Focussing problems

[Robert L Jarvis]

Can anyone help, I use a 300D, 300mm lens with a 1.4 tc, I also wear spectacles. I have found that when using manual focus and getting the image sharp in the viewfinder the same image when looked at on the computer is slightly out of focus. I wondered if this was to do with wearing spectacles. In other words although the image is sharp in the viewfinder to me wearing glasses, the camera is not seeing exactly what I do as for me to see it, the image is giong through another layer of glass and is being corrected.

I hope I have explained this clearly and it makes sense. Has anyone else encountered such a problem or does anyone have any idea how to overcome this when focussing manually?

[Keith Reeder]

Robert, have you adjusted the diopter so that it's right with your specs on?

[Robert L Jarvis]

Hi Keith, I thought that I had but it is something I will have to recheck, not sure if it would help though. Is the camera seeing what I am seeing because the camera does not have specs.

[normjackson]

Unless the camera is in some way knackered my understanding is that the camera sensor or film should be "seeing" whatever appears on the ground glass (or cleverly etched) screen that sits above the viewing mirror and below the pentaprism (or pentamirror or whatever). That image on the focusing screen can be thought of as a 2D image which you need to try and get as clear a view of as possible if you intend using it to judge focus. The optical system for looking at this small image inside the camera is independent of the camera lens and consists of lenses in the eyepiece designed to make the image appear at a comfortable distance away (hence short sighted people tend to need additional dioptre lenses, or adjustment of dioptre settings if they don't want to use their specs). The whole image will always look blurred in the viewfinder if you don't have these settings correct. On the other hand if things look sharp in the viewfinder, they should look sharp in the final image. One caveat to this is that focusing screens can give the impression of greater depth of field than will appear in the actual picture and some focussing screens are worse for this than others.

[Neilj]

Robert,

Try this test. Set your camera on a tripod and then autofocus on something like a magazine page. With the image now in focus so far as the camera is concerned you should be able to set the dioptre adjustment so that you see the image sharp in the viewfinder. Try setting it first without glasses and if you can get it sharp put your glasses on and see if there is a difference. On my camera I can see a sharp image both without and with glasses, without having to change the dioptre setting. When you have decided which combination gives you the sharpest image, focus the same way when out in the field. The autofocus should take care of things, so if the viewfinder is sharp and you still get a slightly blurred image you'll have to think of camera shake. With manual focus it can be quite difficult to achieve critical focus, the eye seems to be a little more forgiving than the camera so you need to get it exactly right. Then try some test shots on a static subject at about 50 ft.

[Robert L Jarvis]

Thanks guys, I have found that my dioptre setting was way off. I have done the tests as recommended and matters seem to have improved. I shall be going to Leighton Moss tomorrow so field tests here I come. Will let you know.