Upgrade from Canon SL1: 90D vs 6d Mark II? or other ideas?

[DavidE1]

I've used the Canon SL1 for wild flowers, butterflies, and birds fairly successfully for years and generally use it with a Sigma 100-400 lens.  It's been a good combination for low weight, long walks and usually obtaining decent to excellent results.  But in a recent bird trip to Costa Rica, I found the SL1 really struggled in nearly constant low light conditions with ISO 1600 being marginally useful at best (and I had to use flash in the jungle quite often, not ideal these days).  I did got LOTS of great photos but I'd like to get even more!  I do not own a full frame camera.  I've read that the 7d II is the 'gold standard' for many birders but it's old and I've also read nearly all the Canon crop sensor cameras are not known for their low light abilities including the Canon 90D. In C.R. I was blown away by the Nikon D500 ability in next to no light at ISO 4000.  I'm considering the Canon 90D or the Canon 6D Mark II.  I may have to resort to just renting them both and test each out myself, but I thought I'd reach out and get some opinions.  I realize photography is about trade-offs, weight, cost, crop/full frame sensor, etc.  Despite my emphasis to keep weight low, I'd like to return to C.R. and get better results even if it requires more heft. Many thanks for any replies.

[Tronhard]

There is a trade-off between crop and FF sensors, as you suggested.  As a long lens shooter you get the narrower Field of View (FoV) that is captured by the smaller sensor, giving an effect similar to shooting with a longer focal length lens.  The difference in what is captured is associated with a sensor crop factor (how many times the crop sensor's diagonal measurement would have to be multiplied to be the same size as that of a full-frame unit) and for Canon APS-C sensors, that value is 1.6. 

Essentially, when using your 100-400 lens, what is captured is equivalent to that which would be achieved by a 160-640mm lens of a Full-Frame body.  If you move to a FF body, you will lose that narrow FoV advantage at the long end - although you will get a benefit on the wide-angle end if you shoot in that range. 

Now, you can obviously crop the image after shooting it to achieve the same Field of View, but there will be a significant cost in terms of the pixels left after that cropping process.  Essentially to get the actual pixel difference, you divide the sensor resolution by the crop factor squared:  thus your sensor pixel count would be reduced by a factor of 2.56.

To some degree you do pay for that in terms of low light performance.  However, the sensor on the 90D is a significant improvement over previous APS-C sensors.  Yes, you may get better low-light capability on a GOOD full-frame sensor - certainly the 5DIV for example.  

Your comment about renting a couple of bodies is really the way to go to see how they work with your styles of shooting - no-one can really judge that for you.

[DavidE1 · Accepted solution]

Thanks, Trevor, for the prompt response, it helps, especially on full frame.  Maybe consider current 5Ds vs 6Ds.  Also reaffirming the obvious:  trying gear first, which I've not done in the past.  I love drooling over gear at a distance, but in reality, I've been extremely conservative as to actual acquisition as my experience is skill beats gear full stop.  Again, thanks for your comments and suggestions.   

[Tronhard]

Glad you have a plan of campaign now.  Definitely worth doing some shooting with rentals before committing to buy.  Perhaps you can post what your final decision is once you have reached one!