How to take sharp picture with wide open aperture

[spidercrown]

Dear CSers,

I like to get some advice from you on how to get sharp picture with wide open aperture when using 50mm prime lens.
Usually, when taking more close up portrait, it is easy to get very sharp image even shot at f1.4 (on the focus area of course). The close up as in half body or quarter body type of shots.
However, when I try to take full body shots, and intend to have bokeh background, the subject always turn out to be a bit blurry (usually shot at about f/2).

Do you have similar experience? Is this behavior normal for more distance subject?

Will telephoto lens help in this case?

Thank you.

 

[swifty]

Sounds a bit odd. Your DOF increases as your shooting distance increase when you stand further back to take a full body shot compared to half or quarter body.
So it should be easier to get a sharp image (of the in-focus area) compared to the very thin DOF when you shoot close up.
Can you post an example with the camera settings ie. aperture, shutter speed, ISO.
It could be any number of things causing the blurriness. Focus, shutter speed etc.

Using a longer focal length lens helps to increase isolation of your subject by decreasing DOF.
So it helps to give you more of a 'snap' because there's a bigger difference between the sharp in-focus area and the blurry not-in-focus area.
But you still have to get the subject in focus and not moving in the first place so it still comes back to your initial problem with the 50mm that we haven't determined the cause of yet.

 

[Mythmaker]

Primes usually focus shift at f2. Micro af adjusment to find a balance between f1.4 and f2.

Some primes also have caliberation issue at 3m and above (Sigma 50mm f1.4 comes to mind). In this case send it in for recaliberation.

 

[spidercrown]

Sounds a bit odd. Your DOF increases as your shooting distance increase when you stand further back to take a full body shot compared to half or quarter body.
So it should be easier to get a sharp image (of the in-focus area) compared to the very thin DOF when you shoot close up.
Can you post an example with the camera settings ie. aperture, shutter speed, ISO.
It could be any number of things causing the blurriness. Focus, shutter speed etc.

Using a longer focal length lens helps to increase isolation of your subject by decreasing DOF.
So it helps to give you more of a 'snap' because there's a bigger difference between the sharp in-focus area and the blurry not-in-focus area.
But you still have to get the subject in focus and not moving in the first place so it still comes back to your initial problem with the 50mm that we haven't determined the cause of yet.

I don't have picture with me now, I'll try to upload some example when get back home .

Primes usually focus shift at f2. Micro af adjusment to find a balance between f1.4 and f2.

Some primes also have caliberation issue at 3m and above (Sigma 50mm f1.4 comes to mind). In this case send it in for recaliberation.

Thanks for the info. However, if the lens is re calibrated, would it cause the near focusing to shift instead?

 

[swifty]

However, if the lens is re calibrated, would it cause the near focusing to shift instead?

Focus shift is a lens design problem so if that is your issue, unfortunately you can't fix it, only work around the issue. Some lenses suffer from focus shift worse than other lenses. Its usually the very fast lenses like the 50/1.2's that are most prone.

Can you get accurate focus at f1.4 whilst shooting full length but it goes blurry when you stop down to f2? That would be an example of focus shift.
But generally, the issue gets better as you increase the shooting distance since you increase your DOF and this compensates for some of the shift.

 

[Turbonetics]

If u are shooting nearer ofcourse the subject will be sharper but the DOF is lesser if using same aperture. But take note that the subject and shutter speed must be firm enough to capture the image instead of a motion blur image.

 

[Mythmaker]

Thanks for the info. However, if the lens is re calibrated, would it cause the near focusing to shift instead?

Yes, if you caliberate using micro af adjustment in the body.

But the calibration I am referring to for the 3m is different from the focus shifting's calibration using micro af adjustment. For the latter, it is a defect in the lens itself that it cannot focus properly from 3m to infinity. Therefore have to send it in to Sigma (in the case I mentioned) for recalibration/fixing.

