[fotofinken]

Recently bought a Sigma 120-400mm paired with my K50, and tested longest sharpness from wide open throughout the f-stops today... So I took some shots of a numberplate on a parked car, approx 115 meters away.

I had the camera on my velbon tripod, SR off , and remote shutter with 3 sek. delay. Focus is not changed between shots (backbutton). I took 2 shots on the same settings, a couple of seconds between shots.

What I found was somewhat inconsistent sharpness on the same settings. While I think the sharpness is OK on some of the pics, most of them seem quite soft... So the question is what is causing this softness? I think I have ruled out camera shake and autofocus...perhaps not good enough?

What else can be the culprit? ... "Shutter shock"? ...Earthquake?


The first pair of pics is at 400mm: F5,6, Iso 160, 1/500s

second pair at 400mm: F11, iso 200, 1/160


Edit: Number is blurred out...

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[stevebrot]

What else can be the culprit? ... "Shutter shock"? ...Earthquake?

I will take a quick guess. Direct sun on pavement may result in variation in air density. Sometimes that changes very quickly. That is why it is usually best to not evaluate lens sharpness or AF at longer distance.

Addendum: Even a gentle breeze can defeat your tripod. There is also some risk of vibrations through the ground when in areas with heavy truck traffic. What model Velbon are you using? Is it safe to assume you are using the tripod foot on the lens?


Steve

[fotofinken]

It may have been changing air density, as the sun was shining at the moment...

There is no traffic, and albeit some wind, the shots were taken from inside our house and out through an open window. It may have been vibrations from my kitchen floor, but I was sitting perfecftly still...even held my breath while I took them ;-)

Don't know why I mix them up, but it is a Induro AT 213 tripod, with ballhead, not velbon...and mounted on the lensmount.

[pentageek]

As far as I can tell the upper edge of registration plate is not horizontal in picture no. 3, albeit others are also slightly tilted to right - are you sure your rig is stable enough?

[fotofinken]

As far as I can tell the upper edge of registration plate is not horizontal in picture no. 3, albeit others are also slightly tilted to right - are you sure your rig is stable enough?

That is because of cropping in lightroom :-)

The files are "untouched" (apart from cropping and anonymisation) jpegs from camera raw.

[BigDave]

You shot at F5.6 and F11. Wide open (or mostly so) and at the edge of where diffraction will start. It would be interesting to see duplicate images at F8 or so, where the lens may be its sharpest.

Kudos on the air density option. Shooting in a vacuum has its advantages!

Regards,

[fotofinken]

You shot at F5.6 and F11. Wide open (or mostly so) and at the edge of where diffraction will start. It would be interesting to see duplicate images at F8 or so, where the lens may be its sharpest.

Kudos on the air density option. Shooting in a vacuum has its advantages!

Regards,

Yes, I chose those two pairs of pics (f5,6 and f11) because the difference in sharpness was most visible.

I did not shoot at f8, but at f7.1 and f9.

I thought those would be the sharpest, but they are not...in fact far from it. I shot between 30-35 pics and only a few are so sharp that I almost can read the telephonenumber on the reg. plate. (like pic no. 3 above is among the sharpest at f11). Most are OK'ish, and some are really soft.
So the softness/ sharpness is not dependent of f-number, which I supposed they would be, but is inconsistent through the whole aperture range. I even took some f18 which are among the best.

So varying air density is possibly the best explanation so far :-)

However I read about "shutter shock" or vibrations due to the shutter movement...can it potentially be a problem?

Attached f7,1, Iso250, 1/500s

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Last edited by fotofinken; 04-03-2017 at 02:55 PM.

[35mmfilmfan]

Another approach might be to choose a fast shutter speed, and adjust the ISO.

[ramseybuckeye]

So varying air density is possibly the best explanation so far :-)

However I read about "shutter shock" or vibrations due to the shutter movement...can it potentially be a problem?

You should not get shutter vibrations using the remote and 3 second delay, and your Induro AT 213 tripod should be rock solid. I have read somewhere that when you change apertures your should refocus, I don't know about that but.... Did you focus from liveview or the viewfinder. Is the autofocus button on AF.S or C?

You might try autofocusing in liveview, then switch to manual focus and press the OK button to magnify and see how close it is, you may need to fine tune the AF for the lens.

[stevebrot]

However I read about "shutter shock" or vibrations due to the shutter movement...can it potentially be a problem?

Shutter shock simply means blurring due to inadequate camera support. At least, that is the practical explanation. Yes, in theory, resonance within your K-50's chassis might flex that structure enough to introduce blur due to mirror or shutter blade movement with mirror slap being a more likely culprit than inertial forces from the shutter blades. FWIW, the chance of that being the source of your problem is slim.

That is because of cropping in lightroom :-)

????? Are you doing rotational adjustment with the crop tool? I suggest not doing anything beyond the initial import, using either the embedded or Adobe Standard camera profiles.

Moving forward, here are my suggestions to limit sources of variability, assuming you are interested in assessing shutter induced softness:

• Make sure your tripod is rock solid, meaning:
  • Not at full leg extension (at least one level back)
  • No center column extension
  • May consider wrapping wrist weights or similar around the tripod spider
  • Camera/lens combo balanced on head
• Don't try and break distance records. A lens test target at 10 meters distance for 400mm focal length should be fine. Make sure the target is aligned parallel to the sensor. Bob Atkin's version 2 chart is a good choice: Camera Lens Testing- Sharpness, Chromatic Aberration and Distortion - Bob Atkins Photography (a little more than 1/3 down the page).*
• Intended subject at center of frame
• Manual focus using magnified live view and tape the focus ring before switching back to the regular viewfinder
• Remote shutter release using 3s delay and RAW capture
• TAv mode with aperture at f/8 for all tests (sharpest aperture for you lens per
• Shoot a minimum of five shots at each of the following speeds: 1/15s, 1/30s, 1/60s, 1/125s, 1/250s, 1/500s, 1/1000s, and 1/2000s. Keep lighting constant, if possible.
• Import into Lightroom using default settings and do no additional edits

Steve

* Version 2 resolution target download at: http://bobatkins.com/photography/technical/lens_testing/reschart2015(9).zip

[fotofinken]

Thank you all for commenting! :-)

It seems my testing was a bit flawed....

I quickly took some new pics today, and this time the distance is approx 20meters to my focuspoint. It is a small sticker on a lightpost across the street.

This time things behave as expected, a bit soft wide open, and sharper when stopped down.... and most important consistent sharpness on same settings!

I even cropped it same size, and untouched jpegs from raw...

2 pics of each f-stop....Settings: f5,6 -f8 - f11

[pentageek]

This time things behave as expected

So I guess the case is closed (to the relief of lens' owner