focus issues

[stephenb]

Took this shot on Fri evening, whilst most of it appears sharp the motorway sign is blurred. I tried auto and manual focussing but all the pics are the same, other than getting there when it is light to focus then waiting how can I get around this.
ISO 100, F9, 27secs, using remote release and mirror lock up.

http://static.flickr.com/80/245372099_53078bae34_o.jpg

[Dark Star]

Its not because the sign is moving slightly is it? Affected by the traffic whizzing below? Or wind? (the weather, not you!). Just a thought!

John

[Dena]

Were you using a tripod? Can you show a 100% crop, it is hard to tell here if it is in focus or if the light reflections are glowing.

[Marik]

First thoughts are, as it's only the sign area, that indicates not enough Depth of field?

Although I may be wrong and there might of been a man moving it

[CT]

I think it boils down to what point you actually focused on. Depth of field extends in both directions from the point at which you focus - backwards towards infinity and towards the camera, being greater as the aperture size gets smaller. Focus too far into the distance and the foreground will be OOF, while if you focus too close, then DOF wont extend to the far distance (infinity)

In practice this means focusing somewhere in the mid distance, and using the focus lock button before recomposing your shot, but in all honesty you might just as well focus manually for this type of shot.

Sadly, we don't get a DOF scale on lenses any more which was an enormous help, but we still have the DOF preview button, which granted, isn't a lot of use in a dark scene like that, but does give a good guide to DOF in better light.

In this instance it could just be the reflective nature of that sign causing the problems as Dena points out.

[WetSparks]

If you have a look at the shot using the bit of canon software 'zoom browser', there is the facility to view the used auto focus points :)

[stephenb]

i tried AF on the sign and it locked on but was still blurred, most of the shots were using manual focus. I have looked at it at 100% and most of it is in focus so as suggested it must be that the sign was moving cos the supports apear quite sharp.
i dont know how to do a 100% crop so can't post one sorry.

[oldgit]

Was the bridge moving?
I mean can you feel the trucks passing underneath?
That could be your problem

[Marcel]

i dont know how to do a 100% crop so can't post one sorry.

Just open the image in Photoshop, and zoom to 100%.

Then crop what you see...that's a 100% crop :)

At first impressions I thought it might be slight movement from where you are. I take it you were on a bridge?

[Gandhi]

Could also be diffusion/defraction in progress? can nvere remember which it is, but I've had problems with bright areas looking soft when the lens is stopped down due to this effect. the lampost is sharp but the light is mushy!

[HIMUPNORTH]

God this is the third time Captain Hyperfocal has struck this week! I will have to put my cape on shortly :lol:

Learn about hyperfocal distances which is what CT is talking about. Basically focus close with a small aperature and you are playing safe (gross oversimplification!!!!) but as Gandhi points out you will have a potential diffraction problem.


Use this chart to work out where to focus:

http://www.dofmaster.com/files/35mm_28_100x.gif


Edit: This is for 35mm but assume will be close enough for DSLRs

[HIMUPNORTH]

Even better reference here (http://en.wikipedia.org/wiki/Circle_of_confusion)

[WetSparks]

Learn about hyperfocal distances which is what CT is talking about. Basically focus close with a small aperature and you are playing safe (gross oversimplification!!!!) but as Gandhi points out you will have a potential diffraction problem.


Ahh the old Airy ring problem :nono:

But then this is all hypothetical/irrelevant, cos in a post on another thread stephenb said that he had found the source of the problem to be that, the bridge on what he was standing was being vibrated be other traffic (lorries) rolling across it. Might as well have given the tripod a good kick :eek:

[oldgit]

See someone else was listening/agrees with me.... can I say "nahh nahh nee naa naa" now?

[CT]

Could also be diffusion/defraction in progress? can nvere remember which it is, but I've had problems with bright areas looking soft when the lens is stopped down due to this effect. the lampost is sharp but the light is mushy!

As you know Spencer, a lens gives better resolution and sharpness when stopped down a little, with the sweet spot fairly universally assumed to be F8, although it can be either side of this depending on the particular lens.The reason is that the light is being concentrated through the central part of the lens where it gives better sharpness right out to the edge of the image.

It's tempting to assume that stopping down further must improve matters , but it isn't so - when the light (and the image) is being transmitted through a very tiny aperture opening, far too much of it gets deflected off the diaphragm blades, and scattered in the lens, bouncing back and forth off the lens elements causing light diffusion and loss of sharpness. It's this which is referred to as diffraction.

[CT]

Ahh the old Airy ring problem :nono:

But then this is all hypothetical/irrelevant, cos in a post on another thread stephenb said that he had found the source of the problem to be that, the bridge on what he was standing was being vibrated be other traffic (lorries) rolling across it. Might as well have given the tripod a good kick :eek:

But if that was the cause, then the whole image would be unsharp - which it isn't. :D

[Arkady]

I remember reading about this somewhere - something to do with halation during long exposures and the way that the pixels react to light during long exposures.
During tests with the D1x we found that exposures longer than 16 seconds started going blurry.
What you're getting in effect is a 'bleeding' of information from one pixel to the next (though that's not exactly what's happening it's easier to explain that way).

I'll get back to you with the exact causes, but interestingly a similar thing happens with film when photographing bright areas against dark surroundings.
I'll look it up in Langford's Advanced later on...

[Jimmy_Lemon]

*looks around to see if anyone else got lost after the chart*

[WetSparks]

*looks around to see if anyone else got lost after the chart*

Dohh it's easy. If your more than 17' high, point yer camera a bit to the right :nuts: :thumbs:

[Arkady]

*looks around to see if anyone else got lost after the chart*

This is basic stuff guys - we teach this to students before we let them even touch a camera...

[Double_Agent]

Ahh maybe thats where the difference is Arkady.

You teach this to your students before using whereas the majority of us I would say are learning as we go along and have not studied Photography before.

[Arkady]

OK fair enough, but why not buy a cheap (£10) book on basic skills at the same time as the camera? The bookshops are full of them.

[fraggle101]

It's easy guys.. its all to do with the angle of dangle being equal to the protrusion of the throb of the knob..:cuckoo: u guys are just thick.. :bonk:


Im joking by the way!! lol...

[fraggle101]

Arkady, do u teach em that the camera should point naturaly at the subject, that the focus point should align correctly, FOCUS picture should be correct and the shot should be released and followed through with minimum effort?? :woot: 20 yrs on and still got it!! lol.. well.. nearly.. hehe..

[wildtracks]

I like the chart, is there an easy way to convert this to 1.5x DSLR, or better still does anyone know of a digi equiv version of this. Saves £20 on buying a DOF cardboard disk thingy.

[dazzajl]

Holy thread revival Fraggle. :lol:

Arkady is right about the way the camera responds to long exposures and very bright surfaces and there will be some of that going on here.

I'd bet on most of the softness on that sign being simple movement. It's a great big sail of a thing and there's no way it's wasn't moving a little.

[rich_eason]

You'd be suprised how much big structures like over bridges, buildings etc move. If they didnt have some flex they'd fail and fall over.

Next time you are in a large busy NCP car park, stand still and you can feel the the concrete floor "bouncing" with the motion of the cars over it.

I'm not sad...just used to work in structures!:bang: