Nikon D7100 file sizes vs image quality

▼ My new Nikon D7100 has a 24 megapixel sensor. I shot some test photos at 24, 13.5 and 6 megapixels with low, medium and high compression to see what the tradeoff between image quality and file size is. Have a look at the results.

These are the options for saving your photos on the D7100:

• raw:
  • 12- or 14-bit
  • lossless or lossy compression
• JPEG:
  • full 6000x4000 resolution, 4496x3000 or 2992x2000
  • fine, normal or basic compression
  • size priority or optimal quality

So how big do the JPEGs get? According to the manual:

	6000x4000	4496x3000	2992x2000
Fine	8.2 MB	5.0 MB	2.7 MB
Normal	4.1 MB	2.5 MB	1.4 MB
Basic	2.0 MB	1.3 MB	0.7 MB

Don't forget that 24 megapixels means 12 million green pixels, 6 million red pixels and 6 million blue pixels, while in a JPEG every pixel consists of three subpixels for the three primary colors. So a 24 megapixel JPEG actually stores 72 megapixels the way the camera makers count them. (Nikon even says the 640x480 LCD display on the back has 1.23 megapixels even though 640x480 = 0.3 MPX. They do have four subpixels per pixel, though.)

I wanted to see what the difference between 4496x3000 (13.5 megapixels) and 2992x2000 (6 megapixels) is at different JPEG compression levels. For this, I used the following scene, which should be fairly challenging to the JPEG compression scheme with lots of fine and contrasty detail. (35 mm f/1.8G DX lens at f/5.6.)

The red square indicates the part of the image shown below. When the page first loads, you get the 6000x4000 version. I'm not sure if this is the sharpest possible image, though, and I'm not sure what the compression setting was. Probably fine, as the file was 19 MB. The different versions of the file are all zoomed to different magnifications so the image remains the same size on the screen. Click the links below the image to load the different versions.

Comparing the 24 megapixel version with the two smaller ones using fine compression, it looks like the 24 MPX version encodes more data than the sensor and lens can deliver, the 6 MPX version less and at 13.5 MPX the file resolution and lens/sensor resolution seem reasonably well matched.

At 13.5 MPX, if you look hard you can already see a hint of JPEG compression artifacts in the fine version. They're easy to see in the normal version, and pretty bad in the basic version. At 6 MPX, they're bad in the normal version and hideous in the basic version.

Note that with "optimal quality" (i.e., JPEGs are allowed to be larger if needed to reach the selected quality level) and this chosen subject, the file sizes (as reported by the MacOS Finder) are a good deal bigger than suggested in the manuall:

	6000x4000	4496x3000	2992x2000
Fine	19 MB	12.0 MB	6.1 MB
Normal		5.9 MB	2.9 MB
Basic		3.0 MB	1.4 MB

So I think I'll shoot most of my photos at 13.5 megapixels normal, but I'll make sure to go to 24 MPX normal or 13.5 MPX fine when I want the best quality. I actually shoot raw+JPEG, where the raw files go to one SD card and the JPEGs to another. So if needed for one reason or another, I can always get the raw file (14 bits, lossy compression). However, I'm not much of a tinkerer after the fact, so with my previous DSLR, for 99% of all photos, I kept the JPEG and threw away the raw version. Photos take up half of my laptop's 250 GB SSD as it is, and 24 MPX raw files are 20 - 28 MB a piece.

My apologies to readers who aren't into photography, by the way. I'm sure there will be a few more posts like this one in the next two weeks or so.

Permalink - posted 2014-02-23

DSLR 101

▼ This is a post about the aspects of DSLRs in general and the Nikon D7100 in particular that those of you who haven't used a modern digital single lens reflex camera may not know about.

Before the days that cameras had LCD screens on the back, seeing what you were about to photograph was somewhat of an issue. The simple solution was to have a viewfinder with its own little lens above the "real" lens. But that doesn't show you exactly what's going to end up on the photo. A single lens reflex (SLR) camera, on the other hand, lets you look through the actual lens you're going to use to shoot the photo through a mirror that redirects the image to a ground glass that you look at through the viewfinder. When you press the shutter button, the mirror flips up and the shutter opens to take the picture.

Of course these days a much simpler way to accomplish much the same thing is to show the image that the sensor sees on the LCD screen. When you press the shutter button, the shutter first closes, then opens to take the picture and finally opens again so you can frame your next photo.

