Comparing images of full frame against APS-C at same (large) focal length

Question

About me

As an enthusiast photographer, I am currently working with a Canon 80D with the following lenses:

• Canon EF-S 15-85 mm f3.5-5.6 IS USM
• Canon EF 70-300 mm f4-5.6L IS USM

Only a small fraction of pictures are taken with other lenses.

I am thinking about switching to the new mirrorless Canon RF lineup, preferably the Canon R6. Of course, I am well aware of the consequences. In that case, I should at least replace the lower end lens with an appropriate EF or RF one – probably the RF 24-70 f2.8 or 24-120 f4.

Problem

Today, I evaluated all my photos (by EXIF data) to find out about my past shots. It turns out that around 25 % of my images were taken at the focal length of 300mm, a lot of them when travelling or observing animals (mostly in zoos). Indeed, I appreciate the quality of the EF lens and the large focal length.

Will I be disappointed after a switch to a full frame camera, because I will not get the same frame at 300mm? Or will the image quality that much better so I will not worry about the loss of the pixels?

Assuming the crop-factor of 1.6 at Canon and the 20 MP Canon R6, I would need to crop an image down to 12.5 MP in order to get the same image area as with the 24 MP Canon 80D.

Notes

I know I could solve this problem by buying an additional lens with higher focal length or by buying the R5 which has more pixels.

I am also aware that this is not a classical Q&A question, but I would love to hear some input from other photographers about this dilemma.

Answer 1

I looked at your focal length statistics. You seem to use the EF 70-300mm lens almost always at 300mm. If you select a full frame camera without a long lens, your ability for narrow field of view reduces markedly.

Your 12.5 MP calculation is incorrect. The correct formula is to use the square of the crop factor: 20 MP / 1.6^2 = 7.8 MP. You probably won't want to have so little megapixels. While EOS R5 with more megapixels could reduce the megapixel count problem, the lens sharpness may not be adequate for EOS R5, and it's a couple of thousand euros more expensive than the EOS R6, and you can compare the price difference to the cost of a 600mm f/11 lens (the 600mm f/11 costs far less than the price difference).

My recommendation, if deciding to switch to mirrorless, would be the following kit:

• EOS R6 as the camera
• Buy a 24-105mm f/4L lens. You lose slightly focal length range at the long end, but you can use the 70-300mm lens for these use cases.
• Keep using the 70-300mm telephoto for the cases where you need intermediate range
• Buy a 600mm f/11 lens. Yep, f/11 is dim, but full frame f/11 is about equivalent in its light gathering ability to crop sensor f/7.1. Then when you take into account that the 300mm lens is equivalent to 480mm on the crop camera, when photographing a distant subject that won't fill the frame, you're using only 64% of the sensor area that you would be using with the 600mm lens. Thus the 600mm f/11 is equivalent in its light gathering ability to a 300mm f/5.6 lens used on a crop camera, when you crop its field of view to be similar to 600mm f/11. So when photographing e.g. a small bird, you gather as much subject light with 600mm f/11 than you would do with 300mm f/5.6 used on a crop camera.

If you want to save a bit of money, even a 24-105mm f/4-7.1 would be better (gathers more light) than a crop sensor 15-85mm f/3.5-5.6.

With a crop sensor camera you realistically can't achieve more than 400mm focal length with a limited budget (which is 640mm full frame equivalent) and reasonable aperture (teleconverter is a possibility but it has effects on the f-stop), and the Canon's cheapest 400mm option (the f/5.6) isn't stabilized. With RF mount, you have a choice between 600mm f/11 and 800mm f/11.

Answer 2

There are a lot of factors, not the least of which is lens capability. However for long shots, pixels-on-target is a significant factor.

A number of camera club members have expressed delight with their new mirrorless micro 4/3 cameras. If long reach is a priority, they support some excellent capabilities at reduced cost, size, and weight. Conversely wide angle shots can be harder.

Consider your use and desires.

Answer 3

That's a somewhat subjective question. BUT.. Technically full frame is better in all respects. If you're worried about losing magnification, you shouldn't worry. The benefits of truer dof, high iso performance and low noise on full frame are much greater than gaining a little magnification on a crop sensor.