Why does big aperture cause shallow DOF?

[GreenTea]

I've tried to understand this and read a lot about it, but I still don't see the logic...

[crisscross]

Do you need to understand it? Just take it as a fact of life, like it getting darker at dusk. Also longer focal lengths progressively give less DOF at constant aperture. Do a few tests using the DOF calculator in the Tutorials https://www.cambridgeincolour.com/tu...h-of-field.htm for the sort of settings you use, or if you have a mac, download the DOF calculator free widget.

[GreenTea]

Ah, ok. I thought I was supposed to understand the logic. I'll just learn the rule by heart. Thanks!

Another question. I have a Fuji Finepix semi-automatic camera, and I shoot the same photo (in A mode) with apertures 2.8 and 8 (biggest and smallest I have in this camera) and get exactly the same DOF results. Does this mean the camera has a very small sensor or am I doing something wrong?

[crisscross]

Another question. I have a Fuji Finepix semi-automatic camera, and I shoot the same photo (in A mode) with apertures 2.8 and 8 (biggest and smallest I have in this camera) and get exactly the same DOF results. Does this mean the camera has a very small sensor or am I doing something wrong?

I expect Dave will pick this up later & has indeed done some explanation of DOF on compact cameras. The apertures are not equivalent to DSLR ones with same number, but ratio of light being admitted at different ones should be.

Again from experience, the effect you observe seems probable from my previous experience of Nikon E4500 & Pana FZ7, though there should be some difference in DOF on closer objects; for 'Landscape' mode both cameras would just focus at infinity, whereas even with a DSLR you have to pull back a bit for the moon!

[Colin Southern]

Perhaps this thread might help in some way?

[Dave Humphries]

Hi GreenTea,

I would guess you Fuji has a crop factor of around 4.5 to 6 depending on sensor size, we'll say 5 to make the maths easy.
By crop factor, I mean the difference in lens real focal length compared to the 35mm equivalent.

The thing is, this doesn't only apply to the focal length, but also to aperture, so f2.8 x 5 = f14 and f8 x 5 = f40.
So as far as DoF is concerned, your camera might be really f14 to f40, both of which will return a LOT of DoF. So as Chris says, it might be hard to spot.

I would recommend the link Colin posted too.

To work out your real criop factor (the easy way);
Fuji tend to mark their lens' zoom control with the 35mm equivalent figures, mine says 28-300mm, but really it is 6.2-66mm, from the front of the lens. Take ONE of these figures and divide by the other, 28/6.2 = 4.516, say 4.5. Clearly, 300/66 should give the same answer, and it is (almost).

Cheers,

[GreenTea]

Thank you very much everyone! Now I have something to start working from.

[johngie]

Imagine that the light from a point on the subject leaves the lens as a cone, converging to the point on the sensor at which it is focused. If the cone has a large angle (wide aperture) the placement of the sensor is very critical - slightly in front of the focus position or slightly behind it means that the "point" is very blurred. But with a small aperture the cone is much narrower and there is more tolerance in the sensor position before it blurs seriously. Does that make some sense? It means then that the position of the subject is less critical with a small aperture than with a large one.

Hope this helps! (It's my first posting to this forum).

(I would add for the purists that I've really described depth of focus here, but the logic is still true for depth of field).

[Dave Humphries]

Hi John,

Welcome to the CiC forums.

Well I think I understood it, what you are describing is what is shown in the diagram in this post (as linked from Colin's post #5 above).

If you've got a moment, could you post an intro about yourself here on the Intro thread please.

Many thanks,