Lenses: Understanding the Numbers

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 1.8 must 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…

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ariana says:

Kenton, I think you misunderstood my use of equivalent.. what I meant is that if you put a 50mm lens on a crop body you will see the same as (the “equivalent” to) putting an 80mm lens on a full frame camera.

Kenton says:

Just one correction to your post. A 50mm on a full-frame is indeed the closest thing to what the naked eye sees. However, on a crop sensor the equivalent would not be an 80mm, but instead would be more like a 35mm. The reason being that your crop sensor is already zoomed in closer then the full-frame to begin with, hence the term cropped. I just thought it would be nice to clear that up.

Glory says:

Love this blog!!! great info… I don’t know much about photography… just trying to take beautiful pictures of my daugthers(15, 5 years & 1 month) :) anyone have a rebel xsi? (I don’t have one yet, but I’ll get one) I want to buy a lense for it but don’t know which one… any recommendation? I would like to take like does newborn pics… Thanks!

Angie says:

I just stumbled across your blog. I absolutely loved this post!! It made lots of sense! Thank you!!

Ashley says:

oh bummer! good to know about it not working with the mark II…that was a beauty all right! :)

ariana says:

Ashley, I don’t still have the sigma, I LOVED that lens.. had to sell it when I got the mark II because it only works with crop sensor lenses. But it’s an amazing piece of glass, I miss it!

Ashley says:

great job explaining! I so want the sigma 30mm do you still have it? I don’t see it on your camera bag list??? Your work is beautiful & I sooo appreciate the time you take to explain everything!!! You rock!

Alex says:

I just stumbled across your blog and looove all the posts you have about photography. I love all things photography and have done a few senior girls on the side for fun and now love photographing my daughter (although with a 4 week old and trying to get into the mommy routine I rarely take out the camera!). Thanks for posting all of this info! Your baby is just beautiful :)

ariana says:

Rebecca, thank you so much!

Alex, I know just how you feel, it’s so hard, but worth it later to remember those precious first months..

Rebecca says:

THANK YOU! I just love your blog. Your photos of Jasper inspired me, some months ago, to invest in a DSLR. (I have a 7 month old.) I had NO idea of the quality difference between a “real” camera and and a point-and-shoot. My photos now are amazing. People think they’re done by a professional. And I’m just letting the camera go on auto! I feel bad that the photos of my two older kids aren’t as beautiful. Your explanation of lenses is really useful. I just bought the Nikon prime 35mm/1.8 and LOVE it…did this b/c of your beauts of Jasper with your prime. I’m really just so grateful to have found your site!

ariana says:

Sarah, that is such a tough question! I think I would have maybe focused less on the body and gotten a used one and put my money towards lenses. I certainly wouldn’t buy the kit lens, I would buy the body only if I were to buy new. I am torn between recommending the 50mm vs just going for something like the Sigma 30mm. The reason is the Sigma is so much more money but I feel like I outgrew the 50 pretty quickly because I wanted a wider angle. It didn’t help though that my 50 was slightly defective and not as sharp as it should have been until canon adjusted it!
I think that it sort of depends on the investment you are wanting to make initially. If you only have x amount of money, invest in the body and the 50 first and then as you get more funds upgrade from there (similar to what I did). If you have the funds now, skip the 50 and go right to something like the Sigma. I hope that helps!

Sarah says:

I find myself reading your photography posts sometimes even though I still only have a little Canon Elf point-and-shoot! Now that Finn is here, though, I’ve decided that it’s about time I invested in a nicer digital, largely based on what I’ve seen and read on your blog. If you could go back in time with what you know now, would you do anything differently in terms of the order of your purchases? What should I invest in first? What should I avoid? You might say that a lot will depend on my preferences and lifestyle, but I can tell you that this time around I’m more interested in quality than convenience. I’ve still got a point and shoot for those impromptu, quick-pull-the-camera-out-of-your-pocket moments; what I want NOW is artful, better-quality shots that I can plan for… especially close-ups!

