Aperture and DOF - Quest for knowledge

Mentalblock

Mentalblock

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Neil
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OK then……………..

I understand that a small aperture is a large F number and a large aperture is a small F number.

I understand that a large aperture lets in more light.

I also understand that aperture plays a part in DOF and that a small aperture (large F) has more in focus/ greater DOF than a large aperture (small F), but I do not understand how.

So, the questions is:
- Do I need to understand how or should I just be happy with what I know, ie. Will understanding how the aperture size versus DOF/ focus help create better pictures?
 
Mentalblock said:
- Do I need to understand how or should I just be happy with what I know, ie. Will understanding how the aperture size versus DOF/ focus help create better pictures?
Click to expand...

Probably not! Do you know how every aspect of your car works? Does it make you any worse as a driver for not knowing?

I like the technical side and the understanding of how things work, but it sure as hell doesn't help my creative side!

I think as long as you know how to achieve what your vision is it shouldn't matter too much.

Just my opinion.

Thanks

Rob
 
I would say you don't need to know; as long as you know what to do to get the desired effect you want, that's all good.

Though as a side note; you could relate the term "circle of confusion" to how the aperture works. By using a smaller aperture you remove more of the overlapping "circles of light" that hits the sensor, and thus the image appear more in focus.

Having just re-read what I wrote I'm not sure it makes any sense at all...
 
From my limited knowlege I find most people have their camera set to Aperture Priority most of the time as I do. the reason I use AV mode is so I can make an easy decision based on the amount of light and how much DOF I want and I leave the camera to do the rest. I also usually leave the ISO as low as it can go!
 
I don't think you particularly need to know how it works, but it's vital you know what effect a given aperture will have on the finished image, control of different apertures is one of the most creative tools we photographers have.
 
Mentalblock said:
OK then……………..

I understand that a small aperture is a large F number and a large aperture is a small F number.

I understand that a large aperture lets in more light.

I also understand that aperture plays a part in DOF and that a small aperture (large F) has more in focus/ greater DOF than a large aperture (small F), but I do not understand how.

So, the questions is:
- Do I need to understand how or should I just be happy with what I know, ie. Will understanding how the aperture size versus DOF/ focus help create better pictures?
Click to expand...

The image forming light is shaped like a cone, coming to a point of sharp focus. With a large aperture (low f/number) it is a wide like an icecream cone. With a narrow aperture, it is long and thin like a spear.

If you slice an icecream cone, the slices get rapidly bigger as you move away from the point. These are the circles of confusion and so long as they remain small enough, the image still looks sharp. If you slice a spear in the same way, you need to move much further from the point before the slices get much bigger.

In other words, with a narrow aperture your subject can move further from the point you have focused on and still remain acceptably sharp.

Does that make sense?
 
Hoppy, that's the best explanation I've ever read. I understand it perfectly now! :clap::thumbs:
 
Great explaination Hoppy - except that if you slice an ice cream cone you get ice cream dumped in your lap!
 
Well the way I work out aperturee and DOF is consider cyclinders;

Assuming a picture contains a 'volume' of light then if you have a wide aperature (low f number) then if you do the maths then the length of the cyclinder is short (hence short DOF); taking the same picture with a narrow aperature means a longer cyclinder hence a deeper DOF.

Simple but effective; best done with pictures though..
 
EdBray said:
Not strictly true!

See here! This doesn't obey any of the above controls!
Click to expand...

I just recently read about T/S in an old photography book and I releaised that there is so much more to them than making everything look like a model or keeping converging lines of a building straight.

The ability to decide that the narrow plane of focus when set at a low F-stop is not paralell to the sensor opens up so many possibilties for creative photography. (as perfectly demonstrated in your photographs of the batteries)

:thumbs:
 
Any recommendations of reference material (websites, books etc) about DOF, aperture, focal length , distance from subject, and how they all work together?

Ultimately I would like to be able to "size up" a particular scene or situation, decide exactly what I want in terms of depth of field, and be able to adjust my aperture, focal lenght, and move to the correct distance to achieve what I want - all without having to think about it too much (i.e. I want it to become second nature).

ianbarber said:
Just remember these three things..

DOF is controlled by

1) Size of Aperture
2) Focul length
3) Distance from the subject
Click to expand...
 
ianbarber said:
Just remember these three things..

DOF is controlled by

1) Size of Aperture
2) Focul length
3) Distance from the subject
Click to expand...

Dredd123 said:
Any recommendations of reference material (websites, books etc) about DOF, aperture, focal length , distance from subject, and how they all work together?

Ultimately I would like to be able to "size up" a particular scene or situation, decide exactly what I want in terms of depth of field, and be able to adjust my aperture, focal lenght, and move to the correct distance to achieve what I want - all without having to think about it too much (i.e. I want it to become second nature).
Click to expand...

I think the best website is www.dofmaster.com It's all on there.

For things to become second nature, you just have to do it a lot. Remember what Ian said above. That's it really, although 1 and 2 come under the combined heading of magnification (as does format).

For example, if you frame up a subject like a portrait, then move back with a longer lens and frame it exactly as before, DoF stays the same (though the field of view changes).

On format, while that is obviously fixed by the camera, you should know what smaller formats deliver greater depth of field for a given framing and f/number, eg full frame gives less DoF than crop format by a factor of about 1.2 stops. The exact difference is f/number x crop factor.

Bearing that in mind will give you accurate settings for your 1D, and also explains why DoF changes if you crop the image in any way, as that's also an effective change in format.

Finally, I'll just say that DoF in practise is a moveable feast. While the science is exact, because it has to be, it is based on a lot of subjective assumptions about what consitutes sharp and what doesn't, and what is an average print viewed from an average distance by someone with average eyesight ;)
 
Thanks Hoppy. I've printed out a couple of DOF tables to study offline so I can understand the pattern of what happens to DOF and hyperfocal distance for different focal lengths and apertures.
 
Dredd123 said:
Thanks Hoppy. I've printed out a couple of DOF tables to study offline so I can understand the pattern of what happens to DOF and hyperfocal distance for different focal lengths and apertures.
Click to expand...

The controlling factors of DoF are very simple - image size and f/number.

If you make the image bigger on the sensor, either by moving closer or using a longer lens (or by using a larger format, which is obvioulsy fundamentally bigger to start with) then DoF gets smaller. And vice versa.

Then there's f/number. That's it :)

By and large, if the image remains the same size on the sensor - by whatever means - DoF remains the same. People say that you get less DoF with long lenses, but you don't per se. They make the image bigger, that's why, but if you move back to maintain image size, DoF is the same.

One thing that does change with longer lenses is field of view, as a function of the greater shooting distance. That can sometimes give a visual impression of less DoF, because it cuts out a lot of distracting background. The effect of that is the main subject tends to stand out more and the impression of greater subject isolation is often mistaken for a DoF effect.

Of course the final output size also has an effect on image size, ie size of the print. But in practise that doesn't make much difference as you tend to moderate that with viewing distance. That is, you look at small prints close up, and poster sized images from a much greater distance, so it cancels out.
 
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