Determine "effective" focal length

Capturing
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So, started out with the Z50II with the 18-140mm kit lens, and I’m having a blast so far, but… I’ve known from the start that I wouldn’t end up with just the one lens, and I thought I’d go with something longer (to get more birding in), or something shorter, as I’ve shot a few pics that I’m quite happy with on my phone, with something that’s 35mm-equivalent to 18mm (so that’d require a 12mm on my Z50II).

However… Simply looking at “what length did I shoot most pictures at” is misleading, because I’m an avid cropper. That tells me that A. I probably need to use my feet more and compose better, but B. also that getting an ultrawide-angle first might not be my best fun-per-spend. I might have better use for something in a length that the kit lens could cover, but a lot faster.

So, to get to the point of this wall of text: I need to calculate an “effective” angle from a cropped picture, keeping in mind that it’s not exactly the same thing and all that.

Let’s take this one, for example:

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image747×1151 167 KB

That’s a portrait-oriented picture that I’ve cropped into landscape orientation to tighten the shot and get rid of an overblown sky. It was shot at 18mm, but I might as well have taken the shot in landscape at… what length?

How do I calculate this? Original shot was 5568×3712, the crop is 3045×2030. Just calculating the ratio between sizes (5568/3045) is roughly 1.8, but that doesn’t seem right, I’ve chopped more than half the image off. Ratio between the left side of the original and the left side of the crop (so the longest vs the shortest pixel lenghts) is a ratio of about 2.7, which feels more on the mark, but at the same times feels like an odd way to go about it.

Am I overlooking something? Any tips/tricks here?

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I am curious …

  1. why not make the perspective correct when capturing already? e.g. when you crop this much towards the edge of the frame you might have a lot of distortion from the lens.
  2. why does the effective focal length after cropping matter at all?
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If I understand correctly: You want to go through your photos and figure out what focal length lens would give you the same field of view (or is it angle…i forget) without any cropping?

I recall looking into the same question a while back. IIRC you would need to divde the long edge of the original frame by the long edge of the crop and multiply that by the focal distance. Since you are trying to get a “landscape” orientation, you need to transpose sides.

BUT…doing it this way only scales on the long edge, so your vertical edge of the crop might be significantly different. It might be narrower or wider depending on the crop size. You could try and calculate on the short edges, but the long edges might be wonky after that…In other words, if the crop is a different aspect ratio than the sensor you are out of luck. You will still be cropping.

I recall getting the calculation off the web, so take everything I said with a grain of salt.

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I think that is what @dnzm is asking. Sometimes it’s not easy to “zoom with your feet.” But @darix does have a good point.

As a quick exercise:

From his numbers I get a crop factor of ~ 1.8 which would indicate a focal length of 18 mm x 1.8 ~= 33 mm. If you wanted a prime for this, a 35 mm lens would be close.

Crop Factor

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I’d like to, but I’m still somewhat bad at this, basically. :wink:

Partly out of curiosity, partly because it might play a role in the selection of a next lens. If it turns out that most of my images are around 20-25mm (just pulling a number out of thin air here), that’d mean that I’d look towards that range.

Exactly.

Yeah, I think that’s exactly what I’ve been doing, which in the above example, gives me “it would have been the same image, more or less, with a 50mm without cropping”. Which is quite a difference from the 18mm it was shot with.

Yeah, I understand that — once I start cropping XPan landscapes out of a 2:3 portrait shot, all numbers will be off.

Also, as @darix already pointed out, different lenses will have different levels of distortions, so it’s never going to be a 100% clear answer. This is just to get a ballpark figure.

OK, this gives me some confidence that my initial idea wasn’t that out there, and no worries, I will be taking any numbers that come out of it with several grains of salt…

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As @Doug-Phoenix said, it should be roughly 33mm (so a 35mm would be in the ballpark)

Your Long side in the original is 5568 and 3045 (I am guessing from visual inspection) in the crop.

(5568/3045) x 18mm = 32.9mm

When I said you need to transpose, I meant if you keep the WxH consistant. Using the example photo --To keep things neat and tidy the original would be 3712 x 5568 and the crop 3045 x 2030. You need to transpose the original to be 5568 x 3712 (which is how you wrote it, but can be easily confused when applying to other images)

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With respect I feel the question you are asking is flawed. you have an 18-140 lens on a crop sensor camera. 18 is a wide angle and 140 is a medium telephoto . Why not just zoom in on the sample picture here to get your composition correct in the camera. Turn the camera in the horizontal rather than switching orientation in cropping. I suspect the zoom would be mid-range around 40-50mm for this shot. It seems you need to focus on composing images correctly in the camera to improve your photography skills because you by your own admission have to crop often.

I personally travel with a 16-300mm Sigma lens on my Canon R7. If I need wider than 16 mm I normally get away with panorama shooting rather than switching to ultra wide lenses. If 300mm is too short then the R7 has lots of pixels so I can crop a lot and still get a good image. A 600mm lens might be nice for bird photography but I am very happy with the 16-300mm range. I aim to keep cropping to a minimum in processing.

