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Lenses and Aperture

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Lenses and Aperture

Part 3

Have you ever wondered why some lenses are so expensive? There are many reasons for the cost of a lens. Here are some of them.

What is a “Fast Lens” and why are they expensive? Does a fast lens focus faster?

The term is a reference to the maximum f-stop (the lower f number or widest opening of the lens). It is not necessarily about faster focusing: a lens with a wider maximum aperture is described as being “faster” than one with a smaller maximum aperture. At a given ISO (light sensitivity) you can achieve a correct exposure at a faster shutter speed with a wider aperture than with a smaller aperture. Basically, the shutter does not have to stay open as long to let in the same amount of light, so it’s faster. In the series on shutter speeds we saw that faster shutter speeds stop motion, making it possible to get sharp results when shooting fast moving objects.

If you’ve ever priced lenses you will have seen one lens for a couple hundred dollars and another lens with the same focal length and from the same manufacturer at twice the price.  For example a 50mm f 1.8 is about $200 and a 50mm f1.4 is over $400. You may ask why a 0.4 of an f-stop (which is actually a full f stop) could justify this huge price difference. Here is what one manufacturer, Nikon, says about it:

“The wider the maximum aperture, the more sophisticated the lens design, and the more expensive it will be. For example, a lens with a maximum aperture of f/2 will cost more than a lens with a maximum aperture of f/4.”

The key words are “sophisticated lens design”. And it is true! Just look at the back element (glass) of those two lenses and you will see the difference.

Lenses and Aperture

Lenses and Aperture> Compare the size of the rear lens of these two lenses. Notice that the f1.4 has a bigger rear lens.

What is the difference between a Zoom lens with “Variable maximum f number” and a “Fixed f number”?

Zoom lenses are beneficial because they allow for a range of different focal lengths without the need to carry multiple prime (fixed focal length) lenses. This lets the photographer quickly zoom in and capture the shot, then zoom back for another at a wider angle. While this is a wonderful advantage to have, there are optical limitations that should be understood when choosing a zoom lens. Here is another explanation from Nikon:

“All lenses have a maximum aperture, or lens opening, used to capture light. On most zoom lenses the maximum aperture will change as you zoom. As you zoom, the optics move to focus at the new zoom setting. These zoom lenses are said to have a “variable” aperture. To achieve the widest possible aperture, you need to be at the widest possible zoom setting.

All NIKKOR lenses list the widest possible aperture on the lens barrel. Zoom lenses that have a variable aperture will show the maximum aperture range. For example, “f/3.5 – f/5.6” will be noted on the lens barrel as 1:3.5-5.6. The 3.5 and the 5.6 are referring to the maximum aperture the lens can achieve for each end of the zoom range.

Some higher-end lenses can maintain the largest aperture throughout the entire zoom range, so only one number is detailed. These fixed aperture lenses utilize more sophisticated lens elements than variable aperture lenses, and are also heavier than variable aperture lenses. As such, these lenses are generally more expensive than variable aperture zooms.”

Lenses and Aperture

The fixed aperture zoom lenses are more expensive than variable aperture zoom lenses

Does a variable maximum f number really affect my photography?

Indeed it affects your photos if you use the maximum f number. For example, you are shooting a baseball game with a 70-300 f4.5-5.6 lens. Your manual exposure is as below:

ISO 400

Shutter speed 1/1600s

Aperture f4.5

As long as you shoot at 70mm where the lens allows f4.5 you’ll get the correct exposure. However if you zoom in for a tighter shot the maximum aperture will change to f5.6, which means the image will be underexposed by 1 stop. You’ll need to either slow down the shutter speed or increase the ISO to compensate for the new f number. If you slow down the shutter speed you may get some motion blur; increasing the ISO can take time to dial in a change and you will miss the shot! Here is a sample of what can happen when the maximum f number changes.

Lenses and Aperture

Shooting with 70-300 f 4.5-5.6 at 70mm and f4.5 )Maximum aperture

Lenses and Aperture

Same scene and setting, but zooming to 300mm, which changed the maximum f stop to 5.6. the result is an underexposed image

To recap: some zoom lenses on digital SLR and compact digital cameras often list a range of maximum apertures, relative to how far one has zoomed in or out. These aperture ranges refer only to the range of maximum aperture, not overall range. A range of f/2.0-3.0 would mean that the maximum available aperture gradually changes from f/2.0 (fully zoomed out) to f/3.0 (at full zoom). The primary benefit of having a zoom lens with a constant maximum aperture is that exposure settings are more predictable, regardless of focal length.

I hope you’ve enjoyed these introductory discussions about lenses and the role of aperture. If you have questions or comments, please send me an email.

Our new Digital Camera Workshop for Beginners is scheduled for November 4 and 7. This is a two part workshop which covers everything you want to know to start your adventure in the wonderful world of photography.  Check out the course page for full details!

Ted and the Omnilargess Team 

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