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I'm currently shooting with a Canon EF 1.8 50mm. I like this lens — it's pretty sharp and the shallow depth-of-field at F1.8 is great.

But I always face a problem using auto-focus: almost every shot is not sharp when taking photos at f/1.8. So I have to stop it down to 3.5 or even more to get the subject really sharp.When taking shots of people in low-light, this is useless. I have to do manual focusing (in live-view) which takes to much time for quick shots of people.

Because of this I'm thinking about switching to the Canon EF 50mm f/1.4. Here are my questions:

  • Has anyone experience with auto-focus (maybe even in comparison to the 1.8)? How precisely does it work, especially at 1.4?
  • What is the difference between 1.4 and 1.8 (has someone an example picture with 1.4 and 1.8 to see the depth-of-field and may also the exposure times?)
  • Would you recommend I switch at all, or stick with the 1.8 since it doesn't make much sense for my purposes?
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1  
Just a thought - focusing shallow depth of field shots can be challenging, even with the best lenses, however their are techniques which can help. For example, when shooting a portrait at a wide aperture, you can set your camera to use just the centre auto-focus point. Aim the centre point at one of the subject's eyes, focus, and then recompose your shot. You will stand a better chance of getting accurate focus. Your camera's centre auto focus point is purposely made more accurate than the outer ones, to allow for this kind of technique. –  Andy Holt Sep 24 '12 at 15:50
    
Before you buy a new lens, check out a depth-of-field calculator like this one: dofmaster.com/dofjs.html As @AndyHolt says, it can be very shallow -- much more so than you expect, at first. In my experience, you shouldn't expect to have two people in focus at a reasonably close distance and f/1.8, and even with just one person, it's very easy to have the nose in focus and the eyes not, say. Look at the math and see what it tells you; maybe some part of the picture is in focus, just not what you expect. –  khedron Sep 25 '12 at 5:53
    
That being said, my 50mm f/1.8 can take its sweet time trying to find a focus, and in some situations (pictures of a black cat) it's completely impossible. Manual focusing (through the view finder, for me) is preferable in those situations, but I'll admit, I almost never get it super sharp that way. The 50mm f/1.4 may be less prone to hunting; it's worth playing around in a camera store, or renting a lens. –  khedron Sep 25 '12 at 5:55

4 Answers 4

up vote 5 down vote accepted

One thing to note about the 50mm F/1.8 is that it's a fairly cheaply constructed lens. If you touch the focus ring you should notice that there's some slack in it whereas if you use other lenses of a higher quality you'll notice that they are much more tightly constructed. While the slack in that part of the focus ring probably isn't directly relevant to the precision of the AF, it's a good indicator of the overall quality of the build of the lens and if you can imagine that same kind of precision on the AF motor then as you use AF you can see how it won't be as crisp as you might expect.

Two things you can do: use a smaller aperture (few lenses are their sharpest wide open) and refocus. If your focus has to travel very far then the momentum may carry it beyond where the camera intended for it to stop. So get focused once, then release the focus button (probably your shutter button) and refocus. The second time it shouldn't move very far, but probably will still adjust, and will hopefully come out sharper.

Also keep in mind that AF sensors are not perfect. The CEO of LensRentals.com has several blog posts talking about AF which are good reads if you're into details. The short version: even with the best cameras and best lenses (calibrated for each other) and under the best conditions, AF can still show an alarming amount of variation. Consider that you're using probably the cheapest lens on the market (though it has an amazing price:performance ratio, it's still quite cheap) and probably not the best camera available and you can see how you start noticing the imperfections.

Pat yourself on the back, your skill is outpacing your gear.

EDIT: Link to LensRentals.com blog - http://www.lensrentals.com/blog/2012/07/autofocus-reality-part-1-center-point-single-shot-accuracy

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Thanks for sharing this useful information. I already tried the thing like focusing first time, then refocus, but most time it doesn't change anything regarding to sharpness. However, I think you made the point that it doesn't make much sense to upgrade to the F1.4. Thanks for that! –  TSGames Sep 23 '12 at 11:42
    
If precise focus is critical then AF quality really doesn't matter because nothing you do will give you precise, reliable AF. However, AF does improve with higher quality bodies/lenses. So you may not find the upgrade useful, but others (especially those that rely on AF more and aren't worried about being super exact) may consider it a worthwhile upgrade. –  tenmiles Sep 23 '12 at 14:21
    
