If you own a standard compact, the word “aperture” probably does not come up very often. You’ll probably have “portrait” and “landscape” modes. Fancy compacts and DSLRs, however, have a “PASM” program dial. The “A” stands for aperture.

Aperture is simply the size of the hole through which light passes in the lens before it is saved as an image. Every camera (except pinholes) allows you to change the size of that hole. Adjusting the aperture does two things: it varies the amount of light when taking the picture and changes how much of the scene behind and in front of your subject is in focus, which is known as depth of field.

When shooting a portrait, it’s usually a good idea for the subject to stand out sharply against a less sharp background (i.e., a narrow depth of field). If you accidentally shoot a portrait in landscape mode, the subject tends to be lost in an image where everything is equally sharp, as the depth of field is much wider. The important thing is to switch off the “auto intelligent / camera decides everything” feature when you shoot portraits.

If the hole is bigger, more light enters the camera when taking the shot, which means you need less time to take it: a wider aperture means a shorter exposure time. This is particularly useful when taking pictures of moving objects (sports, children, pets, etc.). If your compact has one, try the ‘sports’ or ‘portrait’ mode in these situations. With a DSLR, set the mode dial to “A” and dial the f/ number to its lowest value.

Conversely, when the hole is small, less light enters when taking the shot, which means you need more time. A narrower aperture requires a longer exposure time, but more of the scene is in focus. This is useful for landscapes and interior/exterior architecture. Remember that moving objects (trees on a windy day, for example, or people) may be blurry. You’ll probably need a tripod or some sort of solid support.

Drawbacks:

When shooting “wide open”, especially outdoors, you can easily burn out the sky or the brightest areas because there is too much light coming in. There is usually a lot of “vignetting” a.k.a. “light fall-off” (corners are darker than the centre) when shooting at the lowest aperture value. Geometric distortion (buildings bent, people oddly shaped) occurs more often at maximum aperture.

At the other end of the aperture scale, images can become slightly blurry (referred to as “soft”) owing to a phenomenon known as diffraction. In general, it’s better to avoid the highest aperture value.

Slightly techie stuff (which you can probably ignore)

In photography, aperture is the size of the hole divided by the focal length of the lens. If the focal length is 50 mm and the maximum diameter of the hole is 25 mm, the aperture is stated as f/2. This is why the larger the f/number, the smaller the hole. The hole at f/2 is bigger than at f/2.8 or f/5.6. Think of “f” as “fraction” or as “1 divided by…” if that helps.

Lenses are rated by focal length and by maximum and minimum aperture. If you have a camera where you can change the lens, chances are the lens is a zoom; you can change the focal length. Unfortunately, in all but the most expensive lenses, the maximum aperture (lowest f/number) also increases when you zoom in. One of my lenses is a 15-85 mm, 1: 3.5-5.6, in which the maximum aperture ranges from f/3.5 at 15 mm to f/5.6 at 85 mm. In practical terms, this means that you need more time at the long (tele) end of the zoom because the hole is smaller and therefore lets in less light.

You can counteract (slightly) the need for slower exposure by increasing the camera’s ISO sensitivity. Here, the camera sensor records more light, so the exposure time can be shorter, but also adds more “noise” (grain) to the picture. You can also use a tripod, but this won’t help much with moving subjects.

Examples

Below are two captures of the same subject taken at different apertures.

In the first, taken at f/2.8 (the widest aperture available with that lens), notice how everything behind and in front of the spout is out of focus. This is an example of a narrow depth of field.



In this one, taken at f/13, both the foreground and the background are still in focus along with the spout. This is an example of a wide depth of field.



Click here for the technical definition of aperture.

I adapted this post from something I was teaching kids here and updated it with recent photos. I hope this helps people experiment beyond the “full auto” mode, but it certainly isn’t intended as authoritative. Feel free to add to / correct this thread.

I, for one, appreciate this a lot. Clear and concise, thank you.

Thank You for your post. I have owned my camera for almost 4 years now and I have always set it on the anti-shake and attempted to get the photo I was after. Time to purchase a tripod and play with the settings.
Your post is extremely clear and quite educational.

Thank You,
Karman ♥

this for sure will help me a lot!
I hate reading manuals, so this helps a lot

Stefan, this is the clearest and most concise thing I've seen - a wonderful "Cliff notes" for my camera! I'm afraid that I've not experimented much with my new one, but you've made me want to try it! Thanks for the images and the info!

