# What focal length gives a "normal" field-of-view on APS-C cameras?

I wish to purchase a "standard angle-of-view" prime lens for my Canon Rebel, which has an APS-C sized sensor. Various articles note that the popular "nifty 50mm" lenses are a little too much of a telephoto on these cameras to work well as an all-purpose walk-about prime.

What focal length should I look for that would have a "standard" angle of view similar to the unaided eye?

Using the 1.6 crop factor certainly works, but it might be interesting to work it out from first principles, too. The "normal" focal length is generally considered to be close to the diagonal of the image area (sensor, film, whatever). For 35mm film and "full frame" digital, this is about 43mm - 50mm is the closest common focal length for reasons that are interesting but probably not relevant here apart from indicating there's some range for variation.

So, another way to determine which lens is to find the dimensions of Canon's APS-C, and apply some Pythagoras:

sqrt( 22.2^2 + 14.8^2 ) = 26.68


So for a Canon APS-C, you might consider anything from 24mm* up to about 35mm as a good choice for a "normal" lens. If you wanted to get as close as possible to a 50mm, then the 30mm mentioned is likely a good choice, which we can see by comparing the ratio of 50mm to the theoretical 43mm:

50mm / 43mm = 1.16
35mm / 26.7 = 1.31
30mm / 26.7 = 1.12 – closest to 50mm
28mm / 26.7 = 1.04 – closest to theoretical normal
24mm / 26.7 = 0.89

• As an aside - this 'measure the diagonal' is how they determine the crop factor in the first place: 43mm / 26.7mm = 1.61. The two methods are equivalent, I just think it's interesting to know the theory too. Jul 23, 2010 at 23:41

30mm. Sigma make an excellent 30 f1.4 that would work well as a normal prime. http://www.amazon.com/Sigma-30mm-Canon-Digital-Cameras/dp/B0007U0GZM

• I can second the Sigma 30mm f/1.4 -it's a great lens. I have the Canon 50mm f/1.8 II as well, though, and I tend to treat them interchangeably. I usually throw one or the other in my bag. If you don't mind the build quality of the "nifty fifty" it's just as sharp, not much different as far as the angle of view, and a whole lot less expensive. Jul 22, 2010 at 21:09
• just be careful as it tends to back/front focus. You may have to bring it back to the store to replace or just use microadjust on your camera. Jan 23, 2012 at 4:52

The "focal length multiplier" to get the equivalent angle of view for your camera is 1.6, so to get the same angle of view as a 55 mm lens, you should get a 55 / 1.6 = 34 mm lens.

• Why the downvote? If you don't explain what it is that you think is wrong, it can't improve the answer. Jul 22, 2010 at 21:09
• I didn't downvote, but I suspect whoever did finds "focal length multiplier" to be a harmful term. Feb 17, 2011 at 14:10
• Also, you don't address the last part of the question ("What focal length should I look for that would have a 'standard' angle of view similar to the unaided eye?"), although that hardly seems a reason to downvote. Feb 17, 2011 at 14:11

The "standard" or "normal" focal length that gives a similar field of view as that of the human eye is between 45-55mm depending on who you ask.

On an APS-C sensor the equivalent range would be range would 28-34mm.

If you are looking for a Canon Lens two reasonably priced prime lenses are:

If you are ok with third party lenses:

• You could also add the slightly more expensive Canon EF 28mm f/1.8 USM. I've got it and it's a lovely lens. Aug 10, 2010 at 15:50

The crop factor/focal length multiplier of 1.6 Guffa mentions applies to Canon cameras. This answers the original question but it doesn't apply to all cameras. If you're interested in the crop factors for other brands, a list is here:

http://en.wikipedia.org/wiki/APS-C#Multiplier_factors