does the shape of the aperture stop determine the shape of the image circle

[rookie1010]

Hello,

Does the shape of the aperture stop determine the shape of the image circle?
so if the shape of the aperture stop is a hexagon, we end up with a image hexagon?

 

[ElFenix]

don't think so. it does determine the shape of out of focus highlights.

 

[OdiN]

Technically it does. Try this:

Get two black pieces of cardboard/construction paper - anything that light won't be able to shine through. Cut out a hexagon in one sheet - not huge, make it small like one in your camera might be. Shine a light through it onto the other paper. If it's super close - you will see the shape of the hexagon - however, moving the paper the light is shining on away should make it much less defined. The aperture in the lens is far enough away that it won't (or shouldn't) affect the image circle that gets shone on your sensor. Maybe it won't be an absolutely perfect circle - but close enough that you can't tell.

Though yeah the OOF highlights, lens flare refractions, stuff like that you will be able to see it.

 

[rookie1010]

thanks for the reply,

do cameras have non-circular apertures since to my imagination it is difficult to produce a perfect varying circular aperture ?

 

[ElFenix]

Originally posted by: rookie1010
thanks for the reply,

do cameras have non-circular apertures since to my imagination it is difficult to produce a perfect varying circular aperture ?

good lenses have circular blades but the aperture will only be circular at its widest. the blades will start to form corners stopping down until eventually it forms some sort of pentagon or something. because, as we all know from calculus, a curve on a small enough scale is a straight line.

 

[OdiN]

Originally posted by: rookie1010
thanks for the reply,

do cameras have non-circular apertures since to my imagination it is difficult to produce a perfect varying circular aperture ?

They are not perfect circles, no. The cheaper lenses may use only a 5-blade aperture. Higher quality lenses can have 8-blade apertures and use a curved blade aperture which approximates a circle much better.

 

[rookie1010]

thanks for the replies,

so i am thinking on the right track.

the non-perfect aperture does not have an impact on the shape of the image circle which remains circular. but the edge of the circle deteriorates and this would lead to vignetting?

 

[fuzzybabybunny]

Originally posted by: rookie1010
thanks for the replies,

so i am thinking on the right track.

the non-perfect aperture does not have an impact on the shape of the image circle which remains circular. but the edge of the circle deteriorates and this would lead to vignetting?

To decrease vignetting you want to stop down the aperture (make it smaller). You'll get the most vignetting at large apertures.

Large apertures = fuzzy borders on the image circle
Small apertures = well defined borders on the image circle

I might be wrong about the following:

The vignetting that you see at large apertures is this fuzzy border that is encroaching into the corners of the sensor. This fuzz gets recorded by the sensor and you have vignetting in your picture.

When you stop down, the image circle borders become more defined and crisp, and there is not as much, or no, fuzzy border to end up as vignetting in your picture.

 

[OdiN]

Sounds right to me. It can also be caused by lens optics - it's common for less expensive lenses to display vignetting at wide angles - especially on a full frame camera.

 

[rookie1010]

thanks for the replies,

can not the non circular/perfect aperture cause issues.

i mean the imperfections can be viewed as mechanical obstructions which can result in vignetting.


i suppose a small aperture results in a smaller image circle, since a smaller aperture is more restrictive from an angular perspective?

 

[OdiN]

Try my paper trick I mentioned 😛

Cut out a large octagon and try it.

Then cut out a small octagon and try it.

 

[rookie1010]

so shape of the aperture does not matter

 

[AndrewR]

Originally posted by: rookie1010
so shape of the aperture does not matter

Not true. See above -- a circular aperture (so called or actual) produces more pleasing out of focus (OOF) elements. It's a VERY subjective thing though, but certain lenses are praised for their rendering of the OOF areas while others produce "harsh" OOF areas that detract from the main image.

Depending on the photography subject, it may or may not matter.

 

[Fardringle]

In extreme cases, it can make a difference. Most of the time you probably won't even notice whether your aperture is round or not unless the lens itself has problems (as OdiN mentioned).

Here's an example of what happens to out of focus items with a round aperture: Circular aperture

Here's what happens if your aperture is altered dramatically 😉: Altered aperture

 

[rookie1010]

thanks for the replies and for the excellent pictures, now i understand what the effect of a non circular aperture is

i guess the night-mode on a camera results in the aperture size increased to allow in more light?

 

[fuzzybabybunny]

Originally posted by: rookie1010
thanks for the replies and for the excellent pictures, now i understand what the effect of a non circular aperture is

i guess the night-mode on a camera results in the aperture size increased to allow in more light?

Depends on the camera. To do night shots:

1. Increase aperture size.
2. Increase ISO
3. Increase the time the shutter is opened (1 sec, 10 sec, 30 sec, etc)

Night modes in cameras can do any of these three or any combination of these three.

 

[AndrewR]

Originally posted by: fuzzybabybunny

Originally posted by: rookie1010
thanks for the replies and for the excellent pictures, now i understand what the effect of a non circular aperture is

i guess the night-mode on a camera results in the aperture size increased to allow in more light?

Depends on the camera. To do night shots:

1. Increase aperture size.
2. Increase ISO
3. Increase the time the shutter is opened (1 sec, 10 sec, 30 sec, etc)

Night modes in cameras can do any of these three or any combination of these three.

While I've not done very much of it, I have read (ha, makes me an expert! 🙂 ) that for night shots, you shouldn't bump the ISO on long exposures because it increases noise. It makes sense to me, though perhaps a little counterintuitive. The last time I shot fireworks (perhaps a particular case because of the brightness), I in fact did keep the ISO at 100 for 3-5 sec exposures with good results.

Perhaps an astrophotographer can chime in on the subject. 🙂

 

[fuzzybabybunny]

Originally posted by: AndrewR

Originally posted by: fuzzybabybunny

Originally posted by: rookie1010
thanks for the replies and for the excellent pictures, now i understand what the effect of a non circular aperture is

i guess the night-mode on a camera results in the aperture size increased to allow in more light?

Depends on the camera. To do night shots:

1. Increase aperture size.
2. Increase ISO
3. Increase the time the shutter is opened (1 sec, 10 sec, 30 sec, etc)

Night modes in cameras can do any of these three or any combination of these three.

While I've not done very much of it, I have read (ha, makes me an expert! 🙂 ) that for night shots, you shouldn't bump the ISO on long exposures because it increases noise. It makes sense to me, though perhaps a little counterintuitive. The last time I shot fireworks (perhaps a particular case because of the brightness), I in fact did keep the ISO at 100 for 3-5 sec exposures with good results.

Perhaps an astrophotographer can chime in on the subject. 🙂

It really depends on the situation and the effect that you're going for. If you want to freeze the fireworks you use a higher ISO. If you want to have the fireworks stream, you want a lower ISO.

The OP was talking about "night mode" being a pre-configured shooting option on some cameras. What the night mode actually does in relation to the three points above depends on the camera manufacturer. I personally have no idea. It's best to just do it manually depending on the shooting situation and what you're trying to capture.

 

[OdiN]

It's possible that a night portrait mode also may put the camera's flash into second curtain sync mode if it's supported. What this does is fire the flash off right before the shutter closes, vs. right when it opens. What this allows is for ambient light to get exposed, and then at the last instant, the flash fires to freeze the subject. So you end up with maybe some blur in the background, but a sharp main subject. I use this a lot for creative exposures at night say at a wedding reception. You can get streaks from lights in the background and a sharp subject.