Focus adjustment

[Anteaus]

Hi all. In my efforts to learn more about my hobby I ran into something interesting I thought I would share or hopefully incite comment. Most of the experienced photographers here are probably going to find it humorous and basic but hey if another person can learn from my experience it was worth it. I what I write is incorrect, please let me know.

I shoot Nikon FX and I recently took some shots indoors at a birthday party. I was using my 50mm f/1.4G without a flash, so naturally that meant shooting mostly wide open. After reviewing the photos I noticed something interesting. Everyone knows that DOF can be razor thin when shooting at f/1.4 so I never really put much thought into it when some of my photos weren't perfectly in focus. Unfortunately, the more I looked at photos the more dissatisfied I was because I know my focus system well and it didn't make any sense with some of the photos that they were OOF. Soft is understandable, but OOF was not.

So today I decided to take some test shots at f/1.4 and noticed my focus was in fact off. I dialed in some compensation (gotta love that feature) and suddenly everything I would shoot was tack sharp at f/1.4. So I was in the clear, or so I thought.....

Well, that when I learned that as you stop down with the aperture, you're focus point actually shifts forward also. This isn't defect in either the camera or the lens, but the nature with how light interacts in the lens. Ironically, it isn't much of a problem with slower lenses, but if you have a very fast lens then the focus adjustment can be noticeable different between wide open and say f/8.

For most lenses that operate smaller than say f/2.8 it isn't much of an issue because DOF will usually increase faster than the focus shift, so it isn't noticeable. Also, lens at are f/1.8 and slower aren't as noticeable...it's only the very fast, such as f/1.4 and f/1.2, and only when you are wide open.

So what does this mean to me? Well, my factory focus default is great for everything over f/2.~, but as I open it up the focus point becomes noticeably off until at f/1.4 it is off more than the DOF is wide, thus causing focus issues with photos that should never have had them. The good news is now I can compensate for it. The bad news is that adjusting for f/1.4 means focus is off at smaller apertures, so I need to take out the adjustment when I'm in those circumstances.

Now my next question is for Nikon: With all of the innovations made in D-SLR bodies recently, why don't we have in-camera lens focus profiles that can adjust the focus setting in real time based on the lenses aperture setting?

Anyways, that's a giant wall of text. Have a good evening.

 

[JoeBleed]

Not sure about nikon, but I don't think canon had a focus adjustment feature untill maybe a couple of years ago. When I found mention of it, my first thought was, "wow, this is somewhat new?" After reading how much it could change over the apature range and looking at the feature on my 7d my next thought was the same as yours. At least you can set different settings for different lenses instead of just a global/body wide adjustment.

Seems like this would be easy to emplament.

 

[NAC]

I agree with you OP. The fact that modern DSLR can still have drastic focus issues is just... frustrating to say the least.

Apparently focus adjustment should also be entered separately for each focus point.

Sigma's USB doc allows adjustments to be entered into the lens for different focal lengths and focus distances. I don't believe you can enter adjustments by aperture.

MagicLantern has automated focus adjustment entry by using liveview, and apparently Canon created a patent for the same functionality.

Seems to me that mirrorless systems are becoming faster every year, and they don't suffer from the same systematic focus inaccuracies. DSLR days are numbered if they can't automatically correct focus inaccuracies in the future. I prefer optical viewfinders to electronic viewfinders - but most of all I want in focus pictures!

 

[JohnnyRebel]

Seems to me that mirrorless systems are becoming faster every year,

Yes, my Nikon V1 focuses faster and sharper than my D7000. The hybrid system in the V1 is super.

 

[ElFenix]

mirrorless don't suffer the same focus inaccuracies because they (generally) focus in an inherently different way than SLRs do. SLRs (generally) use phase detect, which is basically a means of making a rangefinder using opposite edges of the lens aperture as the rangefinder base. it determines the range to the subject, then moves the lens to that position. the benefit to this is that it's fast. it uses one look to decide both how far to turn the lens and which direction. the downside is that it's not accurate enough for wide open superfast lenses, (particularly medium focal lengths at shorter ranges) because the rangefinder base is short. the dof can easily be narrower than the margin of error.

 

[Throckmorton]

mirrorless don't suffer the same focus inaccuracies because they (generally) focus in an inherently different way than SLRs do. SLRs (generally) use phase detect, which is basically a means of making a rangefinder using opposite edges of the lens aperture as the rangefinder base. it determines the range to the subject, then moves the lens to that position. the benefit to this is that it's fast. it uses one look to decide both how far to turn the lens and which direction. the downside is that it's not accurate enough for wide open superfast lenses, (particularly medium focal lengths at shorter ranges) because the rangefinder base is short. the dof can easily be narrower than the margin of error.

Wow, I didn't know that was how it worked. I thought the phase detectors worked by detecting contrast and adjusted the lens until it was optimized, like with contrast detect.


My A35 seems to have focusing issues. I just bought an A77 though and it has focus adjustment. I think focus is off with the Sony 18-135 kit lens, so I need to mess with it.

 

[ElFenix]

Wow, I didn't know that was how it worked. I thought the phase detectors worked by detecting contrast and adjusted the lens until it was optimized, like with contrast detect.


My A35 seems to have focusing issues. I just bought an A77 though and it has focus adjustment. I think focus is off with the Sony 18-135 kit lens, so I need to mess with it.

the AF sensors themselves do detect contrast, but that's only so that it can see something. the sensors having different contrast alignments (out of phase) means the lens isn't in focus, how far different and which direction correspond with how far the lens has to turn and in which direction. once the lens has turned, the camera again checks the contrast alignments to determine if they're in phase.

but that's not why phase detect is inherently poor for focusing superfast lenses at medium focal lengths. it's the short rangefinder base. a leica has the same rangefinder base regardless of the lens on it, so it's capable of very accurate focus for subjects a short distance away with mid range lenses.


if it worked the same way there wouldn't be any need for contrast detect optimized lenses. main sensor contrast detect works by moving the lens a little bit and looking for contrast improvement.