A futher little test shows the Sigma 120-300mm f2.8 has serious focus breathing issues. This is from 25 ft.
The Canon EF 300mm f/4L IS USM Lens first.
Now the Sigma 120-300mm f2.8 at 300mm at the same distance.
You can try to decide whether the filters were on or off. And, whether the IS/OS is on or off ! Tought isn't it. Is it different than you thought or believed?
"It does support the reviews that show the Sigma isn't quite 300mm at the ong end."
Yes, I have read that. I don't know the exact distance when it does show a full 300mm picture. That will be the subject of a new test. If you were to use this lens for close distance work the Canon EF 70-200mm f/2.8L IS II USM Lens would be a better choice.
Bob Atkins says that a good way to measure field of view (which will give you focal length) is to shoot two stars of known degree separation, such as Orion's belt. A little math will get you the field of view, and hence the focal length.
"Without knowing exposure settings, this is an exercise in futility"
The exposures are the same as if that makes any difference. The camera is the same. The distance is the same. In future cases I invite you to refrain from replying since you have nothing constructive to offer.
Something is amiss... this is mathematically too narrow to have been shot from 25' away.
Ernie, did you crop this?
I stumbled onto something when I started to reply that you can find some photography calculators on this page:
But I have some Scotch packaging tape and I measured a roll... they have a diameter of about 11cm (4 3/8").
So when I did the math for a 300mm lens on full-frame camera at 25' away... I get a MUCH MUCH larger field of view.
At 25', a 300mm lens would have a dimensional field of view of 3' wide by 2' tall (feet... not inches). And since the field of view in these photos is somewhere on the order of 6" tall (inches... not feet), I realized we're off by a factor of about 4x.
Could the dimensional field of view calculator I linked above just be using incorrect math? I've used this website many times in the past and I got accurate results... so unless they updated their math recently, that's doubtful.
I happen to own a Canon EF 300mm f/2.8L IS USM... so I grabbed that lens, attached to my 5D III body, and grabbed a roll of Scotch packaging tape. Turns out I can't get this angle of view with that lens. My angle of view is closer to what the website suggests (several feet across). To get this angle of view, the math suggests the focal length would need to be closer to 1200mm.
In order to get that angle of view with my 300mm lens... the tape would need to be inside the minimum focus distance (for my lens it's about 7' away).
"Ernie, did you crop this?"
Yes, both were cropped 200% in PS. Here is the uncropped shot. And, it may have been 23' or 24' ! My deck is 30' across.
Again, keep in mind, I am solely interested in how lenses work in real life, real world situations. I could care less about how they do in the lab or how pretty they make a chart or graph look. Not many people shoot charts and graphs. I have another little test where you just might be surprised with shots comparing the 150-600 zooms vs the 100-400mm. When you actually use this stuff it becomes very difficult to tell which one did the best on paper.
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