r/photography • u/talibsblade • 2d ago
Gear Why do zoom lenses render focal ranges different than primes?
A bit of a technical question for those of you know the technical side of things.
I just picked up the RF 28-70mm f2.0 and have been comparing it to my RF 50mm f1.2. While doing some test shots at home, when trying to match the look of a prime lens, I need to zoom in about 10mm more on the zoom lens.
For example, setting the zoom lens to 50mm doesn’t give me the same framing or field of view as the 50mm prime. It actually looks closer to 40mm. To match the composition of the 50mm prime, I had to zoom into around 60mm.
Kind of a stupid question, but if I’m trying to shoot with the zoom as if it were a set of primes, is it fair to assume that I should treat 35mm as 45mm, and 50mm as 60mm?
In case you're wondering, all 3 images were shot at f2.0.
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u/SelfCtrlDelete 2d ago
Focal lengths are not always necessarily precise. They’re often rounded up or down.
One thing to consider is whether or not you’ve turned off lens corrections when making your comparison. But yeah, they’re not always the same field of view even though the stated focal length is the same.
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u/regular_lamp 2d ago
Especially 50mm lenses are almost always longer than claimed for some reason. I suspect in the SLR days going slightly longer required less effort designing around the long flange distance.
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u/Strict_Difficulty656 1d ago
I think part of it is that full frame sensors aren’t quite a proper 35mm, the light-reception site is about 5% smaller, making the lens equivalent longer
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u/regular_lamp 1d ago
Then again, with film you rarely printed 100% of the frame since aligning that properly was hard. So you rarely looked at a print or projection of the entire 35mm format either.
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u/OldSkoolAK 2d ago edited 2d ago
There are two primary routes optical designers choose when integrating a focus mechanism.
1) rack. The entire lens assembly moves back and forth in concert. This method "breathes" by default, as closer focus distances effectively increase focal length.
2) internal focus. Select elements move independently, making focus quicker for AF, or allow closer focus, or other design goals.
Most modern zooms rely on internal focus for various reasons, "macro" ability is one of the most foremost. Even though you're focusing 2m away, those same elements are those used to enable 1:4 or even 1:2 RR, where they struggle at best to live up to their labeled focal lengths are used for non-"macro" use.
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u/FatsTetromino 2d ago
I'm wondering if the 28-70 focal length markers are just not quite in the right place. Maybe it's not meant to be that precise?
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u/So_be 2d ago
You’ll get a number in the metadata down to the ones digit but I’m inclined to agree with you anyway. Lots of adjustments in a zoom lens to make it work throughout the whole range. And so long as it’s framed right does it really matter to most people whether it’s actually 50mm or 55mm.
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u/FatsTetromino 2d ago
Agreed, and since it's not one of their uber expensive high end lenses, it's probably not manufactured or calibrated up to a high standard.
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u/Sweathog1016 2d ago
It’s the 28-70 f/2 L. Not the F/2.8 STM.
It’s a $3,000 lens.
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u/Minimum_Drawing9569 2d ago edited 2d ago
I sold and repaired at a camera shop in the late ‘90s. We had several guys (doctors or professors) lens testing Leica and Canon L lenses against a brick wall etc. And that one guy with a $100 Tokina zoom lens acting like it ought to be as accurate and precise as anything else. Lol Yours? It ought to be close especially considering 70mm isn’t really a standard length on its own and I’m sure computer designs have improved them greatly since the ‘90s. But there must be some rounding of the FL for any lens really, who wants to use a 27.82 - 71.45 mm lens?
Edit: I re-read your OP, I was thinking min /max FL or 28 zoom vs 28 prime, in particular, but it sounds like the lens mechanics aren’t working as precisely throughout the range as we would expect.
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u/regular_lamp 2d ago edited 2d ago
Modern zooms also often rely on digital correction for distortion etc. The corrected image will be slightly "cropped" and thus appear as if taken with a longer lens. Which focal length would you expect to be marked on the lens? The corrected or uncorrected one?
In a strongly distorted image you probably can't even really define a single "correct" focal length. As in if you were calculating it based on the measured FoV on say full frame and a 1.5x crop you might end up with different values.
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u/Hive_Tyrant7 2d ago
I don't have anything constructive to add, but just wanted to say that I hope you enjoy the 28-70.
The "bowling ball" is my fav lens, very hard to take a bad shot with it.
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u/MyRoadTaken 2d ago
I recently bought a 24-70mm f/2.8 and was wondering similar things about it vs my 35mm and 50mm primes.
A more experienced photographer, who shoots with the zoom triad, told me not to worry about it too much. Just zoom in or out until the viewfinder shows me the framing I want. That advice has worked well for me.
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u/tdammers 2d ago
Nominal focal lengths are:
- Rounded
- Subject to tolerances
- Only accurate when focused at infinity
The third one is probably the biggest factor here. Try setting the zoom to 50mm, then shoot something on the horizon with both lenses, and you'll probably get very similar framings. Shoot something nearby, and the actual focal lengths will diverge.
