wide angle lens and aperture

rubbertree · Apr 23, 2008

I just bought a Sigma 10-20mm and am learning all I can about using a wide angle lens to it's fullest potential. I have a question about aperture choice though... I just read that wide angle lens can give sharp images foreground to background no matter what aperture you use (but especially tack sharp with f/22), how do you decide what aperture to use with it, if you are not going to use f/22? I mean, if they all give you a sharp image front to back (is that really true?) Does it then become a choice made because of the speed you need?

Sarah Elizabeth · Apr 23, 2008

you can not achieve bokeh with a wide angle? *scrathes head*

Sorry I don't have the answer but now I questioning

Mike_E · Apr 23, 2008

Hi, try going here http://www.cambridgeincolour.com/tutorials.htm and poking around.

christopher walrath · Apr 23, 2008

rubbertree said:
I just bought a Sigma 10-20mm and am learning all I can about using a wide angle lens to it's fullest potential. I have a question about aperture choice though... I just read that wide angle lens can give sharp images foreground to background no matter what aperture you use (but especially tack sharp with f/22), how do you decide what aperture to use with it, if you are not going to use f/22? I mean, if they all give you a sharp image front to back (is that really true?) Does it then become a choice made because of the speed you need?

The aperture controls the amount of light entering through the lens and exposing the film. The larger the number, the smaller the aperture diameter and vice versa. This is because the f-stop number is the denominator of a fraction that is the lens's focal length divided by the actual aperture diameter. Of course you know that, used in conjunction with shutter speed and film speed, this controls exposure. Aperture also affects the depth of focus in your subject meaning the depth of space in front of the camera that in reasonable to sharp focus. Wider lenses have greater depth of focus and smaller apertures also have greater depth of focus. Your camera should have a DOF preview switch that, when set to an aperture, will allow you to preview the depth of focus. You may find that anything from f/11 on up would probably be sufficient with such a wide angle lens. The only limit to you aperture in relation to speed is the ability to hand hold. The general rule of thumb is to hand hold no slower than 1/ focal length. So you're talking 1/15 to 1/30 which should give you great exposure latitude.

JimmyO · Apr 23, 2008

Your kinda correct. Wide angles are tricky. I just suggest playing around with it to get a feel for it

Helen B · Apr 24, 2008

As you stop a lens down (ie increase the f-number) the depth of field (depth of focus is a different thing) increases - that is easy to understand. There are, however, other effects that affect sharpness.

Some of the lens' aberrations - the faults - decrease. This makes the lens sharper, even for those points that are in perfect focus. The improvement is usually rapid as you stop down one or two stops from wide open, then much more gradual. This means that even if you have enough depth of field when the lens is wide open, it will probably be best to stop down two or three stops. This varies from lens to lens.

One of the other things that happens when you stop down is that diffraction increases. This softens the image - turning what should be points of light that make up a sharp image into patches of light that are roughly circular (they aren't circular, but they can be considered as if they are). The area of the patch grows as you stop down. This means that the points in the image that are in sharp focus become soft. The diameter of the diffraction circle (known as the Airy disc) is in direct proportion to the f-number.

It makes good sense not to stop down too much. With 35 mm format, stopping down beyond about f/8 may make diffraction noticeable - it depends a lot on the quality of the lens, the resolution of the film or sensor and the degree of final enlargement.

The smaller the format, the sooner diffraction will be noticeable. This is mainly because the smaller the format, the greater the degree to which the captured image will need to be enlarged.

If you need very deep focus, some diffraction will be acceptable. It affects the whole image, so it looks even. You don't get patches of sharp focus and patches of softer focus - that would make the softer parts more noticeable.

This all means that apart from aesthetic concerns about what is in focus there are also technical reasons to prefer certain apertures.

Best,
Helen

"This is because the f-stop number is the denominator of a fraction that is the lens's focal length divided by the actual aperture diameter."

