FF vs. APS-C - Aperature Behaviour

[elmo_2006]

I was recently in a forum and a member there mentioned that an aperature of a given lens behaves differently on different sensor sizes.


His example was based on the 85L f/1.4. The moment you putthis lenson an APS-C body the lens behaves like a 136mm f/2 lens. Is this correct (I understand the difference between FF and APS-C, it's the f/xx value that's got me wondering between the two)?


So confused!

[neuroanatomist]

The crop factor applies to aperture in terms of depth of field. So, an 85mm f/1.4 lens when used on APS-C has the equivaslent focal length as 136mm on FF, and has the equivalent depth of field as f/2 on FF. That applies for the same framing as FF, and the reason for the apparent effect on DoF is that to get the same framing on APS-C, you're further from the subject, meaning a deeper DoF. Sensor size does not affect exposure, so your f/1.4 lens on APS-C is giving your the shutter speed of f/1.4, not f/2.

[Jon Ruyle]

Sensor size does not affect exposure, so your f/1.4 lens on APS-C is giving your the shutter speed of f/1.4, not f/2.

Let me clarify this.


This statement is true in some sense but false in another. It is true in the sense that , if you use, eg, iso100 with both sensors and the same f number (ie, set your camera to f/1.4 for both cameras), the shutter speed will be the same.


However, it isn't the "iso number" that we care about, but rather how much noise we will get in our pictures. In the above scenario, the ff camera will have less photon noise. Alternatively, the ff camera could have used a higher shutter speed and gotten the *same* amount of photon noise as the crop camera. In this sense, f/1.4 exposes more quickly on larger sensors. Put another way, if you put the 85 f/1.2 on a crop camera and put the 135 f/2 on your full frame camera and set the camera isos so that photon noise is the same for both cameras, you'll get the same shutter speed (not exactly- 1.4 * 1.6 is not exactly 2, but close enough). The iso setting will be higher on the ff camera, but the photon noise will be the same.


Thus I think it is correct to think of cropping as changing the "effective f number" the same way it changes "effective focal length" (ie, multiply f number by crop factor to get "effective f number"), because cropping changes both DOF and exposure speed in exactly the same way as multiplying the f number by the crop factor.


John knows all this (after all, it's been hashed over dozens of times on this forum), so it is not my intention to correct him, but I think his statement is easy to misinterpret.

[DavidEccleston]

Jon, I think the noise issue is a separate argument. Sure FF tends to get less noise, but that

[clemmb]

If a crop camera had the same sized pixels on the sensor as the full frame, the noise would be the same (though your resolution would be low!), so the crop factor doesn't have a directly correlation to pixel noise.
<div style="clear: both;"]</div>

Same pixel size but different noise.


Am I missing something in your statement?


Mark


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[Kamelot]

Sure FF tends to get less noise, but that's due to more photons hitting
the a single pixel over the same time, as the pixels are larger. If a
crop camera had the same sized pixels on the sensor as the full frame,
the noise would be the same (though your resolution would be low!), so
the crop factor doesn't have a directly correlation to pixel noise

Nope, you are not right. Here was discussed several times about sensor size and what depends on it.


And clemmb shows for you good camera comparison.


FullFrame sensor will have less noise not due to larger pixels, but due to larger surface of sensor = larger sensor, which can collect larger amount of light.


Imagine, you have:

1. 10 buckets with 10 <span class="HW"]litre capacity each<span class="HW"]
2. <span class="HW"]25 buckets with 5 <span class="HW"]litre capacity each.

And you place each type of bukets one nearby others. You have 2 areas with different size of buckets and of course different size of covered area. When the rain is starting, think, which area of buckets can produce larger amount of watter?


I think you will do the math []


<span style="color: #ff0000;"]Edit: <span style="color: #ff0000;"]and it is due to overal larger capacity of buckets (pixels) = larger sensor.

It's the DOF increase, due to changed camera to subject distance
required to get the same framing, that makes people talk about the crop
factor applying to aperture, as they are directly linked.

So tell me, when here http://www.dofmaster.com/dofjs.html i select:

• 5D, 135 mm, F/2 and distance 5 meters, and i get 16 cm of DoF
• and with 7D, i select 85 mm (for same framing), F/2 and 5 meters distance, i get 26 cm of DoF.

Which number (Depth of Field) is smaller (more narrow): 16 cm or 26 cm ? []

[DavidEccleston]

I

[neuroanatomist]

DoF is determined by four factors: aperture, focal length, subject distance, and circle of confusion. Only the last one is different with sensor size, but I

[DavidEccleston]

I'm adding more just to get back to the original argument.


As I mentioned, the noise should be identical in the middle of the frame, at a pixel level. Therefore nothing to do with effective apertures, a seperate argument. The extra size gives you less noise in prints to due extra pixels, shrinking the noise, or letting you downsample it away. I'm not saying there is no advantage to FF, just that the noise advantage has nothing to do with effective apertures.


With these same two cameras, though, adjusting your distance, or your lens, to produce the same framing, will have different DOF. This is an effect which is visible at the pixel level. This effective aperture change affects the ENTIRE FRAME. This is a FF advantage due to effective apertures.


edit: slight rewording of noise summary.

[neuroanatomist]

RE: I