f stop and crop sensors

Started May 11, 2010 | Discussions thread
Crocodile Gena Senior Member • Posts: 1,017
How so?

acsmith wrote:

If your photograph will be correctly exposed at f/8, 1/250 sec., ISO 100 with a FF sensor it will also be correctly exposed at that exposure on a APS-C or a 4:3 sensor. Your DoF will be equal to f/11 if you have a APS-C sensor or f/16 for a 4:3 sensor.

This is true, and not in conflict with anything I said in my post above. Here, let's be more specific. Consider the two following scenarios, both with the 24-105 / 4L IS mounted on a 7D and 5DII:

7D: 31mm, f/4, 1/50, ISO 100
5DII: 50mm, f/6.3, 1/50, ISO 250

These two images will will not have the same exposure (the density of light onthe 5DII sensor is less), but they will have the same apparent exposure (that is, they will appear to have the same exposure), the same DOF, same framing, and roughly the same noise (because, while the exposures are not the same, the total amount of light falling on the sensors is the same).

However, the 5DII image will render more detail due in part to the lens being sharper stopped down, and in part due to the larger sensor's lesser magnification of the aberrations of the lens.

Now let's consider another situation:

7D: 31mm, f/4, 1/50, ISO 100
5DII: 50mm, f/6.3, 1/20, ISO 100

In this case, the exposures will be the same, as will the framing and DOF. However, the 5DII image will have less noise (since 2.56x more light falls on the 5DII sensor), but be more succeptible to camera shake and/or motion blur, so this is not always an option.

Next situation:

7D: 31mm, f/4, 1/50, ISO 100
5DII: 50mm, f/4, 1/50, ISO 100

Here, both the framing and exposures are the same, but the 5DII image will have a more shallow DOF, softer edges (but a sharper center), more vignetting, and less noise (again, because more total light falls on the 5DII sensor).

Some, of course, would argue what noise matters at base ISO. Well, that depends on how the image is processed. It is not unusual, for example, when pushing shadows, for noise to be an issue. On the other hand, the 7D sensor has less read noise than the 5DII sensor, so there is some back-and-forth going on here. Regardless, we can repeat all the above scenarios with four stops higher ISO (substitute ISO 1600 for ISO 100, and ISO 4000 for ISO 250) and nothing changes.

So, in what way is ignoring all of the above a good thing? If a person is shooting multiple formats, or considering moving from one format to another, these are all rather important considerations.

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Bob
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