eyeballs, diopters, focus & focus issues

[djtowle]

One focus issue I've been having with my recent Canon slr cameras has been; what seems to be sharply focused in the viewfinder, doesn't seem to be in focus when I get home and review the images. This has been both with manual focus and auto-focus shots. I've read a lot on back-focus, front focus, COF, margin of auto-focus error, etc. But today I wonder if it is something different. Follow my thoughts below, comments, corrections to my thinking welcome:

Just got my eyes checked, age is begining to take it's toll and my eyes are begining to have close focus issues. Thinking about the diopter adjustments on the camera/eyepiece; and then about the image in the viewfinder. At first thought, I couldn't fathom how a diopter would help, you have scenery (far away, di-opter would make it worse) and the information displayed in the viewfinder (close, need a diopter to read). The eye doctor didn' t know the answer.

At home I did a couple of tests, and the diopter seems to affect both image and data focus equally. How could this be? I drew two conclusions (assumptions).

1. The camera is projecting the data (f-stops, exposure etc) at a reasonable eye focus distance, ie greater then 24" (like a Heads up display in a fighter plane). I considered this less likely.

2. That the viewfinder image is being projected onto the focusing screen, ie it is close to the eye, and therefore my eye would/might benifit from the di-opter and it would affect both then viewfinder info and image equally.

Can anyone confirm that #2 is 'how it works'?

Thinking more about this more, the focus screen, and reading this about auto focus cameras:

The AF system sensors are located in the floor of the mirror box. They receive the image through the semi-silvered mirror, which is then reflected downward by a secondary mirror hinged to the back of the main mirror. This forms a virtual focusing plane that is supposed to be at the exact same plane as the sensor (a point of possible miscalibration).
Complete article here:

http://www.fredmiranda.com/forum/topic/241524

and having a few focus issues now and then, which in the past I attributed to operator error, but now I am begining to wonder?

It would seem that to achive good focus on the camera (autofocus mechanisums, AI, etc aside.) 3 things need to be in perfect alignment. i.e. distance from the lens.

1. The main sensor. (makes the image)

2. The Auto focus sensor in the base of the camera.

3. The Focus Screen (didn't think of this before)

So what are the chances of a viewfinder/focus screen being out of alignment. Thus causing my apparent focus point to be off. ie In the viewfinder I see the eyes of a subject in focus, but on the print the ears are in focus?

Time permitting I am going to try a few tests soon.

[lancea]

Hello. The other thing that affects focus is the mirror. It has to be in exact alignment.

Not sure if for your example you used manual or AF. If AF, are you looking at which focusing points are being used? Was the focus point over the ears or eyes, and was there sufficient contrast at those points to give an accurate focus?

A dioptric lens is never going to affect focus. If your viewfinder image is out of focus the extra lens will help you to see that it is, but will never bring it into focus. You are correct on conclusion 1 - the viewfinder image (including data) is projected into the distance (not sure how far). There's an easy way I can tell that: I'm shortsighted and taking my specs off makes everything fuzzy. If the projected image was close I would see it better without my specs! But what you say about benefiting from a dioptric lens/adjustment is also true. You need to be able to focus at that distance so you can adjust the focus correctly.

I've worn specs for most of my life and have never used an adjustment on the camera. It's too much of an inconvenience when I look away from the camera. The on-camera adjustment is really only useful if you've got a slight eyesight problem.

Faces aren't the best way to check focusing. You need something with high contrast. Do a search in these forums for "focus test" and you should find heaps of methods. As to the chances of your focus components being out of adjustment - don't know. If you haven't already try a search on "viewfinder focus".

Hope that's of some help. I'm no expert in this area except as a long-term eyesight-challenged photographer ;)

[tim]

Do this focus test (http://www.canon-dslr.com/Canon_Jan05/Canon_SLR_Focus_Test.htm), on a tripod, mirror lock up, cable release, then post your results. Sorry in bit of a hurry right now.

[Slaggie]

for setting diopter adjustment, you set it so the focusing squares in the viewfinder are sharp. Fiddling around with my camera yesterday and couldent figure it out. Went to the book. Cant begin to answer your other questions. George:)