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Old Jun 3, 2013, 5:12 AM   #1
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Default aperture size, focal length and crop factor

I've been using an IXUS 220 HS :
f/2.7-5.9 24mm - 120mm (35mm equivalent)

I'm in the process of trying to decide what lens to buy for a 700D and wanted to check my understanding....

I thought I would go with the Canon 18-135 STM or the better 15-85mm both are f3.5 to 5.6

Having looked at some of my previous pictures details I can see I am often using 4mm (24mm) and when indoors f2.7. This has made me reflect slightly on my choice of lens.

1) Am I right in thinking that at 4mm (IXUS) and 15mm (700D) will give me the same field of view given they both equate to 24mm?

2) Ignoring DoF I thought that the aperture size is directly comparable i.e.. the ixus lets in significantly more light at f2.7 (vs the f3.5). Then I saw f15.2 - f33.2 (35mm Equivelant), is that more about DoF or does it only let in f15.2 of light when fully open?


It seems very rare that I shoot much beyond 24mm, even when I take a picture of a person it's 24mm. e.g. in front of a landscape or across a table sort of distance therefore fills the frame.

3) So given that is my "style" should I be focusing on the wide end or is there a fundamental difference between what I will shoot with a DSLR over a point and shoot?
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Old Jun 3, 2013, 6:40 AM   #2
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1) Am I right in thinking that at 4mm (IXUS) and 15mm (700D) will give me the same field of view given they both equate to 24mm?
Yes. 4mm on your IXUS and 15mm on the 700D are the actual focal lengths of the lenses used to obtain the same angle of view, given the difference in the size of the image sensors. The 35mm equivalent focal length is so that the actual focal lengths are comparable with one another, because they have identical angles of view.

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Originally Posted by daninmanchester View Post
2) Ignoring DoF I thought that the aperture size is directly comparable i.e.. the ixus lets in significantly more light at f2.7 (vs the f3.5). Then I saw f15.2 - f33.2 (35mm Equivelant), is that more about DoF or does it only let in f15.2 of light when fully open?
First, Aperture serves two purposes. It governs the amount of light that passes through the lens, and so is one of three factors that determine exposure. It also governs the depth of field for a given sensor size.

Second, the sensor in your IXUS 220 HS is so small that, at a focal length of 4mm, a subject distance of 4 feet, and an aperture of f/2.7, the depth of field is from less that 2 feet to infinity.

At the same 35mm equivalent focal length (15mm) and the largest aperture available on the 15-85, at a subject distance of 4 feet, the depth of field will from about 3 feet to about 6 feet.

So the same aperture size gives the same exposure on both cameras (given the same shutter speed and ISO), but because of the difference in the size of the sensors, the depth of field will be very different. I would suppose that going from the 220HS to the 700D will require a major change in how you use the camera's settings to compose your shots.

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3) So given that is my "style" should I be focusing on the wide end or is there a fundamental difference between what I will shoot with a DSLR over a point and shoot?
With the 700D, you'll get less image noise with the same exposure settings. And there's always the 12MP vs. 18MP thing. But the depth of field you're accustomed to with the 220HS will be very different with the 700D.
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Old Jun 3, 2013, 8:06 AM   #3
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So is the 35mm equivalent aperture value for the ixus all about DoF and nothing to do with exposure?

I'm more concerned about my field of view than DoF although I appreciate that with the same wide angle DoF will be different especially when at f2.8 or 3.6.
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Old Jun 3, 2013, 9:07 AM   #4
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So is the 35mm equivalent aperture value for the ixus all about DoF and nothing to do with exposure?
aperture is aperture. When you see/hear "35mm equivalent" it is about FOCAL LENGTH - not aperture. Focal length is the attribute that manufacturers "normalize" by stating a "35mm equivalent".
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Old Jun 3, 2013, 9:10 AM   #5
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But it explicitly says f15.2 - f33.2 (35mm Equivelant).
I can only assume this is for the DoF calculation in relation to focal length.
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Old Jun 3, 2013, 10:38 AM   #6
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Here's what's going on.

Your IXUS 220 HS has a zoom lens with a focal length that varies from 4.3mm to 21.5mm. It has a maximum aperture that varies with the focal length from f2.7 to f/5.9. That is, at 4.3mm the aperture is f/2.7, and as the focal length increases the f-number increases until at 21.5mm the aperture is f/5.9.

SLR lenses actually contain a Diaphragm that varies the size of the opening that allows light to get through the lens to the image sensor. Diaphragms give SLR lenses a variable aperture. As the diaphragm opens and closes, the depth of field changes. As the diaphragm opens the DoF gets smaller, and as the diaphragm closes the DoF gets larger.

That's not what happens in your IXUS 220 HS, or most P&S cameras for that matter.

Instead of changing the size of the opening, your camera flips a 3 f/stop neutral density filter into the light path, limiting the amount of light that gets through the lens. While SLR lenses have a variable aperture, your lens only has two exposure settings: either with the ND filter, or without it. And while the exposure changes, the DoF does not because the size of the opening doesn't change.

What you know about "aperture" and "depth of field" from your P&S camera, does not relate to the aperture and the corresponding change in depth of field you'll experience with a dSLR.
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Last edited by TCav; Jun 3, 2013 at 11:57 AM.
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Old Jun 3, 2013, 10:56 AM   #7
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So is the 35mm equivalent aperture value for the ixus all about DoF and nothing to do with exposure?
Exactly the opposite. The f-numbers for the IXUS are all about exposure and nothing to do with DoF.
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Old Jun 3, 2013, 2:02 PM   #8
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Have you considered trying various settings?
Pick an object.

Set the aperature at f2 shoot.
Set the aperature at f4 shoot.
Set the aperature at f8 shoot.

Compare.

The best way to learn is to use your equipment.
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Old Jun 3, 2013, 4:57 PM   #9
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Hello, Dan;
I think there may be some confusion surrounding the attempt to equate apertures to a 35mm equivalent. Aperture is is the lens diaphragm opening expressed as a fraction. (F/2.7 for example) This is a measure of the amount of light which will be allowed through the lens at that setting, and determines exposure (in conjunction with shutter speed and sensitivity)
Apparently, your camera manual is also giving you the DOF equivalent for the aperture settings as well. So, yes, you are correct in thinking the( f15.2 - f33.2 (35mm Equivelant)) is related to depth of field. (it has nothing to do with exposure, though)

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Old Jun 3, 2013, 5:31 PM   #10
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Originally Posted by Flying Fossil View Post
Have you considered trying various settings?
Pick an object.

Set the aperature at f2 shoot.
Set the aperature at f4 shoot.
Set the aperature at f8 shoot.

Compare.

The best way to learn is to use your equipment.
The user of a Canon IXUS 220HS (300 HS in the US and Canada) can't change the shutter speed or "aperture". They are determined by the camera's autoexposure system. The user can fine tune the exposure by adjusting the ISO and exposure compensation, but that's it.

There's no aperture ring (or other control) that allows the user to select an "aperture". The camera chooses either to use the single built-in 3 stop ND filter or not, along with the shutter speed, to properly expose the image.

Your advice would work well when daninmanchester gets his 700D, but he's powerless to try it while he still has only his IXUS 220 HS.
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