Guest Post – Understanding the Numbers – Becoming Mom

Today’s guest post is from Ariana with Becoming Mom.  I met Ariana at the Adobe Girls Getaway earlier this year and we instantly bonded.  She’s an awesome mom, photographer, and she also created the Photographer’s Wall Display Templates. You must check them out if you’re a photographer!

Thanks Ariana for this awesome and informative guest post! Enjoy!

When I got my camera about a year ago, I knew I would need to get a better lens than the kit lens, but I wasn’t sure exactly which one.  I would read amazon reviews of this or that lens, but found it hard to comprehend exactly what was different about them because the numbers didn’t mean anything to me yet.

Fast forward a year later and I may not understand the MATH behind focal length, or maximum aperture, but just like a newly acquired language they MEAN something to me now that they didn’t then.

If it confused me at first, chances are it confuses some of you, so here is a lexicon of sorts of what the numbers actually mean when comparing lenses.

Let’s take some of the lenses I own as an example, in chronological order of when I bought them.

First, there is the kit lens that came with my rebel.

18-55mm f/3.5-5.6 IS Lens

There are two bits of essential information here.  The first comes in the numbers before the “mm” (milimeters) which is the focal length.  The second are the numbers that come after the f which are known as the maximum aperture.

Here we have a focal length of 18-55 and a maximum aperture of 3.5-5.6.

Why are there two sets of numbers for each separated by a hypen you ask? Because this is a zoom lens! The first set of numbers represents the focal length when zoomed all the way OUT, and the second  numbers represent the lens in it’s zoomed IN state.  So if you take the kit lens and zoom all the way out you are at a focal length of 18mm and your maximum aperture is f3.5.  If you take the lens and zoom all the way in, you are at 55mm and a maximum aperture of 5.6.

It is much simpler when we are talking about a non zoom or “fixed” lens, commonly referred to as “prime” lenses (a prime lens is just a non zoom lens).   So let’s take the almost universal first upgrade lens, the Canon 50mm 1.8.  This lens is a fixed focal length of 50mm and has a maximum aperture of 1.8.

Ok, so now that you know what the number are  telling you, what in the world do they mean?

Let’s start with maximum aperture because it’s a bit easier to understand.  Aperture is basically how wide the “hole” in your lens is when you take your picture.  Think of the aperture like the iris of your eye.. it needs to get bigger to let more light in when it’s dark and it needs to get smaller when there is a lot of light. Aperture is the same. You control the size of the opening of the lens to determine how much light to let in.  The other aspect of how much light to let in is shutter speed (which is how long the camera gathers light for, expressed  in fractions of a second…the longer open the more light comes in). We won’t address shutter speed because that doesn’t change from lens to lens. What DOES change is how wide open the aperture can go from lens to lens expressed as half  f-stop numbers or scale.

Those numbers are:  1.0     1.2     1.4     1.7     2     2.4     2.8     3.3     4     4.8     5.6     6.7     8     9.5     11     13     16     19     22

The tricky part is that the lowest number = the wider maximum aperture = the most light!

So my Canon 50mm 1.8 lets in a ton more light than the kit lens even when the kit is zoomed out and is gathering it’s most possible light it is still only at 3.5, which is much  less than the 1.8.  And my Sigma 30mm 1.4 lets in even more light than that.  There is also a whole bunch of tricky math that I don’t understand but the amount more light that the 1.4 gathers than say the 1.8 is somehow exponential, so each intervals between these numbers represents a lot more light for your money.

And so now you understand why so many people upgrade immediately and never use their kit lens!

If you zoom all the way in on the kit lens, the lowest aperture you can use is 5.6. That sucks. Unless you are outside in full light you would pretty much HAVE to use a flash to gather enough light.  And that’s why even though it’s almost the same focal length “range” as the kit lens, I bought the  Tamron AF 28-75mm f/2.8 Lens. Even though this is also a zoom lens, notice it only has on f number listed: 2.8.  It has what is called a “fixed” maxiumum aperture of 2.8, in other words you can use 2.8 throughout the whole range of it’s zoom, not JUST when it is zoomed all the way out.

Ok, so now that we know about maximum aperture or f number, what about the numbers before the mm (focal length?)

This gets really complicated and to tell you the truth I don’t understand the math of it too much. Here’s what’s important to know though.. the lower the focal length the more you will see in your image. The higher the focal length the more zoomed in your image will be.

If you’ve ever looked into the viewfinder of a camera and noticed that the scene through the camera looked different through the viewfinder than what you see when you look up at the scene with your naked eye, you were experiencing how focal length effects the “zoomed in-ness”. (I just made that term up!)

On a full format camera a 50mm lens is about equivalent to what you see with your naked eye. If you get a lower number than 50, it is what’s known as a “wide angle” lens because it’s will actually show you MORE of the scene than your naked eye. Really low focal length like at 18mm or so actually gives you distortion (a fisheye lens is an extreme example of this).  But you can fit so much more in the picture! Which makes sense why landscape photographers prefer wide angle – they want to get as much of that beautiful landscape in the frame as possible.

What about the other end.. what about say the  focal length of the 85mm 1.8 that I have been lusting after? Well, this will be zoomed in compared to what your naked eye will see.  When you get into high numbers like this they are  known as telephoto lenses.  Some are soooo zoomed in that when you look in the viewfinder it’s like looking in a pair of binoculars!  These are the lenses that private investigators and paparazzi use!

Now here’s where it gets tricky again. Remember when I said on a full format camera 50mm is like what your naked eye sees? Some of you who own the Canon 50mm 1.8must have thought I made a mistake because as you know, when you look through the viewfinder you ARE zoomed more in than “real life”. That is because most of us don’t have full format cameras!  Only the MOST expensive DSLRS are full format. Most of us have what is known as “crop sensor” or “crop body” cameras like the Rebel.   On a crop body camera a  50mm becomes the equivalent of an  80mm lens. The how and the why gets pretty technical, but all you need to remember is the number “1.6″. That is the number that you have to multiply the focal length by to find out what the focal length equivalent will be on your DSLR.   (50mm x 1.6 = 80mm).

So 80mm is pretty zoomed in.. and that’s why if you have the 50mm and try shooting your baby’s cake smash photos you will find yourself having to back up, and back up, and keep backing up and maybe even run out of room when trying to get the baby, the baby’s hat, the cake and everything you want in the scene to be included without cropping some of it out.  i.e. this lens isn’t “wide” enough to fit everything in the frame which is why I ended up buying the Sigma 30mm f/1.4.  30mm x 1.6 = 48mm, or roughly the equivalent of a 50mm on a full frame camera, which as I said earlier is similar to what the naked eye sees.

To drive the point home even more, if you ever upgrade from a crop body like the rebel to a full frame camera all your lenses will technically have the same focal length as before, but they will ACT like they are totally  different!

Are you still with me? It’s a little confusing I know.. Ok, maybe a lot confusing and I probably didn’t explain it well at all.  But hopefully some of this makes sense to someone and maybe I have helped you start to understand this strange language of lenses just a little bit more.

Let me know if you have any questions.. and if you are a “real” photographer who understands this stuff better than I do, let me know if I made any mistakes!

Happy shooting…