Perspective Distortion, Aperture, Depth of Field, and why you want 51 focus points.
I got my hands on a Nikon 85mm f/1.4 and I think it may be the most gorgeous lens ever.
I dare say it is my new favorite lens.
Why is this lens so ridiculous awesome? For starters, it is the in the perfect range of portrait lenses. For portraits, you want to use a lens between 85mm and 135mm in order to get the correct amount of perspective distortion. If you use something like a 12mm lens, perspective distortion will make things that are close appear too close and things that are far appear too far. If you use something with an effective focal length greater than 135mm, you’ll have to stand farther away, which causes telephoto compression — essentially making the image appear more “flat” and flattening out the face in an unpleasing way. 85mm – 135mm is the range that causes enough distortion that you take attention away from certain things (like the nose, for example), but not so much distortion it looks weird.
The 85mm is the range where it works for both full frame cameras and cropped cameras. On my old camera, this lens would still be 128mm and it would still work as a great portrait lens.
On top of all that, the lens is known for its bokeh, which creates nice pleasing backgrounds and helps keep the attention on your subject. Plus, it is ridiculously sharp at f/1.4.
But enough about that. A full review will come up one of these days…
A while ago, I wrote a basic primer on exposure. I talked about f/stops and shutter speeds and I confused a ton of people and it was just a big mess.
For those who didn’t get confused, here’s a basic continuation of that post…
We’re going to focus on aperture. If a large aperture gives us better bokeh and allows more light in, why wouldn’t we just use the largest aperture all the time?(1)
The reason is depth of field. I’m not going to go into all the things that affect DoF — focal length, f/stop, and subject distance — that’s for another post. I’m just going to go over why we have multiple f/stops and why you don’t always want to shoot at the lowest.
This is the basic pillar: lower f/stop (bigger aperture size) = smaller depth of field. What is depth of field? It is amount of an area that is in focus at any given point in time. Your camera can only focus on one set distance — the further away you get from the point, the blurrier things appear. The “in focus” part is called depth of field.
In this video, Wall-E is in focus. Santa is in front. The snowman is in back. And I added a Christmas tree in the background so you can see how amazingly gorgeous the bokeh on my new (and soon to be sold) lens is.
In the upper left is the current f/stop. I’m going from f/1.4 to f/16 and you can see how Santa and the snowman gradually get more and more in focus. You can also see how the out of focus points (bokeh) become smaller and smaller. The easiest way to observe the growing depth of field is to look at the carpet. You can see the gradation the best there. My DoF grows from .026 inches to 3.36 inches.
So if you’re taking a portrait, you would generally want a smaller f/stop so that all the focus is on the subject. If you’re taking a photo of some sort of landscape, you probably want a higher f/stop so that the entire landscape is in focus.(2) If you’re taking a large, group portrait, you probably want a medium f/stop so that the everybody is in focus but the background is still a little blurry.
Which brings me to my next point — sharpness. Most lenses are not optimally sharp until you stop down 2-3 stops from their maximum aperture. If sharpness is critical, you would never shoot with at f/1.4. Another reason not all lenses are made at f/1.4 or you wouldn’t want to carry around low f/stop lenses all the time? Size, weight, and price. Because they (by definition) have a larger hole, that requires more glass, which requires greater engineering, more precision machinery, adds more weight, etc.
And, finally. My last (and mostly unrelated) point — why on earth would you want 51 autofocus points on a camera? I mean… can’t you just focus and then recompose?
Because when you’re dealing with a DoF of less than 1/5th of an inch, the last thing you want to do is focus at one place and shift your camera.
Such a ridiculously nice lens. So now that I’ve got a portrait lens, who wants to volunteer to be a model for me over break? I’m in San Diego 12/17-1/7 and then back to New York for the rest of my break. (1-8-1/22)
Hurry. I gotta sell it soon.
Footnotes:
Great write up…i felt like i learn something new.