Confused about apsc aperture on APSC vs FF.

Started 4 months ago | Discussions thread
GaryW Veteran Member • Posts: 9,617
Re: Aperture is Focal Length / Entrance Pupil

ThrillaMozilla wrote:

GaryW wrote:

Dirk W wrote:

Ace of Sevens wrote:

Dirk W wrote:

Yes it collects less light, but it doesn't matter for the expose because the sensor is equally smaller. When I crop a part out of a picture, the remaining part has still the same brightness as before.

It does matter for the exposure. The ISO would be the same for the same image brightness, but the image on the smaller sensor is made with less light

The TOTAL image on the smaller sensor has less light, but PER AREA it's the same (if the exposure is the same).

so therefore has a lower exposure.

My understanding is that exposure is for a given area. So, same ISO, same aperture, same shutter, should give you the same exposure. Same amount of light per area. Larger sensor has more area.

In real terms, this means it will appear noisier when the images are displayed at the same size.


When you make the image, You scale that image that was made with half as much info up to the same size as the one made by the larger sensor, so the signal to noise ratio will be lower.
It may be easier to think of in analog terms. ISO 400 110 film & ISO 400 35mm film are the same film. The factory makes it all in one sheet & they just cut it down for whatever application is needed. If you make prints from the 110, they will be a lot grainier than the 35mm, though because you are blowing up way less film to the same size. The same principles apply with digital sensor size.

I thought the higher noise of the aps-c was due to the fact that the pixels are smaller and make "less use" of the light,even if the light as such would be at the same level.

That's right -- if two sensors of different sizes have the same total number of pixels, the smaller sensor has smaller pixels. It's receiving less light per pixel, which is less signal (thus more noise). In terms of output, you're magnifying those smaller pixels to match the same print size of the larger sensor, but those smaller pixels had less light to work with.

But ONLY if depth of field or shutter speed doesn't matter!

I assume you mean that you have to swap lenses to match the FOV.  When you change the focal length, you change the DOF, if you maintain the same aperture setting.  You can change the aperture setting and ISO and leave the shutter speed the same.  Of course everything matters!    At least if you're attempting to exactly match the results between the two systems.

FF has a wider range of possibilities and CAN receive more light per pixel under some conditions, but there's a price: either narrow depth of field or longer exposure times.

Most consider narrow DOF a feature.  And the differences are in the extremes, but it's those extremes that people continue to run to, in order to justify the additional cost (and size and weight...) of FF.

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Gary W.

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