Blur background

[Stock-Photos]

Can some please explain why a 2.8 lens blurs the background so much more, wide

open, than does an F4 lens, wide open. I have Canon's 24-105 F4 IS. I'm a bit

dissapointed with it's inability to put the background out of focus when

shooting wide open. Portraits is the application I'm referring to.

 

Does a lens being and F4 mean its DOF is greater?...even when shooting wide open?

 

TIA. J.

[John Crowe]

Exactly! Depth of field increases as you make the aperture smaller, that is making the aperture go from f1.4 to f16.

 

 

Also as you open a lens to it's maximum aperture (smaller number) one must also consider "bokeh" which refers to how the out of focus areas are rendered. Some lenses do it better than others, making these areas very soft and well blended.

 

 

Typical portrait lenses are in the 80 to 135 range with apertures from f1.2 to f2. Telephoto lenses are also capable of these effects, making it possible to isolate the subject from the foreground/background. The 200/1.8, 300/2.8, and 400/2.8 are very good at this.

 

 

Yes f2.8 in the range of your f4 zoom is better and f1.4 to f1.8 is even better. You may want to invest in the not too expensive 85/1.8 or even 50/1.8 (if you are using a 1.6x body) to go with your zoom. Good luck.

[bueh]

For resolving all those photographic formulas and depth-of-field questions, try the freeware <b><a href="http://www.tangentsoft.net/fcalc/" target="_NEW">f/Calc</a></b>.

[ben_ballard]

A pinhole camera has no lens and allows you to place the film at any distance you want from the pinhole, and yet everything is in focus. The light reflecting from your subject is bouncing off in all directions. But from any point on the subject you are shooting, only a very slim beam of light can go through the pinhole and hit the film. What the film records is the collection of those single sharp beams of light coming from every point in the photo.

 

A regular camera at f22 is close, but not quite the same. An f22 aperture is much larger than a pinhole. So going back to the pinhole, if you took another picture, this time with a much bigger pinhole, now you are letting in not just those single narrow beams of light, but slightly wider beams. So the light from a single point in space is going to show up on film as a small circle. Every point in space will do the same, and the result is blur.

 

In a regular camera, with f22, it isn't blurry because the lens takes that wider beam of light and focuses it back to a single point, but this depends on the distance and the width of the beam of light. With f22, it's still pretty narrow, so it can focus everything at all distances. At f4, it's much wider, so it can only focus the wider beams into a point for a certain distance range. That range is the depth of field. At f2 that depth is even narrower, and so on. The larger the opening, the shorter the depth of field, and also the more blurry the other distances will appear. There's a noticeable increase in the degree of blur between f4 and f1.8.

 

Does that make sense? Apologies to anyone I completely bored, but this was my best shot at "the way things work" for this question.

[bobatkins]

Here's a detailed article on background blur

 

http://www.bobatkins.com/photography/technical/bokeh_background_blur.html

 

It's actually something of a complex subject and isn't quite the same thing as DOF. There are times when an f4 lens can give you more background blur than an f2.8 lens for example, though for a given focal length, the faster (f2.8) lens will always give more background blur than the slower (f4) lens if both are used wide open (f2.8 vs f4).

[marknagel]

Here's a quick pic I did a while ago.

 

m<div></div>

[davyjo]

Check these two photos taken with a Sigma 105mm macro.

[davyjo]

The second one.

[davyjo]

Try again. Shutter .3" f/45<div></div>

[davyjo]

1/800" f/2.8.<div></div>

[Stock-Photos]

I guess my question was not clear, but a few responders understood it.

 

I was not sure why the wide open optics of a 2.8 max aperture lens blurs background more than a lens with max aperture of F4, if all other conditions are equal.

 

Must be a complex explanation, as mentioned. Thanks for your assistance. J.

[beauh44]

Hi J. I'm not good with the physics explanations (our own "Dr Bob" Atkins is though!) but there are some things you can do with your 24-105 lens that'll help you get better background blur. This is a generalization but wide angle lenses typically have quite a bit more DOF than telephoto lenses do. So if you shoot wide open at 24mm with your lens it's going to be pretty tough to get very good background blur (if much at all). But if I take my 500mm f/4L IS lens and use it at f/4, the background is all but obliterated, so focal length of the lens comes into play as well as lens aperture in terms of DOF. Also the subject's distance to the background.

 

So... If you want your 24-105mm lens to get pretty good background blur try zooming all the way out to 105mm. Place your subject as far as possible from its/his/her background but pretty close to you (these will be closeups, obviously!:) and try shooting wide open. That'll help and you may be pleasantly surprised at the background blur you get. If you shoot the same subject wide open at 24mm though... you'll see pretty much everything going on around them at least a bit without much blur at all.

 

This is why f/4L lenses typically are much less expensive than their f/2.8 (or less) cousins. It's harder and more expensive for Canon to make a lens that's really sharp wide-open at f/2.8 (or less) than it is to make one that's sharp wide-open at f/4.

 

Check out the price difference between the 17-40 f/4L and the 16-35mm f/2.8L. They're very close sharpness-wise. You pay quite a bit for that little extra light that's let in and the better control over DOF with the 2.8L zoom. While aperture is a huge factor in determining background blur the focal length comes into play quite a bit as well. Good luck!

[terencechong]

Taken with EOS 5D & Sigma EX 50/2.8 lens<div></div>

[terencechong]

Another...<div></div>

[terencechong]

Another: from my earlier digital days..<div></div>

[mark u]

You may also find it useful to read the discussions by Paul van Walree on background and foreground blur in his depth of field article and on bokeh and the contributing factors:

 

http://www.pinnipedia.org/optics.html

[thomas_hardy1]

"From the question submitter

J. Harrington USA (Massachusetts) , dec 17, 2006; 05:06 p.m.

I guess my question was not clear, but a few responders understood it.

I was not sure why the wide open optics of a 2.8 max aperture lens blurs background more than a lens with max aperture of F4, if all other conditions are equal.

 

Must be a complex explanation, as mentioned. Thanks for your assistance. J."

 

 

 

I think all of the above answers addressed this question. This is kind of a simple answer without the use of physics etc... Wide open a lens with an aperture of f4 has more depth of field than a lens with an aperture of F2.8. This means that if you focus on a subject with both lenses, the F4 lens will have more area behind it in focus than the F2.8 lens.

 

This is part of your original post. "Does a lens being and F4 mean its DOF is greater?...even when shooting wide open?

 

TIA. J. "

 

Yes, compared to F2.8, F4 has greater DOF.

 

Your lens you just mentioned ONLY opens to F4. If you take the lowly 50mm F1.8 set to F2.8, and compare the two wide open the cheapo 50mm will blur the background more than your 24-105 F4, zoomed to 50mm.

[terencechong]

Hands...<div></div>

[terencechong]

another...<div></div>