Er, the f-number* is the focal length divided by the entrance pupil diameter (which many people call the aperture, but it isn't the actual aperture).
*The term 'f stop' is less well defined.

rubbertree · Apr 24, 2008

thank you all and thanks for that link Mike_E.
I understand the technicalities behind changing f/stops and what it accomplishes, my issue was that I read (rather, I listened to a podcast) that said with a wide angle lens, all apertures will result in a sharp image. Not really true though, is it? It was just a little misleading, the way the podcast stated that. I've got it sorted out now though!
Thanks!

christopher walrath · Apr 24, 2008

Hey, Helen. As I stated I am always learning. In ALL seriousness, what is the entrance pupil and what is the actual diameter? These are things I want to know and you seem to have a little better grasp than I. Could you explain for me please. Thank you very much.

JerryPH · Apr 24, 2008

rubbertree said:
I just read that wide angle lens can give sharp images foreground to background no matter what aperture you use (but especially tack sharp with f/22)

Wow, where are you getting that info from? If its a friend or someone, beat them with a stick, will ya? They sure are not helping you out. :lol:

Take a look at THIS picture from a 10-20mm and you tell me if its not sharp everywhere from 3 feet in front of the lens to infinity. Would it surprise you to learn that this picture was taken at F/5.6? Most lenses will lose sharpness as you go to either extremes so you don't want to be shooting there unless you have very specific goals.

I am not going to repeat about what it takes to make or avoid bokeh... its repeated here so many times, my fingers hurt (lol). A wide angle lens will create background blur in exactly the same manner that a prime or telephoto does, using the same rules over and over.

rubbertree said:
how do you decide what aperture to use with it

By knowing what and how aperture works, by knowing what you want out of a shot and by adjusting the camera to your needs. Different apertures will give you different results, and its not always to raise or lower shutter speed.

I think you would benefit from reading through UNDERSTANDING EXPOSURE by Bryon Peterson.

rubbertree · Apr 25, 2008

Got that book, read it cover to cover, know it by heart. Of course it is standard photography 101 (about the aperture and shutter speed choice, that is). That's why I was so surprised to heard this about a wide angle giving sharp image no matter what aperture. I just bought last week and this is the first time I've ever used a wide angle lens, we had an enormous blizzard this week and I have not really had the opportunity to get out and really use it yet.
Obviously that is not true, it follows the same rules as aperture and shutter speed as every thing else.
It was a podcast on another forum that said this, I'll go back and find the exact one.

Garbz · Apr 25, 2008

Is a "Sharp Image" exactly what you heard? Wide angles give a larger depth of field at a given aperture than say a lens with a midrange field of view. But sharpness is entirely up to the optical design of the lens and diffraction.

uplander · Apr 26, 2008

Great thread here! I mined a lot of info here. Some of it I learned years ago and simply forgot.

rubbertree · Apr 26, 2008

Garbz said:
Is a "Sharp Image" exactly what you heard? Wide angles give a larger depth of field at a given aperture than say a lens with a midrange field of view. But sharpness is entirely up to the optical design of the lens and diffraction.

I'd have to go back and listen to the podcast again but he says just because of the way a wide angle is made, regardless of the aperture, the foreground to background sharpness will be much greater and he said even with a large aperture you will still have a sharp background, something that is not obviously true with a regular lens. He repeats it several times in the podcast!

Garbz · Apr 27, 2008

foreground to background sharpness = Depth of Field.
This is one thing that is best explained with an example:

The following 4 images were taken at 28mm 70mm 105mm and 200mm. Each at f/2.8. In each case I stepped back to try and keep the fence post roughly the same size:

Now as you may be able to see here the background itself does not get any more or less blurry, but because of perspective it get rendered larger and smaller. I haven't got a ultra wide angle lens. But the point here is that the background itself does not get blurrier, just the perspective makes the individual details behind the subject smaller the further from the lens you get.

The end result is that even at wide open apertures the perception is that the image is sharp all the way, and as such the Depth of Field is significantly larger.