MLU, focus lock and focus shift

[lightwriting by swapan]

<p>Does the focus shift to the new area where your AF point moves after you have recomposed and use the MLU (mirror lock up) ?</p>

<p>We all know that once you lock the focus by half pressing the shutter, it remains locked till the finger is lifted off it. When you use MLU, finger has to be lifted (after 1st push which lifts the mirror) to push the shutter again to take the picture. I knew that the focus should not shift at that time as the mirror is already up and the AF sensor is not getting any info!</p>

<p>Lately, I have seen multiple pictures where my subject is soft or OOF after recomposing and the info. on computer shows the AF point at the recomposed area, not on the original area(say a face) that I focused. The face is blurred but the background, where the AF is now, is in crisp focus</p>

<p>Shake was not an issue. They were shot with camera on a sturdy tripod with cable release. Subject did not move either! Shutter was between 1/4 and 1/60. Had aperture of 5.6 to have acceptable DOF.</p>

<p>What an I doing(thinking) wrong ?</p>

<p>I feel like a dumboo !</p>

<p>thanks</p>

<p>Dholai</p>

[bobatkins]

<p>I've no idea but you are doing something wrong.</p>

<p>The AF zone is selected when you 1/2 press the shutter. When you fully depress it the mirror goes up. When you press it again the shot is taken using the originally selected AF zone. The camera does NOT refocus.</p>

<p>I'm some someone may comment that recomposing with focus lock can cause a focus shift (though even that is debatable - <a href="http://www.bobatkins.com/photography/technical/focus_recompose.html">http://www.bobatkins.com/photography/technical/focus_recompose.html</a>), however even if ther was a slight focus shift that wouldn't shift the selected AF zone, the position of which you can check on image playback.</p>

<p>I'm basing this on my EOS 7D, which I know works this way. I can't see any way any EOS DSLR could change the focus point with the mirror up because the phase detection AF system can't work when the mirror is up and so can't reselect a focus zone. If the AF zone is moving it has to be doing it before the mirror goes up.</p>

<p>Maybe if you were in an AF tracking mode (AI/servo) focused on a point with the shutter 1/2 dpressed, first moved the camera and <em>then</em> pushed the shutter the AF zone could shift, but it can't (and doesn't) shift in one shot AF mode.</p>

[sarah_fox]

<p>Honestly, I think you're better off not using MLU when photographing people, as they can move on you (even when trying to stay still). Also 1/4s is a dreadfully slow speed to be photographing a person. You're far better off with a bit more light or a higher ISO, which will give you a faster shutter speed.</p>

<p>If you really feel strongly about the MLU, try shooting with liveview instead. Use the 10x magnification to focus, and then trip the shutter with a cable release. As I recall, the exposure starts electronically and then is terminated with the second curtain. Therefore it's even better than MLU from a standpoint of minimizing vibration, and you can monitor your critical focus right up until the time you trip the shutter.</p>

[lightwriting by swapan]

<p><strong>Bob,</strong><br>

I agree- the camera does NOT refocus but then, why is it happening?<br>

I dont remember noticing it in the past-even though I always use this technique.<br>

I am going to try the old trick, move to manual focus once I achieve the lock and then shoot and see what happens.<br>

Also, why does the AF rectangle is on the area where it was AFTER re-composition NOT where it was initially when focus was obtained.<br>

<strong>Sara,</strong><br>

The question is does it actually happen and if so, why does it happen when it should NOT happen or am I doing something wrong? Whether to use MLU or not is NOT the question.<br>

I have tried with inanimate objects with the same result.</p>

<p>BTW, This was with an 1Dx and 24-70L MK II combination.</p>

<p>Thanks<br>

Dholai</p>

 

[peter_mckone]

<p>DPP can show you which focus point was active on your raw file.</p>

[bobatkins]

<p>The 7D shows it on the camera when you review the image. On mine the AF zone does not move when using MLU and recomposing. The AF area is always where it was initially.</p>

[William Michael]

<blockquote>

<p><em>Subject did not move either!<strong> Shutter was between 1/4 and 1/60.</strong></em> What am I doing (thinking) wrong?</p>

</blockquote>

<p>How are you so confident, that you did not capture Subject Movement when using such slow Shutter speeds?<br />People sitting or standing "still" are (usually always) actually, moving.</p>

<p>WW</p>

<p> </p>

[sarah_fox]

<p>Well, there's also the question of how accurate the focus might have been in the first place. I mean, 1/4s at f/5.6 means there isn't much light for focusing!</p>

<p>Soooo... A patient comes to your office for a fasting blood sugar test. That person is holding a super-sized cup of coke from Wendy's. There's a big spot of catsup on his shirt. You ask him, "Did you fast for 8 hr as I instructed you?" He burps and then answers, "Uh... yeah!" So you test his blood sugar and find it is high. What are your conclusions, doctor?</p>

<p>I think this may be somewhat the same thing. Unless you can replicate the problem with good light and a stationary subject, and unless you can confirm what focus points are active when you do the MLU, I don't know that you can conclude much</p>

<p>Also, I think we need more data to diagnose your issue -- or at least I do. If I understand correctly, this is what you are doing:</p>

<ol>

<li>You set your camera to auto focus. (Is that one-shot? Servo mode? What?)</li>

<li>You seat your subject (please, only inanimate objects -- no people holding Wendy's cups) perfectly still in front of the camera, which is on a tripod. You place an AF point over where you want to focus. (Are you selecting just that single point, or is the camera free to use any AF point it finds?)</li>

<li>You half-press the shutter button to AF (do you see the appropriate AF point flash?)</li>

