# Why don't you just step back a bit for a crop body? compared to a FF.

Started Apr 26, 2010 | Discussions thread
A bit o' physics

Crocodile Gena wrote:

The reason the noise would be the same is because since the crop camera is 1.6x further away than the FF camera for the same framing (but different perspectives), the light reaching the lens will be 2.56x (1.6^2) times less intense. Thus, it needs an aperture with 2.56x the area (1.6x the diameter) to collect the same amount of light for a given shutter speed.

And, once again, if the FF camera is at 50mm f/1.4, the crop camera is once again SOL because it cannot get an aperture large enough to make up for the greater distance.

Huh???

Are you saying exposure changes as the camera moves away from the subject?

No. I'm not sure how you divined that from what I wrote. I said nothing about exposure at all.

As you move away from the subject, intensity decreases and thus requires a larger aperture? I'm going to have to see some proof of this theory!

The intensity of light reaching the observer is inversely proportional to the square of the distance the observer is away from the source.

For example, if you are twice as far away, the intensity of the light reaching you will be 1/4 as much. So, if the light is passing through a hole (aperture) with a particular area, then if you were twice as far away, you'd need an aperture area four times as great (which would require a diameter twice as large, since area is proportional to the square of the aperture diameter) to gather the same amount of light in the same amount of time.

The inverse square rule does apply if you are using a camera mounted light source. In the case of any sort of daylight photography, exposure is the same whether you are 1 foot or 100 feet from your subject because the light source is 96,000,000 miles away. Last time I shot my kids playing soccer in the sunshine, I don't recall the opposite side of the field being dark or having to adjust my manual exposure setting as they moved closer or further away.

Once again, I am not talking about exposure. Exposure is the density of the light on the sensor. I am talking about the total amount of light that falls on the sensor. The reason is that the image noise is a function of the total light, not the exposure.