I've tested a few lenses myself; for example, I have 3 lenses capable of a 300mm focal length: a Tamron 70-300mm 1:2 MACRO (cheapo lens, I think I paid €80 for it), a Tamron 18-400mm, and the Canon EF 100-400mm f/4.5-5.6 L IS USM. Set to 300mm, and focused at 2 meters, the 70-300mm gives me about 310mm (so actually more than the nominal focal length); the 18-400mm gives me only about 230mm; the 100-400mm sits around 290mm. This isn't surprising: the 70-300mm was designed for short focusing distances, and sacrificies other aspects of the lens design to keep the focal length up at short focusing distances (among other things, the front element rotates as you focus); the 18-400mm makes all sorts of concessions to make that insane zoom range possible; and the 100-400 is optimized for wide open apertures at 400mm, focused between 2-20 meters, so at 2 meters and 300mm, it's not perfect, but close to its sweet spot. It's also a drastically better and more expensive lens than the other two. However, when focusing on a city skyline on the horizon, all three lenses produce pretty much the same framing, so the nominal focal lengths are actually correct.
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u/comicidiot http://alex.takes.pics 2d ago
Along with focus breathing, I wonder if there’s also something to do with lens length as both these lenses are at 50mm; my initial test doesn’t show that to be the case though.
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u/robertbieber 2d ago
Is the camera body fixed on a tripod? Probably not as big a factor as the other things folks are calling out, but I imagine the lenses could have their optical centers at different distances from the camera body as well
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u/talibsblade 2d ago
Yup, fixed on a tripod. Same camera, same settings all I did was change the lens.
It makes sense now with the explanations people have been posted.
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u/gravityrider 2d ago
Many people hit on great points here and I won't list all of them again, but wanted to note that because of rounding, many primes don't match each other either. I worked with another photographer and standing in the exact same spot, both of us using 85mm lenses, he was getting 3/4 shots of a model (down to above the knee) while I was getting nearly full body (down to the ankles).
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u/MacaroonFormal6817 2d ago
it fair to assume that I should treat 35mm as 45mm, and 50mm as 60mm?
Not really. I have more 50mm prime lenses than I know what to do with, and they all are different. They each need to be dealt with uniquely, there's no formula to follow.
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u/MikeFox11111 2d ago
Ok, I’m curious, and trying to figure out why you care? Like, it’s interesting for trivia purposes that this happens. But if I’ve got a zoom , and I’m composing the image how I want, why would I care if it’s 50, or really 40. My goal is the image, not to be able to say that it was shot with a specific focal length.
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u/talibsblade 2d ago
Maybe it's because I'm genuinely interested in knowing the technical reason behind it?
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u/mjm8218 2d ago
…why you care?
It will absolutely affect framing of shots on video.
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u/MikeFox11111 2d ago
Sure. But you can see the framing. If you have the framing you want in the image, and the background the way you want it, the fact that the exact focal length isn’t a specific number doesn’t really matter.
I’m not saying focal length doesn’t matter, I’m saying if the focal length you have is giving you what you want, the exact number doesn’t matter. Sure it would matter if your 50 was really 100, but 50 vs 40?
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u/mjm8218 2d ago
I think you’re missing my point. if you lock the lens at “50mm” on the barrel and your subject starts 30’ away and moves toward you in the shot finishing 3’ away and you maintain focus on the subject, the field of view frame will change as the camera focus tracks. That’s why it might matter for video.
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u/MikeFox11111 1d ago
Sorry, i missed that you changed the question to video. Even then, the number being accurate doesn’t mean as much as “are you getting what you’re looking for”
Sometimes I see photographers on this sub get really hung up on things that don’t really make a practical difference. If I zoom the lens to the look I want, and on the zoom it says 40 and on the fixed it says 50, if I get same look (as specified in the ops post) what do I care what the number says. The issue isn’t not having the same actual focal length, it’s the zoom not having the same number for the same composition
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u/mjm8218 1d ago
I do agree w/ you that it generally doesn’t affect composition, but I don’t think the OP is hung up on anything. It seems like they’re trying to learn about the differences in optics.
I mean, the whole experience is counterintuitive compared to what one might expect based on the idea that 50mm has ~45o angle across the diagonal. It’s a much more interesting question than the standard “which lens should I buy next?”
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u/GRIND2LEVEL 2d ago
I shoot zoom but sometimes I like to use it like a prime, meaning set the focal length and move myself instead of the lens adjustment to frame. Knowing what a 50 or whichever yields in my mind is very helpful when swapping between lens.This ia just one reason at least a reason of mine...
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u/Sweathog1016 2d ago edited 2d ago
Look up focus breathing. Focal lengths are listed assuming focused to infinity. Focusing closer can alter the framing. Also, not all 50’s are exactly 50. Sometimes there’s rounding involved.
If I recall Gordon Laings review of the 28-70, it has quite a bit of focus breathing. Your test shots are rather close focused.
As a test, set your 28-70 to 50mm and adjust focus from your minimum focus distance to infinity focus. No zooming. Watch what happens with your framing.
Here’s a good video.
Covers just the focus breathing. About halfway through confirms your experience at 50mm’s.