Depth of Field Testing DSLRs

If you really want to dive in and understand DOF and focus concepts, I suggest you setup your camera on a tripod and shoot a series of tests that show where the DOF falls for any given fixed focus point when the f-stop is varied. В When you do these tests and review the results you will be amazed at how accurate the printed DOF tables really are. В In my view, there is nothing that replaces hands-on learning of these fundamental optical principals.

HereрЂ™s how to run the DOF tests:

1. From www.dofmaster.com prepare a correct DOF table for a given focal length lens that you use-I would recommend something between 50-85mm on a DSLR as being suitable.

2. Get a clean sheet of paper or if you want to be fancy prepare an excel table as a form to keep your test data. В The headings from left to right are: frame number, focus distance, f-stop, near focus, far focus, hyperfocal distance, comments. В At the top note the date and the focal length being tested as well as the resolution setting of the files being produced.

3. Get two cereal boxes or similar as focus targets. В I use cereal boxes because they have a UPC code and a bunch of other clear text printed on the back that makes a dandy focus target. В I setup my targets on a rail of my porch but you can use any long flat elevated surface that you can level the camera at.

4. Grab a 25рЂ™ tape measure.

5. Tripod the camera and TURN OFF the autofocus. В On the camera, set the file resolution at the highest possible setting-if your camera shoots RAW, all to the better. В Set exposure to full manual.

6. Place a cereal box facing and level to the camera at 10рЂ™ in front of the lens, zoom the lens to maximum and MANUALLY critically focus on the target. В Do whatever you have to do to get the cereal box or other focus target dead sharp.

7. Without touching the focus ring, zoom the lens back out to the focal length to be tested, but DO NOT TOUCH THE FOCUS CONTROL AGAIN WHILE SHOOTING THE REST OF THE TEST. В You will only be manipulating the f-stops to vary the DOF.

8. Set the f-stop to the smallest f-number available, i.e. 1.4, 2, 2.8 or 4 depending on how fast your lens is. В To clarify, the smallest f-number is the widest lens aperture. В Determine a shutter speed that will give a clean exposure of the cereal box and shoot a couple of test frames of the target at 10рЂ™. В Once you re satisfied of your starting exposure, delete all other frames in memory to get a clean start on frame count. В Because youрЂ™ve already set the prime focus point, you can remove the cereal box from view.

9. Now grab your already printed out DOF table from www.dofmaster.com В and read down the left column (distance) to 10рЂ™ and laterally over to the column with your starting f-stop, letрЂ™s say f 4. В In my table at 10рЂ™ distance and f-4 I see near focus at 9-рЂ˜-2рЂЭ and far focus at 10рЂ™-8рЂЭ transcribe this data to your data form or whatever is appropriate for your starting f-stop. В Continue reading right from smallest f-number to largest and transcribe all those numbers to your form. В Once you have done this you should have a near and far focus point for each f-stop that your lens can produce at a given focal length with the lens focused at 10рЂ™. В Note-there is a column on your data form called рЂЗhyperfocal distanceрЂЭ that will not be used in these tests-so ignore it for now. В Now youрЂ™re ready to shoot the tests!

10. Get out your measuring tape and setup the targets at the near and far focus points indicated for your starting f-stop in your data table. В Remember, do not change the focus setting on the camera, only the f-stops. В Make the exposure at the noted f-stop and record the frame number on your data chart. В Reset the cereal boxes at the distances noted for each given f-stop on your form, dial in the f-stop and shoot a frame for each setup.

11. When done and satisfied that your data table is accurately filled in, download all the images to your computer. В Whatever processing you plan on doing to these images, do it in batch mode so that the change is made consistently across all frames. В I recommend you set the рЂ˜sharpeningрЂ™ function to about midrange, but leave everything else alone. В You could increase contrast if that helps, В but donрЂ™t overdo it.

12. If you have performed the tests correctly, you should see a series of images, one for each f-stop value with the cereal boxes in sharp focus at the near and far focus points but everything else outside the area covered by the DOF increasingly unsharp. В You should also see that overall DOF increases as you stop down the lens. В The proof of the increasing DOF is that the boxes stay in focus despite the fact that they are simultaneously moving both closer to and away from the camera as you reset their positions for each of the f-stops tested. В Pretty cool, eh?

13. Remember what I said about the DOF being distributed 1/3rd in front of the point of focus and 2/3rds behind? В If you want to prove this, reshoot the test with the near and far boxes being move around as before, but add a third UNMOVING box at the point of prime focus, i.e. 10рЂ™ for this test. В If you pay careful attention, you will begin to see that no matter where the near / far focus points fall, that the distribution of the DOF is always approximating 1/3 in FRONT of and 2/3 BEHIND the point of prime focus.

Now you understand not only how to read the DOF tables but what they mean, how the DOF is distributed and how it grows or shrinks depending on one of the three parameters, lens focal length, f-stop and subject to camera distance. В Any easy way to reminder the relationship is that a tripod has three legs and so does the DOF concept.

Read and understand the next and final article in this hands-on series is рЂЗDSLRs and Hyperfocal DistanceрЂЭ and you can consider yourself a Jedi Focus Master. В See you there.

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