Lenses

I don’t think there is any question asked more at internet photography forums than” What Lens should I buy” and I don’t think there is more confusion than the answer that people get. There just seems to be a general confusion about lenses and why do we even have more than one or why do we have this one over that one. So I thought I would take a moment and talk about lenses. They are after all, our camera’s eyes. 

Crop Factor

Before we talk about anything, let’s take a moment and talk about “Crop Factor”. When camera focal length is expressed they are expressed in a length that would be in play on a 35mm Film Camera or a “Full Frame” digital camera. However camera with a smaller sensor APS-C or 4:3rds cameras have a crop factor to them that must be taken into account to understand how you will actually see using this lens. 

A 50mm lens on a Full Frame Camera or an APC-C camera is still a 50mm lens; however, the APS-C camera with its smaller sensor only sees a smaller area of the lens area. This is the crop factor. For most Canon’s this is 1.6x for most Nikons this is 1.5x and for 4:3rds camera this is 2x. So we multiply the Focal length times the crop factor and that would tell us what the lens would look like on a Full Frame camera. In other words it will have the same Field of View (FOV) 

So in our instance a 50mm lens on Canon 60D will have an effective FOV of  an 80mm lens on a Canon 5D MKII 

All the images in this tutorial have been shot on a Full Frame Camera (Canon 5D) and are expressed in full frame focal length. So you if you want a lens that looks like a  particular FOV that I have taken a photo of, make sure you know it will look different on your cropped sensor camera. So do the math to find the focal length that will look like it looks on your camera. 

Focal Length

Why do we have different focal length lenses? The first reason that comes to most people’s minds is “Magnification”. We have a subject far away so we want to bring it closer with a telephoto or we have something large in front of us and we want to fit it all in with a wide angle. But as I will discuss in a bit, that may not be the real or most important reason we choose different lenses, but it could be. 

Here are what different focal lengths look like standing in the exact same spot shooting the same subject.

We can see, if we wanted to photograph the entire cliff area from where we were standing, we would need a wide angle lens (17mm). But if we wanted to shoot the Lifeguard Tower and were unable to get close to it (As may also be the case when shooting wildlife that we wouldn’t want to scare away) we would need a telephoto lens (200mm). 

And that maybe all you need to know, but it doesn’t tell the whole story because that is not how we may actually or we SHOULD actually be shooting and what the real and major differences may be in Focal Length. 

The real difference between different focal lengths is not so much about Magnification as it is about Field of View and Perspective. 

Field of View is the width of the View that the angle of view provides with which the lens sees. A wide angle lens has a Wide Field of View, and a Telephoto lens has an increasingly narrower Field of View. We can see that clearly with the images above. and the Diagram below

Perspective is: the perceived relation ship of the background in our image relative to where our subject is. If we have a subject 30 Feet in front of a brick wall and we keep them at that distance. That distance never changes. But depending on Focal length, the perceived distance from subject to background will change as we increase focal length. 

So now knowing these two things, lets look at what happens to the look of an image when we not only change focal lengths but move accordingly to keep our subject framed exactly the same in every image (equal magnification) 

So looking at our images what do we see? Besides some angular distortions which I will discuss in a bit, the look of our subject remains relatively the same. But look at our background relative to the subject. As we change focal lengths from wide angle to telephoto the amount of background visible gets smaller and smaller. Also the perspective gets closer  from subject to the background as we move from Wide Angle to Telephoto

These are two things that you should keep in mind when choosing one particular lens over another as they can change the look of your image entirely. 

Focal Length Ranges and suggested focal lengths

These are only suggestions as I have shot Portraits with 24mm lenses and shot landscapes with 200mm. I have included the ranges in both Full Frame and cropped equivalents in parenthesis. 

Wide angle lenses: 14mm to 35mm (8mm – 24mm)

Normal Lenses: 35mm to 60mm (24mm – 40mm)

Mild Telephoto: 70mm to 180mm (50mm – 120mm)

Long Telephoto: 200mm to 800mm+ (120mm – 500mm+) 

Suggested focal lengths (NOT hard and fast rules) 

