How to Choose a Mac
The top-end July 2010 iMacs include the following array of choices, with prices ranging from about $1699 to about $2199 for a model with the base 4GB of memory and hard drive.
- 2.93GHz quad-core Intel Core i7
- 2.8GHz quad-core Intel Core i5
- 3.6Ghz dual-core Intel Core i5
- 3.2Ghz dual-core Intel Core i3
Since the CPU cannot be upgraded, understanding CPU performance (clock speed and CPU cores) is critical for making an investment in your new Mac. (For in-depth coverage on this subject, see Shootout: 8-core 2.93GHz or Quad-Core 3.33GHz? at MacPerformanceGuide.com). I also advise getting the 2TB drive option, because it is not simple to upgrade the internal hard drive.
The choice is easy for photographers doing certain activities: most RAW-file processing software (eg Adobe Lightroom and CaptureOne) will easily “peg out” dual CPU cores. Also, many Photoshop operations can utilize more than two CPU cores, at least for short periods. So the choice is strongly in favor of the quad-core models for many operations. Similar issues apply to video and sound users.
The dual-core 3.6GHz model can be a good choice: it will run some tasks faster than the quad-core models. But as soon as those two CPU cores are “pegged”, that’s it. Think of the 3.6Ghz dual-core model as being roughly equivalent to a 3-core Intel Core i5 2.8GHz model, once clock speed is taken into account, but performing better on tasks that are not efficient, like saving or opening files in Photoshop. Thus, the choice is not cut-and-dried.
Operations that benefit from quad-core:
- Importing images into Lightroom and similar programs;
- Batch-processing RAW files with most RAW converters (especially Lightroom, Aperture, and PhaseOne CaptureOne);
- Many Photoshop operations, including Smart Sharpen and Gaussian Blur.
- Previewing images in the Finder and other programs.
- A variety of specialty software for focus stacking, image scaling, making panoramas, etc.
Operations that benefit most from a fast clock speed:
- Interactive use eg with a brush, or when drawing, retouching, etc.
- Saving and opening files in Photoshop.
- Panoramas in Photoshop (Auto Align and Auto Blend are single-threaded, using one core only).
It’s not a clear-cut decision, but when more cores are needed, the performance benefits can be substantial for quad-core instead of dual-core. Keep reading for more on clock speed and cores.
The “rev limit” — clock speed
The clock speed (eg 2.93GHz) is the fastest rate at which computer instructions execute— that’s assuming enough memory, and that the disk is not involved, either of which idles the CPU. For a lot of software (even in 2010), the clock speed is the primary determinant of “performance”.
A 3.6GHz model runs 28% faster than 2.8GHz eg 7.7 seconds instead of 10 seconds. This is quite noticeable, especially with interactive use. There is a human threshold factor that can annoy if too slow, but whether 7.7 vs 10 seconds matters in general is up to you.
Baking cakes — CPU cores
Four chefs cannot bake one cake faster than one chef. But four chefs could bake four cakes in the same amount of time that one chef could bake one cake. That analogy is key to understanding why more CPU cores are not necessarily better: not all tasks are divisible.
We’ve seen that clock speed is a “rev limit”. But the iMacs CPUs contains contains either two or four CPU cores. These cores can work together in parallel to speed up certain computing jobs. They cannot “bake a cake” faster, but if the software is well written, and the job is to “bake 4 cakes”, then a 4-core iMac get that job done in 1/2 the time that a 2-core Mac Pro could (at the same clock speed). In reality, it’s not usually that scalable, but that’s the idea.
Alas, it is the software application that determines how many CPU cores actually get used. The application must be written to take advantage of those CPU cores, and few programs are written well enough to take advantage of more than 2-4 cores, with many falling short of even that.
Efficient use means that 4 CPU cores will finish the job in half the time that 2 cores will. That is often not the case, but many programs that cannot use 8 cores efficiently can use 4 cores reasonably well (as compared to two cores).
Many programs are still “single threaded” for some tasks, which means they can use only a single CPU core to get the job done, leaving the remaining cores completely idle. For example, saving a file in Adobe Photoshop™ CS5: only one CPU core is used, so it can take minutes to save a large file (for a workaround, see a tip at MacPerformanceGuide.com).
General recommendations
I purchased in iMac recently, and for me it was a no-brainer: the quad-core 2.93GHz Intel Core i7 model. That is the best model for most users looking for a high-performing iMac. Especially considering additional system costs (eg memory and AppleCare, backup drives, etc), it does not make sense to skimp on a processor that cannot be upgrade.
The large majority of users are very well served by a quad-core machine. Users with multiple programs running (and doing real work all at the same time) should definitely be looking at a quad-core machine. See how to use Apple’s Activity Monitor to view cpu core usage at MacPerformanceGuide.com.
For certain types of peak interactive performance (eg working with brushes, drawing, etc), as well as a “sweet spot” in terms of CPU cores, the 2-core 3.6GHz machine might be better suited to some jobs. However, its 22% clock speed advantage over the quad-core 2.93GHz model evaporates quickly with RAW converters capable of using 4-6 CPU cores.
For the ultimate performance with the iMac, the souped-up MPG Pro One is the way to go.
Mac Pro alternative
For Adobe Photoshop and Lightroom, RAW converters, etc, a quad-core CPU cores is plenty for most tasks, and most of the time, some of those cores will be idle. However, certain operations in Adobe Lightroom, or RAW converters like PhaseOne CaptureOne Pro can use more than 4 cores effectively. Thus, the 6-core 3.33GHz Mac Pro is a “sweet spot” which will satisfy those programs, and generally outperform anything possible with an iMac quad-core.
Other considerations
With too little memory, the CPU will be stalled— your Mac runs slowly. With the iMac, most users should find 8GB adequate, but to “set and forget”, going to 16GB is advised, especially since Mac OS X uses extra memory for caching. Don’t forget a high performance solid state drives as another huge plus.