SLRs have been around for more than half a century. There were many innovations over the decades, most notably measuring the light and thus the correct exposure through the lens and autofocus. The two big brands are Canon and Nikon. Canon started from scratch with the Canon EF autofocus SLR system in 1987, so everything since 1986 is compatible, everything before isn't. Nikon came up with its F-mount in 1959, but over the years continuously refined the interface between the camera and lenses. If you really want to, it's possible to mount a lens from 1959 on a modern Nikon DSLR, although for the really old lenses a modification is necessary or you'll damage the camera. Lenses from 1977 and later will fit without trouble, although many compatibility caveats apply, sometimes even with lenses that are still sold as new today!

Because the light doesn't reach the sensor until the shutter opens, (D)SLRs need separate focus sensors to be able to autofocus. These special focus sensors allow DSLRs to focus very quickly, especially considering that parts of relatively big lenses need to be moved. The difference is especially notable when you compare shooting with a DSLR using the optical viewfinder with using "live view", where the LCD screen on the back of the camera is used as an electronic viewfinder and the focus sensors can't be used.

What surprised me when I got a Nikon D90 four years ago is that the mid- and high end Nikons can actually show certain information overlaid over the optical viewfinder image: you can have grid lines turned on and off, and when half-pressing the shutter button so the autofocus system gets triggered, the active focus point or focus points are highlighted. Apparently, there's a black-and-white LCD superimposed over the viewfinder image. (It gets illuminated in red briefly in low light.) In addition, below the image, the selected shutter time and aperture and some other settings are displayed in green or white. So you can change the most important settings without ever taking your eye off of the viewfinder.

Speaking of settings: of course even high end DSLRs offer an "auto" mode, where the camera takes care of all the settings. But my new Nikon D7100 also has no less than 31 buttons, levers, switches and dials that let the experienced photographer tell the camera exactly what he or she expects from it. And suddenly the "all auto all the time" approach of the iPhone camera seems very appealing...

I'm sure some people get a DSLR and use it in auto all the time. And there's nothing wrong with that. But the trouble with letting the camera select all the settings is that you're going to miss some shots because the camera selected the wrong setting. For instance, with the iPhone, it's nigh impossible to get decent backlit photos: if you're trying to take a photo of a person or an object in front of a bright sky, the iPhone sees the bright sky and makes the person in front of it way too dark. You can tap on the person to indicate that you want to have him or her in focus and correctly exposed, but even then the phone still looks at too wide an area and overexposes.

With a DSLR (and also many point-and-shoot cameras that offer more manual control), you can use spot metering, exposure lock and/or exposure compensation to correct or avoid this problem. (And Nikon's matrix metering is very good, it often avoids these problems to begin with.) Although tap-to-focus on an iPhone is pure magic, a DSLR can track moving objects with its autofocus system. But that mode doesn't work so well for some static subjects, so you need to change autofocus modes between shooting sports and landscapes or portraits.

So what it boils down to is that a DSLR gives you many more tools to get the more difficult shots that simpler cameras just won't give you, but the downside is that you have to be forever vigilant that you're in the right exposure and autofocus modes—or at least, not the wrong ones. But there's no rule that says you have to master each and every one of those 31 buttons.

Permalink - posted 2014-02-22

Railroad nostalgia in Rotterdam

▼ A couple of days ago, I posted this picture:

Turns out, this railroad bridge from the 1920s across the Meuse river in Rotterdam has its own documentary film: De brug (Wikipedia link) or The Bridge, by Joris Ivens, 1928. (It's 15 min and silent). Watch it on Youtube or Vimeo, but not on the Internet Archive, because their copy is much lower quality.

In 1993, the bridge was closed and most of it was removed after a new tunnel had been built. As you can see in the photo, the remaining part of the bridge is left in the open position. I wasn't a very frequent rail traveler back then, but I still remember that the "luchtspoor" (something like "air track", link is a nice history in Dutch with many photos) provided a very nice view of Rotterdam, most notably this building:

The Witte Huis (White House) is a 43 meter, ten story high building from 1898. At that time it was the tallest building in Europe.

A couple of additional photos: this is the old Hofplein station, back in 2009 shortly before it closed. It was the terminus of the Hofpleinlijn, a second rail link between The Hague and Rotterdam, which was the first electrified rail line in Holland in 1908.

For some strange reason they built this station within easy walking distance of the Rotterdam central station. In 2006, it was converted to a metro (subway/underground) line, and a few years later it was connected to the rest of the Rotterdam metro network so this terminus was no longer needed.

Speaking of the Rotterdam central station, this is what it looked like in 2003.

It's really too bad I don't have a better photo, because I liked the 1950s style architecture. It's been completely rebuilt the past years (all of the largest train stations in the country have been under extensive construction the past decade!) and the new look isn't half bad either. So I guess I'll have to shoot some new photos and show how all of this looks today.

Permalink - posted 2014-02-09