Here an interesting site to learn (and do) about aperture and shutter:
http://www.photonhead.com/simcam/shutteraperture.php

Here's a working link to the site Ryana just mentioned....
SHUTTERAPERTURE

And here's a different one...
DOF calculator

Thank you for this post!

Stefan......thanks for all the info! Keep it coming.....

" Selecting The Sharpest Aperture"

I'd like to contribute as well

First of all - a quick correction. "Geometric distortion (buildings bent, people oddly shaped) occurs more often at maximum aperture" - no, aperture stop does not affect the distortion, however focussing distance (near vs. infinity) might. It is true, though, that maximum aperture will exhibit all flaws that lens has. Apart from vignetting, the CA's (chromatic aberrations - colour fringing) will be maxed out, front/back focusing (if any) will become apparent, also most lenses will have poor corner resolution. While it's fine with portraits, as corners usually don't matter, it's a bad idea to shoot landscapes wide open.

Typically the lens' sharpness graph would look like bell shaped curve - relatively low on maximum aperture, increasing when you start closing the aperture, then around f/11 - f/16 diffraction kicks in and resolution starts decreasing again. Generally it is accepted that the maximum sharpness is between 2 and 3 stops down from the maximum aperture. I.e. if your lens is f/2.8, peak should be reached between f/5.6 and f/8. There are exceptions of course - most of macro lenses are sharp wide open.

It's important to understand that each aperture stop (f/1.4, f/2, f/2.8, f/4, f/5.6) means that the amount of light passing through the lens is divided by two, i.e. the light difference between f/1.4 and f/2.8 is not two times, but four.

Regarding the diffraction - simple explanation. Once the hole (aperture) is small enough, light starts to bend and the ray that passes through the hole becomes defocused.

Also, one more thing: although depth of field is a combination of aperture and magnification (i.e. technically 50mm f/2.8 at 2 meters will have the same DOF as 100m f/2.8 at 4 meters) it's only part of the truth. Longer lens has narrower viewing angle, so the background becomes more "expanded" and we perceive it as more blurred. So the pleasant "portrait" look depends not only on aperture, but on the focal length as well.

And the last one - if you have a fast prime lens (e.g. 50mm f/1.4) and want to shoot portraits in the broad daylight, it's worth buying an ND (neutral density) filter to decrease the light by a couple of stops, otherwise, like Stéfan already mentioned, you will overexpose badly.

Sooner or later I will have to learn about all these things if I want to get better quality pictures...and I want.
Your discussion only makes me want to learn them sooner rather than later ;)

Agree with Rokas, If I may add Geometric distortion often occurs at wider than 50mm Focal Length lens and went crazy in ultra wide lens (10mm-14mm) or fish eye lens. But that's what makes ultra wide lens fascinating for some fotog, the distortion makes the clouds look longer and almost 3D and you can make subject faces look funny too.

In most cases ND filter is unnecessary since you can always compensate wider aperture with higher speed to avoid overexposed pic (that's why we call it FAST lens) or just simply stepped the aperture down to get the desired pic. And me..I just love the bokeh fast lenses give. Fast lens Rocks!

@ Rokas: thanks for the valuable input - it's perhaps more technical than I intended for this introductory level, but there are some good points. If I may correct your correction (sorry!), it's true that distortion isn't contingent on aperture alone, but the way most lenses are constructed today ensures that lens aberrations (distortion, CA, etc.) occur more at maximum aperture than at the lens' sweetspot, which is why barrel and pin distortion is more prevalent at either aperture extreme. I was trying to keep it simple... ;-) Nyquist is for another time.

I think I'm somehow starting to understand that aperture and shooting speed thing.
though for now i'm still using the automatic settings because I'm not secure in using A, M, S and P mode.. I might loose the subject or the moment. so when i become confident i'll surely apply these techniques!
I'm glad, there are still people that like to teach us somethings on photoblog, really thanks for the thread stéfan! and to all other that added some links and info to this important thread for photographers that are just starting

Ok, this one might be helpful:

http://toothwalker.org/optics/distortion.html

I use AV during the day and TV at night, the photos are good and with this helpful thread I just begin to understand what is what Thanks Setfan