Kimberly says:

I tried to comment on this earlier from my iPhone but I lost it before I could post. :(

This is a really nice, clear explanation for those who are trying to figure it out! I’d add that focal length is very simply the length/distance in millimeters from the camera’s light sensor to the end of the lens. So a 30mm wide angle lens is quite short while a telephoto 100mm lens or 200mm lens will be much, much longer. That’s about as technical as even a serious hobbyist needs to really get! :) Longer length = closer view of the subject. It’s the reason (combined with the 1.6x crop factor that my camera also has)that I struggle with the 85mm lens… while I adore it for its beautiful portraits, low-light capability and bokeh, I have to be quite far from my subject which is challenging even in my relatively roomy house. That being said, I love that I can borrow it from my FIL on an as-needed basis.

The zoom lens that came with my 40D is actually a pretty nice piece of glass. It’s the Canon 28-135mm f/3.5-5.6 and while it doesn’t stop down nearly as far as my 50mm f/1.4, it still takes some very nice, sharp photos, even with less than bright light. I use it quite a bit. And I find that even with the smaller max aperture, I still get some pretty decent background blur in the f/4-5 range. (In my photography class, my friend – the instructor recommended starting out at 5.6 and working down from there as the light dictates… you can still get very nice bokeh at that f-stop, and that way you give yourself better odds of nailing your focus.)

I don’t mind “zooming with my feet” so I use my 50mm the majority of the time, but there are times when that focal length just doesn’t work. I think my next lens will be a more high-end zoom, but I go back and forth on the primes, because I love the quality for the money… it’s a slippery slope and an expensive habit, as you well know! ;)

Would you mind sharing the photographer’s blog that you mentioned? Oh, and I agree with Michelle Louise and I’m a GREAT fan of digital-photography-school.com, as well! :)

ariana says:

Kimberly, thanks for adding your knowledge!!

The photographer I was talking about is Jasmine Star. She has a wonderful FAQ category on her blog where she is very open and shares so much:

ariana says:

That’s pretty much it Lou!

Colleen, wow, I’m so glad! Probably it’s understandable because I just barely understand it myself so can’t get too lost in the technicalities that are gibberish to me as well :)

Alicia, you do more with a point and shoot than I’ve seen many people do with fancy cameras. When you do get a DSLR (hopefully before you win the lottery LOL!) lookout world!

Meredith, which lens is it?? is he being facetious? I think there are a whole slew of husbands out there that hate me for the millions and sundry ways I’ve found to spend their family’s money ;)

Meredith says:

Very good explanation. And my husband would really like to thank you for showing me exactly which lens I MUST have before travelling to Texas for his brother’s wedding, at which I will be the unofficial (because they’re not having a photographer, but maybe she’d take a few, since she has the camera and the experience) photographer.

Alicia says:

wow – what fab technical writing skills you have!

I’m kinda like Lou, only way worse – because I’m still operating with a humble point-and-shoot (Kodak! jeez) and a really tight budget, and besides all that, have not done a dang thing to learn anything about photography and getting a decent pic.

But I enjoyed reading about this anyways. I’ll definitely buy all your camera bag recommendations soon as I hit that lotto jackpot!

Colleen says:

Thank you, thank you, thank you!! I have taken three photography classes (two in HS and one in college) and none of them explained all of this better than you just did! I just bought a Canon Digital Rebel (just the XS not the XSi, but still, it’s awesome!) and figuring all of this stuff out has been confusing and frustrating to say the least.

You just explained to me in five minutes what I havn’t been able to figure out for years! All of your photography posts are extremely helpful and I always come away with a million lightbulbs going off above my head.

Did I say thank you yet? :-)

Thanks Ariana! Thanks Michele!

I think I understand, lol. It seems that prime glass is better than zoom glass, and that you’re likely to get bigger apertures, too, thereby allowing more use of natural light, which I’m sure beats flash.

However, if you’re just an amateur mommy photographer and/or don’t have much money, I’m thinking you’d get more bang with a zoom. Sure, you’re sacrificing quality, but you have more options. This wouldn’t be the case if I could afford individual prime lenses at all different focal lengths. But I’m guessing that’s not an option for most moms who still want better quality than they’d get with a point & shoot but can’t afford to buy 5 different lenses.

The software alone — PhotoShop, Lightroom — is expensive enough!

And you’re so right Ariana –if you had multiple photographers, if you were a pro at photography, if you weren’t photographing a baby who just as soon is crying as smiling, switching between lenses would be cake. But that’s not super practical, not unless you’ve “set-up” a photo shoot of sorts.

