Understanding effects of imperfect eyesight, Glasses, Dioptric adjustment on Focus

[rollercoaster]

Hello everyone, It has been two years since I bought my DSLR (Canon 500D) and I have been happily shooting away. Yesterday I got a new lens ( Tamron 18-270mm DI VC PZD if you are interested) and was looking to buy an eye piece extender too. I am always bumping my nose with the LCD on the camera and making it oily.

When looking at the Canon site I came across the Dioptric Adjustment Lens (-4 to +2) which I guess are for people with imperfect eyesight (like me). Looking at them I remembered the Dioptric Adjustment thingy built into the viewfinder of my camera, which I had set all the way anti-clockwise as that seems best and forgotten all about it. My prescription glasses are (Cyl, Axis), Right: -2.00, 10deg and Left: -2.50, 175deg).

Anyway, enough history. What I want to know is,

• Does imperfect eyesight have any effect on the perfection on focus (mostly manual focus)?
  
• Does wearing glasses/contact lenses have any effect on focus sharpness?
  
• When not wearing corrective eye gear, do we "see" the actual sharp focus?
  
• Does using Dioptric Adjustment mean that we can focus better without using our glasses and conversely Dioptric adjustment done for naked eye should be undone when wearing corrective eye gear?
  
• Should I get the -4 Dioptric Adjustment Lens

As you can tell I am totally confused. It happens to me sometimes.

Regards.

[Tzetsin]

I've got extremely bad eyesight so I can understand what your trying to say. I think what it comes down to most is that you need to be able to trust your autofocus. Alternatively, If focusing is paramount I set up the tripod, activate the live view, zoom in 10x and manually focus. At 10X magnification on a 3.5" screen, even people with the worst eyesite should be able to pull off tack sharp images.

Another thing you can do to ensure focus is to keep your aperture a bit narrow increasing the DOF. This will work pretty good provided it doesnt interfer with your photographic "vision".

As for the Diopter, i haven't had much use for it other than just setting it to where i can see best and leaving it at that. I don't think its really ment to be a corrective lens replacement. (or perhaps its just not strong enough to get anywhere near my prescription)

-Jason

[sk66]

The diopter adjustment is meant to allow you to focus the "focus screen" without wearing glasses.
On Canons the adjustment range is larger at +1 to -3. You should be able to set it so you don't need your glasses looking thru the viewfinder.

If it is not set correctly it will definitely affect manual focus thru the viewfinder.

I set mine for use while wearing my glasses since that's how I use it.

[rollercoaster]

Hi Guys, sorry for the late reply. I got caught up in work.

I understand what we can do but I want to understand the physics/optics of this.. Technically, what happens in these cases..?

[sk66]

"Technically" the diopter adjustment changes the magnifying power of the eyepiece so that the focus screen is in focus....it functions exactly like a (variable) lens in your glasses. It only affects the image as viewed on the focus screen. If it has enough range to get to the same setting of your eyeglasses then you don't need your glasses...if you wear your glasses then you (shouldn't) don't need the diopter adjustment.