I'm finding it tricky to recomposing an image especially when shooting with large aperture when dof is really shallow. For example:



Taken at f2.8 and manually recomposed. You can clearly see that my dog is not perfectly in focus (sharpest) but the grass, to the lower portion of my dog, is shar[ because I moved the lens downwards.

I dun think my camera body has enough resolution to shoot things in center and crop...should I be using a different focusing point instead?

Please don't tell me my lens is front focussing, if you do, you are clearly a bigger n00b than I am

no harm giving it a try, isn't it?

the easiest mistake one could make. You're using center focus point rite? So what's the truth? Does focusing and recomposing lead to focusing errors? The answer is yes...and no! Yes, in theory (and in practice) there will be a focusing error, but no in the sense that the error may be small enough that you'll never notice it. Especially to those who focus and then tilt the camera upwards/downwards or sideways to recompose.

Let's say, you focus your dog in the middle, the distance of your dog to the sensor is X. Once you recompose, your subject's distance is not X anymore thus the sharpest point is the grass and not your dog. Take a look at those FTM lens, noticed the distance indicator? You focus, the distance is locked and then you recompose.


yes, use a different focusing point instead of the centre. Unless you can keep your subject distance b4 focus and after recompose the same.

This post has been edited by nairud: Jun 7 2007, 08:42 AM

I know about focusing error from recomposing, and this problem has nothing to do with focusing error in that regard. In fact, like you said, focusing error of such degree is negligible at this focal length/magnification size...this isnt macro

The problem is in fact the sharpest point is at the center (center point focus). When I pan the lens downwards, the focus is still locked at the specific distance but the subject is no longer at the center....and the center point is now the grass...I could step my lens down to get an overall sharper image but that would defeat the purpose...I want my bokeh

damn D50 doesnt have auto focus point selection like the D80 and etc.

Which focus point are you using? D50 has 7 focus points iirc. Where did you focus b4 recomposing?

at f2.8, the DOF is still shallow and the slightest recompose could result in that pic of yours.

Actually D50 has only 5 focus point. I use center focus point when taking the picture as I mentioned in my previous post.

Yup, shallow DOF is awesome, but too bad I cant manually recompose it without moving the sharpest focus point...well unless the subject is really tiny.

no AF-C, AF-S selections?
When I do recomposing, the locked subject remain as the locked subject
D80 have those exposure/focus lock too

D50 doesnt have them?

Get DSLR proffesional body with focus points all across the viewfinder
they will be no problem
no need recomposition

Pro poison promoter
Very consistent in effort

Based on these pictures, can use "AF" mode (not sure if equivalent to AF-C or AF-S though), focus on doggie, then switch to "M", and recompose... hehe



But actually i see a AE-L/AF-L button on d50. Maybe need to adjust some in-menu settings as well to make sure it's locking focus...



This post has been edited by wlcling: Jun 7 2007, 10:18 PM

hmmm.. didn't realise abt diz.. thanks nairud

Yeah honestly....nikon professional body...problem solved

My AE-L/AF-L button is set to lock exposure. The shutter button locks the focus, so there is no need to use the AE button to lock focus.

The problem also has nothing to do with AF-S/AF-C. AF-C would always attempt to focus the subject within the selected AF point if the subjects moves away (best combo with dynamic), but it doesnt allow for recomposing when the AF point is set to center because once the lens is moved to focus on the grass...which is the center, the body will automatically focus on the grass. You just can't recompose with AF-C, AF-S is the only way.

Anyway, the solution is to select the proper af point. How I wish the AI is smarter...I think even the 350D's AI-servo is much smarter than the D50/80

Traditionally, DOF will appear two-thirds behind the focus point and one-third towards the camera. You have most interestingly focused nearer. This effect could be used to get the background more OOF behind the dog, allowing it to stand out more.

I use center AF point most often, and point one-third into the subject to maximize DOF with a bright aperture. However, I shall adopt your idea from time to time.

you sure it's not your lens that has center defect? I believe the background should be evenly OOF form the focus point if it is in the center.

I've wanted to pick up something around the S5/D200 range body by end of the year but while I was looking at S5's price with the grip, maybe it'd be better off to just get the D2x(s) for a little more $$.

just a little more $$? hmm

I'm still scratching my head over your problem. The DOF range shouldn't really have moved that much from your dog (if in fact your dog was focussed on correct in the first place) with so little tilt/pan, and especially so given the 1/3:2:3 rule of DOF span. I only see this sort of errors with moving subjects.

Maybe somehow the shutter button is too sensitive and even in AF-S it decided to send a command to the lens to refocus?

Try panning/tilting the lens more off-centre than in that picture you posted, and in the same sort of scene where the background is something that slopes or varies in distance away from you like in that field of grass. If the sharp region consistently lies in the centre of the frame, then you've got a refocussing-when-you-don't-want-it problem.

AlamakLor: Are you saying that my lens has a defect if the DOF is spread 2/3rds behind the subject and 1/3rds towards the camera? It's a normal characteristic of depth of field.

Lay a ruler on a table, 0cm mark towards you, and shoot it from 45 degrees. Point at the 10cm mark and shoot with a 50mm lens wide open. Notice that the 5cm point and the 20cm point are equally in focus.

(At this point, you'd also find out if you have a front-focusing problem or not.)

Then, look at the DOF markings on the 50mm lens. Notice how the distance doubles for an equivalent turn.

More AF points does not make it any easier. I could use the center point all day. I like where my Sony A100 puts the 9 points; all along the rule-of-thirds (which, incidentally, setting a focus point along the rule of thirds will help with hyperfocal focusing).

Even if you had a D200, you might be cursing its other, less sensitive AF points. At that point a Pentax K10D with 9 out of 11 points being cross-type sensors will appeal.

If there's no front-focusing, then cjtune's theory of it refocusing might be true. You could also focus at a distance, then outstretch your left hand in front of the camera.