<li>You recompose the picture with your finger holding the shutter button down halfway, holding the focus.When you recompose, are you shifting the camera only a little bit? If you use a somewhat wide angle and shift the camera quite a bit, you will have a focusing error, due to the geometry of the lens. If you focus using the center point and then recompose to move your point of focus towards the edge of the frame, your focus will end up behind the subject, not on it. (Perhaps that's the problem?)</li>

<li>Now you press the shutter again to snap the picture.</li>

<li>You examine the image in the computer and find that the focus is accurate at the location of the AF point AFTER your shift to recompose the image. Is the focus ALWAYS accurate at that point, or is it just accurate sometimes? What happens if you move the subject farther from or closer to the background? Is the focus still accurate on the background?</li>

</ol>

<p>Finally, although I realize I was not answering your question, I was offering what I thought might be very helpful advice. I took time out of my day to do that for you. (That would be similar to you giving me free medical advice because you thought I might be unaware of some health issue.) When I see that someone is using MLU to photograph a human subject (I presume a live one) at 1/4s, I sense that something is being done very wrongly. I simply thought you would like to know.</p>

[ruben leal]

Swapan, this may be a silly question, but when you say "Also, why does the AF rectangle is on the area where it was

AFTER re-composition NOT where it was initially when focus was obtained.", you mean:

a) If focusing with, say, the center AF point, you expect it to show where it was before re-composing, or

b) If focusing with, say, the center AF point, the image shows a different AF point, say a lateral one?

 

Perhaps is my poor English, but, in your answer to Bob, I understood you are in scenario 'a'.

[Robin Smith]

<p>Obviously, if you focus using the center rectangle and then recompose the image but don't refocus, the camera/DPP will show only the center rectangle and it will have moved to be over the spot where it ended up after you recomposed the image. DPP shows which AF rectangle you used when focusing and this does not necessarily coincide in the resulting image with where it was before recomposition. It would have to be a genius of the camera to do that, and it would be misleading too.</p>

[peter_e]

<p>Great idea, though, a focusing rectangle that locks onto and tracks a moving object! Should be doable in live view. A feature that we definitely need for the EOS 5D mk iv!</p>

[sarah_fox]

<p>^^^ would be awesome for video! :-)</p>

[lightwriting by swapan]

<p><strong>Bob,</strong><br /> I think I got it. Thanks for your article.<br /> MLU has nothing to do with it.<br /> Elementary physics of lens and focal plane!<br /> It is the change of the focal plane which moves behind or forward depending on where the initial focus is, that makes the subject go OOF and it is worse with lenses at short distances where DOF is shallow.. Also, I think the improved quality of the sensor and the lens exaggerate the problem being less forgiving!<br /> I am going to stop "focus lock and recompose" technique that I learnt so long ago ! I am going to use selective AF sensor(s) to focus my subject in such a way that I dont have to recompose.<br /> thanks again for your nice article.</p>

<p><strong>Sarah</strong>,<br /> Thank you for your time<br /> You nailed it right in # 4<br /> I disagree with you though when you said using MLU at 1/4th sec and shooting a human has something very wrong in technique. I have hundreds of portraits done this way, dozens of which have been blown to 40 X 60 inches without a problem! Just FYI, whenever my shutter is between 1/4 and 1/60, I use MLU -100% of the time.<br /> Also, 1/4sec at f5.6 indicates an EV of 7, the camera can go down to -1, meaning that it could AF correctly in 1/256th amount of light that was available to me at that time!(7 stops less light). So, that was not an issue either!<br /> <strong>Ruben,</strong><br /> I was in scenario one, you are right. Now I know, I should not expect the AF point to show where it WAS but will show where it IS !<br /> Robin,<br /> Are you kidding me ? MY camera IS a genius and more than that!<br /> Joke apart, why would it be misleading ? It would just show where I focused initially and would be better for me to evaluate my precision and accuracy along with the result!<br>

Wont be so easy for AI servo though- I want it only for single shot!</p>

<p><br /> Thanks again everybody- A puzzle solved for me!</p>

[sarah_fox]

<blockquote>

<p>I am going to stop "focus lock and recompose" technique that I learnt so long ago ! I am going to use selective AF sensor(s) to focus my subject in such a way that I dont have to recompose.</p>

</blockquote>

<p>Either that, or try out the liveview. (I'm assuming the 1DX has this?) It's really better than MLU. Then you can focus manually at 10X magnification anywhere in the frame that you choose. Do yourself a favor, and give it a try. You'll be sold on it.</p>

<p>I do have to insist that 1/4s is way too slow a shutter speed for portraiture. I've viewed your portraits, and I do see motion blur in some of them. It's subtle, but it's there. I do a lot of candid photography, so I often can't use flash and often have no choice in available light. As a rule, I wouldn't go any slower than 1/15s for a stationary subject, 1/100 for a subject in not very spirited motion, and 1/500 for someone moving fairly quickly. </p>

<p>For formal portraiture, I can choose my light, and I don't shoot anywhere near this slowly. Where possible, I use modified, off-camera flash. I know many people consider flash a dirty word, but it looks very natural if done well. One can learn quite a lot about off-camera flash from the strobist blog, lighting 101 (which you can google).</p>

<p>Anyway, listen to a professional photographer: If you do nothing else, try focusing and shooting from liveview! You'll like it very much. It will eliminate your focusing errors (giving you more accurate focus than AF, and placing the focus exactly where you want it), and it will give you less vibration than even the MLU.</p>

<p>Glad we solved the mystery for you! :-)</p>