What the F-Stop, Indeed!

As many of you know, I enjoy listening to podcasts on my drive to and from work, and this afternoon was no different. On one of the podcasts I listened to today, I happened to hear what had to be the most convoluted, confusing, and downright confounding explanations of the f-stop that I've ever heard. I'm pretty sure that at least one crucial part of the explanation was even wrong. Very wrong. WTF-stop, indeed! So, let's go ahead and take a look at the term "f-stop" to see if I can make some sense of it for you, as it's actually deceptively simple. Maybe.

f-stop: In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil (in photography, the aperture or iris).

Being a ratio, or fractional number, the f-stop is usually expressed as something like f:4 or f/5.6.

So there you have it. Makes perfect sense now, doesn't it? No? Read on, then.

F-stops control the amount of light passed through the lens

To put some concrete numbers behind this, a 50mm f/2 lens, has a maximum aperture of 50/2, or 25mm. When set to f/2, this particular lens would be said to be "wide open," meaning the aperture is at its maximum opening, allowing the maximum amount of light to pass through the lens. If we set this particular lens to f/4, as an example, the aperture will be closed down (or "stopped down") to 50/4, or 12.5mm. This, obviously, reduces the amount of light that can pass through the lens. By applying this same concept, we can figure that a 25mm aperture on a 100mm lens works out to f/4.

Oh, Lawdy, we've used algebra!

If this is still confusing, think of the aperture or iris like a very precise valve in a water pipe. Closing the valve makes a smaller opening, restricting the amount of water that can flow.

Now, here's where the math can get hinky, and where people start to get confused. The math involved is weird because we're dealing with the area of a circle, and that makes some of the numbers start to look funny. I say that someone else figured out all this math (using algebra and geometry and physics and stuff), and handed us the results, and they work, so let's just go with their results from here on out.

With that said, there's a sequence of numbers that are the "full f-stops," and they are the numbers that set up the ratios that are the key to understanding the whole thing. Here's the sequence for our hypothetical 50mm lens:

2, 2.8, 4, 5.6, 8, 11, 16, 22

These are arranged from the largest opening to the smallest, and I think this is where people start to get confused -- the bigger number let's in less light? What? But remember, it's a ratio that describes how far the aperture is open with respect to the focal length of the lens. Getting back to our 50mm f/2 lens, the numbers break down like this:

f-stop aperture diameter
2 25mm
2.8 17.86mm
4 12.5mm
5.6 8.93mm
8 6.25mm
11 4.55mm
16 3.13mm
22 2.27mm

The next bit that's important to know is that each full f-stop you stop down cuts the amount of light passed through the lens by half. In other words, the f/2 lens transmits 100% of the available light when set at f/2 (wide open). Setting the aperture to f/2.8 passes half the available light. Stopping down another stop cuts the light in half again, passing only one quarter of the available light. Going down to f/11 passes just 1/32 of the available light through the lens.

Conversely, each stop you open up doubles the amount of light passing through the lens. So, if you're at f/11 and open up to f/8, you go from 1/32 of the available light passing through the lens to 1/16 of the available light passing through the lens -- double the amount of light.

So, to re-cap the subject, the f-stop is the ratio of the focal length of the lens to the diameter of the aperture. The aperture controls the amount of light that can pass through the lens. Smaller apertures mean less light, larger apertures mean more. Each full f-stop step down cuts light in half, each full f-stop up doubles the amount of light.

The guy on the podcast rambled on about this for at least five minutes!

F-stops control depth of field

Wait, what? It does more than just control light? 'Fraid so.

I'm not going to dive too deep into this right now, but the aperture can also be used to control something called "depth of field." What's that all about?

Depth of field: In optics, particularly as it relates to film and photography, depth of field (DOF), also called focus range or effective focus range, is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image.

We've all seen brilliant landscape photographs in which everything from the rocks in the near foreground to the distant mountains appear to be in sharp focus. A photo like this is said to have a deep or large depth of field. On the other hand, a portrait where the subject eyes are in sharp focus, but the background is nothing but a fantastic blur of color, is said to have a shallow depth of field.

There's a lot of math involved in figuring out exactly what will appear sharp and what won't, but the basic gist is that a small aperture will cause more depth to appear sharp (our landscape), while opening the aperture up will cause less to be sharp (our portrait).

The podcaster took another five minutes to describe this.

There's something called "hyperfocal distance" that can tell you what will and won't be sharp, but we'll talk about that another day.

Aperture-to-Lightroom Migration Follow-up

It's been a while since I commented on my switch from Aperture to Lightroom, so I thought I might fill you in, briefly, on how things have gone. In Part 2 of the saga, I had mentioned that I had lost and found a few thousand images, and that many of them were on my laptop. Last week, I got decided to try out one of the features that was a big part of my reason for switching to Lightoom -- cross-platform compatibility. It was amazingly simple to bring the images from the Lightroom library on my Acer laptop into the library on my iMac, and along with them came all the edits, adjustments, tags, and metadata, as well as the externally-edited images. The whole process took about 40 minutes. In a word: Sweetness!

