About Lenses and depth of field.

I mentioned in a “About Zoom lenses – Pt1” a discussion at DPreview forums regarding depth of field and how zoom users in particular would go about determining how to approach the subject.

For most people reading through the various comments it would seem that no one seemed to have a particularly good grasp of the process. The keyboard warriors among them sallied forth with more math’s than I can remember from my science and physics classes.


So what do I mean by this? Well its pretty clear that the vast majority of people replying to the question on the forum have vastly different interpretations to the meaning of depth of field and how to calculate this. Especially when applying to zoom lenses.

If I’m shooting wildlife, action sports, rodeo for example I wont be worrying about depth of field. What I will be concentrating on is the action. How the subject looks in the viewfinder, composition, where I am in relation to the subject, and lighting.

Worrying about what might be in or out of focus in the background or foreground is largely immaterial.

If I have the luxury of having my camera on a tripod and my subject matter is fixed or only moves at the speed of an arthritic turtle then I will be more likely to consider fore & background elements at what should be in focus.

To better understand this process I have included the link below to Photography Online‘s latest episode. For me at least this YouTube Channel is by far the best photography program I have watched and I’ve watched  hundreds of photography channels, and no I have no affiliation with them, although my family does live on the Isle of Skye.

In this episode there are two outstanding sections that deal with lenses and how to use depth of field for your images. The first part starts at 0.53 seconds and the second section that deals with DOF and selecting aperture and starts at 18.26 minutes. Don’t let the topic headings put you off.  Watch the video, in fact watch the whole program its more than worth it.

In the second link below Marcus shows how the various parts that make up focal length actually relate to each other. This is another really good demonstration in practical terms. It also demonstrates one of the misunderstandings when people talk about DOF, typically confusing DOF with perspective. The section about lenses in the second video starts at 1:30.


About Zoom lenses – Pt1.

Weighty Issues.

What lens should you use?  Its a common question and one that’s often seen on photography forums and often opens up a nightmare discussion.

Case in point is the current discussion in the Fuji X system forum at DPreview. Click this link to see the discussion. The forum post topic is the following question – “How do zoom shooters actually calculate their shots?”

The topic heading is in itself somewhat confusing and for many may be a little difficult to make sense of. Essentially what the member is asking is, how do you determine the depth of field based on the lens being used and distance from the subject. Right about now  many of you are no doubt going “WTF!!” Understandably so too.

The question is about how to balance those regions of the photograph to create the perception of depth in the image. What is going to be in focus in the foreground, mid ground & background areas.

For this exercise lets say we have a subject 3 meters from the camera. Then we have to decide what is to be in focus.  Obviously the subject needs to be in focus, but perhaps both the foreground and background should be somewhat out of focus. If  your lens has a native f ratio of 1.6 then you can have really well blurred background and foreground. If you select f4 then a little more of the foreground and background will be slightly more focused. If you then select f11 or f16 then everything in the image comes into sharp focus.

That’s why many photographers love to use fast Prime lenses that have typically f1.4 to f2.8 fixed focal ratios. Fixed aperture Prime lenses are without doubt some of the nicest lenses available when you are relatively close to your chosen subject, but they come with a few caveats. They are generally large lens in terms of size. Heavy or in some cases extremely heavy, bulky, and generally are extremely expensive. For most of us, trying to justify the purchase of fast prime lenses just isn’t in our budgetary constraints.

The size and weight issues continue when people consider buying premium grade telephotos lenses as well. Take for example a long time favorite the Canon 70-200mm f4 lens. At 979 grams in use plus the weight of a camera body, you have a  hefty piece of kit hanging around your neck. Consider that the Canon 600D is 575 grams the total weight of the EF 70-200 plus the 600d is just over 1.5kg or about 3.5 pounds.  Now add to this other equipment that you may have in your camera bag and all of a sudden those much touted fixed aperture primes are not so much fun to carry around.

Another fan favorite is the EF 85mm f 1.4 L IS USM lens. At 970 grams it too is a big piece of kit. So while these bulky, heavy expensive lenses are favored by pro photographers, and dare I say it – camera snobs, the reality is that for 99% of us these lenses are not something we are ever truly likely to own.

In the next section we will take a look at what is available that is lighter, less expensive and perhaps more versatile than the Pro class lenses, what some of the tradeoffs might be and how we can work around these perceived shortcomings.