50mm 1.8f slightly off focus??

METALBLADE

Hello all!
So i just watched a video on fine tuning your focus on a lens. So i figured why not give it a try! Now i just got my D750 back from the flare fix a few weeks ago and so i dont have results of before vs now. Anyways so i printed out the focus chart, layed it flat on a table and shot in the center as described at 1.8 while shooting at a 45 degree angle. I find that for this lens i need to push the focus up all the way to 20 in order to get proper center focus with this lens. Now mind you i just got in a tamron 70-200 2.8 and that only needed a +9 adjustment. I then tried the nikon 24-85 3.5 (which isnt as good to compare i know because its at 3.5 but i wanted to compare anyways) so then tried at and it seemed to need +7.

So my question from what i see is:

A) since there isnt much of a change in fine tuning for the rest of the lens i am to assume that the 50mm is more of the problem then the camera is?

Has anyone else had this problem at 1.8 on their 50 or is this super out of spec?

C) Suggestions as to what to do

Thank you all so much for your time!

Ironheart

Are you sure you are perpendicular to the target?

METALBLADE

I am 90% sure. The chart i use you focus in the middle at a thicker line and then it has measurements on the left and right. When i was doing it i check both sides and they were very similar. If i wasn't straight on one side would have been more in focused then the other. But i could be wrong ive never done it before. I also put it on a chart that is at a 45% and put the camera at 0 and tried it again and got similar results but it was more at +16 as apposed to the +20

Paperman

I was never able to get the fine tuning right with the f1.8D on focus charts There seemed to be no consistency - I would get it right for +5 and then it would need + 15 the next few shots so I gave up after a while.

The lens is quite crappy till you hit f2.8 ( which provides enough DOF to cover the focus issue ) so really no reason to worry about slight back/front focus at f1.8 as using it wide open creates more problems than just focus.

And I had the problem on a D300; the lens will probably perform worse on a D750. You know the focus mechanism is quite cumbersome ; the whole lens shakes and all. It would be a miracle for it to get the AF spot on every time.

METALBLADE

Thank you @paperman I am guessing my G would have similar results as the D then.

WestEndFoto

Your G should be much better. I have no issues with my 50mm 1.4G.

Ironheart

My 1.8G focuses consistently. My 50mm 1.4D focuses a little less so.
How far away are you from the target?

Paperman

I was talking purely 1.8D ... The G is a totally different lens.

FlowtographyBerlin

The +20 adjustment isn't a problem, unless of course you feel you'd need even more and can't go beyond 20.

I don't know my 50/1.8G adjustment value by heart, but it could be +17. I've always had to adjust all my lenses, back with Canon as much as with Nikon today, and it's not a matter of the price of the lens. Some people are lucky with this and don't need to adjust much or at all, some aren't.

Do keep that advice by @ironheart in mind, if you don't align the chart properly, your adjustment will be flawed. You can use the grid and guide lines in the viewfinder.

funtagraph

I suspect - but don't know from experience - the mechanical parts of the lens can cause the problem of inconsistency, especially if you change the direction to come to AF, either from infinity or closest, but don't mix that directions. Geared focus drives do have more play and more flexibility than direct driven ones like the G-types with SilentWaveMotor.

METALBLADE

Thank you all for your feedback. From what i see overall im not going to overly freak out about it and let it ride since it is working just fine even though i am up at +20 or so. I think if i did it with a 2.8 stop i would have more consistent results but i wont be able to tell yet since i just shipped my D750 back to Nikon because they messed up installing my grip when i brought it in for the recall. O well we will see!

Thank you all so much this is such a great community!

Msmoto

I have found that the AF adjustment can change slightly as I change lenses or shoot at a subject distance which is drastically different from the test procedures. And, sometimes I can see such minimal differences I think i am only guessing at the best image.

So, i will do the AF adjustment multiple times, then try to pick the value which is in the middle of my testing.

For the OP's lens, I would suggest removing/replacing the lens redoing the AF adjustment and see if he same value is determined. Do this about three times and if the 20 is the value, go for it.

Ironheart

Also, it would still be good to know how far away from the target you are. As msmoto points out how close or far away may affect focus fine tune as well.

METALBLADE

Thank you all for your responses again!

@ironheart i was as close as i could get so that it would auto focus.

Ironheart

You should be at 50x the focal length, so that's 2500mm or 2.5m or 8 feet.

funtagraph

"as close as I could get" for sure is the wrong distance. How would you exclude the possibility of being already too close for AF? Generalizing to 50× focal length? 25× til 50× I saw as well. My recommendation would be: as close to your most used working distance as possible. Because our expensive Nikon glass only allows one AFMA setting (Autofous micro adjustment) sometimes you will have to decide. Or to use LiveView at aperture/distance combinations you could not set up correctly. The fast the lens, the more critical the whole process.

The always cheaper Sigma lenses allow not less than 4 different distances at 4 different focal lengths, if it's a zoom. A nightmare to do but after that it's a paradise to use. I feel definitely on the safer side with those lenses.

