How do I find out my diopter size?

I wear glasses and have very short sightedness. I have my Canon 450D set to -3 which is as far as it will go. To be able to see anything without glasses I want to know what diopter size I would need to choose to make it sharp.

Any idea how? Do I just get a -3 diopter and push from between 0 and -3 to find the sweet spot with the addon?

EDIT: Actually I am wondering if a -4 would be better since I could have a little more scope and dial it back to -1 on the adjustment with glasses on. Or doesn't it work like that?

Do you have a copy of your prescription from your optometrist? for reading strength?

The typical camera is designed to have your eyes focus to 30-36" (not sure which distance Canon uses). If your eyes are corrected to distant vision, you could simply go into a drug store and try over the counter reading glasses of different strenghs while trying to read text at 30" away, to get an approximation of the diopter strength you need for reading at 30"

Wilt wrote in post #9649991
Do you have a copy of your prescription from your optometrist? for reading strength?

The typical camera is designed to have your eyes focus to 30-36" (not sure which distance Canon uses). If your eyes are corrected to distant vision, you could simply go into a drug store and try over the counter reading glasses of different strenghs while trying to read text at 30" away, to get an approximation of the diopter strength you need for reading at 30"

Drug store reading glasses are + diopter correction for people who are farsighted, the opposite of the thread starter. I am nearsighted like the theadstarter and without my glasses or contacts, the adjustment on my 5D doesn't even get me close. My contacts are -4.25 (left) and -5.50 (right). So it the camera's -3 isn't cutting it for him, his eyes are probably at least as bad as mine. He may also have astigmatism that won't be corrected by the viewfinder.

Have you thought about contacts? They're a lot easier to work with than glasses when shooting, at least for me.

mike_d wrote in post #9650177
Drug store reading glasses are + diopter correction for people who are farsighted, the opposite of the thread starter. I am nearsighted like the theadstarter and without my glasses or contacts, the adjustment on my 5D doesn't even get me close. My contacts are -4.25 (left) and -5.50 (right). So it the camera's -3 isn't cutting it for him, his eyes are probably at least as bad as mine. He may also have astigmatism that won't be corrected by the viewfinder.

Have you thought about contacts? They're a lot easier to work with than glasses when shooting, at least for me.

I am near sighted and my vision is corrected with contact lenses. Nevertheless, I do have to use PLUS lenses to correct my vision for reading purposes. Meaning that a similar plus setting goes on the back of the camera viewfinder adjustment. (Actually now my vision is monovision -- left eye reading, right eye distance -- and I know he optometrist said my left eye uses +1.0 less correction in order for it to be set for reading distance for computer work.)

You need to correct your vision for distant first, then set the diopter value for closer. If you contact for right eye is -5.5. assuming that +2 is needed for reading distance, the net correction with no contact in your right eye would be -3.5 for the viewfinder.

Wilt wrote in post #9652385
You need to correct your vision for distant first, then set the diopter value for closer. If you contact for right eye is -5.5. assuming that +2 is needed for reading distance, the net correction with no contact in your right eye would be -3.5 for the viewfinder.

Do you know how much the 5D's viewfinder can correct? I tried it last night with no glasses or contacts and it wasn't enough to create a clear image. I could barely even see the unlit AF points with either eye. My reading vision is still OK without needing reading glasses in addition to my contacts.

Consult the owner manual, in the specifications section. A 40D goes from -3.0 to +1.0

mike_d wrote in post #9654234
Do you know how much the 5D's viewfinder can correct? I tried it last night with no glasses or contacts and it wasn't enough to create a clear image. I could barely even see the unlit AF points with either eye. My reading vision is still OK without needing reading glasses in addition to my contacts.

The built-in diopter of the 5D corrects to -3.

However, in my 40 years of being grossly nearsighted and using a camera, I have always found it an intolerable bother to take off my glasses to see through the camera and put them on again as soon as I lower the camera to see beyond it. Simply getting contacts (which I wore for a long time) or getting used to using the camera with glasses has always been preferable to doing spectacle-shuffling all day long. Of course, nowadays surgery is an option, but it was never an option for me (pupils too large).

RDKirk wrote in post #9654364
Of course, nowadays surgery is an option, but it was never an option for me (pupils too large).

Large pupils are an issue?

I hope my pupils are normal sized then, as I was hoping for a permanent fix to 20/400 vision (almost surely worse now).

But I definitely agree with your sentiment. I just got used to the glasses, and I have the indents on my nose to prove it.

emilysium wrote in post #9655803
Large pupils are an issue?

Yes, imagine wearing a contact lens that's smaller than your pupil. Light would get past the edge of the contact and make its way, uncorrected, into your eye. The result would be halos, starbursts and other visual artifacts that wouldn't look very good.