In this article we’re going to discuss about the aperture of the lenses, how this is affecting the resulting image and how can you put it at good use to get better photos.
- 1 F-number
- 2 Brightness
- 3 The smaller the F-number, the brighter the image
- 4 Stops of light
- 5 Why should you care about stops of light at all?
- 6 Misconception: Lenses with larger physical apertures are always brighter than the ones with smaller physical aperture size
- 7 Misconception: The larger the lens’s aperture, the wider the image
- 8 Bokeh
- 9 Good vs. bad bokeh
- 10 What is a good indicator of the blur thickness?
- 11 Depth Of Field
- 12 Try before buy
- 13 Wrapping it up
The aperture of a lens determines how bright the lens is, how long the depth of field is and also how the blur in the out of focus areas is rendered. Photographers use to specify the aperture using a single number, called the F-number.
The F-number or F-stop is usually also part of a lens’s name and it’s featured on the lens itself.
Unlike the physical aperture of a lens which is how big the lens opening actually is, the F-number is a clever way of telling how bright the lens is. You could not derive that from the physical aperture alone, because the quantity of light concentrated on the sensor depends on the focal length as well.
If the lens has a smaller focal length, it concentrates closer. Therefore, the resulting density of the concentrated light is higher because closer means smaller and when the same amount of energy is packed on a smaller area, the energy per square mm is higher.
But that just means we have to account for two things to fully compare lenses in terms of how bright they are: physical aperture AND focal length combined. Wouldn’t it be great if there would be just one measure to do the job?
Luckily there is: The F-number represents the ratio between the focal length and the physical aperture and it’s an indicator of how bright the lens is. That’s really all you need to know. Really.
It would be pretty silly to tell someone you bought a 100 mm lens that has a physical aperture of 50 mm and a 300 mm lens with a 150 mm physical aperture. By enumerating these attributes, you’re not really conveying the brightness equivalence of the two without undergoing some math.
It makes more sense to say you have two lenses, a 100 mm and a 300 mm respectively, both f/2.0 (100/50 and 300/150). Doing so, you communicate that both lenses are equally bright.
The smaller the F-number, the brighter the image
For instance, an f/1.4 lens is brighter than an f/5.6 lens. Always. Regardless of their focal lengths and their physical aperture. Read on to find out why.
A lens will usually specify the minimum F-number right on it and this number can only be increased when using the lens by decreasing the physical aperture of the lens. This can be done by using either the aperture ring if it exists or electronically from within the camera.
Decreasing or closing the aperture is called stopping down the lens. It’s “down” although the number increases because the physical aperture gets smaller and the quantity of light gets cut with every stop of light that is being increased.
Stops of light
When photographers want to communicate that a certain quantity of light has doubled, they would say that a stop of light was added. Admittedly, if the quantity of light has halved, a stop of light was subtracted. For example, 3 stops means 2 * 2 * 2 = 8 times brighter.
Great. So that means when you set your exposure compensation to +1EV, the camera knows what to do in order to make your resulting image 2 times brighter.
Ok, perhaps a body can double the ISO or the exposure time in order to make the image exactly 2 times brighter, but how do you have to change a lens’s aperture to double the light?
To double the quantity of light on a lens, you would have to divide the current F-number by 1.4 and dial that F-number instead.
For example, by opening the aperture from f/2.8 to f/2.0, you gain one stop of light. In other words, you double its brightness. Why 1.4? Is is some kind of magic number? Yes and no, it’s simply the root square of 2. But why 2? Because 2x means one stop of light. Then why root square of 2 and not 2 directly? Because the concentrated quantity of light depends in the area of the lens and area is a bi-dimensional size.
Think about it this way. If you buy a TV which is 2 times wider and 2 times taller, do you get a 2 times larger TV in terms of its surface size? No, you get a 2 * 2 = 4 times larger TV. To get a 2 times larger TV, it would have to be √2 (~1.4) bigger on each dimension. Same with the surface size of a lens.
So by making the lens physical aperture 1.4 times larger, the lens will capture 2 times more light, hence 1 stop more. Therefore going from f/2.8 to f2/0 you gain one stop of light. Going from f/2.8 to f/1.4, you gain 2 stops, to f/1 it’s 3 stops and so on.