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Aperture to Lightroom: The Pleasure and The Pain Part 2

Last week, I posted that I had started to migrate from Aperture to Lightroom, and that I had all my images exported and ready to bring into Lightroom. That import went relatively smoothly. About 42,000 images, as expected. I then realized that I had images on several external hard disks that needed to be part of the collection, too. So, I spent large parts of Saturday, Sunday and Monday moving all that data around and importing those images into the library. My library quickly ballooned to over 90,000 images, and many are duplicates!!! And, performance was beginning to suffer as well.

I quickly decided that there were a number of older projects' images that really didn't need to be in the main image library -- those we took for various weddings and several commercial jobs that I've done. Those images were all moved into separate Lightroom libraries which, along with removing a bunch of stuff that I just knew was crap that I would never need or want again, took up almost all of yesterday. That all moved about 23,000 images out of my way.

This morning, I went in to start tackling the duplicates and suddenly realized that about eight months worth of images were missing. Bloody hell! Did I delete them accidentally? A quick look in Aperture revealed that they were missing from there as well. While I know that I didn't shoot a lot during that period, I did shoot, and some of those images I was quite happy with! I also noticed that my edits to my past three shoots were not in Lightroom. This was getting disturbing. And I was getting tired, so I decided to step away.

It then occurred to me that those missing edits were actually already in Lightroom, but stored on my Laptop, so that worry was over. But, what about all those missing images?

This morning, I went in search, and found the missing pictures hiding out in a separate Aperture /iPhoto library file on a shared network drive. So, now I'm back into the export/import business for another 3,800 images.

Does anyone remember where I put my sanity?

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Aperture to Lightroom: Data Migration of Near Epic Proportions Part 1

Okay, maybe that's a bit dramatic. But it's sure beginning to feel that way.... Apple Aperture

I've been using Aperture ever since I got my iMac, which was about two years ago. At that time, I moved all of my digital images from a "folders and files" disk hierarchy into my Aperture library, and continued to add and catalog every image since. My library contains a little under 42,000 images in an image library that takes up almost 300GB -- most of which are probably garbage.

When Lightroom 4 came to the Adobe Cloud, I decided it would be a "good thing" to make the move. This was not an easy choice, as I've invested in several sets of "presets" for Aperture, as well as having done alot of image editing in Aperture. And, while the results of those edits will be retained, there will no longer be "back links" to the original files.

Obviously, there must be compelling reasons to switch, right. Why, yes, of course there are. Lightroom is cross-platform, and through the Cloud, it will eventually be quite easy to move images collected in the field on my Acer laptop to the iMac (and back, if needed), including any post processing. There are a number of plug-ins and add-on items for Lightroom. And going back-and-forth between Lightroom and Photoshop is a snap.

So, what's the big deal? Open Lightroom and import the pictures, right?

Not so fast, buddy! Seems that when I set Aperture up, I elected to store my images "in the Aperture Library," which means that all 42,000-ish images are stored in a special kind of Mac directory called a "package." While that's an easy format to deal with for some Apple software, Adobe programs can't look inside a package to see the individual contents.

After a fairly exhaustive search of the Aperture "documentation," and several sources on the web, I came to the conclusion that the process of moving the images was going to be a long one, and it was going to be just that: a process with several steps, none of which would be particularly quick.

The first step is to make the "Master" images become individual files on my hard disk and have them be "Referenced Masters" in Aperture. Fortunately, this is pretty easy to do using the "Relocate Originals" command in the file menu. And, it only took twelve hours. Yeah, twelve. But, the result was that all my original images were back to the old folders-and-files on the disk format and out of the previously huge Aperture library file. The Aperture library still contained my "Versions" (information about my edited images and any files that were created from the Masters using external editors), references to the original files, thumbnails and various other metadata.

Of course, I wanted those finished images to be available to Lightroom as well, so I had to export those. I chose to make them export as Photoshop PSD files, stored alongside the Masters. Obviously the records of all the individual adjustments would be lost (or, rather, invisible to Lightroom and Photoshop), but I'd have uncompressed, full-resolution versions of the images for later use. If there's a need, I can always go back into Aperture to see what I did -- all the images still appear in Aperture as they always did, but now they're references to the disk files. That took nine hours.

The final step to getting the images out of Aperture was to marry the various metadata in Aperture to the files so it would be preserved in Lightroom. There are two ways to do this -- write the metadata into the files or write it out as XMP "sidecar" files. I elected to write the metadata into the image files using the "Write IPTC to Master" option on the Metadata menu, as I've never had really good luck using sidecars -- they get out of sync or lost or just not read, and that becomes a pain. That process ate up another nine hours or so.

At this point, I have all my image files ready to import into Lightroom. Only thirty hours of processing and about four hours of research were required. Tonight, I'll start bringing all of the images into Lightroom. Again, I'll have the option to incorporate the images into a database, or leave them as folders-and-files. You get exactly one guess about which path I'll choose this time...

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