advice on portrait lens

hi folks,

wanted to ask all the bokey pros a basic question. i m planning on going for a nice 50mm portrait lens. should i go for the f/1.8 or the f/1.4? how much difference will it make to the amount of DOF i wanna achieve?

cost is a limiting factor. the 1.8 is 1/6 the price of the 1.4. is it worth the money?

ill be going either for canon or sigma.

cheers.

I have a 50mm f/1.8. I am pretty happy with it. Personally, I would not take a portrait at f/1.8 as it would get me very little area of the face in focus, unless thats what you intend to do in most of your shot, thats something I would not want. For portraits I set the aperture to 2.2 or 2.8.

Anirban

The other factor to consider is how 'heavy-duty' you require the lens to be. If it's going to get a lot of use and is going to be off and on your camera a lot, then remember that the 1.8 is not really built for the rough stuff. By being 6x more expensive, the 1.4 is going to be much more robust. But if you treat the 1.8 with kindness adn gentleness, it will reward you.

I also use f2.8 for portraits with my 100mm f2 (on a full frame camera). f2.0 is too hard to manage unless the subject is absolutely not moving! The bokeh seems to be fine at either aperture.

Hi Anirban,

As I see it, the aperture; 1.8 or 1.4 is not the deciding factor, since you'll be VERY unlikely to use it wide open - and even if you do, it's not that much difference to either DoF or exposure.

So it comes down to build quality as Donald discusses.

Another factor, regarding bokeh, is how many blades does the iris (aperture) have? That'll be largely what determines how good the bokeh is. I am not familiar with those lenses, so you'll have to research that yourself, or someone here might be able to advise what theirs have.

Good bokeh usually comes with say; 8 or more blades (cheaper lenses have 5).
Blade shape matters too; curved ones should give better bokeh.

Cheers,

Hi Anirban,

What kind of portraits are you talking about?

50mm on a crop-factor camera is equivalent to 85mm on a full frame, and that's too short for just head and shoulders, but OK for 3/4 length shots.

If you're just shooting head and shoulders portraiture then I'd suggest looking for something around the 85mm mark (135mm equivalent on a FF) (I use an EF135mm F2.0L USM (on a FF camera) almost exclusively in the studio for head and shoulders portraiture). The EF85mm F1.2L USM II has a venerable reputation for quality (I have one and it's great, but I seldom use it for portraiture).

hi all,

thanks for the advice. considering what you folks are saying about the 1.4 not making a big difference, i think ill go for the 1.8 and take good care of it. :D

@Colin - i actually mean to do some general bokeh shots, not people. still life images, machinery, inanimate objects with shallow DOF. i have attached a simple pic i clicked with my regular 18-55. hence i want the lens only for the larger aperture to get shallow DOF.

http://i33.tinypic.com/sg38yg.jpg

cheers

BOKEH vs. Depth of Field...

Bokeh is the subjective rendering of the out of focus areas whereas depth of field is the objective area between the nearest point of acceptable focus and the furthest point of acceptable focus.

Sometimes these terms are incorrectly used interchangeably, however they should not be.

You cannot correctly say "a large bokeh" like you can describe an extended depth of field and you cannot say "smooth or ragged" depth of field" like you can say smooth or ragged bokeh.

Depth of field is controlled by several objective factors: f/stop used, point at which the lens is focused, focal length used and the size of your sensor in the case of digital or the size of your negative in the case of film. The size of the image acquired by the camera is used in determining depth of field because larger originals do not need to be enlarged as much as smaller images to produce a final copy.

Bokeh is controlled by how perfect a circle the aperture forms; which is again usually controlled by how many blades are used by the diaphram . As an example, the aperture of a Canon 50mm f/1.8 lens is formed by a five bladed diaphram while my 90mm f/2.8 Tamron Macro lens uses eight blades and can form a more perfect circle.

The result is that the out of focus areas of the Tamron are subjectively smoother and more pleasing to most people. Ragged bokeh can compete with the subject for the viewers attention.

