Does FF glass on a crop sensor affect apertures?

oO Stevie Oo · 11 Dec 2011 at 01:08

Howdy

As I'm new to canon I've just been having a play with it there and noticed something, when looking down the lens on my 70-200mm f4L with the aperture at F/4 and pressing the DOF preview button, the aperture is noticeably bigger than my kit crop lens 18-55mm at F/4 and doing the same thing In comparison to the sensor in the body.

Does this mean anything or am I looking at it wrong? Lol
I.e does a FF lens let in more light when compared to a crop lens @ the same F number?

oO Stevie Oo · 11 Dec 2011 at 01:14

Just realised its the viewfinder I'm seeing through the lens not the sensor lol
Still does it mean anything?

An Exception · 11 Dec 2011 at 01:20

^^^
No it's the same, although at the same framing and same aperture, you will get 1.3 stops shallower DOF compared to a Canon crop, and approx 1 stop shallower DOF compared to a Nikon Crop.

oO Stevie Oo · 11 Dec 2011 at 01:40

Sort of what I ment as a wider aperture would affect DOF lol
Ah well cheers dude

An Exception · 11 Dec 2011 at 05:27

oO Stevie Oo said:
Sort of what I ment as a wider aperture would affect DOF lol
Ah well cheers dude

Two thing affect DOF, aperture and focus distance.

Example:

A 550D focusing on a subject 10m away using a 50mm lens at 2.8, will have the same DOF as a 5D focusing on a subject 10m away using a 50mm lens at 2.8.
However the framing or field of view will be different as the FF camera will achieve a wider view. So if you want to achieve a similar framing as the 550D you would need to move closer to the subject. It's only the fact you can be closer to the subject that causes a difference in DOF.

Also perspective will likely be different, however if you were using a zoom lens, instead of moving closer you could zoom in instead, this will achieve the same perspective as the 550D, however the DOF will still be shallower.

Andy90 · 11 Dec 2011 at 10:46

Depth of Field

F-Stop

Aperture

Some 'light' reading (excuse the pun).

Ksanti · 11 Dec 2011 at 12:18

f/4 is a ratio, with "f" standing for focal length, giving the physical diameter of the aperture.

If you put your 70-200 at 70mm f/4, you'll see the aperture is a lot smaller (70/4 = 17.5mm) than at 200mm (200/4 = 50mm), and this is what you're seeing with the kit lens as well e.g. 18mm f/4 = 18/4 = 4.5mm diameter. This is why longer lenses tend to be 'slower', as you start getting to things like 300 f/2.8 lenses you have a hell of a lot of glass (minimum 100mm diameter) and that gets very expensive and very heavy very quickly.

In terms of DoF, that's governed entirely by physical diameter and focusing distance. The 70-200's f/4 will give a narrower depth of field than the 18-55 as the aperture is around 4 times the size at each end, but you've also got to remember that the 70-200 will be giving a much narrower field of view, so you'd have to get back to fit in the same amount, basically counteracting the narrow DoF of the large sensor.

f/4 on a crop isn't going to be giving you a huge amount of DoF capability except right out at the long end, mind you. If you want to get started looking at that, look at the Canon 50mm f/1.8.

An Exception · 11 Dec 2011 at 13:00

^^^
Focal length doesn't affect DOF, 50mm at 2.8 will have about the same DOF as 200mm at 2.8 if the image is framed the same, or as close as it can be due to perspective being wildly different.