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jueves, 28 de febrero de 2008


Extraido de la página de Canon.

Digital SLR - Focus Test Chart & Method
Used to evaluated the quality of Canon Digital SLR's, this test is your guarantee your camera is performing to a reasonable standard. The test takes the form of an 2 Adobe PDF file (at end of page, see 'Make Your Own Test Chart'), which can be printed on most A3+ printers (inkjet, laser etc), then easily assembled. Then you need a tripod, camera and lenses, a long enough room (50mm lens test 2.5m, 200mm lens test 10m).

Example Photographs
The below 3 photographs show, good focus, front focus and back focus, using the same camera. Most interesting is the varience of focus, between 2 shots, one after the other, with the same lens, the same target, distance etc. Yet one is front focussed outside of any reasonable speicifcation (or measure of sharpness), the other is back focussed again outsdie of a reasonable sharpness. Two shots in a row, completely different result - this is a problem many users have no appreciation exists.

Note the 50mm f1.4 test shown, will give some shots in focus (as well as some front and some back focussed). This causes people to think the camera is ok and they are the problem - when one shot looks sharp and the next looks blurred. In fact it's just the level of tolerance of the camera system, in that the autofocus varience per shot, is so great - and clearly looking at these images not suitable for the job.


OK - Canon EOS 20d with 70-200mm f2.8 lens at 70mm - Focus OK, note the zero's in the middle on the sides of the photo, are sharpest, and the numbers moving away from the zero (up to about 6) in each direction are reasonably sharp. This is because the camera managed to correctly focus this shot - but see further down, for what can happen next, when focus accuracy is not good enough.

Front Focus - Canon EOS 20d with 50mm f1.4 lens - Focus is front biased out of specification. If shooting a wedding group, of people 3 deep, you would have to focus on the back row, or even behind them to end up with all three rows in focus. But many photographrs would focus on the front the row, and everything behind that point would be unacceptably blurred. Even worse, change to another lens or body, and the point of focus might be different again.

Back Focus - Canon EOS 20d with 50mm f1.4 - same equipment as above, but the shot before is back focussed, showing varience per shot, many users might be aware of focus issues, but have no idea that the problem (focus accuracy) typically changes every shot, even when all else remains the same. One only needs to use a decent camera body, to realise the 50mm 1.4 can work perfectly, thus the 20d would appear to be the weak link.

An alternative test, more real life, but a different camera to above.
Back Focus - Canon EOS 10d - the three phones are at different distances from the camera. The left phone is closet, the right phone furthest from camera. The middle phone is the point focussed on. The middle phone has ended up ouside the depth of field, because the camera has back focussed, and thus the rear phone is the point of sharpness in the photo. Most worrying is how blurred the front phone is due to the erorr

Make your own Focus Test Chart
The full details will follow shortly including test procedure, and additional setup photographs etc.

The pictures above show the test chart - 2 piece - in practise. This works well with EOS 10D/20D and 1d and 1d Mark 2. Also with Nikon, Fuji etc, where crop is 1.25x to 1.6x.

Initially I have enclosed the files you will need to print. The end plates shown below, are to be cut out according to the lines on the print out, but allowing extra for the tabs when the end plate is folded on the line, and then assmebled as below.

The target is to be printed out on A4 paper, and you should add little tabs around the 2 triangles, so when cutting out, you see the shape above, with the little tabs.
The ruler, or measuring guide, is to be printed onto A3 plus size paper.

The target should be vertical and parralel to the camera's plane of focus. The measuring guide, should be at an angle of 30 degree's. The end plates are at 60 degree's to make this very easy to achieve. It is not that critical, and can often be eye-balled if you are failry good at judging horizantals - any commercial property photographer should be fine with this.

More Test Details - added 04/05/05

You can test for yourself using this 2 piece chart with perpendicular focus target and angled measure rule grid. Test at x40 or x50 focal length (e.g. 50mm is 2m-2.5m, 200mm is 8m-10m), 4500-6500k, 6-12ev, tripod, cable release.

Focus once each time with one shot, and do 4 SHOTS, 2 from nearest focus and 2 from infiitny focus - you'll be shocked at the results. Light should be 4500 to 6500k, luminence 8-10ev, tripod, one shot, cable release. take 2 shots with lens starting from infinity. and 2 shots with lens starting from near focus. For fun you can do 2 shots, where you focus 3 times before taking the shot - this is a little more accurate but not what you often do in real life.

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