Focus Issue 50 f1.8

[jabtas]

I was messing about today and decided to test the sharpness at different aperture with my Canon 50 f1.8.

Camera on solid tripod, self timer, subject was a 6 inch engineers rule, focus confirmation on the number 3

I tried several shots and here are 2 100% crops from f1.8 and f5.6 These shots were taken at a distance of approx 16 inches and I realise that at this distance the DOF at f1.8 is almost non existant.

It appears I have a slight Back (or front) focus problem, is this type of problem a camera or a lens fault.

I am not worried as such, cos this doesn't show in the "real world", I was just surprised that it was actually present.

BTW The sharpnes at f5.6 is fantastic

http://homepage.ntlworld.com/jabtas/IMG_0466.jpg



http://homepage.ntlworld.com/jabtas/IMG_0467.jpg

[Andy_T]

Just 2 thoughts ...

did you have the ruler at an angle or parallel to the camera?
You have to set it at an angle to be able to judge where it focuses.

Do not focus on '3', but rather at something else (paper target, lens box etc.) that is perpendicular to your lens. You can not say if the camera focused on the '3' or somewhere else because the focus area is larger than the area indicated in the viewfinder.

The lens should be tack sharp at f/2.8

Best regards,
Andy

[jabtas]

Thanks for the reply Andy

The rule was set parralel to the camera, as I was just testing the sharpness and didn't expect any focus issues.

I will cary out a few more test with the target set at an angle and see what the results are like, but I am currently getting ready for work and will have to tryit out later.

[tim]

Try using a proper focus test.

After doing the test with mine and sending it to canon for calibration, they said it was fine, but the focus motor isn't very accurate so it mises focus sometimes. The solution they suggested was to get the 50mm F1.4 USM, which I did, and works a lot better. If you use the advanced search to search my posts from 3 or so months ago you'll find a couple of long threads about it.

[Juan Zas]

Tim

Did you try taking photos increasing the f/stop? Try also at consecutive closing f/stops until you reach 5.6 .
In the two most open wide, normally it´s soft

[jabtas]

Hi Guys

Thanks for the feedback..

I am wondering, is this kind of focus problem usually asociated with the lens or the camera. The reason I ask is that I have recently upgraded from a 300D to a 20D, and haven't had much time to use the 20D since having it delivered last week BUT I do have some tack sharp shots taken over the last year with the 300D and the same 50 f1.8 that I took the pics with.

As you can see from the shots that I took, the lens is incredably sharp at f5.6. The 100, 64 and 50 figures in the image are fractions of an inch, which is the reason I used that rule as a sbject for a sharpnes test.

I will have to try some more shots with a focus chart and invetigate further.

I hope it is just my imagination running riot, and not my new 20D that is at fault

[tim]

Tim

Did you try taking photos increasing the f/stop? Try also at consecutive closing f/stops until you reach 5.6 .
In the two most open wide, normally it´s soft

I did a proper lens test, the lens misses focus. Even wide open it should focus accurately, by setting it to F5.6 you're just increasing the DOF so that your subject is more likely to be acceptably focused.

It's a cheap nasty lens, you get what you pay for.

[robertwgross]

I am wondering, is this kind of focus problem usually asociated with the lens or the camera.

I agree. Or, is that a question?

If it is a focus problem, then it might be a problem of the lens, or it might be a problem of the camera body. If you move one lens around from camera to camera, and if you move the camera around from one lens to another lens, you will see whether the problem stays with the lens or with the camera.

---Bob Gross---

[tim]

Hi Guys

Thanks for the feedback..

I am wondering, is this kind of focus problem usually asociated with the lens or the camera. The reason I ask is that I have recently upgraded from a 300D to a 20D, and haven't had much time to use the 20D since having it delivered last week BUT I do have some tack sharp shots taken over the last year with the 300D and the same 50 f1.8 that I took the pics with.

As you can see from the shots that I took, the lens is incredably sharp at f5.6. The 100, 64 and 50 figures in the image are fractions of an inch, which is the reason I used that rule as a sbject for a sharpnes test.

I will have to try some more shots with a focus chart and invetigate further.

I hope it is just my imagination running riot, and not my new 20D that is at fault

It's a lens issue, not a camera issue. Any lens is sharp stopped down, what makes a good lens is being reasonably sharp wide open. I took plenty of sharp shots with the 50mm F1.8, but its focus problems also meant I missed quite a few shots too. With the 50mm F1.4 I get more keepers, you can't afford to miss shots when you're being paid to shoot.

[ed2day]

I don't see any evidence of a focus problem from those pics. Looks pretty much like normal softness at 1.8 to me when you blow it up to 100%. Like Andy said, it should be sharp by 2.8. You also need to take multiple pics. I also agree with Tim--the biggest difference I see between my 50 1.8 and 1.4 is the repeatability of the focus. Most the time it's not a problem but if you are trying to nail the focus at 1.8 maybe 1out of 3 will be a little off.

[mbellot]

I tried several shots and here are 2 100% crops from f1.8 and f5.6 These shots were taken at a distance of approx 16 inches and I realise that at this distance the DOF at f1.8 is almost non existant.

Not to pick nits here, but my 50 1.8 (Mk II) shows a minimum focus distance of 1.5 ft (0.45m), which would put your target too close for reliable focusing.

[jabtas]

Not to pick nits here, but my 50 1.8 (Mk II) shows a minimum focus distance of 1.5 ft (0.45m), which would put your target too close for reliable focusing.

Good point

But I only guessed at the distance, and I definatly had focus confirmation so at last the camera thought it was in focus.

I have since carried out a focus test (target at 45 degree) using the same lens and both of my 300D and 20D bodies, and I can confirm that my lens does indeed suffer from front focus at f1.8, which evens out as the aperture closes until it is non-existant at f2.8.

I am also releived to report that the front focus is consistant between the two bodies, so that does seem to confirm that it a lens issue and not a camera issue (sigh of relief).

Thanks to everone for listening to my concerns and offering advice

[Juan Zas]

I have tested all my lenses for focus issues when I have had the bad focusing problem with my original 70-200 f/4 L. I Tested all with the pattern from (I have found this one was the best):

http://www.canon-dslr.com/

When I received my new 70-200, I tested it and this one rocks !!!

But I am going to the point related to this post.

Checked all my lenses, all are fine but I have noticed my 50 mm f/1.4 USM has a little front focusing at f/1.4. The rest, perfect. I don´t think my lens is bad. I read anothers reviews and it looks like Canon 50 mm lenses open wide trend to be sligthly fron focusing.