Why should you care about stops of light at all?
Well, with each full stop of light you gain from your lens, you get the option of either halving the ISO, yielding less noise, either halving the exposure time, yielding less shakiness.
For instance, if your current lens is 18-55mm f/3.5-5.6, by getting your hands on a 50mm f1/8, you get a 9.68x (5.6/1.8=3.1 3.11*3.11=9.68) brighter lens than yours at 55mm f/5.6. That means you can either shoot in pristine ISO100 instead of ISO1000 or 1/200 sec instead of 1/20 sec exposure time given the same lighting conditions. That’s a major difference!
Misconception: Lenses with larger physical apertures are always brighter than the ones with smaller physical aperture size
I’ve heard this a lot and it seems to be one of the most popular misconceptions regarding lenses. And it’s probably because it seems so hard to believe that a lens with a 70 mm physical aperture could be brighter than a lens with a 2 mm aperture. After all, the 70 mm lens is larger, so more light gets is, hence it must be brighter, right?
It’s like saying that the lens on the back of your mobile phone is brighter than the fat, long one hanging on the neck of a professional sports photographer.
Well, here’s the thing.
Let’s take for instance the huge, one of a kind Sigma 200-500 mm F/2.8, which is an absolute monster in terms of appearance featuring an astonishing physical aperture of ONLY about 500 / 2.8 = 178.57 mm. That’s almost 18 cm in diameter. You can fry eggs on it.
And let’s take a smaller, 20mm f/1.4. This one is only 50/1.8=27.77 mm in diameter regarding its physical aperture. Both lenses can be mounted on the same camera, both can cover the same sensor. But the smaller will produce brighter images. About 4 times brighter.
Because this smaller lens produces a smaller image on the sensor since it’s considerably wider at 20 mm. Smaller means denser and denser means brighter since brightness is directly proportional to the concentrated energy per unit.
The sensor is able to catch a lot of this projection (because the projection is small). It’s 20mm, it’s a wide lens. It picks up and projects more of the environment on the sensor than the 500mm which picks also a lot of light due to the bigger physical aperture but produces a far bigger image too. And bigger means less density of energy.
Its projection is bigger and bigger means less density. Less density means less concentrated energy per unit => darker image. Not to mention that a lot of its projection does not fall on the sensor and it’s basically lost.
So there you go, bigger lens does not automatically mean brighter. Check the F-number for that. Lower means brighter. Always. Unless the transmission factor of the glass itself is a problem, but that’s not something to bother with at the moment.
Misconception: The larger the lens’s aperture, the wider the image
I can understand why this is easy to fall into, perhaps because one might think that since a lens is essentially a hole, by making the hole larger, you would be able to see more angle. This is true for windows, but that’s just not how lenses work.
Read here why aperture has nothing to do with how wide the resulting image is.
Another way the F-number influences the resulting image is how the out of focus areas look like.
The way the blur is rendered by the lens is called bokeh and it’s a subjective measure. Not all lenses are created equal and the blur can look in many ways, have different thickness and be rendered more or less pleasant, therefore each lens has more or less its own type of bokeh.
There are many types of bokeh and the subject is very controversial even among professionals, but for now we’re going to keep things fairly simple.
The bokeh depends on many things, but mostly on the aperture of the lens. The larger the aperture (low F-number), the thicker the blur and vice-versa. The rounder the aperture, the smoother the blur, hence the nicer the bokeh.
Smooth and creamy bokeh of Canon 135mm f/2 L @f/2 on Canon 5D Mark III
Good vs. bad bokeh
In the end it all comes down to personal preference. Some prefer the swirly bokeh created by lenses such as the Helios 40-2 85mm f/1.5 or its smaller brother Helios 44-2 58mm f/2, effect created as an optical artifact due to a poor design, some prefer the creamy, soft blur created by lenses like the Samyang 85mm f/1.4 or the Canon 135 f/2 and other prefer a flat, perfectly even blur like the one created by the infamous Canon 70-200mm f/2.8.
