Blur front and back but keep middle focused

[Gerald]

Hi Darren and all the other members! Thank you for the great site and information!
My wife recently started a photography course at the national photography collage here in South Africa. One of her exercises is to have the front and back of the image blurred while the 2nd subject must be in focus. The setup is 5 cans in a row, slightly in front of each other, and the 2nd can from the front must be in focus while the first can and the rest behind the 2nd can must be out of focus. In the training/course manual it explains how to do it, but it seems like there's something missing, we just can get this right!!? We have a Canon EOS 400D body and a Sigma 70-300mm lens. Please can you explain EXACTLY what we must do to get this right?
Kind regards, Gerald.

[DarrenT]

You are going to want a large aperture as a starting point. Remember that a small aperture (big f number) makes more of the image in focus and a large aperture (small f number) makes less of the image in focus.

Moving on you need to think about focal point. The closer you focus to the camera the smaller the area of focus is. So if you focus at infinity at f5 the real distance that is in focus might be 5ft, but if you focus 5" from the lens at f5 then maybe only 3cm is in focus.

The next stage is the focal length if i have this the right way round (possibly not!). If you shoot at 70mm the depth of field (are area in focus) will be greater than if you shoot at 300mm.

So you are looking to shoot with a lowish f number and fairly close to the camera at the far end of your lens.

here is an example i took just the other day. f2.8 42mm and focused fairly close to the lens, but not as close as possible (apologies if i posed this wrong, im about to re read the rules to make sure i got it right!)



hope that helps.

[Lakitu]

What a good explanation

[Gerald]

Thank you Darren, I'll try what you said. We've tried a similar way, but cannot get the "middle" part of the picture focussed. I set our camera on "m" (manual setting), change the F value to the lowest my lens can go, compensate for the light (by changing the shutter speed) and when taken, the photo is either more or less the same blurry or clear effect (front, middle and back) I've also tried manual focussing, but this focusses the whole picture (in the viewfinder)...
Will it be ok if I post (show you) the page that explains how to do this. It's at home and i'll be able to broing it tomorrow.

[mdruziak]

This is an exercise in depth of field. It is also a good way to test your lenses to see if they are focusing on the right point.

I like this site to help explain depth of field: http://www.dofmaster.com/dofjs.html It will also help you get your cans set up the right distances apart from each other.

Here is what I would do:

1. Set your lens to 200mm and move in as close as you can to the cans and still focus. Your lens has a minimum focus distance so don't get too close.

2. Set your camera to Av or M. Set your aperture to f2.8 or as large (small number) as you can get it. The combination of long focal length and large aperture will minimize the depth of field.

3. Set your shutter speed to provide proper exposure. Depending on what shutter speed you wind up needing, you may need to put the camera on a tripod.

4. Select the focus point that is closest to the can you want in focus and use it to focus on the can. You may need to focus and recompose or use the center focus point to focus and recompose.

5. Shoot the picture.

[DarrenT]

Originally Posted by Gerald

Thank you Darren, I'll try what you said. We've tried a similar way, but cannot get the "middle" part of the picture focussed. I set our camera on "m" (manual setting), change the F value to the lowest my lens can go, compensate for the light (by changing the shutter speed) and when taken, the photo is either more or less the same blurry or clear effect (front, middle and back) I've also tried manual focussing, but this focusses the whole picture (in the viewfinder)...
Will it be ok if I post (show you) the page that explains how to do this. It's at home and i'll be able to broing it tomorrow.

It should be fairly easy to replicate, you can try with anything that is packed in, a keyboard is a fav of mine when trying out the DOF with a new lens. because i know how big my keyboard is i can easily see just how much in real terms is sharp at various settings.

If your lens is the 70-300mm Sigma DG Macro then i beleive the minimum focus distance is 1.5m from the sensor. That means that ideally you want the camera 1.5m from the second can to ensure that the first will be OOF.

Dont forget that things will get more out of focus the further out of the DOF area they are. So you could try just moving the cans slightly further apart. Shooting at 200-300mm will make the cans look like they are closer than they really are anyway

I'll give a similar thing a try this evening with my 70-300 and post some results.

[Gerald]

Thanks guys... I'll try this tonight and let you know by tomorrow.

[Gerald]

Hi.
I cannot paste/copy the scanned page I promised because it's looking for a URL which I haven't got. I tried yesterday but no avail, I still can't do it...
Can anyone please explain to me what and how to do this from the very first step to after taking the shot?

Regards, Gerald.

[Iguanasan]

If you follow mdruziak's instructions and remember to be at least 1.5m (about 4 1/2 feet) away from the cans, as DarrenT said it should work. Is it possible that you are too close to the cans for the lens to focus on any of them? Move farther back and see when they come into focus.

Also, here's a video on how to post: Posting Photos to DPS Forums using Flickr or Picasa and don't forget the FAQ section: http://digital-photography-school.co...vb_faq#faq_img

[Jim Poor]

Originally Posted by Gerald

. . . I set our camera on "m" (manual setting), change the F value to the lowest my lens can go, compensate for the light (by changing the shutter speed) and when taken, the photo is either more or less the same blurry or clear effect (front, middle and back) I've also tried manual focussing, but this focusses the whole picture (in the viewfinder)...

Are you using a sturdy tripod? I ask because if you are using a slow shutter speed, you are going to get blurry pictures by hand-holding.

Blur can come from missed focus, camera shake, and subject motion.

What focal length are you using? To get a minimum DoF, you should be at the 300mm end of the lens you mention.