Nikon 50mm 1.8 G miss guiding camera at times

[mrinmoyvk]

I recently bought Nikon 50mm f1.8 G lens and have tried few shots with it. In some shots (obviously AF on) I have noticed that camera stops focusing considering that it is the optimum level it could focus and takes the picture. However focus is either little in front or back of the subject where I had placed the focusing point (I always use single point focus). This never happened with my 18-55 kit lens.

Any specific reason this would be happening?

[Stagecoach]

Hi,

You do not say which camera you are using but I'll throw in a few ideas based on what you describe;

a) As you say this 'concern' did not happen with your kit lens is it that you are now using the 50mm wide open at f/1.8 with a shallower DOF and an inherent problem is now more noticeable?

b) Have you an image you can post that shows your problem?

c) Have you undertaken any controlled test of the AF? As a basic one put the camera on a tripod, wide aperture, using the centre AF point and shoot a subject such as below (brick wall, fence etc) where you can clearly see the result.



Grahame

[mrinmoyvk]

Please find answers below to your questions.

Camera: Nikon D5200
a) Yes, I am using 50mm at 1.8 most of the times, some times I narrow it down to 2.8 never below that (whats the point is narrowing it down further) I do not think the problem is now more noticeable as almost all the shots with 18-55 were amazingly sharp around 35mm with f4

b) posted the image (focus point was on the left most pink colour candle)

c) not yet

Note: Focusing issue does not happen every time.

[Shadowman]

Please find answers below to your questions.

Camera: Nikon D5200
a) Yes, I am using 50mm at 1.8 most of the times, some times I narrow it down to 2.8 never below that (whats the point is narrowing it down further) I do not think the problem is now more noticeable as almost all the shots with 18-55 were amazingly sharp around 35mm with f4

b) posted the image (focus point was on the left most pink colour candle)

c) not yet

Note: Focusing issue does not happen every time.

Looks like the camera was still trying to acquire focus when this was shot.

[AlwaysOnAuto]

From all my playing around with my D7000, I've found the DOF is pretty narrow at f1.8 compared to f4.
What part of the candle were you focusing on?
The candle appears to be 'deeper' than the DOF.

[Stagecoach]

Please find answers below to your questions.

Camera: Nikon D5200
a) Yes, I am using 50mm at 1.8 most of the times, some times I narrow it down to 2.8 never below that (whats the point is narrowing it down further)

The use of narrowing the aperture further is to give increased DOF, if required, and theoretically increased sharpness as most lenses are a bit softer wide open.

I do not think the problem is now more noticeable as almost all the shots with 18-55 were amazingly sharp around 35mm with f4

I do not understand the relevance.

b) posted the image (focus point was on the left most pink colour candle)

Looking at the image posted the camera to subject distance (assumed the flame of the left candle) would have been near enough 1 mtr.

Using a 50mm FL at f/1.8 you would of had a DOF of just 26mm which is much less than the diameter of the candle holder.

The plane of sharpest focus would depend upon exactly what the AF system decided to 'lock' on to which on a rounded glass object or flickering flame could vary considerably.

So from the image posted this is not something that you could consider demonstrates an AF system that is not functioning correctly.[/QUOTE]

c) not yet

If you do, ensure you use an area for which there is no confusion for the AF system.

Note: Focusing issue does not happen every time.

I suspect this may be due to the times when it is OK it has a good target.

Grahame

[mrinmoyvk]

Yes, the candle was withing 1mtr from lens.
I have my camera set to settings that will capture the image only if focused and also to single focus, I very well remember that the focus point was on the edge of candle holder which is near to camera.
I will try to get some test shots on tripod and then will get back to you.

Thanks anyways for the help.

[AlwaysOnAuto]

Are you aware of how the AF works on your Nikon?
Reason I ask is, when I went from a Coolpix 995 to a D80 w/AF, I could not get a decent picture to save my life. Some would be in focus, others not. And for no apparent reason either. I finally educated myself on 'how' the AF works and it is rare that I get an out of focus picture now using the AF.
I suggest you find out the 'how' of the AF system so you can use it to your advantage.