Swirly bokeh of Helios 44-2 58mm f/2.0 @f/2.0 mounted on a Canon 5D Mark III
Smooth and soft bokeh of Samyang 85mm f/1.4 @ f/1.4 on a Canon 5D Mark III
Distracting bokeh of Vivitar 135mm f/2.8 @f/2.8 mounted on a Canon 5D Mark III
However, very few seem to prefer a distracting, non-smooth bokeh which seems to compete with the subject for attention by featuring more details than necessary.
The bottom line is that you need to consider the quality of a lens’s blur before purchasing a lens. Do not fall into the trap of buying a lens exclusively for a convenient focal length or a generous aperture. Do take in consideration the bokeh.
This aspect is too overlooked even by some professionals. Before purchasing a lens, do your homework and research the targeted lens. Does it produce nice blur? Is the blur soft, creamy, dreamy, smooth? It’s a go. Is the blur distracting? Does it look like a pattern rather than an out of focus image? Then it’s a no.
In order to close or open the aperture, a lens uses blades which make a hole of a variable size depending on how you want your F-number. The number of the blades and their shape influences how nice the blur in the background is rendered. More blades means softer blur because the hole they make is rounder.
Also other aspects of a lens influence the quality of the blur, but we’ll not get in very much detail right now.
What is a good indicator of the blur thickness?
I’ve always asked myself that. If you have to make a choice between two lenses, your goal being picking up the one with the thicker blur given the same framing of the subject, which one should you choose?
The indicator is the physical aperture of the lens. Since the F-number is focal length divided by physical aperture, if you divide the focal length by the F-number, you get the physical aperture. This will tell you how much blur you can get out of the lens. If you need accurate numbers, you’ll need to take into account many other things, but if you need to “guesstimate”, this will do just fine.
For example, if we compare two lenses with different focal lengths, a 50mm f/1.4 and a 200mm f/5.6, most likely, given the same subject framing and a fairly far background, both will blur the background just as much.
Why? Because their physical aperture is 50/1.4=200/5.6=35.71 mm. And it’s the physical aperture which determines the pattern of the blurred areas due to convolution. Convolution is not something to explain in few lines, so we’ll skip it for now.
Of course, the DOF will be wider for the 200mm, resulting in more of the subject being in focus, but the thickness of the blur in the background will be approximately the same.
Depth Of Field
The Depth Of field, or DOF for short is a range in your scene which is in acceptable focus. Therefore, just like the bokeh, this is also a subjective measure.
The F-number affects the size of this range. A higher F-number gives a higher DOF and vice-versa. That’s related to the fact that higher F-numbers make the blur thinner, therefore the area in which the subjects can be considered in focus becomes larger.
For you, this means it’s usually harder to focus with an F/1.4 lens than it is with an F/4 one EVEN if you have auto-focus. On a 50 mm F/1.4 lens, the DOF is so short (or shallow), that it’s almost impossible to make a portrait from 1 meter and be able to catch the eyes and nose both acceptably sharp. You have to make a trade-off.
So what do you do in that case? Either get farther from the subject and refocus, case in which the DOF will become larger, either use a longer lens (like 135 or 200 mm) and shoot even farther if you really need a close-up of the subject. Or simply stop down the lens to enlarge the DOF, but you’ll make the background blur thinner and perhaps you might not want to do that on a portrait.
Try before buy
Of course that would be great, but not all the time you can get your hand on the lens you plan on buying.
So whenever you can’t test the lens, make use of the awesome DOF & bokeh simulator tool from dofsimulator.net. I found it extremely helpful and actually helped me deciding between some lenses I purchased.
I’m sure there are a lot of DOF & bokeh simulators out there, but I personally found this one rich in the set of features if offers.
Wrapping it up
To summarize, the aperture is one of the most important aspects of a lens. Some thing about the aperture:
- It is specified by the F-number which gets lower as the aperture gets wider.
- It determines how bright a lens is.
- It determines how narrow (shallow) or how wider the Depth of Field is.
- If determines the characteristics of the out of focus areas which appear in blur.
- It determines the quality of the out of focus areas, hence determines the bokeh.
It does not:
- Determine how wide the produced image is. That depends on the focal length and sensor size.
Ok, hope you found this article helpful. Until next time, have fun with your gear, go out and shoot as much as you can, create stunning images and should you have any questions or just thoughts, just pop them in the comments or address them to me directly.
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