Two other considerations: 1) light low enough to require F1.8 is probably also getting close to the limits of the AF system in gneral - one option is to use the AF assist (red light) on an external flash. If you disable the flash but leave it powered, you can still use it for AF assist. May still be too annoying for portrait work though. 2) I've had better luck on my 40D with the 30mm-f/1.4 and selecting a specific AF point rather than letting the AF system guess at which of the 9 it should pick. Doing this on a 550 may be more difficult since it doesn't have all the extra control dials. –  AngerClown Sep 24 '12 at 2:26

The F1.4 is about a half stop faster than the F1.8. Your exposure times will be a bit faster. If you have the person's head filling the frame, there is no possibility that you can have both the eye and the ear in focus.

You should expect that you will have to stop down to F4 or more to get even a reasonable depth of field.

The depth of field at either 1.8 or 1.4 is tiny at most distances. What are you shooting? and what AF mode are you using?

Does your body allow adjustment of micro-focus?

Have you tried another body?

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Hi, and many thanks for your answer! I have the 550d body, it doesn't allow the adjustment of micro-focus (afaik). So far I didn't have the opportunity to try the lens with another body, but I think it doesn't have to do with this since other lenses are working fine when focusing (it simply seems like the auto-focus motor doesn't have enough "stops" to get always the right focus). When people are about 2-3 meters away the face is pretty sharp, and this is what I'm looking for (like capturing without flash to keep the light). –  TSGames Sep 23 '12 at 1:13
    
Forgot to mention, I use Single Shot AF and it also occurs on static subjects (trees, plants, ...). –  TSGames Sep 23 '12 at 1:49
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Each lens and camera body are manufactured to within certain tolerances so it's possible that while both are within tolerances, when combined they don't focus as correctly as they might on another body/lens. –  tenmiles Sep 23 '12 at 7:52

With @tenmiles already covering the quality of auto focus I will try to add something about your last point.

Because of the quality mentioned by @tenmiles the answer is most likely Yes; the EF 50/f1.4 will probably focus better, but not perfect, as mentioned in said post. The build quality is many times increased. However if it's actually worth it, not so sure.

I am a user of the EF50/f1.8 myself, the focus can indeed be tricky sometimes, but with this knowledge I shoot a few extra shots and so far it as worked out for non-critical shots.

Unless you are shooting very critical things exclusively with the 50mm (though it is 80mm on your crop factor camera) I might consider saving the money and buying another lens altogether. For not much more there is the EF100/f2.8 USM Macro (this would be 160mm on your camera, without IS) or the EF85/f1.8 USM or even the EF70-300/f4-5,6 IS USM.

Perhaps one of these lenses will help expand your photography more than changing to the 50/1.4. The one thing in common on all these lenses is the USM (Ultra Sonic Motor), the fact that it is Ultra Sonic doesn't matter for the sake of auto focus. However it does indicate that the motor used is of much higher quality and will probably be better.

Oh and you could even get the EF20/f2.8 USM or EF28/f1.8 USM for roughly the same money, if you haven't got a lens to cover that range already.

Multiply the lens by 1.6 for your camera if it is an EF lens. EF70-300 is 112-480 on the 550D. This is called crop factor and occurs when the lens is designed for full frame sensor camera but the camera used has a sensor smaller than that.

Hope all that helps, good luck!

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I think you're right. May I should really go for some lens like the EF100/2.8. I already have zoom-lenses (18-55 and 55-250) so I think this is the best choice since I want to shot in low-light and want to get some nice bokeh. I know about the crop-factor on the APS-Sensor so I think I should check first if the 100mm is suited for my needs. Thanks for all your hints on that! –  TSGames Sep 23 '12 at 11:45
    
@user1289222 Don't forget, the camera shops are very helpful, go there and ask them if you can try it out. Feel the weight, the build quality. And any slack in the focus. And of course try framing some subjects and such. Good luck! –  Alendri Sep 23 '12 at 11:53

I would note that having to stop down to obtain optimal sharpness is true for most lenses. I often shoot my 50mm f/1.4 USM at f/2.0 or f/4.

Although the lens is only 1/2 a stop or so brighter, that means a little more light is let into the camera body whilst composing/focussing and you may find this helps with the AF sensor being able to more accurately lock on to something in low light.

Also, no-one has yet mentioned the fact that the autofocus mechanism on the f/1.4 uses a micro USM motor. This results in a much faster focussing than the non-USM of the f/1.8 version.

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