11-23-2018 12:49 AM
I have two Canon telephotos, a zoom and a prime, and I noticed a difference in focal length that really surprised me.
My two lenses are 1) a Canon EF 100-400mm f/4.5-5.6L IS II USM, and 2) a Canon EF 400mm f/5.6L USM.
I shoot mostly at the top end of the zoom range when I use that lens so I really care about reach.
It slowly dawned on me that I wasn't getting the same magnification as when I used the prime.
So I did a more controlled test at home: I put the camera plus zoom on a tripod, zoomed all the way out to the max, and took a photo of a yardstick across the yard. Then I swapped in the prime lens, leaving the tripod in the same position.
Sure enough, the image that I capture with my 400 prime is about 6% more magnified than what I get with my zoom!
I redid the test a couple more times to be sure, and indeed I got the same results. I assume it's the zoom that has a low effective focal length, and not the prime that is too high, but I guess I don't know that for sure.
So my question is this - is this something that everyone already knows? How can 400mm not be 400mm? Is it only my lens that does this? And if so, is it fixable? What the heck?!
Here are the two test shots showing the difference between the two lenses, plus an image I combined in photoshop illustrating the difference:
11-24-2018 04:09 AM
I say ignore the small difference and enjoy the lens.
The actual focal range of zoom lenses are never exactly as advertised. This is true for zooms made by nearly all manufacturers. In fact, Canon just may be better than most when it comes to accurately covering the rated zoom range.
11-24-2018 09:26 AM
"Sure enough, the image that I capture with my 400 prime is about 6% more magnified than what I get with my zoom!"
How close were you. Zoom lenses have focus bloom when you get near the close focus limit. One zoom lens for example, the older version of Nikon's 70-200mm was actually only able to manage 137mm at close focus. It is probably the worse at that I can remember. Of course primes, typically, don't suffer from focus bloom as much.
Bottom line, it is quite normal just some lenses are worse than others.
11-24-2018 10:00 AM
Telephoto zooms are measured at infinity, and they do tend to "shrink" at closer distances, so your numbers are not out of line. Here are some ways to measure focal length, including a clever way using stars at a known angular separation:
11-24-2018 10:38 AM
"Some have asked if the 100-400 L II's focal length is really 400mm on the upper end. This is a fair question because it seems that manufacturers sometimes take the liberty of rounding off/up the specified focal length numbers in zoom lenses. In this case, the lens set to 400mm frames a test chart from 42.86' (13.063m) while two of Canon's prime 400mm lenses (the 400 f/2.8L II and 400 f/5.6L) frame the chart at 44.87' (13.675m) and 44.54' (13.576m) respectively. If I take the average of those two comparables, I get 44.70' (13.625m) (using the metric numbers for calculating). Using that number, the 100-400 II's max focal length seems more like 383mm or just slightly wider. Especially with distortion potentially effecting this calculation, that number is close enough to 400mm that few will care about any actual difference. Canon has claimed to be within 5% of the specified focal length number, adding credance to the calculated figure's possibility. "
11-24-2018 11:23 AM
You are likely running into "focus breathing".
Lens values (and values in photography in general) tend to be rounded to the nearest meaningful value. Even f-stops are usualy not strictly accurate (but so close that the rounding error doesn't impact exposure value in any significant way).
As others have mentioned, the focal length of a lens is typically established by measuring it when focused at "infinity".
If you look through your lens and manually adjust focus from ifinity down to closest focus distance and back to infinity ... look at the field of view (even though it's blurry) and you'll notice the angle of view is slightly shifting as you focus. This is called "focus breathing" and it's normal.
Many lenses control this pretty well and the breathing is limited to maybe 5% of the focal length (in other words 380mm on a 400mm lens ... and you're pretty close to that). But some lenses are fairly extreme (called "heavy breathers") where the true focal length can change by 30% or more.
Keep in mind you are using a lens which is primarily designed to photograph subjects at a distance.
11-24-2018 01:28 PM - edited 11-24-2018 02:32 PM
Thanks for all the responses, with a shout out to John Hoffman for your detailed numbers.
Regarding focus breathing, this should affect both lenses equally, no?
I shoot mostly birds, and I chose this distance as it is in the range that I typically target. A medium to smaller sized bird will already require a crop in post-processing as you can see by looking at how much 5 to 6 inches would fill in the sample images I posted. Any farther away, and even more crop would be required.
So from what I'm gleaning from all of your comments - one should expect to sacrifice reach at these distances by using the zoom; and the published specs are a bit loose and are not 100% applicable to my scenario.
Definitely something to be aware of when choosing a lens!
11-24-2018 02:00 PM
"Regarding focus breathing, this should affect both lenses equally, no?"
No it doesn't. It is an individual lens issue. Some lenses have very little focus blooming or breathing. Like I mentioned the Nikon 70-200mm was horrible while the Canon version (70-200mm) is very good with very little. Even primes suffer from this although it is very small.
Plus you have the ad department that tends to be optimistic when assigning numbers. A word about FL and trying to calculate it, it is way more difficult to do so on a camera lens than it is for just a convex or concave piece of glass, a lens if you will. Now add to the fact you have a zoom lens with all the correction lenses which can move inside. FL is the convergence of parallel rays of light. In order to do this zoom lenses have to preform some magic.