Depth of Field/Bokeh question

[TMR Design]

I have been doing some experiments with lighting and have gotten mixed results. The one thing I can't figure out for the life of me is why on some shots I will have the subject in the foreground and the background will be nicely blurred and on others the background will be clean and in focus.

My setup is this:

Dark backdrop hung on the wall.
Subject is 6 feet in front of the backdrop.
Camera is 8 feet in front on subject.
Lens is wide open.
Long end of the lens. In this case it is an A620 at 140mm.

With the exact same setup, what other factors will affect the blurred background? I am going crazy trying to consistently achieve the blurred background. This is partially due to the fact that the backdrop I am using is a charcoal colored flannel bed sheet. There are a few light creases in it and I can see when in the pics that at times the creases are blurred and can't be defined and in other shots it looks like I have a sheet with creases hanging on the wall...lolool
It is consistent with the same subject but I can't figure out why different subjects affect whether the background is blurred.

[dunker]

only thing i can think of is that the camera is changing the f/stop somehow? try it in AV mode that way the f/stop gets locked?

[Mike Reynolds]

can you post a sample photo?

[TMR Design]

Hey Dunker. I am in Manual mode so I know the shutter and f-stop are staying the same.

This is another related thread where you can see the examples of what I am describing.
http://www.photography-on-the.net/forum/showthread.php?t=216922

[dunker]

hmm only other thing i can think of is that maybe the auto focus is slightly varying on each shot, set manual focus to 8feet for each shot?

[TMR Design]

Ahh ok maybe that is it and the focus is finding a spot or an area it is averaging. That will be first thing on my list to try tomorrow.

[Stefan A]

The 3 keys to blurred background are large aperture, subject far from background, and large focal length. Since your zoom and aperture are limited, the only other thing you can control is the subjects distance to the background. But keep in mind that although background blur is possible with a P&S, it's probably not going to be as good as an slr. The largest aperture on a P&S is roughly the equivilent of f/11 on an SLR - which you would never use on an slr for blur.

Stefan

[TMR Design]

Thanks Stefan but I have taken many pictures, both telephoto and macro that I have able to achieve a great blurred background. The macros seem to do it well. I just played a bit a shot this one and although you can make out the creases they blend in and give it some texture which I don't mind. In the other thread I pointed to earlier there is an example with a potted plant and you can see the creases in the background very well.

I am going to have to play some more. All the tests I am doing have the lens wide open ( I did not realize that was f/11 on an SLR...sheesh!).

[Moppie]

Are you sure the camera is staying in Macro mode?
I know on my A80 if I alter anything, or goto view photos it often changes

[TMR Design]

Hey Moppie,

Yes I am sure. I have seen those times that the camera jumps out of either Macro or Manual Focus but this was not one of those times. I am going to have to do some careful testing.

The other side of all this is that I took this same lighting setup and created a wonderful way to do macro of items for cataloges, ebay, etc..

Check it out.
http://photography-on-the.net/forum/showthread.php?t=217597

[Pixel9ine]

Is your background lighting consistent? A darker background will give the impression of being more "blurred" than a well-lit one.

Also, as you noted above, macro mode on these cameras results in a fairly respectable blurred background.. combine both macro mode with a lens zoomed in all the way, and you will have a very decent effect (though with a very limited Field-of-View).

[Moppie]

The other side of all this is that I took this same lighting setup and created a wonderful way to do macro of items for cataloges, ebay, etc..




Iv done the same thing shooting a friends a minitures, it works, but proved to be a fiddly set up, especialy trying to get different angles of different sized minitures.

I know have a giant soft box, made out of A1 sized sheets of tracing paper with a card board and wooden dowling frame.
I can easily light anything inside the box from either outside, or in (shooting into the roof, or a wall) useing the 550 on an optical trigger.

http://img.photobucket.com/albums/v31/Moppie/20Dlighttent.jpg

(Thats a borrowed 20D, which works even better with the E-TTL on the flash).