I have the same camera as you have, the Canon 550. I have the old version of the 50mm lens, and also the pancake lens that you mentioned. I do shoot a lot of portraits and when I do it is usually with the 40mm lens, but the new 50mm is probably a good buy.

Hi Hari,

The best way to decide between the two prime lenses is to choose the focal length that you're most likely to favor (and use) rather than being forced to adapt to it.

If you find the field-of-view of a 50mm on a cropped sensor body particularly more appealing, you have several choices in a 50mm prime depending on your requirements and of course, budget:

1. Canon EF 50mm f/1.8 STM
2. Canon EF 50mm f/1.4 USM
3. Canon EF 50mm f/1.2L USM (if you can afford this!)
4. Sigma 50mm f/1.4 DG EX HSM Art

At present Canon only offers one version of a 40mm prime and it's the EF 40mm f/2.8 STM. This pancake lens may not be as fast as the EF 50mm f/1.8 series (including the discontinued Mk II model) as it's one and a thirds of a stop slower, but it gives you a wider field of view equivalent to 64mm on a full frame dSLR. You can fit more into the frame with the EF 40mm f/2.8 STM than any of the 50mm primes listed above.

On the flip side, the EF 40mm has a deeper depth-of-field than a 50mm f/1.8 lens by virtue of its wider focal length and its slower f/2.8 max aperture. Therefore if you prefer a shallower DoF and better background blurring, go for a 50mm prime.

Personally I prefer to use my EF 40mm f/2.8 STM as I find a 50mm kind of tight for a general walk-around prime lens on my 60D. Now if I had a full frame body like the EOS 6D, then I would definitely buy an EF 50mm f/1.4 USM. :o)

For the occasional macro shots, buy an inexpensive extension tube to allow the lens to focus more closely than its natural minimum focusing distance. An extension tube may not allow the flexibility like a dedicated 1:0x magnification, macro lens does but at least you can move the lens closer to the subject - hence larger looking closeup shots. :o)

stratman² (2 many pix!):

Exactly. I feel the 50mm is too narrow for me, but is 40mm that much wider? I am unable to decide this just by looking through the viewfinder on my 18-135mm.

I've not really explored portrait photography, so I feel tentative about this lens. Probably purely for the extra F-Stop I might get the 50 mm 1.8 over the 40mm pancake.

harishankar:

On a cropped sensor body, my answer is a resounding "yes". :-)

A 50mm f/1.8 lens on your 550D will be equivalent to an "80mm, f/2.9" lens on a full frame body (the f/2.9 is if you want to get technical with the aperture viewpoint with crop sensors), while the EF 40mm gives a field of view equivalent to a 64mm lens on an FF camera.

50mm is pretty limiting when your indoors in a small cafe, restaurant or living room - if you want to include as much edge detail as possible.

For this reason I no longer use my old EF 50mm f/1.8 II, preferring the more compact EF 40mm f/2.8 STM or EF 28mm f/1.8 USM when traveling. I also have the EF-S 60mm f/2.8 Macro USM and the EF 85mm f/1.8 USM primes - but they are not the lenses that I would carry along in my camera bag - except if I know in advance that I would be taking people portraits, e.g. a wedding event. :o)

Anyway, if you have decided on a 50mm - go for the current EF 50mm f/1.8 STM even if you're able to find the discontinued EF 50mm f/1.8 II in a store at a cheaper price. The STM version is designed with a new optical formula and is sharper at large apertures than the older model.

There's one caveat with STM lenses though: they consume more power than an equivalent focal length, non-STM version. These are focus-by-wire lenses and power is required even in manual focus mode. Non-STM lenses are also able to "focus" mechanically when they're not attached to the camera body.

So don't be surprised if your 550D's battery goes down quickly when you're having fun with the EF 50mm f/1.8 STM! ;-)