Understanding focus distance of Nikon f/1.8D lens with D5600 Camera

Question

I took some sample photos using my Nikon D5600 camera with Nikon f/1.8D lens. I had to set the focus manually since this lens doesn't provide AF with my camera.

My Question: The album of my sample pics (just 3 pics) is present at this Flickr Link.

In this Pic (Short distance focus = 4.22m), the girl is blurred along with the background. This makes sense to me because the focus point was set somewhere in front of the girl and hence all the things behind that focus point were blurred.

(Nikon f/1.8D 50mm lens, @f/1.8, ISO 100, 1/160s, focal distance=4.22m)

But in these two pics (Long distance focus = 21.13m), I guess that I set the focus point to infinity (or near infinity). The girl was standing at a distance of around 10-15 feets from me. Now my question is, why everything behind the girl is focused? Why are the things beyond a distance of 21.13m are also focussed?

(Nikon f/1.8D 50mm lens, @f/1.8, ISO 100, 1/160s, focal distance=21.13m)

Last Question; I tried so much but I couldn't find any focus point to bring the girl into focus when she was standing at a distance of 10-15 feets, is this normal or did I made some mistake?

PS: I was shooting at f/1.8.

Answer 1

I think you'll really, really benefit from having some basic understanding of depth of field and how focus works with camera lenses. Some of this information can be found at:

• What exactly determines depth of field?
• What is aperture, and how does it affect my photographs?
• What is the "Circle of Confusion?"
• What is "infinity focus"?
• What is "Hyperfocal Distance"?

The basic thing is: although there is one theoretical distance at which a lens is focused at any given time, there is a range of distances in front and behind that distance where the focus is close enough that we perceive objects there as also in focus. There's no hard cutoff for this, because it depends on a lot of different factors, including print size, viewing distance, and simply your willingness to accept something as sharp enough.

That's what you're seeing in your experiments, and reading and understanding the above should make it all clear to you.

Finally, unless the lens is broken, you certainly should be able to get your subject in focus at 10-15 feet with this lens. Unfortunately, it's going to be hard to get precise manual focus with your camera — it is very hard to get right with the optical findre alone, and unlike some competing models, Nikon does not offer focusing aids like "focus peaking". It does, however, offer a live preview zoom which you should use to better see what's in focus and whta is not.

Answer 2

I have some good news for you. The autofocus system on your camera still works just fine, your body just does not have the engine to move the focus ring. If you are willing to do that for your camera, you can still focus just fine. When you are using the viewfinder, look at the bottom left. Here you will find the focus confirmation dot, and (probably) two arrows next to it. They should still work in manual focus mode (otherwise check your settings). The red dot tels you when the focus is good, and when it is not, the arrows tell you which way to turn the focus ring. It is a little bit slower, but focus should be just as good as with autofocus. Don't use the focus and recompose technique though, that does not work well with a large aperture. Also keep in mind that the lens has a very shallow DOF at such a large aperture. There is a very big chance you won't get autofocus spot on anyway without micro adjustments(which i believe are not supported by your body). Also, take a look at options a4 and a5, they are related to the 'range finder', a tool specially developed for this situation.

The most accurate way to focus such a lens is with live-view, and a tripod can be quite helpful with that. You just put it in live-view, and zoom in quite a lot. Camera shake becomes very obvious, but you can focus extremely accurate.

A other interesting tool to understand what is happening is a DOF calculator. You can find a (free) app, or use one online, like this one: http://www.dofmaster.com/dofjs.html
A circle of confusion around 0,02 mm should be fine for your camera, just select a model with about the same number of pixels and sensor size as yours. The calculator is telling me you need to set your focus distance to within 0.4 ft of your subject distance. Good luck doing that by estimating the distance.

Answer 3

I tried so much but I couldn't find any focus point to bring the girl into focus when she was standing at a distance of 10-15 feets, is this normal or did I made some mistake?

I think you made a mistake.

At those distances the lens should be able to bring her into focus, and also you should have between one and two feet of depth of field to play with.

From your shots I suspect you are simply not correctly focusing. This does require some practice manually. I'd really suggest you try doing some simple test shots with on on well lit simple objects. Also check the diopter setting - you may simply not be seeing a properly focused image in the viewfinder because the diopter setting does not match you eyesight needs.

You might be better off in the long run selling the 50mm f1.8 D and getting a (used) 50mm f1.8 AF-S G which will AF on your camera. The 50mm f1.8 D is a false economy if you are not able to use it effectively.