Cropping : sensor size / MP

Started 10 months ago | Discussions thread
Horacecoker Senior Member • Posts: 2,163
Re: Cropping : sensor size / MP

sybersitizen wrote:

Horacecoker wrote:

morinor wrote:

Horacecoker wrote:

Zefah wrote:

... if you took a 42MP image from the A7R III, for example, and zoomed in 100%, then cropped it, you should (more or less) get what would be the equivalent image if you had shot using a M43 21MP camera with the same focal length, aperture, and ISO.

If you do that you lose 1 stop of noise advantage over the M43 sensor. So Full frame's 2 stop ISO advantage becomes 1 stop.

Demonstrated here - you can see the A7Riii is noisier at 1600 ISO when the M43 sensor is set to 400 ISO when if fact, according to the two stop 'rule' they should both look the same.

I've had to lower the A7Riii ISO to 800 to get similar noise levels to the M43 sensor at 400 ISO.

I couldn't quite believe this myself till I had it pointed out to me on the M43 forum.


You are 100% percent wrong as the crop that you have done. You should compare at the same produced size. Switch the image size on the tool to "COMP" or "PRINT" and compare again.

No, that's exactly what I don't want to do.

I believe there's some confusion about what you want to do.

Maybe, but I'm not the one who's confused.

Let me explain, I was thinking of buying the A7Riii for birding ...

In that case, you want to compare the size of the image of a particular bird on each format, and probably also take into account the resolution of that particular bird's image on each format. Correct so far?

Of course, that's exactly what I want to do.

The premise stated by Zefah above was that in both cases you are using 'the same focal length, aperture, and ISO'. Still correct?

No that doesn't apply to me. I want the best image quality as possible on either system. I will try to achieve that by juggling shutter speed, F stop and ISO. Of course this is light and circumstance dependant.

If so, you don't really need to concern yourself with anything else outside your subject of interest, such as the portions of each image that will be cropped away. You are only concerned with comparing equal areas from each image that contain your subject. Isn't that right?


If so, the size of the bird's image will be the same on both formats. In that case you would want to look at equally sized sections of DPR's test scene to compare their noise (and maybe dynamic range) characteristics.

Well it might not be exactly the same but yes it is the size of my subject (ie the bird) and the image quality at that size that is all important.

The problem is, I'm not sure how that can be accomplished when the two cameras record such different total areas. Can someone explain how?

I'm not sure either but I'm trying my best to work it out to my own satisfaction, hence my ramblings on the subject.

Then, because the pixel pitches of the two cameras don't match, that means you would clearly not use the COMP and PRINT switches to examine the detail characteristics. For that, you would compare the two versions at their 'native' resolutions and 100% pixel magnification. The apparent sizes will be different, but you will be concerned only with examining differences in detail.

Yes that is why I know I was 100% right to compare at original size and not use COMP or PRINT.

But again ... to answer Zefah's challenge you have to conduct any comparisons at the same ISOs - not different ISOs.

No, I don't have to do anything of the sort and why Zefah's wants to do that I have no idea.

That's how I figure it. If you see flaws in this interpretation, please point them out.

You're trying to figure out two similar but slightly different aims and combine them together and it's not working too well.

Now ... if you yourself intended something very different (by that I mean anything other than the same focal length, aperture, and ISO) when theoretically shooting birds with both systems, explain in detail what that is.

If I've not already explained it the in above answers and my previous posts, I give up!

I think I've now satisfied myself that using a FF camera with a much larger sensor resolution  to compensate for a lack of focal length at the tele end (400mm) for birds is not the way forward. A smaller sensor camera with much less resolution can yield better results because it gains the required focal length with the crop factor. That is slightly simplistic because the smaller sensor camera must have a top notch lens and that lens has got to be at least one stop faster than the FF lens. It so happens that there's kit out there that have these requirements. Of course we can stick a 600mm F4 prime on a FF camera but the cost and weight maybe too much for most folk. Like I say it's all a juggling act but it's good to discuss these things


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