Canon Digital Rebel T2i (EOS 550D) Review
Autofocus
The Digital Rebel T2i uses a 9-point wide area AF sensor with the points set in a diamond pattern, similar to that of the Rebel T1i and EOS 50D. The focus point can be manually selected, or the camera can decide which AF point(s) to use. There are 4 focus modes: One-Shot AF, AI Focus AF, AI Servo AF, and Manual. The Rebel XSi has a cross-type (dual axis) AF sensor at the center that's effective with all EF and EF-S lenses and which provides enhanced precision with lenses having maximum apertures of f/2.8 or faster The center cross-type AF sensor reads a wider variety of subject matter than conventional single-axis AF sensors and thus increases autofocus speed and accurately. The outer 8 AF zones use linear (single axis) sensors (which differs from the 40D/50D where all the AF sensors are cross type).
AF is generally fast (especially with ring USM lenses) and accurate. The small viewfinder size doesn't help much when using manual focus, but for most users that probably won't be an issue.
Resolution
The resolution of the Digital Rebel T2i matches that of the EOS 7D, just as you would expect since they have the same sized sensor with the same number of pixels. In theory going from 15MP to 18MP should give you about a 10% gain in linear resolution, based on the Nyquist sampling criterion and if all other variables were equal (such as the effect of the anti-aliasing filter, the RAW file demosaicing algorithms and assuming the lens being used didn't limit resolution).
Due to a number of factors including the optical limitations of lenses and the anti-aliasing filter over the sensor, the practical resolution limit is always lower than the theoretical limit. With the T2i and an EF 85/1.8 lens at around f4 I measured a resolution of about 90 lp/mm in RAW images converted with DPP In terms of “line pairs per picture height”, a measure used by many other reviewers, that would correspond to approximately 2700 lpph. Of course exactly what constitutes “resolved” for digital images is something of a subjective judgment, however whatever standard you use, the T2i resolution is very good, similar to that of the EOS 7D and better than that of the EOS 50D and Digital Rebel T1i.
On the subject of resolution, I've seen the statement repeated a number of times that in order to take advantage of the higher resolution of sensors such as the 18MP sensor in the Digital Rebel T2i you need to use the best "L" series lenses, and maybe you even need to use them at their best aperture, in order to see the higher resolution. Well, it turns out this is just not true.
In fact the T2i will show higher resolution than the T1i or EOS 50d with just about any lens, however it is true that to take full advantage of the high resolution sensor of the T2i, and to get the highest possible resolution, higher quality lenses will certainly be be worthwhile.
Exposure
The Digital Rebel T2i uses the same metering system as the EOS 7D, i.e. a 63-zone multi-segment metering (linked to AF zones), plus a center-weighted averaging option, partial metering (9%) and spot metering (4%). Canon specifies the operating range of the Rebel T2i as EV 1-20 (at 73°F/23°C with an EF 50mm f/1.4 USM lens, ISO 100). This is an advance over the 35 zone metering system of the EOS 50D and Digital Rebel T1i.
Like the EOS 7D, the EOS Rebel T2i has a dual-layer sensor, with 63-zones. By measuring not only the amount of light, but also taking into account color and luminosity, this new system is designed to provide better exposure accuracy. Since the metering sensor also has a color measurement function, exposure errors and focus errors caused by different light sources are minimized
Exposure compensation is possible over a +/- 5 stop range on the T2i, though only +/- 2 stops are shown in the viewfinder. The rear LCD screen shows the full +/- 5 stop range. This may be an advantage to those making HDR images composed of multiple shots taken with different exposures.
Auto exposure bracketing with the Rebel T2i is possible over a ± 2-stop range, from -7 stops to + 7 stops. For example, what this means is that you can shoot one shot at -4 stops, one at -2 stops and one at the meter reading, or you can shoot at the meter reading, +2 stops and +4 stops, or you can shoot at -2 stops, the meter reading and +2 stops, or you can soot at -7 stops, -5 stops and - 3 stops. What you can’t do automatically is bracket at -7 stops, 0 stops and +7 stops from the meter reading. You can only bracket over a ±2 stop range, but you can center the bracket over ±5 stops from the meter reading (the range of exposure compensation on the T2i). This sounds more complex than it actually is!