[Dave Humphries]

Yes, the candle was within 1mtr from lens.
I have my camera set to settings that will capture the image only if focused and also to single focus, I very well remember that the focus point was on the edge of candle holder which is near to camera.

In which case, it was a pretty "low light" situation, but with the much brighter flickering(?) flame just off the focus point.

I do think you need a brighter, more controlled, test example to check with.

Could you do me a favour please?
Could you click Settings (right at the top),
then Edit Profile (on left)
and put your first name in the Real Name field
and where you are (roughly) in the Location field?
this helps everyone give you more personal and relevant answers - thanks.

Welcome to the CiC forums, Dave

[inkista]

a) Yes, I am using 50mm at 1.8 most of the times, some times I narrow it down to 2.8 never below that (whats the point is narrowing it down further)

The point would be to get more DoF, or to use the lens at its "sweet spot" (where the lens is sharpest. On this lens, it looks to be around f/5.6-f/8 judging by dpreview's graphing of DXOMark data). Wide open is using a lens at its weakest, particularly when it comes to softness, chromatic aberration, and vignetting. Stopping down can mitigate a lot of that.

We know you got a faster lens to go faster. And most of us the first time we got a fast prime shot with it wide open by default for a good long while. But wide open is an extreme and there are tradeoffs, just as there are tradeoffs with very small apertures.

I do not think the problem is now more noticeable as almost all the shots with 18-55 were amazingly sharp around 35mm with f4

Yes. Most lenses are sharper at f/4 than they are at f/1.8. And a 35mm gives you more DoF than a 50mm.

b) posted the image (focus point was on the left most pink colour candle)

Well, that's a low-light shot (which doesn't help the AF any), with near-macro distances. First of all, realize that the minimum focus distance of the AF-S 50mm f/1.8G is 0.45 m (17.72″). If you were closer than that to the subject, the lens physically cannot focus on your subject.

c) not yet

Just don't think that shooting a slanted ruler handheld in low light counts. Man, the number of "is my lens bad?" posts I've seen with those... Make sure you have a perpendicular target that you are actually perpendicular to (just google up DIY LensAlign and you'll get the idea), are on a tripod, have good lighting, etc. etc. If you're expecting to measure performance with mm accuracy, your test methods better be that accurate, too.

[Stagecoach]

Just don't think that shooting a slanted ruler handheld in low light counts. Man, the number of "is my lens bad?" posts I've seen with those... Make sure you have a perpendicular target that you are actually perpendicular to (just google up DIY LensAlign and you'll get the idea), are on a tripod, have good lighting, etc. etc. If you're expecting to measure performance with mm accuracy, your test methods better be that accurate, too.

Kathy, I would suggest to anyone that considers that they 'may' have an AF problem that the very first thing to do is undertake the most basic, ruler, brick wall or picket fence type test.

The significance of undertaking these basics first is to ensure they are using targets of which the AF should work well and not as per the posted candle image target subject. Depending upon the results of a basic test further testing may or may not be considered necessary.

From all the subject matter I have read regarding AF suspect performance over the years one of the greatest problems is that the basics are so often overlooked and the op is too often steered towards test methods requiring very controlled protocol without an assessment of whether this was necessary beforehand.

[inkista]

Grahame, I thought I was describing the basic ruler test. The problem is folks who slant the ruler, and then think they're aiming at a mark on the ruler and then freaking out because they're 1mm off or something, rather than creating a proper perpendicular AF target to go with the ruler and making sure the camera is lined up properly. Especially with an f/1.8 lens wide open.

[mrinmoyvk]

Hi AlwaysonAuto,

Yes, I do know how AF works. Although I do not know the technical stuff that goes inside the camera, I am clear with basics atleast required to focus on the required subject.