Enjoy your photography and I feel the 18-140 is an excellent range on your camera.

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Presumably because he is looking at getting a new lens. How he could have got the framing in camera by zooming is not relevant to the question of what focal length a new lens would be most useful to him (given his average focal range of his existing shots).

When I was asking the same question, it was because I was dreaming about buying a prime.

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I guess I tried to answer that when I said the shot would have been 40-50mm which his current lens already has. I am also suggesting that panorama stitching can sometimes replace the use of ultra wide angle lenses (just sometimes). I am suggesting longer focal lengths for birding etc. But I don’t feel he is making appropriate use of the lens he already has if he is cropping images as in the sample he provided. Working on in camera cropping through zooming will improve his skill level.

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As to crop factor in a cropped image I would use diagonals rather than long or short sided. So 1.6 times your 18 mm = about 30 mm.

As to aspect ratio, most cropping options allow the User to keep that constant or not.

Hope this helps.

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same here: as long this is given, we will be croppers, means a recalculated focal lengths will not help at all in spite we suddenly could switch to use the correct focal length. me, i can’t…

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Buy a prime lens. No option to zoom will force you to become better at it :slightly_smiling_face:

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TBH unless you plan selling your zoom, I would buy outside of that range. and if you want to practice getting a feel for focal length and forcing you to zoom with your feet. go for either “set focal length in the morning and dont zoom” days. or get one of those cheap Nikon own primes. Because

  • knowing when to move
  • finding ways to make a shot work despite “I wish my lens is wider or longer”. which might involve looking for different angles e.g.
  • at which distance from my subject do I feel comfortable.

All those are skills/experience you can/need to learn. And it will slow down your photography in the beginning.

e.g. a 35mm FF equiv lens can get you very nice personal feeling portraits. but it also means you are really close to your subject. If you arent comfortable with that. an 85mm equiv lens can be better for you with the trade off that your shots feel a bit less intimate with your subject.

( I used to be someone who carried around 24-70+70-200 (2.8) everyday. and now I dont even own a zoom lens anymore)

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It is right, you are comparing lengths/distances, not area.

That said, I don’t see any problems with mild cropping. In most situations you don’t control everything and you just get the shot. Keeping 1/2 of your 20M pixels is still plenty for screen viewing and decent prints.

Also, if you already have a zoom lens that covers the focal length you need, I would only buy something if it is either

  1. smaller,
  2. has wider aperture,
  3. or something extra that you need (IS, weather sealing, etc).

For this particular lens, my understanding is that it satisfies (1) and (3), so maybe you want to get a prime for your favorite focal length.

Personally my favorite is 50mm equiv, so I would get something like the cheap TTArtisan AF 35mm f/1.8 for this camera. Or the 27mm which is a bit wider but perfect for street.

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nikon itself has some rather cheap 2.8 primes to get into it. was really surprising.

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You’re quite right, of course, and this has come up multiple times in the thread as well. There’s no substitute for proper framing. Also, yes, the 18-140 is an excellent lens, I’m having tons of fun with it, the range is great, and the pictures it takes are certainly adequate.

The reason I started this thread, was to find some guidance in how to determine what, if and when the moment comes up, would be a sensible prime length to grab. Not because my current lens is bad, it’s not, but I do want something faster, at some point, and really fast zooms cost an arm and a leg. I’m simply too attached to my extremities at this point in time. :wink:

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Yes, but do you really need them? A f/2.8 constant zoom is considered fast, but even at the tele end (assuming it reaches there) it would give you (6.3/2.8)^2 \approx 5 times more light. Unless you are doing dawn/dusk wildlife, I am not sure it is worth it. Tele gives you a lot of subject separation already.

I think that your “problem” is that Nikkon really nailed it with this lens. Versatile, sharp, lightweight. Yes, there are better lenses, it could have weather sealing, etc, but they found such a sweet spot. It is amazing how far kit lenses have come.

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It’s really fun to see the different responsed and the recurring themes, as well, I really do appreciate them all!

I’m fairly sure that there’s a lot of skill and familiarity with the kit (or, rather, lack of both) on my end, and that I’m not just going to buy my way out of that gap. But as for supplementing the kit I have now, that’s what I’m currently thinking about.

The “limit yourself to one length for the day” idea might be a smart one, actually, the only thing a prime would bring to the table is More Light, which is something I actually would have liked to have in a bunch of indoor shots. I don’t think I’d get rid of the 18-140 in a hurry, it’s just so convenient to have all the range when I’m out and about and not actually aiming for certain shots.

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Your range for the next photo walk is 24mm ( 36mm FF equiv)!

Let us know how it goes.

Oh should we make it a new capture challenge?

Vote here [Poll] Focal length capture challenge

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I forgot one important bit:

when you do such reframing as in the starting post. Try to put this in the back of your mind as a motif idea for the next time. and maybe even remind you when go for a photo walk.

Also consider “what do I want to show with this shot?” what is my focus for it? e.g. for your original framing I would have a hard time to pick what should have been your focus point in the shot. the reframing is a lot better.