why are ISO ratings in DX crop mode of FF Cameras below the rating for FF on the same camera?

[BillN_33]

Just been looking at the DXo figures?

It's a question?

 

[woof woof]

I'd imagine because in crop mode the image would need to be magnified more thus making the noise more visible? Maybe? Dunno, just guessing...

 

[Brachytron]

I'd imagine because in crop mode the image would need to be magnified more thus making the noise more visible? Maybe? Dunno, just guessing...

You're not far off the mark there Alan.

It's because the ISO ratings relates to the total light captured over the surface area of the sensor as well as its inherent sensitivity. Therefore a smaller sensor will have a lower ISO in relation to a larger sensor.

 

[BillN_33]

You're not far off the mark there Alan.

It's because the ISO ratings relates to the total light captured over the surface area of the sensor as well as its inherent sensitivity. Therefore a smaller sensor will have a lower ISO in relation to a larger sensor.

so does this mean that effectively you see more noise on the DX image, (versus the FF images) .... i.e. it is a crop that "magnifies" the noise

 

[seriousrikk]

so does this mean that effectively you see more noise on the DX image, (versus the FF images) .... i.e. it is a crop that "magnifies" the noise

If you view the image at pixel level, no I don't believe it will.

A 100% crop from DX mode will contain the same number of pixels as a 100% crop from FX mode. However if viewing both images at the same output size and distance, you would see more noise in the cropped shot because the noise has indeed been magnified.

You can test this by cropping an image in photoshop then looking at the original and the cropped image at the same size side by side. The result will be the same.

 

[Nawty]

so does this mean that effectively you see more noise on the DX image, (versus the FF images) .... i.e. it is a crop that "magnifies" the noise

It's all about magnification and specifically in this case, as Richard says, the output size.

Magnify ANYthing and you will magnify the noise as much as you magnify the information, that's basic physics.

 

[BillN_33]

If you view the image at pixel level, no I don't believe it will.

A 100% crop from DX mode will contain the same number of pixels as a 100% crop from FX mode. However if viewing both images at the same output size and distance, you would see more noise in the cropped shot because the noise has indeed been magnified.

You can test this by cropping an image in photoshop then looking at the original and the cropped image at the same size side by side. The result will be the same.

yes I have done this

My question arose from looking at the DXo scores versus the D500 and D7200

The D810 is better at FF, but slightly below the D500/D7200 at DX crop - i.e. D810 DX versus D500/D7200 - trying to understand their logic because I have found that a cropped D810 image is less noisy than a cropped down to the same size D7200 image

 

[woof woof]

yes I have done this

My question arose from looking at the DXo scores versus the D500 and D7200

The D810 is better at FF, but slightly below the D500/D7200 at DX crop - i.e. D810 DX versus D500/D7200 - trying to understand their logic because I have found that a cropped D810 image is less noisy than a cropped down to the same size D7200 image

I give more weight to my own eyes than test scores these days and if what you see contradicts what's on a review site maybe you should go with your own eyes too

One thing that I would recommend is sizing your image for how it's going to be viewed and if you do that you might find that noise performance improves.