Full frame cropped v APS-C?

[thebeardedgroundsman]

I know that A7r cameras can be cropped in camera - to "extend" the focal length of the lens, which means the effective resolution is similarly reduced.

So, if a full frame camera has 42Mp and you put a 400mm lens on it and you use it in cropped mode you effectively have a 600mm lens on an 18Mp camera.

An APS-C camera with a 24Mp camera, using the same lens will effectively be 600mm with 24Mp.

Now my question is this. If I use the full frame camera with that lens in FF mode and then crop the resulting photo in editing, to result in the subject ending up the same size in the resulting image, a) what is the final resolution like (more like the APS-C camera or the FF in cropped mode) b) do I get the benefit of keeping the same aperture as the lens indicates c) are there any gains to be had (apart from having the opportunity not to crop so tightly) to be had from the FF camera compared to the APS-C.

In short, do I put my 400mm lens on my APS-C body and keep my higher resolution body for wide to short telephoto shots, or do I do it all on my FF body and keep my APS-C as my spare/light weight option?

(I know Pieter is a great fan of APS-C cameras) 

[Pieter]

3 hours ago, thebeardedgroundsman said:

(I know Pieter is a great fan of APS-C cameras) 

Haha now you're challenging me to give you a sensible answer... Let me try:

3 hours ago, thebeardedgroundsman said:

a) what is the final resolution like (more like the APS-C camera or the FF in cropped mode)

Cropping your FF image to the same field of view as an APS-C camera with the same lens yields exactly the same resolution as putting your FF camera in crop mode. 18 MP that is, for the A7Rii/iii. So if you're planning to put your A7Rii/iii in crop mode, you're likely better off using an A6xxx camera.

3 hours ago, thebeardedgroundsman said:

b) do I get the benefit of keeping the same aperture as the lens indicates

Aperture is a property of the lens, not of the image sensor. Your aperture value, and corresponding exposure, does not change when cropping or using a smaller sensor. What does change is your apparent depth of field and noise performance when using a smaller sensor area. Using a FF camera in crop mode yields virtually the same image as an APS-C camera when shot at the same exposure parameters. In terms of exposure, depth of field, field of view and noise performance.

3 hours ago, thebeardedgroundsman said:

c) are there any gains to be had (apart from having the opportunity not to crop so tightly) to be had from the FF camera compared to the APS-C.

Yes: with a FF camera you primarily have the opportunity to have better noise performance and shallower depth of field compared to an APS-C camera. However, if you put your FF camera in crop mode, all these advantages are lost (except you still have the opportunity to switch back to FF mode).

All in all, I just believe there's a best tool for each job. For my job, it's primarily to provide the best possible image quality in the smallest possible package. FF is too bulky for my job, considering the (in my humble opinion) marginal advantages it provides. A hypothetical A7Cii might tempt me to switch to FF if it at least provides the same controls as the most basic A6xxx camera.

[thebeardedgroundsman]

Yep, having used my A6500 with 400mm lens for wildlife photography, earlier in the year, whilst my A7Rii was being repaired, I found the results pleasing.

So, I'm going to try keeping the A6500 on the big lens and use the A7Rii for my other lenses for landscapes etc.(considering the weight of the lens, I don't think carrying an extra body will be too much).