Simple explanation of the effect of aperture on DOF?

[Helen B]

...
so a 50mm 1.4 lens

1.4 = 50/D D being the aperture the diameter of the first element in the lens

by transposition D= 50 x 1.4 therefore D=70 mm

Oops. 1.4 = 50/D transposes to D = 50 / 1.4, not 50 x 1.4.

D doesn't refer to the first element, but to the 'entrance pupil' - the image of the iris 'hole' you see when looking through the lens from the front. It's not necessarily the physical size of the iris opening, but the size of the image of that opening created by the elements in front of the iris.

 

[Helen B]

OP - This link/video might help you understand Av/A (depth of field) a bit better...

Depth of Field - AdoramaTV from Adorama Learning Center

I'm not sure that it is a good idea to introduce yet another term - 'Aperture Value' (Av). Av is calculated from f-number, but it isn't the same.

Av = 2 * log2(N)

so Av = 0 at f/1; 1 at f/.4; 2 at f/2 and so forth.

 

[bennielou]

Ok, as I'm no techinical hero, I will explain it the way I experience it:

Low numbers mean low depth of focus. High numbers mean higher depth of fucus. The closer you get with a low ap, means the more bokah, and less DOF. Go in reverse for higher.

 

[FlyingScot]

...
so a 50mm 1.4 lens

1.4 = 50/D D being the aperture the diameter of the first element in the lens

by transposition D= 50 x 1.4 therefore D=70 mm

Oops. 1.4 = 50/D transposes to D = 50 / 1.4, not 50 x 1.4.

D doesn't refer to the first element, but to the 'entrance pupil' - the image of the iris 'hole' you see when looking through the lens from the front. It's not necessarily the physical size of the iris opening, but the size of the image of that opening created by the elements in front of the iris.

HUMPH i had corrected that in my post - i just thought it was valuable to leave in my wonky thinking - be nice to me - when i was at skool we still used inkwells and pens with nibs!!!!! and i do believe it is the effective iris size as magnified by the lens a sort of dimensionless number.

 

[Big Mike]

 

[FlyingScot]

nice graphics BUT the last one is a little misleading as the models stay the same size and dont change as the focal length of the lenses changes - in fact looking at the graphic the model in the 24mm line looks bigger than the one in the 50mm line

 

[Big Mike]

What does model size have to do with it?

 

[bennielou]

What does model size have to do with it?

Wide. If you shoot a big girl with a wide lens, she is gonna be bigger.

 

[tevo]

Thank you Big Mike for my new iPhone wallpaper

 

[FlyingScot]

What does model size have to do with it?

you cant be serious surely - shooting from the same distance with a lens of 24mm 50mm and then 70mm what you see in the viewfinder and what you capture as an image will be something quite different and yes this is or was about DoF

 

[Big Mike]

you cant be serious surely - shooting from the same distance with a lens of 24mm 50mm and then 70mm what you see in the viewfinder and what you capture as an image will be something quite different and yes this is or was about DoF

Of course, the image will be different. Specifically, the Field of View will change. But we are not talking about field of view, we are talking about depth of field.

 

[AgentDrex]

Let's say you want a photo of a bug mostly all in focus. You will most likely be very close. You'll want a bigger aperture to have a deep enough depth of field. Around f/8 seems to work most of the time for me. If something in a landscape is a ways off and you want it in focus and the extraneous material softer, you won't need a big aperture, you may be able to get away with f/5.6 or f/4. Now, you'll want to learn the relationship between aperture and shutter speed for correct exposures.

 

[FlyingScot]

you cant be serious surely - shooting from the same distance with a lens of 24mm 50mm and then 70mm what you see in the viewfinder and what you capture as an image will be something quite different and yes this is or was about DoF

Of course, the image will be different. Specifically, the Field of View will change. But we are not talking about field of view, we are talking about depth of field.

I did actually say this is or was about DoF but they are both interrelated they are not standalone concepts and photography is all about what the eye sees so i still say that graphic is misleading remembering that that graphic is i would imagine designed to teach novices. So the introduction of three different focal length lenses is a further complication and without additional explanation liable to confuse tis not nearly as simple as that graphic makes out.

 

[photo_will]

Let's say you want a photo of a bug mostly all in focus. You will most likely be very close. You'll want a bigger aperture to have a deep enough depth of field. Around f/8 seems to work most of the time for me. If something in a landscape is a ways off and you want it in focus and the extraneous material softer, you won't need a big aperture, you may be able to get away with f/5.6 or f/4. Now, you'll want to learn the relationship between aperture and shutter speed for correct exposures.

This is one of the trickiest topics that we have to teach at our school. The aperture topic borders on the 'insanely geeky' and we have to walk a tightrope trying to give enough information without going overboard.
One of the problems has to do with terminology; big, small, wide, shallow, greater etc. This is how we describe it at the school:
Regarding Relative Aperture Numbers; big number (e.g. 22) = small hole, small number (e.g. 2) = big hole.
Regarding Depth of Field; The primary subject is in focus because of the glass in the lens. What appears to be in focus in front of and behind the primary subject is controlled by two things; the effective diameter of the aperture and the distance from the camera to the subject.
Put simply: The further the camera is from the subject, the more the background and foreground will appear to be in focus (a greater depth of field). The closer the camera is to the subject, the less the background and foreground will appear to be in focus (a shallower depth of field).
The bigger the relative aperture number (smaller hole) the more the background and foreground will appear to be in focus (a greater depth of field). The smaller the relative aperture number (wider hole) the less the background and foreground will appear to be in focus (a shallower depth of field)

Whilst it is very tempting to get more technical than this, I can confirm from years of experience that it is very unwise to do so

 

[ann]

And to add to the above. In my class I try to make it even simpler.

The information a professional needs and has, is a far cry from a beginner who just needs things to be as simple as possible.
The information that a photographer with lots of experience has is a far cry from a beginner who just needs things to be as simple as possible.