Q&A

Q&A: What are apertures and f-numbers? [for novices]

2012, Inspiration, Konica Hexanon 60mm f/1.2, Leica 75mm Summilux (Canada 🇨🇦) f/1.4, Q&A, Teaching point

[Note: This post is intended for novice, and not experienced, photographers.]

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In photography, the aperture is the adjustable opening of a lens that determines the amount of light entering through it and onto a camera’s sensor, be it digital or film.  If the aperture is large, a large amount of light will pass through the lens and enter the camera; if the aperture is small, a small amount of light will enter the camera.

Simple, right?  But, it gets a little confusing…

The problem with using the entrance aperture size to communicate a lens’ light transmitting ability is the physical reality that a longer lens will require a larger aperture to achieve the same level of light transmission as a shorter lens.  In other words, if “x” is the amount of light we wish to reach a camera’s sensor, a 75mm lens will require a bigger “hole” (aperture) at the entrance than a 50mm lens to achieve “x”.

To avoid confusion, and to standardize notation across all lens types, f-numbers — instead of actual aperture sizes — are used to communicate the light-transmitting ability of all lenses.   An f-number (also known as an f-stop, or focal ratio) is defined as the focal length of a lens divided by the aperture diameter.

You’ll usually see f-numbers labelled on lenses as a sequence of fractions:

f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, etc.

The important thing to remember is that because the f-number is a fraction, the smaller the denominator, the greater the light transmission.

(please click on the image to view LARGER)

Thus, when considering light transmission, f/1 > f/1.4 > f/2 > f/2.8 > f/4, etc.

In fact, for every step to the left in the sequence above, twice as much light is being transmitted as the preceding step.  Thus, f/1 allows for twice as much light to hit the sensor as f/1.4, and f/1.4 allows for twice as much light as f/2, etc.

Incidentally, lenses that are able to achieve maximum f-numbers of f/1, f/1.4, or f/2 (or even f/2.8) are often loosely referred to as “fast” lenses because they allow photographers, for a given amount of light, to shoot at faster shutter speeds than so-called “slower” lenses (exactly twice as fast for each step up in the f-number ladder).

It follows from the above, then, that large aperture/f-number lenses are useful in low-light situations:

↑Leica M9 and Konica Hexanon 60mm @ f/1.2.

The other main use for large aperture/f-number lenses is to achieve subject isolation.*

Thus a lens of a given focal length set to f/1.4 (for example) will create a greater background blur than when set to f/4.:

↑Leica M9 and Leica 75mm Summilux @ f/1.4

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↑Leica M9 and Leica 75mm Summilux @ f/4.

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I hope you found this useful,

—Peter.

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*The other way to achieve subject isolation is to shoot with a longer focal length.

Q and A: Voigtländer Nokton 40mm f/1.4.

2012, Q&A, Teaching point, Voigtländer 40mm f/1.4 Nokton

This arrived via email this morning and I was about to respond as I usually do, but then realized, I often get this question, so why not turn my response into a post that will hopefully be of help to other readers?

 

Hi. I read your review for the The Voigtländer Nokton 40mm f/1.4. and found it very helpful. I have this lens with a Leica M9 and don’t seem to be getting the results you pictured. Did you alter your M9 shots in Lightroom or do you have the lens detection set a specific way in the M9 for the The Voigtländer Nokton 40mm f/1.4? My shots tend to come out overexposed when I use it open to 1.4. Any suggestions would be appreciated. Thanks and keep up the good work. Your photos and reviews are great.

– Adam.

First of all, thank you Adam for your very kind words.

Now, regarding your questions…  I post-process all of my files in Apple’s Aperture and then do my final colour tweaking in Nikon’s Capture NX2.  On the M9, I have lens detection set to “auto”, but I do not code any of my uncoded lenses, so for lenses like the Nokton 40/1.4 the camera isn’t recognizing anything and is therefore NOT making any adjustments.  For focal lengths longer than 35mm, there really is no need for coding anyway, unless you’re hoping to correct for vignetting or are interested in generating focal length EXIF data.

Finally, I always shoot in manual mode (controlling for aperture and shutter speed) and often manually set the ISO.   I never rely on the camera’s metering.  I’ve done this with ALL of my cameras, including Nikon, Canon, Sigma, etc., because no camera – not matter how sophisticated – truly understands what your “mind’s eye” is seeing when you attempt to create an image, and it’s better that you take full control of all the variables.

I hope this helps.

Thanks again for writing Adam, and Happy New Year,

—Peter.