@Richard

thanks for the clarity. so the best bokeh (conventionally) is the one where the aperture is perfectly circular (more blades) while the ragged bokeh is the one where less number blades leads to less circular.

got. but just one small thing. what kind of difference does it actually make to the image? can you post some pics of either type? i cant seem to figure it out on google.

thanks and cheers

Quote:

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Originally Posted by shutterx

so the best bokeh (conventionally) is the one where the aperture is perfectly circular (more blades) while the ragged bokeh is the one where less number blades leads to less circular.

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Yes.

Quote:

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what kind of difference does it actually make to the image?

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With high-end lenses it's not just the number of aperture blades but also their shape and positioning. With cheaper lenses you'll often get a hexagonal shaped blur of a point that's out of focus, whereas with a circular aperture the blur is truely round. In other words, the shape of blurred items takes on the shape of the aperture.

Quote:

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Originally Posted by Colin Southern

With high-end lenses it's not just the number of aperture blades but also their shape and positioning. With cheaper lenses you'll often get a hexagonal shaped blur of a point that's out of focus, whereas with a circular aperture the blur is truely round. In other words, the shape of blurred items takes on the shape of the aperture.

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This is very true (not that you ever post anything that isn't, Colin). A good example is the Canon 70-200 f4L IS. It has much nicer bokeh than the original non-IS version due to its new, rounded aperture blades. It is amazingly smooth for an f4 lens! It is also a fantastic portrait lens.

Shutter Rx asked, "but just one small thing. what kind of difference does it actually make to the image? can you post some pics of either type? i cant seem to figure it out on google."

Here is a collection of images from 50mm f/1.8 lenses...
http://www.pbase.com/cameras/canon/ef_50_18ii

Here is a collection from 50mm f/1.4 lenses...
http://www.pbase.com/cameras/canon/ef_50_14u

Pbase contains a collection of images from just about every lens out there...

However, whenever you are comparing a more expensive lens with a far less expensive lens on Pbase or any other gallery; you need to remember that more expensive lenses are often used by more experienced photographers which might result in better images through the skills of the user. Therefore, view the images with a grain of salt and not decide on one lens or another just by the quality of imagery posted on Pbase.

The bokeh of a lens is mainly determined by its PSF (point spread function). That's why the bokeh of different lenses differ even when they are shot wide open where no aperture blades play any role.

The number of blades mainly plays a role when you stop down and have highlights in the image that take on the form of the iris. In this case, the more and the rounder the blades, the better.

I'd say smooth bokeh (i.e., a PSF that is as much as possible uniform, avoiding an accentuated out rim) is a more important criterion than maximum aperture. Often, the sharper a lens, the harsher the bokeh. Lenses that have undercorrected spherical abberrations aren't as sharp wide open as their corrected competitors but often have the smoother bokeh. So the question may not be f/1.4 vs f/1.8 but which has the nicer bokeh?

I'd compare images shot with the lenses and make a decision on the bokeh. Note that bokeh can benefit from having the lens stopped down a little so don't look at wide open shots only.

Note that close focusing will make almost any lens look good in terms of background blur. The challenges lie in less then close focusing shots and difficult backgrounds like foliage in sunlight.

Quote:

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Originally Posted by Class A

I'd say smooth bokeh (i.e., a PSF that is as much as possible uniform, avoiding an accentuated out rim) is a more important criterion than maximum aperture. Often, the sharper a lens, the harsher the bokeh. Lenses that have undercorrected spherical abberrations aren't as sharp wide open as their corrected competitors but often have the smoother bokeh. So the question may not be f/1.4 vs f/1.8 but which has the nicer bokeh?

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hey that makes a lot of sense. i tried out a 100mm 2.8 macro. you dont need even f/5 aperture to get huge amounts of bokeh. me being used to an 18-55, slapped it down to f/2.8 to try out my first shots. i noticed insane amounts of flooding and burning in the images.... but is this a trait of only macros?