Nikon 50mm f1.8 Not Focusing??

[rawHulk]

Hello everyone..

I was trying to get some shots yesterday when me and my friend went to an Ice Skating ring.. I had the Nikon D90 + 50mm f1.8. I set the ISO to 400 and also the aperture was at the minimum most of the time. The lens was set to AF mode. I shot a couple of her pictures from up close, at average distance and then a little more longer while she was skating..

My question is.. The long shots had real bad sharpness.. Its like as if I didn't focus properly.. but nothing in the object is focused either.. I even tried to change the metering mode to Centered and Spot metering from its default. That didn't help either.

1. Is this normal for a prime lens?
2. It is only good for a subject that is at a particular distance?
3. Does a prime lens have any restrictions that I should know about?
4. Any help regarding which metering mode to use when, is greatly appreciated.

I admit though.. the close-up and not so far shots were amazing.. I just love this lens..

Thanks for the help in advance..
Rahul.

[inkista]

Two possibilities:

1) The blur could be from motion blur. If your friend was moving quickly, and you were using a slow shutter speed and panning the camera, that can cause blur. Using a faster shutter speed can eliminate this kind of blur. You may have to bump up the iso much higher, depending on the lighting conditions. You also want to watch your handholding technique to make sure that you're not adding even more blur from camera shake.

2) Misfocus.
At f/1.8 (which, btw, is the maximum aperture of the lens, not the minimum), your depth of field is very small, and focus becomes critical. Your autofocus system may simply be choosing the wrong thing to focus on. Forcing the camera to use only a single autofocus point instead of the entire matrix can fix this. You may also not be waiting until the autofocus system has gotten a lock. The green AF indicator lighting up is key. Also, learning to use the shutter button half-press before taking the shot is a good thing.

You'll also want to master the different autofocusing modes. You probably want to use some form of servo mode if you're shooting someone who's moving.

[rawHulk]

well.. if it was blur.. it should have occurred for closeup shots too right ?

Well in this case.. its not a blur.. its just that the picture wasn't sharp.. I hope i had those pics to show what I mean.. but I deleted them.. lol

Regarding focus, I kind of figured that out and hence used Centre weighted or the Spot metering. But I don't seem to have any good shots when she was far away.. like I mean among a group of skaters.. Although she appears to be focused on compared to people around her (beacause of the max aperture that I used).. the pictures weren't as sharp as the lens would normally be in like close-up shots.. Hence my question.. if there's any maximum distance for the lens for the subject to have good sharpness? or is there something else that I am missing ?

[OsmosisStudios]

Im gonna guess its a DOF problem.

[Mr Guy]

I'm still going with motion blur. If they were closer, they were also brighter. Inverse square law and all that. Your eyes are pretty good at lying to you and pretending that light doesn't fall off rapidly. The camera, on the other hand, would be adjusting it's settings.

What mode where you shooting in?

Can you post the EXIF from a long shot versus a close shot?

As always, examples help diagnose.

*Edit* Just realized you said you deleted them. It's POSSIBLE it was a depth of field problem, but even a vaunted nifty fifty is going to lose some sharpness wide open. At 1.8, you should have a foot or so of wiggle room by 10 feet, I'm guessing the close shots were still in that range, and by 15 to 20 feet you have a lot more wiggle room. My money is on motion blur from dim light.

You're going to want to limit the shutter speed and pop the ISO up a bit more.

[rawHulk]

I was shooting in Manual mode completely. I was increasing or decreasing the sutter speed depending on the pictures - I mean if it looks too over-exposed, I increased the shutter speed and vice versa.

Maybe you are right.. I will try to shoot something similar if possible again and follow your suggestions.

Thanks Inkista and Mr Guy for all the help.. Appreciate it.

[PhotoNewt]

Originally Posted by rawHulk

Regarding focus, I kind of figured that out and hence used Centre weighted or the Spot metering...