There’s just so much to learn!

ariana says:

Michele, great point and thank you for that link! It explains it much better than I did :)

KaiRayne says:

Thank you for the refresher. lol I took a photography class in highschool, but that was 14 years ago and I don’t remember much and it certainly wasn’t with digital cameras, which I’m still getting used to. lol

I think you did a good job explaining, and even invented some words along the way. :)

About the prime vs zoom thing that Lourdes mentioned, you frequently get a sharper image from a prime lens at the same price point as a zoom which is a common reason people go with them beyond the wide aperture. The reasoning behind this is less glass in the lens, less moving parts, means they are easier (ie cheaper) to make so a $400 prime lens will probably have better glass etc in it than a $400 zoom lens.

Here is a short article for anyone interested in understanding the crop vs full frame issue and how that impacts your focal length
Basically it boils down to the focal lengths being based on 35mm film size (and the sensor in a full frame camera is that same size) but in a crop body the sensor is smaller, and just takes the center part of the image making the focal length SEEM longer.

ariana says:

Whoa! You guys actually made it through that morass? Yay! :)

Savannah and Kari, thank you :)

Lou, you bring up some good questions, and there isn’t an easy answer. So much of it depends on your shooting style and preference and opinion to some extent.. for example most people believe that prime lenses are the best quality glass period. So it’s better to own say 3-5 prime lenses that cover the approximate range of one zoom lens in terms of quality.. but there is obviously a practicality trade off! Who has time to switch lenses a million times when you have a baby toddling off in another direction?

It also comes down to preference in terms of speelite vs natural light and also primes in terms of your involvement with the scene. I’ll just give you one example of a brilliant photographer who’s blog I was just reading. She says that she prefers to use primes even at weddings because “zooming with her feet” makes her interact with the scene physically in a way that she does not with a zoom. Also, she photographs using really really wide apertures and uses natural light even when most pro photogs would at least use fill flash.

One thing that I didn’t touch on is (as you know!) how hard it is to nail focus using a really wide aperture. I’m not sure how she does it! It’s really hard and that’s where a speedlite comes in handy because you don’t HAVE to shoot wide open.

But, I haven’t directly answered your question yet.. the answer about the speedlite is that while I love my speedlite and can’t recommend you DON’T get one I wouldn’t plan on using your kit lens and liking the results much. The glass on the kit is pretty crappy, and you won’t get much bokeh on the zoomed in side if your max ap is 5.6.

Another thing I didn’t mention is that all lenses are sharpest about 2-3 stops DOWN from their max aperture.. so the kit lens even if you are shooting wide open at 5.6 while zoomed in won’t be very sharp anyway. Image stabilization or not this is a lens I wouldn’t wan’t to use much. I never touch mine.

The Tamron is a great range and also is really sharp.. but even 2.8 isn’t that wide and so I find myself using the speedlite with it indoors most of the time.

As for your question about having a prime in the same range as covered by another zoom.. it’s tricky. Since I mastered the speedlite and can use it with the Tamron I haven’t been using my Sigma as much. But for an all indoor photo shoot I think I would use my sigma because it is a tad sharper and I can get some all natural light shots.

Again, there is no one good answer which is why most people who love photography like we do get pretty addicted to adding to our lens collection!!

kari says:

you are amazing.

Savannah says:

I have been following your blog for awhile. I really enjoy the pictures you take and your photography lessons you give. I now understand the meaning of all the numbers and letters that describe the lens. Thank you so much for taking your time to explain. :)

The numbers really do get all confusing!

Thanks for explaining about the crop body camera.

What I’m still not clear on is when it comes to making the choice between a zoom lens and a prime lens…take your Tamron lens — it covers the 30mm range. But also many other distances. Why then would it make sense to purchase a prime 30 mm lens, too? It is because the prime lens allows more light in with a bigger aperture?

I LOVE that my 50mm lens has such a nice big aperture (now that it’s fixed anyway, lol), but I hate that you have to physically move in/move out to get the shot you want. I’d much rather have a zoom lens. So I’m trying to figure out on what lens I should spend my money next. I wonder too if first it wouldn’t make most sense to get the speedlite…you know, to make my kit lens go further in the meantime.

In the future, I think I’ll want my lens to have image stabilizer (IS) capability as well. My Canon kit lens does not; my friend’s newer one does: I think it makes a huge difference!

So many things to consider! Your opinion is valued.