METALBLADE

That makes more sense. It looks like im going to have to try doing the test again when i get the camera back at about 8 feet away and hopefully it will be a bit better! Thanks for the heads up on letting me know im doing it wrong! Ill have to test the rest of them at proper distances as well then, should be interesting with the 70-200 lol

FlowtographyBerlin

@METALBLADE: Do also keep in mind the focus shift. The focus point will shift as you close the aperture, so if you shoot at all apertures, picking out something in the middle (like f/2.8 or so) might be a good idea. Otherwise, your subject will be in focus at f/1.8 (but wide-open blurry) and out of focus at f/8.

spraynpray

@METALBLADE: Do also keep in mind the focus shift. The focus point will shift as you close the aperture, so if you shoot at all apertures, picking out something in the middle (like f/2.8 or so) might be a good idea. Otherwise, your subject will be in focus at f/1.8 (but wide-open blurry) and out of focus at f/8.

You'll need to expand on your theory there Flow. If METALBLADE focuses at three metres f1.8 he has a DoF of 38cm if he then moves to f8, it is 1.85m - how is focusing going to shift that far?

funtagraph

Even within those DoF distances, there's one distance setting (of the lens) apparently to be the sharpest. But of course, if the sharpest point for f/1.8 is correct, nearly nobody will notice it has moved a bit at f/8. To be honest, I only read about focus shift. I never saw it happen in real life.

But what I did see happen, were different AF adjustment values for different distances (and in case the lens in question is a zoom, also at different focal lengths).

Sometimes I wonder, with all our knowledge about what can go wrong and what can influence focus, how we ever get sharp pictures 8-| In any case, I will stick to AFMA wide open and let the focus shift do it's shameless work. Wide open to me is critical - if I miss sharpness wide open, the picture is wasted for sure. If there's already a huge DoF, I don't care that much baout the most precise focusing and in the rare cases I do, I'd use LiveView anyway.

heartyfisher

I couldn’t understand focus shifting when I first encountered (on this forum) but after some research I think I understand it now. basically what happens is the centre of the lense and the edges of the lense focuses on different points due to weaknesses in the lense design thus the lense output changes at different apertures in the following ways as the aperture goes from F8 to F1.8.
1) The image gets less sharp.
2) DOF decreases
3) The distance of the "sharpest" part changes slightly.
4) Aberration at the edges of the frame are more apparent (comatic and chromatic)

It is the 3rd factor that is the focus shifting.

spraynpray

I couldn’t understand focus shifting when I first encountered (on this forum) but after some research I think I understand it now. basically what happens is the centre of the lense and the edges of the lense focuses on different points due to weaknesses in the lense design thus the lense output changes at different apertures in the following ways as the aperture goes from F8 to F1.8.
1) The image gets less sharp.
2) DOF decreases
3) The distance of the "sharpest" part changes slightly.
4) Aberration at the edges of the frame are more apparent (comatic and chromatic)

It is the 3rd factor that is the focus shifting.

Hi Hearty, your points are all plausible but my point is that I cannot see how calibrating at f1.8 then viewing a shot at f8 (which is what flow said) will ever show a shift of focus.

@funtagraph: Yes.

funtagraph

@heartyfisher your description fits to my idea of "field curvature". "focus shift", as far as I understood it, to me is no change in the lens distance adjusting element (every piece of it's glass elements remains in place) then stop down and find the sharpest point of a given aperture doesn't remain in place.

The AF module isn't calculating that function so in theory I can adjust my lens to be perfect sharp in 3 meters distance at f/1.4 but if I close the aperture to f/8, the perfect spot lies a t 3.25 meters. Although this drift can be measured, it is not very visible to the eye (here I can be wrong, I think) due to increasing DoF.

I guess, FoCal as a tool for AFMA can also do a test run for focus shift and if I get curious enough, maybe I will kill some time with it - but I'm a little afraid, too much "knowledge, what can go worng" will lead to too much pictures going wrong.

Msmoto

As I understand focus shifting as a result of spherical aberration is primarily in lenses faster than f/1.8. An f/2.0 lens will not have this unless of poor quality. And stopping down a lens to f/2.8 usually eliminates focus shift with increased DOF.

I usually AF Fine Tune at about ten feet with lenses up to 135mm, then recheck at three feet as this is atypical subject distance for me. IMO, focus shift has not been seen as a problem for me.

heartyfisher

"Field curvature" and "focus shift" are 2 different things.

Field curvature is the location of objects that are in focus. They are on the a virtual curved surface. its like you are in a big balloon focused on the "wall" which is the inner surface of the balloon which is curved.

Focus shift is for any given point object that is in focus, it is represented by a line perpendicular to the sensor where the object is actually in focus. each point on that line relates to a "ring" on the surface of the lense (made up of a bunch of concentric circles) so as the aperture closes more of the dots disappear. since the larger aperture "rings" contribute to more of the light it overpowers the point of light that is in the center(smaller aperture). So the apparent place that in "focus" can change from larger aperture to smaller aperture. Note DOF is a separate factor and not really related.

Ironheart

Remember that DSLRs focus wide-open and then close down right before the shutter opens. It is during the "closing down" when the focus shift occurs. Depending on how you set your AF fine tune, you can help or hurt this process. Put a different way, you can optimize for different shooting styles or scenarios, while potentially hurting others. E.g. If you optimize for infinity stopped down, wide open close up may suffer.
Nasim does a good job explaining (as usual):
https://photographylife.com/what-is-focus-shift