Problem with manual focussing

[Frank Kolwicz]

In other posts I've described my FD500L/EOS conversion and using it with a 1.4x telextender. I also mentioned the difference between focussing with the optical viewfinder and a magnifier on the LCD in Live View.

There are some things for which the magnified Live View (mLV) technique is simply too clumsy: birds in flight is my big problem right now, but anything moving presents a challenge to it.

The alternative, old-fashioned, traditional, conventional use of the optical viewfinder for focussing with my 5dii, even with the lens wide-open, even without the 1.4 tele, does not seem to be accurate or maybe I just am no longer fast enough in judging good focus as I used to be (my eyesight has been corrected to better than 20-20 by recent surgery). I have to constantly hunt for critical focus with stationary subjects, too, and, when compared with focus via mLV, often find my best optical sharpness to be significantly off from what the LCD screen shows.

Is it possible that the optical viewer is off? The VF screen seems to be sharp enough to judge critical focus without a focussing aid and I used to be successful focussing with a plain screen in my older cameras (F1, A2,) and my previous FD 600L on film.

I've read that digital sensors are more demanding in attaining critical focus than film ever was and wonder if the reliance on auto-focus in modern cameras has let the manufacturers slack off the specs of the accuracy of the optical viewfinder. I know that prior to using this all-manual lens I relied totally on getting AF confirmation when using manual focus with AF lenses and now I no longer have that option.

I've done the obvious test repeatedly: focus on a static, detailed, flat subject with manual focus, then compare with mLV, and the only thing I can say is that tiny changes in focus ring rotation make the mLV image pop in and out of focus, but more substantial rotation is needed when looking for differences with the optical viewfinder. The degree of rotation difference, however, is very small.

I do wish I could implement the focus-confirmation function without having to do a chancy and delicate hack with the lens chip and would even sacrifice my xTi body in case it was hazardous to camera electronics, but I don't think I could accomplish the delicate hand work.

Any insights into this that might help me improve use of the optical VF would be appreciated.

[KeithB]

Do you have the diopter correction set for your old vision? 8^)

Because my eyes are not very good, I tend to not worry about absolute focus, a adjust things until the focus is as good as I can get, even if a bit blurry.  This usually works out OK.

For birds in flight, you will be pretty safe if you just pre-focus at infinity or at a decent hyper-focal distance.  If your distance scale does not work, you can probably calibrate your focus with a friendly neighborhood football field.  100 yards should be pretty close to infinity.  Using Bob's calculators, you should be able to create your own f-stop correction.

[Bob Atkins]

It is certainly possible for the viewfinder screen of be slightly off. I know for sure that back in the days of the EOS film bodies, various thickness shims were available for adjusting the exact position of the screen. I assume the digital bodies are similar.

There are screens available for better manual focus, but unfortunately they are recommended only for lenses f2.8 and faster. There was even a split image screen for some bodies, but I don't think such a screen is available for any of the digital EOS bodies.

If the screen is in the right position, an eyepiece magnifier helps with precision focus.

[klindup]

I think that you can get  split screen focussing on the 1D/1DS bodies.  I have never understood why Canon did not make this an option on all their bodies, it would be at the top of my wish list.  One question though Bob, have you ever used a Hartmann mask to help reach focus?  I realise that it is not an option for action shots but would it work for still subjects?

Ken Lindup

[Bob Atkins]

I've used a Hartmann mask for astrophotography (though I understand the Bahtinov mask is better). However I'm guessing that covergence of the images is much easier to see when looking at the real high contrast point image of a single start then with a typical terrestrial subject viewed on a viewfinder screen.

If the subject is static, I'd just use live view. Seems a lot easier and itshould be more accurate.

If I wanted to use direct optical manual focus I'd do some tests to show the viewfinder screen was offset, then I'd give Canon the data and get them to adjust it. Then I'd do the tests again to make sure they got it right!

[KeithB]

I was just shooting the Thunderbirds over the weekend with my Rebel XS and Signa 150-500.  Autofocus was pretty useless.  I would autofocus on some clouds to get infinity focus and then switch to manual.  The beep when I got the jets over the focus points assured me that I was still in focus.  Occasionally, it would drift off focus - I think I might have switched from 500-150 mm between fly-bys - but would just have to focus again.

This might work out pretty well for birds in flight, too.