Digital Photography Course #2 Focal Length: What Lens to Use?


Before you read this, please go through the first article: Course #1
And here’s a secondary disclaimer: I can’t teach you how to take good photos. I can only try to teach you how to use the tools for you to ‘try’ to take good photos. I’m still trying all the time myself! Besides, a good photo is entirely entirely subjective!

Focal Length
The age old question: “Does size really matter?”
The answer: Yes! Yes it does! Though the real answer is ‘Length’. Or the Focal length of a lens. But before I can discuss focal length, I must define what a ‘lens’ is.
Lenses, work just like your eyes. Light comes through and it focuses it into an image in the sensor. Zoom lenses are typically more complicated, as are wide angles. Think of it as a ton of light coming through, and smart engineers have to bend, refract, reflect, and all the stuff you never really cared for in Physics, to get that image to go through the sensor without distortion.

For example: this is what goes into a Nikon’s 70-200mm.
And part of the reason why that lens costs around $2000 USD.

Lenses come in two flavors: Zooms and Primes.
Primes, are generally lighter and less complex than their zoom counter parts. As such, they generally have higher apertures. For example, the internals of an 85mm.

How to read a lens:
Note it says: 85mm 1:1.2

This means it’s a prime lens. Its’ focal length is only 85mm, and the highest aperture it goes is 1.2.

Note it says: 24-70 1:2.8

This means it is a zoom lens. It’s widest is 24mm, and it can zoom up to 70mm. And the highest aperture it can go, on either end of the zoom scale is F2.8

Take a look at your lenses, they are usually marked at towards the barrel of the lens. Some lenses say: 18-55 1:3.5-5.6
This means at the lower end of the zoom. 18mm. The highest aperture you can go is F3.5 As you zoom in towards 55mm, your highest aperture is now 5.6.

The more you zoom, the less light can come into your lens. It’s much harder for those geeky engineers to defeat physics.

Authors Note: That is why generally zoom lenses with a single fixed aperture are so large and expensive. It takes giant glass to be able to suck enough light in, and
account for the differences at each end of the zoom spectrum. My 24-70mm looks like a mini rocket launcher and weighs a ton. But in actuality, it’s just a 24-70mm lens.

Back to the original topic
Ok, enough about the lesson in lens characteristics. The topic of this course is Which lens do I use? and why
The short answer is, you use whatever the heck you want to use. However, as with every discipline, there are guidelines.

14mm, is a bit wide for my tastes. It does have distortion towards the edges, and thus, I avoid using wide angles for anything other than landscapes, or if I want some wonky effects when shooting people. But on that note, the wider the lens, the larger the separation between the subject and the background is. It’s a matter of perspective

Too illustrate the point.
This is my good friend Optimus Prime, shot at 14mm. Notice the background and subject look pretty separated.

Now shot at 24mm:

Now shot at 200mm:

Trying to keep the subject the same size, I had to physically move backwards. Hence, my perspective has changed. Note, at that distance, using a 200mm lens, the optical illusion is that the background background appears to have moved much closer to the subject, or appears compressed. Contrary to popular belief, perspective isn’t a matter of what ‘focal length you use, but rather…where you place the lens. It’s not really compression, just perceived. Again, it’s just a matter of perspective. Ansel Adams repeats so often in his wonderful book, The Camera: “perspective is a function of camera-to-subject distance.”

So what does this mean in the real world? You can apply this to landscape photography, or portrait photography. When I shoot portraits, babies, dogs, etc. I like using 70mm, 85mm, or even 200mm focal lengths. It makes the subject a bit more pronounced. I want the subject to be dominant.


The same principle can be applied to landscape photography. When you’re shooting a picture of a mountain. Do you want a wider angle shot? The background will be small, less daunting. Or do you want to shoot it at 200mm, and the mountain you’re shooting will appear as if it is much closer than it is. Much more larger, grander, in scale? That all depends on how you want to shoot the scene.

in conclusion
Zooms and primes, each have their place in life. I used to have more zoom lenses, but I’ve since sold most of them off. I only have two left. The 24-70, and a 70-200. The rest of the lenses seen above are all primes. It’s a matter of preference. Zooms are more versatile in wedding or fast past ‘event’ driven photography. Whereas primes, (at least for me), work for everything else. Unfortunately, you need to switch lenses back and forth if you’re dealing with a prime lenses, depending on the situation. A zoom on the other hand, can just be used as is. I personally think zooms make you lazy. You don’t have to move, or think about things as much. In contrast, primes force you to move forward, backwards, trying to get you to think more about the angle of attack. But it takes some getting used to. If you’ve been reading my blogs for the past 2 years, it wasn’t until fairly recently that I was comfortable enough with myself to move to nearly a full ‘prime’ setup.

