is not enough that you know the size of your lens opening. Back to the 300mm vs 50mm example above, 37.50mm is obviously larger than 17.85mm but we also mentioned that f8 is smaller than f2.8 so what gives?!

Well there is another factor that affects the amount of light hitting the sensor and that is distance from the light source. The farther the light source, the lesser the amount of light and that’s why stars look much fainter than our sun. In photography, this distance is the focal length of your lens. The longer the focal length, the farther the rear end of the lens is from the sensor. Another thing that you should understand is that the intensity of light varies as the square of the distance. What this means is that given the same lens opening, light coming from a 50mm lens has four times the amount of light coming from a 100mm lens. Mathematically,

light intensity = (100mm / 50mm)2 = 4

It follows that for the 100mm lens to get the same amount of light as the 50mm lens, it needs to have a larger lens opening. The immediate question is, by how much larger of an opening? This is why knowing your lens opening is not enough to control light. You should also keep track of your focal length. This makes photography so much more complicated than it should be.

And so they “invented” the f-stop. Again, recall that f-stop is a ratio of the focal length and the lens opening diameter. It is very clever because it calculates the light intensity for you automatically no matter what your focal length and your lens opening are. So given this information let’s calculate the lens opening diameter for both 100mm and 50mm at the same f-stop, say, f8:

100mm / 8 = 12.5mm

and

50mm / 8 = 6.25mm

So double the focal length requires double the lens opening diameter for the same amount of light hitting the sensor. Imagine having to change your lens opening every time you zoom in and out. Instead, you just set your f-stop to, say, f5.6 and let the lens handle the opening according to your focal length of choice. And that is exactly what you see when you keep a constant f-stop and look through the front element as you zoom in and out. Easy!

How does f-stop relate to the amount of light hitting the sensor? Each stop of difference is double the amount of light. For example, going from f5.6 to f4 is twice the light intensity and going from f4 to f2.8 is also double the intensity. So going from f5.6 to f2.8 is four times the amount of light and so on.

Let’s summarize what we have discussed so far:

1. The amount of light hitting the sensor is affected by the lens opening (aperture).

2. The amount of light is also affected by the distance of the light source to the sensor (lens focal length).

3. A f-stop is the ratio of #2 and #1. This allows us to easily calculate light intensity because the lens automatically adjusts the aperture as focal length changes. We only have to worry about one parameter instead of two.

From this, it is easy to see why fast lenses, those with wide apertures such as f1.4, are much larger than slower lenses of f4. For the same focal length, the faster lens needs to have a wider opening diameter. This also explains why some zoom lenses have varying apertures and others have constant apertures. Lenses with varying apertures, say, f4 on their widest to f5.6 on maximum zoom, are cheaper because the aperture does not change much going from wide to telephoto and are therefore smaller in terms of diameter and only need smaller glasses. Constant aperture zooms are not only more expensive but also bigger and heavier because they have to open up much wider as the focal length increases. Wider, bigger and more glass. Finally, this also explains why m43 lenses are much smaller than their full frame counterparts. The smaller m43 sensors require shorter focal lengths to cover the entire sensor area and therefore have smaller lens opening diameters.

Before I end this post, let me address a very common misconception. A lot of photographers think that full frame sensors are better than m43 sensors at capturing light because they have larger surface areas. This is not true simply because a sensor without a lens in front of it is useless. Now with a lens in front, we know that an f-stop is the same for any sensor size. A full frame camera requires a longer focal length and therefore a lesser amount of light hits the sensor compared to a m43 camera that requires a shorter focal length because of the smaller sensor. A f-stop of f5.6 in a full frame camera allows exactly the same amount of light per unit area as f5.6 in a m43 camera.

I hope you learned something in this post. There will be more next time.

Keep shooting.

Advertisements

Hyperfocus: How a Cheap Lens Changed the Way I Shoot

For me, photography is all about having fun. I do not want it to be another job. I’m not saying I do not want to be a pro. In fact, I would like to get paid just traveling and taking photos. I may have taken this “fun” thing into a different level because I have grown a dislike for heavy (i.e. expensive) equipment. Yes, a 70-200/2.8 VR2 is nice to have but I seriously can’t shoot with it for 15 minutes straight without having to shake my arms to relieve myself of that tingling sensation behind my wrist. Yes, I’m cheap and that cheapness had me lose a fair amount of money because I am forced to discard my cheap equipment for something a bit less cheaper. And that brings us to the subject of this post: my cheap Sigma 17-70mm lens.

Like I said, I have grown a dislike for heavy equipment and that includes my stupid Nikon D700. So before I went for a holiday in the Philippines, I decided to get a lighter camera. It has to be small and light but it should not sacrifice image quality and most importantly, it should not get in the way of photography. So I bought a Pentax K5 which was not so cheap back then. And because I’m a cheapskate I was forced to buy the cheapest lens with the best possible zoom range because there is no way I’m gonna get another lens. One camera, one lens. That’s it and nothing more. The Sigma 17-70mm fit the bill. Very good zoom range for just about anything. It goes from 2.8 at the widest end and closes down to 4.5 at full zoom. At $339 AUD it was perfect. Not!!!

