What is the difference between motion blur and defocus?

[isline_cleary]

<p>I was out today with my 5D Mk II and 70 - 200mm f4L lens (non IS) photographing moving trains (average speed of 30-40mph).<br>

All of them were unsharp !<br>

I had this problem with my old 5D of late (same lens) but when I first got the camera, I was getting a high keeper rate. Then I was questioning my technique going down the pan, defective AF and wondered even if the lens was at fault.<br>

With the 5D II set to Ai servo, I selectively changed the focus point and alternated between AV and M mode.<br>

Where shutter speeds were too low, motion blur looked fairly obvious. However at higher shutter speeds where trains were only going about 10-20 mph, pictures were still unsharp.<br>

How do I know whether the unsharp pictures are a function of...?<br>

1) too low a shutter speed<br>

2) not enough camera-to-subject distance<br>

3) Ai servo not locking focus correctly<br>

I've a feeling it's No. 3 because static tests using the same body/lens in One-shot mode with shorter camera-to-subject distance gave me sharp pictures. But I would like confirmation please<br>

Assuming I am allowed to post samples, here are three... The camera box is in one-shot mode for comparison </p>

<p> </p>

[chris_gampat]

<p>First off: http://digital-photography-school.com/two-factors-for-perfect-focus<br>

Motion blur is when your subject is moving and everything else in the photo is still. Or it can be that everything looks like it's shaky. Lack of focus, your subject isn't sharp.<br>

Try this:<br>

Aim your camera at a subject and while taking your photo, move your camera around. Then aim it at a subject, take a deep breath in and at the top release the shutter.<br>

You'll be able to see the differences.</p>

[matthijs]

<p>By the way, iIf you look at 100% the 5D-II will show more motion blur than the 5D...</p>

<p>(When all else is equal, the blur is spread over more pixels.)</p>

[robert_armstrong2]

<p>Chris. Thanks for link but may I point out that the camera was in AI servo mode for the moving pics.<br>

As I can't seem to post pics here, I have asked the same question in the EOS lens forum (but the pics don't do justice due to size constraints)<br>

Basically the background is sharp but the slow moving train is unsharp. When in AI servo mode, the autofocus tracks the moving subject and "should" hold focus. But focus is off.<br>

Matthijs, I own both the 5D and 5D Mk II and see no difference. I was hoping the Mk II would have better AF tracking especially with centre point focus (but I tried other focus points too)</p>

[steven_f1]

<p>AI Servo works well as long as the following it true:</p>

<ul>

<li>The subject is moving directly toward you or directly away from you. If you are following a train as it approaches and continue to follow it as it moves away, it will appear to the camera focus system that the train is slowing down as it approaches and then speeds up again. When the camera sees zero velocity (or very low velocity) it might not track very well and can miss focus. Additionally as the train passes the velocity the camera sees will change rapidly. Possibly too fast for the camera to keep up with.</li>

<li>AI Servo works best when you can keep one focus point (preferably the center) on only one portion of the train so that the camera always is focusing on the same point. If you cannot keep the focus point on one spot due to the speed of the train AI Servo may miss. IF you are not following the train (meaning you want the train to show motion blurr but everything else in focus, you should use single shot focus. </li>

</ul>

<blockquote>

<p>Basically the background is sharp but the slow moving train is unsharp.</p>

</blockquote>

<p>I would guess in this case the train wasn't approaching or moving away or toward you very fast and the camera didn't detect motion and probably just focused on a still object. </p>

<blockquote>

<p>the autofocus tracks the moving subject and "should" hold focus. But focus is off.</p>

</blockquote>

<p><br />Were you keeping the focus point on one portion of the train? Such as the very front? Also just as the train goes by you the camera will see a rapid change in the trains velocity and the camera may have a hard time keeping up.</p>

<p>Generally motion can be identified over out of focus by the following:</p>

<ul>

<li>Edges at 90 degress to the direction of motion will be blurred.</li>

<li>Edges parallel to the direction of travel will be sharp.</li>

<li>Edge on non moving objects should be uniformally sharp unless it is at the edge of your depth of field. </li>

</ul>

<p>Also the higher shutter your shutter speed, less motion will be recorded by the shutter. At a sufficiently high shutter speed no motion will be recorded. </p>

<p>If you have an IS lens and you are following the motion of the train make sure to set the lens IS system to its panning mode. If it doesn't have a panning mode you might get better results with IS turned off. IS should always be off when using a tripod. </p>

[neill_farmer2]

<p>Isline, I photograph trains a lot so here are my tips. Use shutter speeds of 1/500th or more for moving trains if you are not trying to purposely get some motion blur. Slow trains can be shot a bit slower. Use one spot focus to pre focus on the place where the train will be when you take the picture. I do this because train headlights send AF crazy. Following an approaching train with a zoom, winding in from 200mm to 70mm and trusting AI servo dosen't seem to work. I have better results sticking to one shot, focus to the 200mm position, take that shot, wind in to 70mm, hit the '*' button to focus on the 70mm spot and take that shot. I use '*' for all my focussing and prefocus on the track or a lineside object at the point I want. If your lens has IS remember to allow a 1/2 second for it to settle before taking the photo. I also use as high an f no. as possible to increase depth of field, this ensures most of the length of the train is in focus and gives a margin of error. My exposures are typically 1/500th at f5.6 - f8. My best results come from composing the shot in good time, selecting the right focal length and being satisfied with one good shot rather than 6 ordinary ones got by blazing away on continous.<br>

If you are panning then technique is all important, it's difficult and requires practice and an element of luck. On FF you will need a lens of 85mm or longer. Shutter speeds around 1/25 to 1/50th. Pick up the train as it approaches and follow through like a golf shot, firing off a series of shots on 'continous'. As mentioned above set IS to mode 2. You will find that due to differing radial speeds as the train passes the whole train will not be sharp. To reduce this effect I generally stand well back from the track, maybe 150 feet or more.<br>

Finally don't get too close. Trains are dangerous, especially fast freights which may have loose ropes or lashing that can inflict serious injury. Train drivers will report trespassers to local police if you are not 'doing the right thing'. Trains can also approach silently, even though it's doing 70 mph and weighs 6000 tons it can creep up on you.<br>

Hope these tips help.<br>

Neill</p>

<p> </p>