It's rare that there is room for a tripod where I shoot and I would be scared of somebody knocking it over when passing me, so it needed to be an IS lens.

I've never shot with a IS lens before, but the idea of being able to stop down multiple f-stops made this one a must-have.

Half the year I shoot indoors in poorly and unevenly lit riding arenas, so I knew a slow lens wouldn't cut it. Despite the camera itself having excellent low light performance I knew I wasn't going rely on high ISO capabilities saving pictures shot with a slow lens, so I opted for a f/2.8 lens

I could have gone for a prime lens, but shooting horses at competitions means that they will sometimes pass a few meters in front of me only to ride to the far end of a 60m arena - so I couldn't live without a zoom lens.

I had been using a Nikkor 70-300mm 4.0/5.6G zoom on my aging Nikon D70s. So, since I was going from a cropped sensor to a full-frame camera I initially looked at something that would go beyond 300mm to make up for the angle of view loss I would get from going full-frame.

So I went looking for a 400mm lens.

Looking for speed (f/2.8) and image stabilization (IS) I had the option of a neat looking Canon EF 400mm f/2.8L IS II USM Image Stabilizer Super Telephoto Lens. There were two downsides though - a) it wasn't a zoom lens and b) the price didn't have a very high WAF (Wife Approval Factor) - even the grey market version didn't make the price sufficiently less painfull.

Zoom lenses that could reach 400mm would be either the Canon EF 100-400mm f/4.5-5.6L USM AutoFocus Image Stabilized Telephoto Zoom Lens or the Sigma 120-400mm f/4.5-5.6 DG APO OS (Optical Stabilizer) HSM AutoFocus Telephoto Zoom Lens.

Both these zoom lenses were affordable, but fully zoomed they were both f/5.6 aperture lenses so they were not an option either.

Looking at the 300mm range was the same - either I could get speed and image stabilization (from the Canon EF 300mm f/2.8L IS II USM Image Stabilizer AutoFocus Telephoto Lens) - or I could get zoom and IS (from the Canon EF 28-300mm f/3.5-5.6L IS USM AutoFocus Wide Angle Telephoto Zoom Lens)

Initially I didn't really want to look in the 200mm range because I felt I wasn't going to be able to capture the horses properly (as in roughly filling the picture) at the far end of the arena since I would go from 300mm to 200mm and from cropped sensor to full-frame.

I went through plenty of pages explaining the different differences between cropped and full-frame, the difference in image quality, the difference in effective focal lenght and the difference in angle of view - and everytime I read another page I was more confused than before.

I ended up using ther Focal Lenght Comparison Tools from Tamron and Nikon (the one from Canon was utterly useless for my purposes)

Playing with these tools I was able to visually compare the difference between 300mm on a cropped sensor and 200mm on a full-frame sensor - and there really is a big difference.

I would have needed somewhere around 475mm zoom on my Mark III to get the equivalent framing or angle of view as I get from 300mm on my D70s.

Or another way to look at it - zooming fully in (300mm) on my D70s I get an angle of view of 5.2 while I would get 12.2 with 200mm on my Canon.

Sadly I couldn't find any tool that would allow me to compare the image quality and pixel count from a picture taken on a 22.3 MP full-frame sensor at 200mm when it's cropped to match the same subject taken with a 6.0 MP cropped sensor at 300mm.

In the end I figured (or hoped) that the improved quality from the lens together with the higher pixel count and the better image quality from the sensor itself would make up for the lack of zoom.

The decision

And so I decided on the Canon EF 70-200mm f/2.8L IS II USM AutoFocus Telephoto Zoom Lens.

While researching the lens it also dawned on me that the downside of not being able to get the same narrow angle of view when fully zoomed in is at the same time an upside when fully zoomed out since both lenses start at 70mm - which on my new full-frame sensor means that I can capture horses closer to the barrier than before (with the angle of view going from 21.8 degree to 34.1 degree).

Could I have gone for the IS instead of the IS II? - sure, but the reviews of the IS II (especially when compared to the 'old' IS) were so great that I convinced myself so much that I was also able to convince my wife that the pictures would get significantly sharper - even at low light  - and who is she to resist better pictures of her horses :-)

Canon EF 70-200mm f/2.8L IS II USM