For most of you this will be a boring review but for some of you this will be very informative. I want to discuss in this post, lens choices as it relates to their focal length and f-stop.

Let’s first establish the focal length. For our purposes here we will define the focal length as the zoom or magnification of a subject. I use this definition as we are all familiar with the zoom feature of a video camera. So with that, the most common focal length you should know is the 50mm primarily because it most closely imitates the focal length of the human eye and therefore is called “normal.” Any number less than 50mm is considered “wide angle” (or zoomed out, less magnified) for example 35mm, 28mm or 24mm. Any number greater than 50mm is considered “telephoto” (or zoomed in, more magnified) for example 85mm, 100mm, or 135mm.

With this in mind there are primarily two different kinds of lenses, fixed primes with a single focal length and zoom lenses with a range of focal lengths.


Zoom Lens


Fixed Prime

I believe it’s good to have a zoom lens in my tool bag to address the different focal lengths that are needed in any given “run and gun” situation, for example 24mm to 135mm zoom or 24mm to 70mm. But for more professional results a fixed prime yields better image quality, better low light performance and more limited or shallow depth of field. My first lens of choice would be the 50mm but I’d also want to have at least one wide angle lens and one telephoto lens in my bag. For telephoto the 85mm also considered a “portrait lens” is very popular as well as the 24mm for wide angle.

Shooting around with these various types of lenses will help you develop a sense for what lens is most appropriate in a given situation. Where you’re shooting your masters shot or in tight or confined spaces or for or where action is happening in the scene, a wide angle lens is suitable. For medium shots and most general use the “normal” or 50mm is a good fit. And for distance or close-up with nice depth of field a telephoto is applicable. Granted in many instances you can achieve the same look with different lenses dependent on proximity of distance but this is only a rough guide, you will have to make the determination yourself based from experience.

Canon’s focal length comparison tool.

That leads us finally to f-stop. More simply the f-stop is the number on the lens indicating the diameter of the lens opening or aperture opening. For our purposes here today adjusting the f-stop number or aperture effectively accomplishes two things.

1) Allows you to adjust for exposure
2) Allows you to control depth of field

But before we get to addressing these two things it’s important to know what a lens is rated at by f-stop or how fast it is. For instance when looking at lenses you’ll notice after the focal length there will be a number. For example 50mm 1:1.8, where 1.8 is the f-stop number that lens is rated it at. The lower the number the faster or better low light performance that lens has and the more shallow depth of field the lens will achieve. 1.2 is super fast, 2.8 relatively fast and 4.0+ is slow, all relatively speaking of course. You will pay $$ more for faster glass or lower f-stop number.

1) So when it comes to adjusting for exposure a larger opening of the iris/aperature is a lower f-stop number (like f1.4 or f2.8) and a higher f-stop number (like f16 or f22) is a smaller opening of the iris/aperature. To expose for overexposure a high f-stop is needed, for underexposure a lower f-stop is needed.

2) For controlling depth of field it’s always easier to illustrate than to explain but suffice it to say a lower f-stop number means shallower depth of field or blur and a greater f-stop number means greater depth of field or more things in focus.

Depth of Field Comparison

F-stop affecting depth of field

There’s a few things left worth mentioning. It’s important to note that depth of field is also dependent on the proximity of subject to the camera and the distance between the subject and the background. “Bokeh” or quality/amount of blur will be more apparent with larger focal lengths like with the telephoto lenses. They will appear more shallow and pleasing as what’s happening is the background behind subject is being magnified therefore making the blur less distinguishable and obvious. Also it’s important not to get really crazy with shooting at really low f-stop numbers to get the shallow depth. For instance if you’re shooting you’re talent at f1.2 you’re depth of field is so shallow that their nose (foreground) is out of focus, eyes are in focus and ears are out of focus (background), clearly you’re shooting with too small an f-stop number. Here is an iphone/itouch app DOFMaster available on itunes that will that allow you to punch in focal length, measurements and f-stops and it will then tell you what depth of field (in distance) you are working with. This depth of field should be determined by what it is you’re trying to focus the audience’s attention on in the scene. It’s therefore advisable to shoot at higher f-stop numbers to include more of the scene, talent, objects or whatever you’re trying to focus on. As a rule of thumb it is good to determine depth of field dependent on what you’re trying to show your audience.

I hope that this information is helpful and we will continue to offer additional write-ups on these subjects from time to time.

Denver Riddle

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1 Comments

Pete JamesMarch 5, 2010 7:03 pm

Thank you, this is the best explanation I have had with regards to the whole new ball game that id HD DSLR’s my journey is about to begin, and I thank you for the education

Regards

Pete

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