[ray]

Ive been reading and looking at a few photos of flowers with blurred backgrounds, all taken with the lens im trying to sell the 28-135, but i can do it am i doing something wrong any advice please on this thanks...Ray

[Chris H]

What you need to do Ray is make sure your lens is wide open, so if it's an F4 - f22 lens you have to have it at the f4 end.

Lower F numbers give less depth of field, or narrower focus front to back.

It appears to be called Bokeh or something very similar

[ray]

Right chris but if i use the zoom i can only open to f 5.6 this is confusing me a bit why is that do you know

[Chris H]

Your Zoom will have a lowest F stop of F3.5 or widest aperture at the 28mm end.

As you zoom toward the 135 end then your lowest F stop goes to F5.6.

What that means is that if you zoom out your lens will have slightly more depth of field or focus than at the 28mm end.

If you want to blur out the background you would have to lower your ISO or slow down your shutter speed to push the lens to the F3.5 to F5.6 end. In effect causing the lens to open up and shorten the amount of picture in focus.

If the lens gets down to F22 say, then you will have more of your view front to back in focus.

To achieve Bokeh you need the minimum front to back in focus that will allow only your subject to be sharp.

Not a perfect description but you should get the idea

[ray]

So really chris i would want a lens with f2.8 would you say

[jake]

What Chris says is right but subject distance and size also come into play. This was taken at F11 but it was only about 150mm away. The stem is already way off focus and it's only about 100mm away. If the stem had been 1m away it would have been that far out of focus it would have been unrecognisable.
The lens was set at 42mm.
All older guys have a moan but depth of field was much easier to tell when every lens barrel had the DOF scale around the outside

[JMitchell]

An f/2.8 lens is ideal for blurring out the background etc, but this means that they are much more expensive. One of the most popular lenses for example is an 85mm f/1.4, but this is a very wide aperture, and generally you pay for the 'fastness' or how wide the aperture is at a given focal length.

A favourite of many is the 70-200 f/2.8 lens, which keeps f/2.8 all through the zoom range, allowing for nice blurred backgrounds, and fast shutter speeds. But again, the newest versions of this lens are very expensive.
The lens you are trying to sell will easily also be able to do this, as long as you keep it at or around the lowest aperture possible (unless you're gonig for a different effect). Getting as close as possible whilst zoomed all the way in will also increase the 'blurring' effect on the background

Hope that wasn't too confusing

[ray]

No not at all john im taking all this advice in, ive been looking at a Tamron 28-75/2.8 on ebay what do you think of this lens

[silkstone]

The background blurring is a really a function of magnification + aperture, as Jake's pic shows. With high magnification - most macros - you can struggle to get all of the subject in focus even at small apertures - f/11 or more. The DoF can be just a couple of millimetres.

It also depends on sensor size, and that's where it can be difficult to make the transition from 'compact' cameras to DSLRs. Smaller sensors give much greater DoF at the same magnification, and focusing is therefore more critical on a DSLR. By the same token, it's more difficult to achieve a blurred background with a smaller sensor.

Here's a link to a DoF calculator I made earlier. There are plenty of others if you do a web search.

http://myweb.tiscali.co.uk/silkstone...h-of-Field.xls

(The 'Circle of Confusion' changes for different sensor sizes, and if you play around with the spreadsheet you'll see there's quite a difference in DoF between the APS-C (1.5x or 1.6x crop sensors) and full frame. The effect is even greater with the smaller sensors on compacts.)

[AQ]

Ray - perhaps what you need to do is some practical experiments. Set the camera up on a tripod with the 28 - 135 and then take a series of pictures of one subject using different apertures and focal lengths . Take a careful note of what is what and then look at the results. That way you should get a good feel for what is going on.

[ray]

Originally Posted by AQ

Ray - perhaps what you need to do is some practical experiments. Set the camera up on a tripod with the 28 - 135 and then take a series of pictures of one subject using different apertures and focal lengths . Take a careful note of what is what and then look at the results. That way you should get a good feel for what is going on.

Thats on my things to do list AQ its as long as my arm