Wednesday, February 16, 2011

135mm F/2 & 100mm F/2.8 L (Subject Isolation)

Even at F/4.5, the isolation is very
easy to accomplish
I've been meaning to do this simple comparison for a while. A friend of mine loaned me his Canon EF 135mm F/2L to try out and I will admit, it's a spectacular lens to use. For portrait work it's sharp, but it does a great job of isolating the background.

I took the opportunity to use the lens for a shoot where I was photographing some headshots for actors. The worst part of using a lens like the 135mm is that you need some room to work with (at least 10 feet), but the best part of using a lens like this, is what I will call Subject Isolation Characteristics, or SIC for short (also known as selective focusing).

I hate using the popular term 'Bokeh', mostly because people overuse the term, but I try my best to reserve it to describe the properties of the background blur, and not about how well it isolates the background. Depth of field is also an equally overused term which can describe shallow or deep depth of field. But most people still fail to clarify what they mean when they use the term. I'm not going to define what bokeh means, but in this article I'm going to talk about only one specific property that affects bokeh, not what makes good bokeh.

I'm not going into too much detail on how depth of field charts work either and rather show a simple comparison between two specific lenses. One is the Canon EF 135mm F/2L USM and the other is the Canon EF 100mm F/2.8L IS USM Macro. Unfortunately I didn't have the 100mm with me during the actor head shots, so I resorted to my rather boring subject of my 7D on a 100-400mm lens.

135mm @ F/2
135mm @ F/2.8
100mm @ F/2.8

So as you can see in these three examples, the 1 stop of difference isn't a massive difference. Of course, the 100mm requires getting in closer to frame up the same shot, I found that the background blur properties are different from each other. On the 135mm F/2 and F/2.8 weren't all that different from each other, but between the 135mm and the 100mm @ F2.8 there is definitely a small difference between the two.

The 100mm is more defined in the background but can compare nicely against the legendary 135mm. Aside from the other two advantages of being a macro and having IS, at the same field of view, both focal lengths have identical working depth of fields.

Focal Length ƒ-stop Near Limits Far Limits Total Working Depth of Field Hyperfocal
135mm @ 10' 2 9.91' 10.1' 0.19' 997'
135mm @ 10' 2.8 9.87' 10.1' 0.27' 705.1'
135mm @ 10'  4 9.81' 10.1' 0.38' 498.7'
100mm @ 7.4' 2.8 7.27' 7.54' 0.27' 387'
100mm @ 7.4'  4 7.21' 7.6' 0.38' 273.7'
* All figures are based on a 5DmkII. Figures from dofmaster.com

So why does the background blur differ in the 100mm if they have the same depth of field? The answer is partially found when you look at the difference in the hyperfocal column. Because of this difference, the background blur on the 135mm is going to be much softer and blurrier than the 100mm (because it requires more distance to resolve therefore closer objects will appear 'blurrier'). It's amazing how much difference 35mm can make.

Is the 135mm a lens to choose over the 100mm L macro? As you can see it's not all that different from each other and it comes down to how easy it is to isolate your subject from the background. The 135mm is much easier to do just that, but so is a 70-200 F/2.8 zoom. So it begs the question, what is the real purpose of the extra stop advantage. I don't have a F/2.8 zoom to compare at this time, but I do know that F/2 and F/2.8 can make a major difference in low light situations.

Another thing to note is the subject distance. Although it does mean you need more space to fit your subject into your frame, it is far easier to frame up something than to move your background further. When you're shooting with a 100mm focal length you still need to consider the subject distance. With a 135mm focal length, that situation is less of a concern.

Of course it begs the question, why not just get a shorter focal length and even larger aperture? For example, one could use an 85mm F/1.2 and shoot wide open but you are faced with a far smaller working distance for subject isolation. What this means is you have a higher chance of your primary subject being out of focus.

Focal Length ƒ-stop Near Limits Far Limits Total Working Depth of Field Hyperfocal
135mm @ 10' 2.8 9.87' 10.1' 0.27' 705.1'
100mm @ 7.4' 2.8 7.27' 7.54' 0.27' 387'
85mm @ 6.3'  2.8 6.17' 6.44' 0.27' 279.6'
85mm @ 6.3'  1.2 6.24' 6.36' 0.11' 644.7'

Another thing not to ignore is the near limits of a particular lens. In this chart the near limit of the 85mm is less than an inch, which means for example if your autofocus is focusing at the distant eye on a 3/4 portrait, there's a good chance if shooting wide open, that your subject's face will be out of focus. With the 135mm lens, the distance is double which makes a huge difference when trying to isolate your background and working with your subject's face.

So what does this really all mean? It seems easy to convince yourself that the wider the aperture the better the lens. This isn't always the case. The real question to ask is, which lens is easier to isolate your subject without introducing potential focusing errors but still maintain a good level of background blur. The SIC of the 135mm is pretty hard to beat, and for why many professionals use this lens, it's about usability. Even at F/4 the 135mm gives me better working distance and better background blur (bokeh) than the 100mm wide open. It doesn't mean I can't achieve a pleasing or similar result with the 100mm, it just means I need to work harder or be more conscious of my subject distance and the background to get pleasing results.


4 comments:

  1. Terrance, great article! Love to read your stuff. Keep it up!! Mark.

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  2. Thanks Mark.

    I enjoy writing about these things as much as utilizing them in the field.

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  3. Awesome! I have been looking at purchasing the 135mm, and this was a great article to help with my decision. Thank you for the great work you're doing.

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    Replies
    1. You're welcome... the 135 is a fantastic lens for your kit.

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