Focal Length and Aperture Explained for the Photography Novice

Focal Length

The focal length of a lens determines its angle of view, and also how much the subject will be magnified for a given photographic position.

Focal length also determines the perspective of an image.

Longer focal lengths require shorter exposure times to minimize burring caused by the shake of hands.

The Focal Length means how much can your camera see.




The aperture range of a lens refers to the amount of light that the diaphragm can let inside the camera to reach the sensor.

Apertures are listed in terms of f-numbers (expresses the diameter of the entrance pupil in terms of the effective focal length of the lens; It is the quantitative measure of lens speed), which are marked on the lens.
On a camera, the f-number is usually adjusted in f-stops.
Each “stop” is marked with its corresponding f-number, and represents a halving of the light intensity from the previous stop. Modern electronically-controlled interchangeable lenses, such as those from Canon and Sigma for SLR cameras, have f-stops specified internally in 1/8-stop increments, so the cameras’ 1/3-stop settings are approximated by the nearest 1/8-stop setting in the lens. The F number can be displayed as 1:X instead of f/X(like in our example).
Lenses with larger apertures are faster because, for a given ISO speed, the shutter speed can be made faster for the same exposure. A smaller aperture means that objects can be in focus over a wider range of distance (depth of field).

Portrait and indoor (sports and theater also) photography often requires lenses with large maximum apertures in order to be capable of faster shutter speeds (and narrower depth of fields) in order to combat the low light problems with no camera shake.
The narrow depth of field in a portrait, as well as in macro photography, helps isolate the subject from the background.
Minimum apertures for lenses are almost as important as maximum apertures. This is primarily because the minimum apertures are rarely used due to photo blurring from lens diffraction, and because these may require long exposure times.

For cases where extreme depth of field is desired, then smaller minimum aperture (larger maximum f-number) lenses allow a wider depth of field.
It’s also good to know that lenses typically have fewer aberrations when they perform the exposure stopped down one or two f-stops from their maximum aperture.

This article is an excerpt fromĀ Complete Technical Photography Manual
(for any camera and any photographer!)
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By Laura

I started photography as a hobby in 2005, during college. My passion slowly became a more important part of my life since 2008. Because of using a combination of my photographic knowledge, with those of internet marketing, I like to call myself a "photomarketer".


  1. Wow, that’s really clear.

    I am not really into photography but of course I have seen those terms just forever; I just had no reason to look in to what they meant.

    It is a waste of time an money for me to use anything but a point and shoot –I have carried a single use camera in my glovebox for years in case I had to document the scene of an auto accident but that is the extent of my skills.

  2. Clear practical description for the novice, but should mention image sensor size and the related multiplier for field of view, or at least say that the specific examples are for a 35mm camera.

  3. Hmmm… This is an ‘okay’, back of the envelope, explanation. However it’s riddled with minor technical inaccuracies that for the most part can be overlooked.

    Except for one… “Focal length also determines the perspective of an image.”

    Sorry, focal length has absolutely NOTHING to do with perspective. The ONLY factor which affects perspective is the camera’s location with respect to the subject. Focal length certainly affects magnification, and a longer focal length lens can produce an image of an object of equivalent size from farther away when compared to a lens with a shorter focal length. But in the process you’ve also changed perspective.

    They only way to change perspective is to move the camera!


  4. I was going to correct the author on his erroneous statement about perspective, but Strider beat me to the punch. Strider is correct.

  5. I think this is a gr8 article explaining the concepts but one question, what is the significance behind your profile photo on the website? It has a certain sadness and mystery to it!

  6. FINALLY! A no nonsense, straight to the point explanation! Thank you! I spent weeks looking for something like this a few months back and never found it.

    Very informative.

  7. the less than 21mm IS NOT an architecture lens. Problem is the massive amounts of distortion, and barreling effect that the wide lens causes.

    The tilt lens is more of an architecture lens.

  8. Let’s see, since this is a beginners description, I don’t think it matters as to the nuances of architectural lenses. This is aimed at those who have the 18-55 kit lens with their 350D.

    As for focal length and perspective… perspective is more than just camera placement.

    Here’s another great website for a more in depth but understandable explanation of f-stops:

  9. I like to think of aperture and exposure in terms of a water faucet. Aperture is the size of the faucet’s opening and exposure is how long you open the faucet. A faucet with a small hole (i.e. large aperture number like f16) means only a little bit of water trickles through. A faucet with a huge opening (i.e. small aperture number like f2.0) means a lot of water pours out. Now, if you were trying to fill 2 buckets of the same size, if you have a small opening you have to leave the tap on longer. To get the same volume of water with a large faucet tap, you leave it on shorter.

    In photography, the “water” is the light and the bucket is your film. To capture a photo where you “faucet size” changes, you have to adjust the “time the faucet is on”.

    Just remember, for aperture the smaller the number (1.0, 2.0, 2.8), the bigger the opening, so the more light flows through quicker (i.e. “faster” lens)

  10. The extreme wide angle is better geared for landscapes and fisheye style photography than architecture.

    It’s better to let beginners know architectural photography is a special discipline that strives to AVOID weird distortions. LInes need to be clean and straight.

  11. just curious as to why when we increase the focus of the lens the aperture of the lens becomes smaller? im using finepix s9600 model. hope anyone can help and explain. im new to photography and i want to learn more.

  12. Nice tutorial on Focal Length and Aperture.

    Still being a novice, how to read lens desriptions like “VF-58CPKS” of Sony, please explain

  13. like the vast majority of novices, i sorta, kinda know what ISO is… but when asked to explain ISO and apertures and f stops, i scramble.

    this makes more sense! thanks!

  14. still confused about one thing: the specs for a digital camera (a Canon)I’m interested in say the lens is “7.3-29.2mm f/2.8-4.1 (35mm film equivalent: 35-140mm)”. why is the digital camera spec 7.3-29.2 expressed that way instead of just as the 35-140mm? Can anybody explain this to me?

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