It wasn’t until the following day after that errant purchase that I noticed that this lens is terribly soft especially at the corners. At 17mm the corners are so blurry unless I stop down to f11 and even at that aperture it is never sharp. Worse, this lens can never get the focus right. I have updated the camera firmware because the K5 is known to have focus issues in the older firmware versions but that did not fix the problem. I have returned 3 copies of the lens but all of them had the same back-focusing issue. I have grown in love with my camera so I can’t return it but on the other hand, I have this lens that I can’t replace because I’m a cheapskate.

What does a cheapskate got to do? I taught myself how to hyperfocus!

First things first, let’s tackle depth of field, aka, how much of our photo is in sharp focus. We all know (I assume we all know) that we can control our depth of field by changing the lens aperture. The smaller the aperture (f11) the deeper the depth of field. If we want our subject to appear to pop out of the frame, we use a bigger aperture (f2.8) to make the background go out of focus in a creamy blur we call bokeh. Every amateur photographer should understand this. It doesn’t stop there. Another factor in depth of field control is the focal length. Longer focal length means shallower depth of field. Lastly, there is distance to subject. The closer the subject in focus is, the more blurry the background becomes. So again, depth of field is controlled by 3 variables: 1) lens aperture, 2) lens focal length, 3) distance to subject. We need a firm grasp of these 3 basic concepts otherwise the subsequent discussion would be tricky to comprehend. Actually there’s another factor which is the sensor size (or crop factor) but let’s not deal with that because we can cheat as I will show you later.

In landscape photography, we usually would want to capture the grand vista and make everything from the foreground to the background in sharp focus. A very common mistake made by beginners is to allow the camera autofocus mechanism to pick a focal point. Depending on how the camera is aimed, it may focus on the horizon and result on a blurry foreground or focus on the nearest rock and make the background go out of focus. Night time photography would be a lot more difficult because your lens would just hunt and fail to focus properly. Sometimes we get lucky and have everything in sharp focus but we want to control this instead of just relying on luck.

This depth of field control is called hyperfocusing. I’ll go slowly this time and try to explain without using any diagrams (coz I can’t).

If the camera is focused on the far horizon (infinity focus), everything from that horizon up to a certain distance between you and the horizon will be in sharp focus. So if you are standing on point A and the horizon is point C, there is a point B between A and C where everything between B and C is in sharp focus. Keep repeating that sentence until you undertand the concept. Move to the next paragraph when you think you’re ready for the next concept.

That point B, is your hyperfocal point. What that means is, if you focus at point B, there is a point X approximately half-way between point A and B where everything between X and C are in sharp focus. Still with me? So we have something that looks like this:

A—-X—-B—————-C

Point B is the hyperfocal point and everything between X and C are in sharp focus. Take your time to digest those concepts before continuing to the next few paragraphs.

The question is, how do you find point B? Others would tell you to focus one-third of the way to your subject. It’s probably a good approximation but that does not work for me. What I do is memorize a few combinations of numbers. What numbers? You probably guessed it already: the numbers that pertain to the 3 factors that control depth of field.

Here’s an example: My lens goes from 17-70mm. I’m shooting landscape so I want to go as wide as possible so I choose 17mm as my focal length. I know that my camera is soft in the corners unless I stop down to f11 so I use that as my aperture. What’s left is the distance to subject and this is where I cheat 😀 Open another tab in you web browser and point to this URL: http://www.dofmaster.com/dofjs.html. Remember that other factor that controls depth of field? Yep, the sensor size. From there choose your camera model. Yours might not be in there just like my K5 so I chose the Pentax K7 instead. And that’s cheat number one. Now enter the values of your chosen aperture and focal length in the remaining fields. In my case that would be 17 for focal length and f11 for aperture. Never mind the distance to subject field. Just click calculate and the frame on the right would automatically give you the hyperfocal distance and that is cheat number two. In my case this number is 4.25 feet. It means that if I focus on something that is 4.25 feet away from me then everything from 2.13 feet (that’s half-way) up to infinity will be in sharp focus. Don’t worry if the subject seems to be out of focus when viewed through the viewfinder. This is normal because your camera does open aperture metering (as opposed to stop down metering of old film cameras) so you are viewing the scene at full aperture, f2.8 which isn’t your real aperture when you click the shutter. Trust that math will save the day.

What I do is memorize the hyperfocal distance that correspond to the my most used focal lengths, say, from 17mm to 24mm. If I’m not quite sure of my numbers, I would compensate by closing down a stop further. For example, my lens has distance markings on the barrel. There’s one for 3 feet and the next one is 7 feet and then 10 feet and then infinity (that drunk number 8 lying on the floor). Supposing that my chosen composition requires me to zoom in to around 24mm to avoid clutter. I’m not quite sure what my hyperfocal distance is for that focal length. What I do is I set my lens focus distance to that 7 feet marker and stop down to f16. Had I remained at f11, subjects near the horizon will not be in sharp focus. By stopping down to f16, my hyperfocal distance changes such that I could get everything between 3.5 feet and infinity in focus. Neat!

So what’s the moral of the story? Do not let crappy equipment hinder your photography. Instead, try to find ways to work around the minor issues. In my case, I learned how to hyperfocus (and I hope you learned as well from reading this post). Everytime I shoot landscape, I never use autofocus. Hyperfocusing is way superior.

Until then, have fun shooting!