Monday, December 12, 2011

Image Quality

In checking into the Pentax 645D, I paid for a subscription to Digilloyd's Advanced Photography and noted his other site on image sharpness.

Below is a list of topics covered. I'm showing you this, not to promote the site (I haven't paid for this section of his site) but simply to point out the many ways an image can be degraded.

  1. Banish Blur, or Blur for Beauty?
  2. Blur by Focus
  3. Blur by Autofocus Error
  4. Blur by Manual Focus Error
  5. Blur by Manual Focus Inconsistency
  6. Blur by Focus Shift
  7. Blur by Focus Lock and Recompose (FLR)
  8. Blur by Subject Movement
  9. Blur by Camera Movement
  10. Blur by Mirror Slap
  11. Blur Caused by System Alignment
  12. Blur Caused by Image Stabilization
  13. Blur from Haze and Refraction
  14. Blur Caused by Lens Optics
  15. Blur and Haze from Spherical Aberration
  16. Blur by Purple Fringing (Axial Chromatic Aberration)
  17. Blur by Red/Cyan Fringes (Lateral Chromatic Aberration)
  18. Blur Caused by the Laws of Optics (Diffraction)
  19. Blur by Depth of Field
  20. Blur from Field Curvature
  21. Resolving to Sensor Resolution
  22. Blur Caused by Anti-Aliasing Filter
  23. Blur Caused by Digital Capture
  24. Blur Caused by Demosaicing
No wonder so many people struggle to get high quality images, and sharpness is only one of several characteristics of a technically good image. Only when you are confident you have satisfactorily controlled every single one of the above issues can you start thinking you need better equipment.

On his site, Lloyd has several examples of commonly used top brand lenses hugely changing focus as one stops down, the difference between focussing on the eyelashes and the tip of the nose or the ears type of difference.

I couldn't get a sharp image with the Phase one camera and P65+ back I borrowed for a workshop - and finally got rid of my centre post, tall ball head and leveling base that stuck up another two inches, and now I get sharp pictures consistently - if there is no wind.

On the 5D2, I have been utterly amazed at how much image blur there is at full magnification live view when the wind is blowing.

Michael Reichmann found that tripods (the best tripods) don't stop shaking for 7 seconds - using mirror lock up and a two second delay doesn't cut it.

With medium format especially, focal plane shutters can add considerably to the instability and I wouldn't be surprised if that wasn't a sig. part of my diff. with the phase one camera.

Camera straps blowing in the wind contribute to movement as do cable releases. Let's not even talk about people who press the shutter button and hope the camera stops shaking by the time the self timer activates.

I have been paying attention to what I stand on. Ground isn't ground - ground can be springy, it can be soggy or even squishy, it can be a connection between your feet and a tripod leg and there goes your image.

I`m amazed at how many people are still stopping down their camera to an f stop that is severely diffraction limited. With a full frame sensor (my 5D2) I usually use f11 and focus blend if I have to. Sometimes I will use f16. it isn`t as sharp, though I think I can pretty much compensate with local contrast enhancement.  At f22, I can`t compensate. With an APS-C sensor, I try to stick to f11 at worst, and with point and shoots, even f8 can be problematic.


TJ said...

Well, for someone like me who works extensively on panoramas, I think it is hard to stabilize the camera according to the established concepts mentioned above. The VR-head alone (without the camera) weighs 2kg (~4.4lb) and tends to tilt to one side, and when the camera is attached, it tilts more and then you would have to fix the tilt manually.
The thing is, we have to think about the final goal as well. I mean, how many large prints are we making to the extent that such vibrations (even micro ones) would be visible? Notice as well, large prints are made so people can see them and notice them from a distance, i.e. the small vibrations from mirror flipping for example can still be negligible in the practical world.

Personally, I don't think there is a concrete way to absolutely stabilize the camera when working on panoramas. I work in live view mode usually (which flips the mirror already) and this is one reason I do prefer to use it, but I use 2 sec delay timer. The other option is 10 sec delay, but I can assure you that the day will be a looooooooong one if I use this option when making panoramas!

Frank Field said...

I certainly can attest to the mirror slap issue. Even using my largest tripod (Gitzo series 3) with rugged ball head (RRS 55), I find I need to wait at least five to six seconds between raising the mirror and tripping the shutter. While my heaviest tripod with best ball head certainly do better than lighter tripods (Gitzo series 2), the mirror slap vibration is still there - you just have to wait. All this with crop sensor Nikon cameras and modest focal length lenses (thus not too heavy). Sometimes I'll rush it in the field, waiting maybe three seconds, and I pay the price in images that are just simply not quite as sharp. I can not imagine using a camera without mirror lock up!