[mrinmoyvk]

Thanks Dave, I have update my profile now.

[AlwaysOnAuto]

If I had focused on the edge I circled in your picture, I know my camera would have had a hard time determining the focus point because of the reflection of the candle sitting next to the one you're trying to focus on.
I think you need to understand the 'how' of the AF system in order to use it correctly.
If I were to take that picture, I would have focused on the far left edge of the candle holder as there is a more defined 'edge' for the focus system to use when it determines the focus point. I'd then recompose the shot while holding the shutter button half way down, which on my Nikon locks the focus.
Only trying to help as I think you'll get a very nice shot once you do get the focus to work for you.

[Inkanyezi]

I recently bought Nikon 50mm f1.8 G lens and have tried few shots with it. In some shots (obviously AF on) I have noticed that camera stops focusing considering that it is the optimum level it could focus and takes the picture. However focus is either little in front or back of the subject where I had placed the focusing point (I always use single point focus). This never happened with my 18-55 kit lens.

Any specific reason this would be happening?

There are several possible reasons why focusing can be a bit off, but as you state, it sometimes is in front and sometimes behind where you supposed you set it, you might not really know where the camera tried to focus, as the focus spot is a bit wider than shown in the viewfinder. Within that focus spot the camera will choose something with better contrast than other parts of what it covers.

Then there is also the matter of hysteresis. No assessment system is ever exact, they all have some overshoot, and this may work against your efforts to get sharpness exactly where you want it. A f/1,8 lens has a very shallow depth of field at close distances, and the AF system works at about f/5,6 with some hysteresis. Thus images taken with a f/5,6 lens may focus sufficiently well, while it might fail with a lens as wide as f/1,8. The hysteresis of the system may be wider than the depth of field of the lens.

If a lens constantly focuses beyond the subject, or in front of it, always, calibration may solve the problem. Not so however if it is sometimes in front, sometimes beyond. But to know whether there is anything wrong with the camera or lens, it must be tested under rather stringent conditions, so that other error sources are eliminated. To that end, we use a focus test setup, which usually comprises one test target, perpendicular to the optical axis of the lens, and a slanted ruler, that is used to determine whether focus falls close or far. Several test shots are taken, which will cater for the hysteresis. If some of them are beyond and some are closer, by a small amount, we consider the system correctly calibrated, but the lens might still be too wide for critical focusing by the AF system.

Problems with focusing calibration stem from the fact that the AF sensors are at a different position than the imaging sensor, and light from the lens is deflected to it by a secondary mirror behind the main mirror. Many a slip twixt the cup and the lip - to have it work according to design, it must be calibrated.

Most critical focusing may be attained by using live view. When in live view, the same sensor is used as for capturing the image. If it is sharp in live view, it will be sharp when you release the shutter. Also when in live view, with a camera using contrast detection AF in live view, there is hysteresis. You might possibly focus more accurately using MF.

With time, you'll learn how to use the f/1,8 lens. It is not quite as easy as the kit lens, but it can be very rewarding, as it offers new possibilities, for low light shooting and for shallow depth of field.

[mrinmoyvk]

If I had focused on the edge I circled in your picture, I know my camera would have had a hard time determining the focus point because of the reflection of the candle sitting next to the one you're trying to focus on.
I think you need to understand the 'how' of the AF system in order to use it correctly.
If I were to take that picture, I would have focused on the far left edge of the candle holder as there is a more defined 'edge' for the focus system to use when it determines the focus point. I'd then recompose the shot while holding the shutter button half way down, which on my Nikon locks the focus.
Only trying to help as I think you'll get a very nice shot once you do get the focus to work for you.

Great tip ....
I should have tried it, will do it next time.
I used to use this tip on my point and shoot as it always focused at the damn center of lens, my bad I relied on the focus point
I had actually focused not on the tip you have highlighted but center of horizontal edge between the flame and the highlighted tip (that would have been even more confusing for camera)