Focus Problem with Canon 75-300 mm Lens

[David]

Hi Guys -I own a Canon 75-300 mm 1:4-5.6 image stabiliser USM lens. This model was manufactured from 1995 to 2005, when it was superseded. I first used it in ca. 1996 with good to reasonable results in bright light conditions on travels in Africa. Since switching to digital cameras, the results have generally been less pleasing, with focusing as the problem. Having purchased a Canon 5D Mark 2 recently, I took the opportunity to test this lens (along with others) with respect to focusing. I used car number plates along about 100 metres of road as the focus subjects. I took all the normal precautions such as tripod, and timer shots. I tested on both AF and on MF via Live View. What I found was that at f22 the focus was a sharp as a pin (but see below), both at 75 and 300 mm. However, as the aperture changed to f8 and to f5.6 so the focus degraded such that at f5.6 it had become unacceptable. As focusing at higher f numbers uses f5.6 as its basis, shots at f22 would normally be out of focus, unless I used the Depth of Field button to stop down and then use AF or MF.

I tried the Microfocus option built into the 5D Mark2, but with no success. I think this facility only applies to current Canon lens that the company has registered with the 5D. But I'm not sure on that. I've also spent some time trying to track down anything similar via Google, but nothing specific has been found.

Does this sound like an irredeemable fault, a feature of the lens, a workshop job, or is there a work around?


Thanks for any help or comments.

Cheers

David

[Colin Southern]

Hi David,

Focus on a ruler at an angle and see if it's back or front focusing.

[Dave Humphries]

Hi David,

Focus on a ruler at an angle and see if it's back or front focusing.

I'd be wary of just doing that, different manufacturers have their Auto-focus work in different ways. So put something alongside the ruler with a high contrast edge that unambiguously defines the distance focused, rather than rely on an oblique view of a scale, which leaves too much 'open to interpretation'.



As you can see from my shot, which is a (cropped, 50%) screen grab showing the focus point, Nikon tend to make the furthest thing within the focus point sharp, but even then, this lens shows considerable back focus (about 2cm behind).

The key was aligned exactly to the 15 cm mark when viewed from above.

This is actually a bad example; it used flash, but I needed one with the focus point on, to show the issue - because if the focus point were on the ruler, there would be an oblique view of 8-9 mm within/under the focus point box.

By the way, if the camera was inverted, it still focused behind, showing the lens was at fault, not a misaligned focus point in the camera.

If you want to 'prove a point' to a retailer/manufacturer, you have to have thought it all through thoroughly and have test shots ready for every 'argument' they put forward

HTH,

[David]

Thanks guys - I'll get back to you once I've had a further go and once I'm back from bonnie Scotland. It's the September Weekend in Alba (as the Scottish PM likes to call it) - a time of shenanigans and houghtmaghandie, drinking, eating and general merriment.

Cheers

David

[Donald]

I a time of shenanigans and houghtmaghandie, drinking, eating and general merriment.

Not in Glenfarg, it's not.

Just let me know where the houghmagandie is. I'll be there.

I think you've been watching too many re-runs of The Wicker Man

[David]

Ah Donald! You need to move down to the land of the Beltie. For our fellow members, the Wicker Man was a cult film from the 1970s (?) featuring the late Edward Woodward, Christopher Lee and Britt Eckland (not the recent Nicolas Cage remake). The film involved a great deal of houghtmaghandie and Edward Woodward's character did not take advantage of that offered by the said Britt Eckland, leading to his incineration. Much of the film was shot near the village of Auchencairn, where my cottage is to be found. The villagers have never recovered and re-enact much of the film at the Wicker Man Festival every July.

Anyway, back to focus problems. Following advice from Colin and Dave I have taken the following two images to illustrate the problem. The first is taken at f5.6 and the second at f22, both at 300mm. The focus point is the numeral 6 on the white ruler.





These are autofocus images, with mirror lock-up and remote shutter release. Manual focus at f5.6 is no better. The problem does not seem to be front or back focusing, but either not focusing correctly or simply very soft focusing.

Does this help identify the problem?

Cheers

David

PS My weekend ended with me making damson jelly, so forget the other!

[arith]

You could try this I suppose.

http://www.northlight-images.co.uk/a...djustment.html

[Dave Humphries]

Hi David,

Far too small to tell, I need to see a 50% or 100% crop from image centre, like mine was. (I assume this is the whole frame)

I would also suggest using something thin, like a pencil pointing at the number 6 and focus on the pencil, not the ruler.

Other things; take at widest aperture, with longest focal length, both for narrowest DoF, to judge where focus is.
Ensure you are on single focus point and static mode if on tripod.
Be careful you are not on or under the lens' minimum focus distance, back off a couple of inches/50mm to be sure.

Try one at f8 or f11 to judge sharpness, no lens will be good at wide open (f5.6) or at f22 where diffraction may well be affecting things.
If you can, set a focal length you can duplicate on another lens to compare against.

Cheers,

[inkista]

Just me, but are you sure it's a focus issue and not just overall lens softness? I have the non-IS non-USM 75-300 III, and its performance at 300mm wide open is just plain soft. Stopping down to f/8 tends to improve matters. If you look at your first test shot--there's nothing there that looks sharp and in focus--if this were front or back focus issue, at least some part of the ruler would be sharp.

[David]

Steve - I found the Northlight image and have applied it to my Canon 50 mm 1:1.4 lens. Worked well - with amazing moire patterns. However, with the 75-300 mm lens I haven't enough space in my study to get the lens as far as necessary from the screen.

Inkista - I'm coming to your opinion that this is a softness issue and not to do with focusing. I agree with your comments re image #1 above. I cannot get anything sharp at f5.6 yet when you look at the image, there is a loss of focus below and above the "6" point. The question then arises as to whether softness can be corrected.

Many thanks for everyone's help.

David

[Nihia]

I'm sorry David (or Rawiri if you might be in certain places in NZ), but I am quite distracted by the Houghtmaghandie. It's a long time since I heard that expression & am so fascinated that am unable to focus on the focus problem.

Nihia

[Donald]

Well, indeed. If a good bit of houghmagandie can't take your mind off a focus problem, then nothing will!

[David]

Nihia - Thank you for your interest in the houghmagandie (in Scotland we always give important matters a courtesy definite article, as in The Macallan - arguably the finest whisky). Regrettably, the old days have passed. Gone are the happy times when you could down six pints of Tennants Lager, eat a fish supper followed by a deep-fried Mars Bar, have a birl with the banshees, and then back for the houghmagandie with equally stoshus partners. Now it's a bottle of Merlot (New Zealand of course) and falling asleep in front of the tele.

Cheery Bye

David