 

[swifty]

Just on focus shift, you need to be careful if you're going to micro adjust your AF to compensate for focus shift.
Read this: http://diglloyd.com/articles/Focus/FocusShift.html
Read the last point on 'Mitigating Focus Shift'.
If you always shoot at f2 and you dial in focus shift compensation for f2, be aware it will no longer be accurate at f1.4. I personally wouldn't AF microadjust to compensate for focus shift.

But anyhow, if you don't have the focus issue at close focusing at any aperture then it isn't a focus shift problem. Focus shift is a difference in focus between shooting wide open and stopped down. With the narrow DOF at closer focusing, any problems with focus shift will be greater compared to shooting further away. So the fact your issue only occurs shooting further away seem to suggest another issue.

 

[spidercrown]

If u are shooting nearer ofcourse the subject will be sharper but the DOF is lesser if using same aperture. But take note that the subject and shutter speed must be firm enough to capture the image instead of a motion blur image.

Noted. The blueness that I see seems like OOF type of blur, but I can't find the in focus plan either from the image. That puzzle me a bit. Do you also have same experience with distance subject when shot wide open? I mean sharpness not as good as close distance subject.

Focus shift is a lens design problem so if that is your issue, unfortunately you can't fix it, only work around the issue. Some lenses suffer from focus shift worse than other lenses. Its usually the very fast lenses like the 50/1.2's that are most prone.

Can you get accurate focus at f1.4 whilst shooting full length but it goes blurry when you stop down to f2? That would be an example of focus shift.
But generally, the issue gets better as you increase the shooting distance since you increase your DOF and this compensates for some of the shift.

Good question, I didn't have the chance to shoot at f1.4 that time as I was shooting on broad day light with ND8 on. I couldn't open up the aperture further. Will try that out when I get the chance.

Yes, if you caliberate using micro af adjustment in the body.

But the calibration I am referring to for the 3m is different from the focus shifting's calibration using micro af adjustment. For the latter, it is a defect in the lens itself that it cannot focus properly from 3m to infinity. Therefore have to send it in to Sigma (in the case I mentioned) for recalibration/fixing.

Noted. I'll try out few more thing including what suggested above to find out more. Worst case then I may really need to send for the fix.

 

[Turbonetics]

Noted. The blueness that I see seems like OOF type of blur, but I can't find the in focus plan either from the image. That puzzle me a bit. Do you also have same experience with distance subject when shot wide open? I mean sharpness not as good as close distance subject.

first of all as i mentioned the further u stand from the subject,the less sharp it gets.
secondly,from your description,it looks like focus shift issue.
base on your example of shooting portraits of a person,we would always want to get the eyes sharp and the eyes is very small so naturally if u stand closer,the eyes looks bigger and its easier for the camera to get more precise focus on the eyes.
and u mentioned your problem arises when u were shooting full body which means u were standing further(if u are using the same lens).
so when u are further away,the small eyes becomes even smaller and even the head is small too so focus shift can occurs especially if the background has higher contrast or subject is wearing colorful clothes or accessories which makes the camera trying to focus on those areas,but it may not be precise on those areas too that's why u don't get any sharp points in the photo.
lastly,a telephoto lens won't help much because if u are shooting full portraits,the size of the subject is similar and u have to stand further. highend telephoto lenses has an additional Spot AF features for Canon(not sure what other brand calls it) which narrow its focus point to get more prescise focus and still it is not 100% fool proof.

 

[swifty]

Noted. The blueness that I see seems like OOF type of blur, but I can't find the in focus plan either from the image. That puzzle me a bit. Do you also have same experience with distance subject when shot wide open? I mean sharpness not as good as close distance subject.

Good question, I didn't have the chance to shoot at f1.4 that time as I was shooting on broad day light with ND8 on. I couldn't open up the aperture further. Will try that out when I get the chance.

This sounds worrying if you're not able to find any plane of focus. But this will depend on your framing and the plane of focus may just not be in view.
Focus accuracy issues means its focusing on the wrong spot but something should still be in focus, just not what you intended.
Also you should leave your ND8 filter or any filter off the lens if you're going to do any testing. I know your scenario needed an ND filter to get your shutter speed within your max shutter speed but you're introducing too many variables of what could be causing the problem.