How can I improve autofocus accuracy with a Canon 500D and Tamron 18-270mm at long focal lengths?
Asked 11/3/2010
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2 answers
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I’m getting a fair number of soft or misfocused photos, especially at the long end of my Tamron 18-270mm f/3.5-6.3 on a Canon 500D. I struggle with manual focus through the viewfinder, so I mostly rely on autofocus.
I’ve read that autofocus performance can be limited at smaller maximum apertures, and I’m wondering whether my lens/camera combination is contributing to the problem. Am I misunderstanding how the AF points work at f/5.6–6.3?
More importantly, what practical steps can I take to improve focus accuracy and sharpness with this setup?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
7
Firstly I don't know about the 500D but you are probably right in that only the central point is working at f/5.6 as light is blocked to the peripheral points. You can get the camera to AF with smaller apertures (up to about f/8 if you're lucky) with the centre point if you trick the camera about what lens is being used by taping the contacts.
Can you post some images of the problem? Are the shots definitely front or back focussed instead of just soft? If so is there any consistency in the front back focussing? Sorry for the barrage of questions it's just difficult to diagnose the problem from description alone. This could indicate that the lens is out of calibration (focussing isn't a feedback loop, the AF takes a measurement and then tells the lens to go there.
Have you thought about manual focus with live view? It may be tricky at the long end due to camera movement (a monopod/tripod will help here) but you should be able to view the screen from a comfortable distance.
At the end of the day, any lens with such a large zoom range will never perform as well as a dedicated telephoto so you will undoubtably benefit from upgrading to something faster.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
15y ago
0
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Your lens/camera combo may contribute somewhat, but the bigger issue is often technique and AF-point choice rather than the camera being unable to focus at all.
On many DSLRs, the center AF point is the most sensitive and reliable, especially as the lens gets slower at the long end. Peripheral points may be less dependable. A good starting point is:
- use single-shot AF
- use a single AF point
- set it to the center point
That usually gives the best accuracy.
Also separate true focus errors from general softness. If images are consistently front- or back-focusing, the lens could be out of calibration. If they’re just soft, that may be a sharpness/technique issue rather than autofocus failure.
At long focal lengths, small errors are more obvious, so careful technique matters more. If possible, test with static subjects in good light and compare results using the center point.
So: you probably haven’t completely misunderstood the aperture/AF limitation, but f/6.3 does not automatically mean autofocus is unusable. Start by using the center AF point and single-shot mode, then test whether the issue is actual misfocus or just softness.
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