Evaluating different lenses with the same focal length

- This week's Practicing Photographer is for those of you who are maybe a little bit new to photography and are confused by something when you go lens shopping. I have three lenses here. I have a 50mm lens, I have a 50mm lens, and I have a 50mm lens. Yes, they're all the same focal length. Why are there so many different kinds of lenses of the same length? These are actually three different brands, this is a Sigma, I'm sorry, a Zeiss, a Canon, and a Sigma. But even just within the Canon catalog, you'll find at least three different 50mm lenses.

What are the differences? When you go lens shopping, how do you sort through the fact that there are so many different choices for a given focal length? You may find yourself frustrated by finally figuring out, oh, this is the focal length range that I like, now I know what lens to go buy, and then there's all these choices. So, I'm going to walk through the differences between these three specific lenses, because they're pretty representative of what you're going to find when you go lens shopping. This is the Canon f/1.2 50mm, by the way, all of these lenses right now are Canon mount lenses, they're designed for Canon SLRs.

This is the Canon f/1.2 50mm lens. It gives the same field of view, because it has the same focal length, as Canon's other two 50mm lenses, the f/1.4 and the f/1.8. The big difference there is that maximum aperture, this can open to 1.2, whereas the other lenses can only open to, the other Canon lenses can only open to either f/1.4 or f/1.8, that's the main difference. However, there's some other stuff that's a little bit different. This has a different optical design that possibly produces a little bit less lens flare than those other lenses.

Thanks to coatings and the way that the lens has been engineered, it's going to fare differently in that regard. So, for the most part, when you go lens shopping, the main difference you're going to see is that maximum aperture number. That's what separates one lens from another. That's what separates one really expensive lens from another. That's also what makes a difference in lens size a lot of times. The bigger the lens, more than likely, the wider its maximum aperture can go. This is a 50mm lens and it's pretty large, and that's because it can open all the way to 1.2.

This lens, by the way, is what you're looking through right now, that's what's fitted on our video camera that we're shooting with. This Zeiss lens is very different from the other two in a couple ways. First of all, it's all manual. I cannot autofocus with this lens, nor can I auto iris, I have to use the focus ring and the aperture ring, so I give up a whole lot of automatic features on my camera. What I gain, is exceptional optical qualities. Zeiss lenses are always extraordinary and they carry a price tag to go with that extraordinary-ness.

This is a Sigma 50mm 1.8 lens, so this lens is slower than the 1.2, but it's much larger, it's very confusing in that regard. But it's because Sigma has worked very, very hard to put exceptional pieces of glass in this lens, and to build up a physical body on the lens that is very, very sturdy. So, really, all I'm talking about when I go for these different lenses are different levels of quality.

Quality is measured in a lot of different ways in a lens. Everything from sharpness from the corners, to the center, to management of optical aberrations and artifacts. Like, chromatic aberration, which is where you see purple or red or green fringing around high-contrast lines. Vignetting, which is where you see a darkening in the corners. Distortion, where straight lines bend on the edges of the frame. And finally, as a lens goes wider, a wider aperture, it might have a difference in the bokeh that it produces, that's the softening in the background as depth of field gets shallow.

So, when you go lens hunting, what you're going to want to do is read a bunch of reviews, and try to figure out what the differences are. You may find that your first filter is simple price point. You just can't afford the top-of-the-line, really fast lens, but that may be OK. I actually feel like, as much as I love this 1.2 50, and I own one of these, it's a great lens. I still think the Canon 1.8 50mm is just about the greatest lens deal in the world and Nikon has an equivalent. This is an exceptional 50mm lens that you can get for around a hundred dollars. So, it's just worth looking into the different elements of optical quality, aperture size, and mechanical features in any of these lenses, because they're going to vary from one to the other.

That's the difference that you're seeing when you see a bunch of lenses with the same focal length. Same thing with zoom lenses. You might see a 24-70 in slight variations and mostly what's going to be the difference there is speed, lens speed, maximum aperture, and whether it's that same maximum aperture all the way across the zoom range or not. So, just pay attention to the details and it's probably going to become clear what the differences are when you're looking at different price points of the same focal length.