• Landscapes: 14mm to 35mm Wide Angle lenses
• Portraits: 85 to 135mm have been the portrait standard, many photographers are turning to 200mm as the go to portrait lens (on Full Frame) Popular Zoom Lenses for portraits are the 24-70mm and the 70 – 200mm
• Macro: 50-60mm if you can get close to your subject i.e. flowers. 100mm or longer if you have a subject that would be easily scared away i.e. insects. Use TRUE macro lenses that have 1:1 capabilities and short minimum focus distances.
• Wildlife: 200mm if you are shooting in a zoo or your backyard. For serious Birding or Wildlife in Nature 400mm+
• Sports: depends on the sport and distance to subject: May be as short as a 70-200, But 300 – 400mm telephoto are the more common
• Automotive: A wide variety depending on what you are shooting, Whole car shots will look good in Midrange lengths 50mm to 100mm. But for certain shots a wide angle looks great. Doing detail shots of parts of the care may bring back out that 50 to 100mm range
• Architecture: Because we usually don’t have the ability to move back, wide angle lenses are standard faire. However if you are serious into Architecture you should look into “Tilt-Shift” lenses. These allow for correction of angular distortion when you are unable to shoot on a level plane to the building/room. I.e. shooting up or down on something. 

Angular distortions

Here is the same image shot at 17mm and also at 105mm. As we can see in the 17mm there is a lot of angular distortion. Sometimes it’s good to use that for a look or whimsy to our photographs. But other times we need to have something look as good and natural as it can. Thinking of this in a portrait role, using a wide angle lens close up to a person can cause distortions in the face that may not be appealing and will make an area closest to the lens look larger than it actually is, i.e. giving someone a big nose.  (Again notice the perspective compression in the 105mm image)

The Depth of Field Semi-Myth

Here are the 3 things that affect Depth of Field (DOF)

• Aperture
• Focal Length
• Distance to subject 

All are absolute truths, however in practice, “The only thing that affects DOF is Aperture” HUH? Why?…

Why? Because we move.  

In practice we keep our subject framed the same regardless of our focal length. We move a distance equal to the change in our focal length; therefore the two cancel each other out. 

Look at these three image shot at: 

50mm at 5’ from subject

100mm at 10’ from subject

200mm at 20’ from subject 

Even though the DOF looks different in each image, in reality the DOF is, for all practical purposes, the same (there can be some slight differences especially as we get closer to the Hyperfocal distance of a lens/aperture) 

But why do they appear different? Why does it look like the 200mm has a shallower DOF? Perspective Compression: Because that lens focal length bring the background into closer view (even though the distance hasn’t changed) we can better see that the background is out of focus, even though it is  out of focus the same amount in the  wider angle shots. 

What to look for in a good quality lens

Lenses are rated on a number of things.

• Build quality: Use of good materials and manufacturing
• General Sharpness
• Sharpness throughout the aperture range: ( Most lenses are sharpest stopped down a few click from their maximum aperture)
• Chromatic Aberrations or CA: This is a condition where all the frequencies of light  don’t align correctly and it is seen as color banding at the edges of objects or edges of contrast
• Edge Distortion: Some lenses will start to distort the image as you move to the outer edges of the frame
• Focusing: a good, fast accurate and silent focusing motor
• Constant Aperture: Better lenses will have a constant maximum aperture throughout the zoom range. Less expensive lenses will have a different aperture depending on how zoomed out you are. You will often see them expressed such as this. 17-85mm f/4-5.6. This lens has a maximum aperture of f/4 at 17mm and f/5.6 at 85 mm. As opposed to say the 24-70 L f/2.8 which has a constant aperture regardless of zoom.
• Constant Focus Lens or a “Parfocal” lens. This is most often found only in high end lenses. This is a function where the lens maintains focus even as you zoom or change focal lengths
• Vignetting: Look for lenses that do not vignette or have darkness near the corners or edges. Some lenses will only vignette at wider apertures so check at all apertures. Vignetting IS easily fizxed though and even some great lenses do do it. So it’s not always a deal breaker
• “Fast Lenses”: Lenses that have a wider maximum aperture are called fast lenses because they let in more light allowing for a faster shutter speed. Most people want them because they allow for a shallow DOF. But they also have other benefits such as working in lower light, a brighter viewfinder and they allow Auto-focus system to work better. On most auto focus systems the outer focus points are sensitive or effective down to only f/5.6 light (Heavy Shade) The center Focus point is effective down to f/2.8 (dusk) but you can only take advantage of that if you have a lens with a maximum aperture of f/2.8 or better
• Bokeh: I have to mention this because it is on everyone’s lips. But better lenses are said to have a better bokeh or quality of the out of focus area. I refer to it as Boke Ehh, more on that later

Zoom Lenses vs. Prime Lenses

There is always a big debate over Zoom Lenses vs. Prime lenses (Fixed Focal Lengths). I really don’t get into it. They both have their place and advantages.  