These are metering modes, will have little impact on focus accuracy. I believe what Inkista was trying to tell you is to use a different autofocusing mode.

For Canon (and also for Nikon as I understand) there are two different autofocus modes you can select, the One-Shot which is good for stationary subjects and the Servo mode which is better suited for moving ones.

In Servo mode the camera locks focus on the subject and tracks movement while you half-press the shutter button. When you fully depress the shutter button, it uses the movement information to predict the position of the subject and adjust the focus distance just before opening the shutter curtain. This produces better results.

In addition to the above, using a 50 mm lens at it's widest aperture (i.e., f/1.8) makes it very difficult to get sharp focus on a rapid moving subject. For once, as stated above, the DOF is quite thin so critical focus is important. On the other hand, at wide aperture you have more spherical aberration which also causes your focused image to appear softer. Reducing the aperture to f/2.8 or so will yield a sharper better quality image.

[rawHulk]

Originally Posted by PhotoNewt

...
In Servo mode the camera locks focus on the subject and tracks movement while you half-press the shutter button. When you fully depress the shutter button, it uses the movement information to predict the position of the subject and adjust the focus distance just before opening the shutter curtain. This produces better results.

Thanks for the tip.. I guess now I know what to do for a moving object..

In addition to the above, using a 50 mm lens at it's widest aperture (i.e., f/1.8) makes it very difficult to get sharp focus on a rapid moving subject. For once, as stated above, the DOF is quite thin so critical focus is important. On the other hand, at wide aperture you have more spherical aberration which also causes your focused image to appear softer. Reducing the aperture to f/2.8 or so will yield a sharper better quality image.

"Spherical Aberration" -- That explains it. I didn't know about it and googled it. It makes a lot of sense why my pics (only pics for the subject that was far away) were loosing sharpness.

Also, I was always wondering why the costly zoom lenses like the nikon 70-200mm f/2.8 had their maximum aperture only upto 2.8. Now.. i guess that explains it.

Thanks a lot.. Greatly appreciate your help PhotoNewt.

This gives rise to another question though. Does that mean that the maximum aperture lenses (more than f/2.8) like the 50mm f/1.4 are only good for close-up and medium distance subjects ?

[PhotoNewt]

Originally Posted by rawHulk

Thanks for the tip.. I guess now I know what to do for a moving object..


"Spherical Aberration" -- That explains it. I didn't know about it and googled it. It makes a lot of sense why my pics (only pics for the subject that was far away) were loosing sharpness.

Also, I was always wondering why the costly zoom lenses like the nikon 70-200mm f/2.8 had their maximum aperture only upto 2.8. Now.. i guess that explains it.

Thanks a lot.. Greatly appreciate your help PhotoNewt.

Most professional (read costly) lenses have ultra low dispersion elements which improve sharpness significantly and can usually perform better wide open than their consumer conterparts. Still some professional lenses need to be "stopped" down a little to reach the highest quality.

There is an interesting lens on the Canon line up, the EF 135 mm f/2.8 SF, it was popular for portraits several years ago when soft focus was the thing to do. The lens has a "floating" aspherical element that can be moved with a ring to create more or less spherical aberration, this caused the focused image to have variable softness.

This gives rise to another question though. Does that mean that the maximum aperture lenses (more than f/2.8) like the 50mm f/1.4 are only good for close-up and medium distance subjects ?

No at all, it means you need to learn to stop down a little when using those to improve quality.

As you can see from my sig, among the lenses I own, I have an EF 50 mm f/1.4, and I use it often. When I use that lens, my initial aperture tends to be f/4, often times I shoot at f/8, very rarely I go below f/2.8 and only if I need maximum DOF or I am on a pinch to get a quick low-light shot and can't get it any other way. The reason is that below f/2.8 the sharpness and color saturation of this lens as well as chromatic aberration (CA) performace deteriorate rapidly.

[rawHulk]

Great.. Thanks for all the help.. Newt

Time to visit the skating ring again.. ..