Alley in Pisa, Italy

by Lars Kotthoff

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My favorite portrait lens is the Nikon 50mm 1.4G. I love the light and what it is capable of. My problem with it is that on my D90 the AF is sometimes horribly slow, and in many cases (mainly low light) I can't auto focus at all. I use AF lock as much as possible, but for many shots I really wish I could just use manual focus. The problem is that I'm usually wrong about whether my subject is in focus or at least completely in focus. So my question is...

Are there any recommendations you can make for training my eye to be sure my manual focus is accurate? Any diopter adjustments or calibrating tools? Any eye exercises to train myself to recognize a true focus?

I know that I would probably have better AF performance on a more expensive body, but for now I think it's good practice to train myself in manual focus anyway. Thanks for any assistance.

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Another point I meant to mention... for regular manual shooters, is there a maximum aperture you tend to use? Should I just say to myself if I'm going manual that going wider than 2.0 would be a waste of my time? What's the widest aperture that you would use in manual? –  Steve the Maker Jan 27 '12 at 8:36
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With the default matte screen, about f/2.8. See my answer below for why. –  Staale S Jan 27 '12 at 13:37
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3 Answers 3

up vote 6 down vote accepted

One of your problems is that the matte screen in your camera (this is what you are actually looking at through the viewfinder, it's a semi-transparent plastic screen that sits at the top of the mirror-house, below the prism housing) is designed to give a nice bright useable image through a slow autofocus lens such as your average f/3.5-5.6 consumer zoom. This is good if you are using a consumer zoom, but very bad if you are trying to focus a fast prime lens. The reason is that the extra screen brightness is bought at the expense of focus accuracy - is unable to show you the small depth of field at larger apertures than about f/2.8-ish. In other words, if you look at the image from an f/1.4 lens through this matte screen, the depth of field you see will be that of f/2.8 and not f/1.4. Again, for a consumer zoom this is entirely irrelevant but for a fast prime it is highly misleading. You physically cannot focus a fast prime accurately through such a matte screen; you are just not getting the information needed.

A solution for manual focusing is to replace the default "fast" matte screen with an old-school, coarse-grained one which will show you accurate depth of field for a faster lens, and more "pop" when things come into focus. You may also get a screen with a split-prism center, or one with a prism-ring round the center.

I'm not sure if Nikon offers such alternative mattes for the D90 but third parties might although installation is likey to be a bit more fiddly in that case. The downsides to a coarse-grained matte screen is that it affects light-metering, and that the view from lenses slower than about f/2.8 will be darker (much darker, in the case of an f/5.6 lens) in the viewfinder. The matte does not affect autofocus performance in any way though.

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+1. I've heard nothing but raves about this company, which produces custom screens for a variety of cameras, including the Nikon D90. katzeyeoptics.com –  mattdm Jan 27 '12 at 14:14
    
(And they also provide installation service.) –  mattdm Jan 27 '12 at 15:30
    
Do higher-end bodies typically come with coarse-grain or split-prism screens? –  Steve the Maker Jan 29 '12 at 6:25
    
No, they don't, as far as I know all DSLR cameras come with some variety of the fast matte screen. The reason being that coarse screens are simply too dark for your typical autofocus lens. Most of the time, a bright view is more valuable than an accurate view! However, on a higher-end body the manufacturer normally gives you the option of buying various replacement screens such as a coarse matte. They usually don't offer this for the el-cheapo cameras; third parties may, but at higher cost and installation is then more fiddly because the camera isn't designed for having the matte replaced. –  Staale S Jan 29 '12 at 9:43
    
For example, to replace the matte on my Canon 1Ds2, I use a very clver small plastic gizmo that comes with the new Canon matte screen to release a locking latch, pick up the old screen, install the new screen and close the latch. Done in thirty seconds! On a consumer body I'd not have such a tool and the latch would be a lot shorter because it is not supposed to be fiddled with... opening the latch would then be a tweezer job, as would the screen dismount and remount. A lot more fuss all around. Still doable, by all means, but fiddly. –  Staale S Jan 29 '12 at 9:47
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At f/1.4 your DOF is going to be very narrow. Even in good light you can easily miss focus, so there's no silver bullet that's going to be foolproof.

The green focus lock light in your viewfinder will still function in manual mode, so as you focus keep an eye on the green indicator. Of course if AF is hunting, the AF lock indicator will probably not be sharp either, but as you focus in and out, you can use it to gauge focus (it will slowly flash, then flicker, then hopefully come on steadily).

Use the centre focus point (it is the most sensitive) and try to focus on something with as much contrast as possible, and edge or line if possible.

If you have a hard time deciding exactly where the sharpest focus is, I find it best to bring it well out of focus, then quickly focus back in, overshoot the mark and basically go in and out quickly and settle on a middle point. You can also stop focusing and lean towards your subject, and away, and try to judge where the nearest point is.

You can also try live view. This can give you a brighter image to work with than the viewfinder, and larger.

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Didn't realize the green light still works in manual, that was also helpful, thanks! –  Steve the Maker Jan 29 '12 at 6:27
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Switch live view on, zoom all the way in (using the live view zoom function, not the lens) and use the enlarged image to judge focus.

If you are using a wide aperture you should probably also put the camera on a tripod to help you prevent camera movement while focusing.

This method had its limitations (for example it doesn't work at all if the subject is moving) but it does work.

I know I can't manual focus at all using the optical viewfinder unless I set an aperture that leaves me a wide margin for errors.

And for your comment, the widest aperture depends on your camera, the focal length and the distance to your subject, just use a DOF calculator and see for yourself - if you are very close to your subject even F/11 will give you a very shallow DOF while if you are very far the DOF will be huge even at wide apertures.

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