Focus problem when shooting with sigma 70-200

AAC

When I used the sigma 70-200 2.8 zoom to shot a speaker's close up on stage 20 feet away from the camera using no flash, the focus was not right (but not blurred) but the exposure was OK.  Looks like hand shaking but I was using tripod.   
I used tripod, auto focus, center metering, "P" setting.  
However, when I changed to wide angle lens (10-20) with same setting, the picture looked OK.
Should I change to manual fouces and tune to shutter (125 or 250)priority ?

echo

你既問題可能係快門太慢, 如果睇相景唔blur 人blur 就增加快門, 有時景深太淺都有問題. 腳架起碼要 m記 190, 用三向頭好過用波頭, except for arca swiss etc..

我以前用死馬 70-200 冇focus problem, 反而用 18-50mm 2.8 就有 focus 問題,  開工影event, 通常影兩張, 一張橫度, 一張直度. 第一張清, 第二張就唔清. 好在我影兩張, 仲有 canon 17-85mm, 死唔晒.

後來做測試, sigma 18-50 MM 同canon body 唔夾, 特別係連拍環境, eg. take 2 photos within 2-3sec in very dim light.
第二日即刻賣左, 當時未有 canon 原廠大光圈 in same range.

用過  15-30 mm 70-200mm 18-50mm 24mm 15mm
只餘 15mm fish eye.

SAZABI

before we question the lens, let's get some answers from you.

what shutter speed was used on the blurry picture(s)?

when you say zoom, so your focal length was close to 200mm? Which body are you using? 1.3 crop? 1.5 crop? or 1.6 crop? When you say setting shutter priority to 125, or even 250, I think it may not be enough if your focal length is, let say, 200mm with a body that is not full frame.

But you said you were using tripod... so what were you shooting?  moving fast or slow?

Ultraman

Besides the factors stated above, did you use "center focusing point" (cuz you said you used "center metering") and recompose afterwards?

AAC

More information for comments :
1) Tripod : "M" - ball head, model unknown, given by my brother.
2) Sigma 70 - 200 f2.8 on Nikon D50, no flash.
3) Shutter speed : 125, 250 and P setting
4) Shotting indoor seminar speakers close up, speakers normally standing on stage and didn't move very quick
5) Room light : regular classroom setting, no spot light.
6) Centre metering

The problem was, when I used wide angle to capture the whole room, it looked fine. But looked blurry in close-up.
Does it matter with sutofocus or manual focus ?  If I increse the shutter speed, the picture is dark. Tune up ISO ?

Thanks.

[ 本帖最後由 AAC 於 2007-12-30 07:04 編輯 ]

echo

想知道咩問題, 只有做測試, 做對焦測試.

樓上 ultraman 講得有道理,  有時影 close up 用中央點對焦, 再構圖個 focus 會唔岩.

如果有相睇就容易知道你問題所在.

shutterbug

how's the stage lighting?  how's the lighting on that speaker?  is there enough contrast on the speaker for the D50 to lock focus in dim light?  you know...I wouldn't expect D50's low light AF is that great...

Catpiano

um .. how come .. more and more focus problem nowaday ??

Is it because we can blow up the pic in our pc ?  

My be ..  need to calibrate ??

I got light front focus problem too ..   

AAC

Well, I tried again yesterday @ hotel function room by setting the shutter speed to 125/2.8 and 90/2.8 with manual focusing and the picture quality improved but not satisfied.   
There was no spot light on podium, just regular function room light. If flash is restricted and no spot light, by using a f2.8 zoom lens, what can I do to make it better?  Tune up high ISO result in noise, lower shutter speed result in blurred picture.

[ 本帖最後由 AAC 於 2007-12-31 13:33 編輯 ]

echo

你要分清問題係乜, focus problem? 手震, contrast 唔夠? 曝光 under? 因為你講 quality 唔好.

我估最大問題係張相 "鬆" 唔夠 "sharp" ? 我估得岩唔岩?

試下收一級光圈用 f4 啦, 可能有改善.