Prime lenses can be sharper for a lower price. Lighter weight, faster, less complicated designs therefore better quality, possibly less CA and Edge distortions. Their downside is that you may need to change lenses more often and also carry more different lenses with you. The downside of changing lenses often is that there is more chance for dirt to get on you sensor which can be a problem in tough terrains (Desert/Beach wind) 

Zoom lens advantages are, more available focal lengths in one lens/ less lenses to carry. Less lens changes. The disadvantages are; heavier weights, more complex designs which can lead to lower image quality. Higher Price 

Years ago it was clear if you wanted higher Image Quality, prime lenses were the clear choice. But with improvement in Zoom lenses lately, there may be little to no difference between quality Zoom and Prime lenses. The choice is clearly up to you. 

For me and the way I shoot and the conditions I shoot under I use a lot of Zoom lenses. I do however lust after a couple primes that I long to have in my kit. 

Image Stabilization

Image stabilization can be a handy thing to have in a lens. It allows for hand holding a lens at shutter speeds about 3 stops lower than you normally could hand hold that same lens without Image Stabilization. But it is not a cure-all for everything. It’s meant to help handholding shooting a relatively stationary or slow moving object.  

It doesn’t help at all on a tripod, in fact it is usually recommended you turn off IS when mounted on a tripod. It also will not help to stop motion of a moving object, only shutter speed or panning can do that. In fact some of the better IS systems have a Panning Mode so that the control gyros inside the lens do not get confused with the panning motion and make the image worse. 

It can be a nice addition to have if you need to hand hold in low light situations, but it is not the cure all for all motion artifacts

My two lens pet peeves 

“I want a lens I can get a lot of Bokeh with”. Bokeh and a Shallow Depth of field are NOT the same thing and the terms are not interchangeable. A shallow depth of filed is just that. Bokeh describes the quality of the Out of focus field or “Circle of Confusion” and no, Bokeh is not another term for circle of confusion either. It only “describes” how it may appear with a given lens. Better lenses are said to have a better or creamier “Like Butah” bokeh than lesser lenses. I think Bokeh has been given much more attention than it needs. If you are spending more time looking at the bokeh than you are the subject, you kind of missed the meaning of isolating the subject in the first place. But that’s me. 

“I always shoot wide open” Please stop, just stop. I’m not sure where this notion started but I suspect it is one of the many”Tog celebrities” that are on the wedding/portrait lecture circuit and are only two steps removed from a point and shoot camera where a shallow DOF wasn’t possible. (Shooting wide open: Shooting at the widest maximum aperture of the lens) But this insanity has led to almost every new photographer that has just purchased a Canon Digital rebel and the obligatory 50mm f/1.8 to scream: “Help, I’m having trouble focusing”. You’re not having trouble focusing, you have a DOF problem and until you understand how DOF works you will continue to have this problem when you “Always shoot wide open” 

Understand this. With a 50mm lens on a Rebel at f/1.8 and you are shooting a headshot of a person 3 feet from your camera, The total DOF (both in front of and behind the point of focus) is: 3/4s of an inch,  That 3/8th in front of the point of focus and 3/8ths behind. Which means if you have a person slightly, just slightly facing away from you and you focus on the eye closest to the camera, the opposite eye will not be in focus. If you focus flat on both eyes, the nose will not be in focus. Seriously, that is your artistic intent? 

Now you add to this handholding the camera with shooting wide open. When we stand, especially with a camera on our face, we have a tendency to sway forward and back. So you combine our swaying with a ¾” DOF and you can easily see why even that eye we focused on is not in focus. 

Now I certainly understand your intent, you want to isolate your subject by using a shallow DOF just like in this image right?

Oh, yeah, that image was shot at f/11. So much for “Always shooting wide open”  

Don’t get me wrong there is a time and place for shooting wide open. But “Always shooting wide open” tells me you do not have a firm understanding of Lenses, DOF and Photography. And also, don’t listen to everything you may hear at those seminars, they may not be any more knowledgeable then you are. 

Thanks for allowing me to get those two things off my chest 

Those are the basics of lenses and why we do use one lens over another.

A few Helpful Links

To Learn more about DOF or actually a DOF calculator go to

DOF Masters

For reviews of Canon, Nikon and Third Party Lenses I really like the reviews that Bryan does

The Digital-Picture.com

If you are still unsure of the lens you really want, I suggest renting it for a couple days and really know for sure

Hope that helps

PT