20d and focus

[wintoid]

I have 2 lenses, a Canon 24-85mm and a Canon 50mm 1.4. I've just upgraded from a 300d to a 20d. Most of my photos are of my 1-year-old daughter, and a lot of them are shot indoors using a Canon 550ex flashgun.

When I was using the 300d, I found that the 50mm lens was way way sharper than the zoom. Since getting the 20d, I've found that the 24-85 is giving me better results.

After much experimentation, I believe that the 50mm lens does not like the red focus assist beam that the 550ex fires. If I turn off the focus assist beam, I get sharp images, and here's a 100% crop from such an image...

http://simon.pietroni.co.uk/sharpeye.jpg

If I turn on the focus assist beam, the image looks OK, but when you go to 100% you can see it's out of focus, and rather speckled. I also find it's front-focused.

Here's what I was focusing on...

http://simon.pietroni.co.uk/notsharpeye.jpg

and here's something closer to me that was (pretty much) in focus

http://simon.pietroni.co.uk/sharpzip.jpg

This is pretty much consistent and repeatable.

Anyone ever seen anything like this before? For now, I've simply turned off the flash's focus assist. It doesn't seem to bother the 24-85mm but the 50mm cannot handle it.

Edit: Just a comment, all these images were taken with the 50mm lens

[wintoid]

Just to further demonstrate it, I took 2 identical pictures using my 20d with 50mm 1.4 on a tripod with the 550ex. The aperture was at 1.4, and the shutter at 1/250. In one case I used the flash assist to focus, in the other case I just let the 20d focus with no flash assist. In both cases I was focusing on the letter "g".

Without the flash focus assist, the letter g is bang in focus:

http://simon.pietroni.co.uk/gnoflash.jpg

With the flash focus assist, the camera has front-focused:

http://simon.pietroni.co.uk/gflash.jpg

[elbirth]

I probably can't be of much help, but just something I was wondering, how did it look in the viewfinder before you shot the picture? Did it appear in focus in both shots but then turned out to front focus in the one with the assist beam?
It looks almost like instead of using the focus point you wanted, it seems to default to another with the beam on.

[wintoid]

I'm afraid my eyesight is pretty bad, and I've not yet got the viewfinder set up satisfactorily for manual focus, so I can't really say what was in or out of focus in the viewfinder :/

It hadn't occurred to me that it might be selecting the wrong focus point. The central focus point went red when the beep sounded, so that implies it had the correct focus point.

[BigRed450]

Well I just took a look at the last pics you posted and I must admitt that both are front focused. You have mentioned that the focus is on the "G" yet the "G" is not in the center of the picture. In this first image the "E" is in sharper focus.

http://simon.pietroni.co.uk/gnoflash.jpg

The same in the second image. However in the second image the "G" is even farther from the center of the picture. This may have an effect on what you are trying to achieve here..

http://simon.pietroni.co.uk/gflash.jpg

I suggest Locking your AF to center point only and try this experiment again. The issue may not be the flash at all...

[wintoid]

Sorry, just to clarify, I was locked on the central focus point. The images shown are 100% crops, and so the position does not tell you anything about what I was focusing on. In both cases the central red square was right in the middle of the letter g.

[elbirth]

In light of what BigRed450 had to say, it does seem that the "e" is in quite sharp focus in both of the pictures. I wonder if it may also be possible that a setting is getting changed somehow such that the depth of field is changing?

Also, it'd definitely be more consistent proof for or against any of this if you placed the camera on a tripod or something and fired the 2 test shots, taking the EXACT same picture, 1 with the assist beam, and 1 without. The caterpillar alphabet thing was moved slightly between the pictures.

[wintoid]

That's what I'm trying to say. These are from 2 tripod shots with the exact same picture. The difference is that I cropped from different areas in order to show where it goes in and out of focus.

I actually think the focus was on the f but because yellow and white are not very contrasty, we see the e as most in focus.

One thing that occurred to me today is that the lens hood on the 50mm 1.4 is really fat, and perhaps the red light from the flash doesn't clear the hood. I'll try this again without the lens hood and see where that gets me.