For multi-purpose shooting, I prefer a 35 or 50mm lens. Landscapes, obviously I’m a wider 14mm or 24mm type of guy. Portraits (single person), I’m a fan of the 85, 70, or 200mm variety. For family or group shots, I prefer 35, or 50mm. Sports shooting, well there’s no matter of preference there. I need all the reach I can get, so I generally grab a 200 or a 300mm prime for that.

So the million dollar question? What lens do you buy next?
Well, here’s a tip. Do not go out and buy this lens: 120mm F5.6
It costs $99,000.00 (yes I have the correct number of zeros). I guarantee if you bring one home, you will get a nice divorce. However, if you do have money to burn, you can impress your spouse by shooting images up to 2 kilometers away!! So the good news, is while your filling out the divorce papers, you can spy on other houses far away, as soon as you get kicked out of yours!

Well before you make your lens choice, you must know about your digital camera. What kind of crop sensor does it have? People who come from a film background really are not used to this. Film that is used is generally 35mm,(the width of the physical film). And lenses, are as they appear to be. However, most digital cameras are 1.6x crop. What does that mean? It means it has a smaller sensor size.

(these are rough numbers, actual numbers vary by manufacturer)
A full frame sensor is: 36mm x 24mm
A 1.6x crop camera is: 22.5mm x 15mm
A 1.3x crop camera is: 28.7 x 19.1mm

The smaller sensor size means: it is cheaper to manufacturer. Silicon to fill a full frame cost a boat load of money, they actually stitch the silicon together. Therefore during the manufacturing process, it is more error prone, and much more costly. To get an idea, one of the first standard full frame digital cameras to hit the market (The canon 5D, cost nearly $3k) at the time. A 1DsMKIII cost roughly $8k, when it was introduced, compared to it’s 1.3x brother, which was only $4k.
Yes, full frame digital cameras are expensive. But…what you gain from them, is more like shooting with film. Your lens, focal length is what it is, not hampered by the dreaded focal length multiplier

To illustrate, here’s a picture I recently took at the Academy of Sciences in San Francisco.

Notice I drew out a rough diagram of the sensor size, and what you would see for a 1.6x crop, or vs a 1.3x crop, vs a full frame.

So if I used my my 20D camera and shot a picture of the frog. I would get this image:

Whereas if I stayed at the EXACT same position, and used my 5D camera, I would get this image:

Nothing has physically changed in terms of distance. But, because the sensor on the 20D is smaller, it seems as if I zoomed in just a little bit. Hence, why they call it the 1.6x crop factor. What this means is, for every single lens you buy. You have to multiply it by 1.6x, and that will be the focal distance that you are ‘really using’.

Lets say you bought a 50mm lens on your shiny new D90 or 7D. You take 50mm x 1.6 = 80mm. That means your 50mm is roughly the equivalent of my 85mm lens on a full frame camera. If you bought an 35mm lens: 35×1.6=56mm. That’s why a 50mm lens may not work too well in a small room indoors. It will, get good portraits though of their faces! But if you want full body, you’d need a much much wider lens on a crop camera. Kit lenses that come with the camera are generally 17-55 mm. Or 17-85. It covers a good range, (remember to multiply that by 1.6 on your crop cameras). I avoid kit lenses because they’re typically too slow for my line of work. F3.5 indoors where they do not allow flash is not fast enough. However, there are Image stabilized (IS) lenses that cover that range, which I hear are pretty good.

Here’s a list of the cameras (to my recollection). I’m sure there are more out there, and I really don’t care. This is just to provide general information.

1.6x APS-C format sensors.
Nikon calls it DX format which is 1.5x crop
Canon’s are just APS-C: which is 1.6x crop
modelsCanon: Rebel xti, 10D,20D,30D,40D,7D
Nikon: D40, D60, D80, D90, D300

Nikon: I don’t believe have a 1.3x crop body
Canon: calls it AP-S sensor
modelsCanon: 1D, 1DII, 1DIIn, 1DIII, 1DIV
Nikon: n/a

Full Frame
Nikon calls it: FX
Canon: just calls it full frame.
modelsCanon: 5D, 5DII, 1Ds, 1DsII, 1DsIII
Nikon: D700, D3, D3x, D3s


About greenbeanfx

Photography is what I do =) If you wish to contact for a photoshoot, send me an email or comment on a blog with your info and I'll get back

Posted on April 26, 2010, in Aperature and tagged , , , , , , , . Bookmark the permalink. 2 Comments.

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