Guessing Focal Length By Looking At A Photo

[httpwww.photo.netbarry]

Yes. There are many examples in 'how to' books and on the internet.

Those do not make it more plausible that guessing the focal length is possible with a reasonable degree of accuracy. So i appreciate the effort, but your link is useless.

Not useless actually and probably more useful then most of this whole discussion. It's not a "how too", it simply shows the same subject at different focal lengths. Sure, without back grounds to gauge compression it is difficult to make any accurate guess of focal length, but it does show how focal length effects the subject.

[q.g._de_bakker]

Not useless actually and probably more useful then most of this whole discussion. It's not a "how too", it simply shows the same subject at different focal lengths. Sure, without back grounds to gauge compression it is difficult to make any accurate guess of focal length, but it does show how focal length effects the subject.

You're turning the subject round to something else. It is not possible to guestimate focal length with any degree of accuracy, for the reasons given earlier. No matter that you can show photos taken through different lenses of known, different focal lengths.

So yes: useless.

 

Background compression, by the way, does not help. That is perspective, i.e. position relative to subject. Not caused by or an effect of focal length.

[httpwww.photo.netbarry]

You're turning the subject round to something else. It is not possible to guestimate focal length with any degree of accuracy, for the reasons given earlier. No matter that you can show photos taken through different lenses of known, different focal lengths.

So yes: useless.

 

Background compression, by the way, does not help. That is perspective, i.e. position relative to subject. Not caused by or an effect of focal length.

 

Mostly just commenting that most of this isl useless anyways, who really cares if you can guesstimate the focal length. I appreciate your remark that compression is a function of position relative to subject, but while true theoretically in practice a 21mm only shows the same compression as a 200mm in extreme situations. In other words in real life you would only come into play with extreme cropping and resulting imager degradation. Though if closer in focal length like a 50 and a 75 or 90, you might not have to crop overly much to create equivalency. I think the link that was given actually is quite useful for showing how different focal lengths depict a subject. And maybe I have given the conversation a twist, but it probably needs it :)

[glen_h]

Again watching an (American) football game, there are some shots low to the ground from one end of the field, toward the other end, which greatly distorts perspective. Looking at the yard line numbers on the grass allows one to figure out the distance.

 

But without the reference of the yard line numbers, it would be hard to know.

[q.g._de_bakker]

Mostly just commenting that most of this isl useless anyways, who really cares if you can guesstimate the focal length. I appreciate your remark that compression is a function of position relative to subject, but while true theoretically in practice a 21mm only shows the same compression as a 200mm in extreme situations. In other words in real life you would only come into play with extreme cropping and resulting imager degradation. Though if closer in focal length like a 50 and a 75 or 90, you might not have to crop overly much to create equivalency. I think the link that was given actually is quite useful for showing how different focal lengths depict a subject. And maybe I have given the conversation a twist, but it probably needs it :)

The uselesness of a guessing app is the subject of this thread. That you perhaps think it boring makes it no different.

 

And no, saying that you can produce series of photos that show what you get using different focal lengths is really, really not more interesting. It's in fact absolutely uninteresting. A textbook sample of a "Duh!!!"-case. Who needs that?

 

No, a 21 mm always shows the exact same compression as a 200 mm. Always.

The image the 200 mm produces is also in the image the 21 mm produces. There is just more subject matter in the 21 mm image.

[httpwww.photo.netbarry]

"No, a 21 mm always shows the exact same compression as a 200 mm. Always.

The image the 200 mm produces is also in the image the 21 mm produces. There is just more subject matter in the 21 mm image."

Right, and like I said, to see that you're going to have to crop the crap out of an image. it given similar distance from the image. And I don't need your permission to add a thought to a thread.

 

"And no, saying that you can produce series of photos that show what you get using different focal lengths is really, really not more interesting. It's in fact absolutely uninteresting. A textbook sample of a "Duh!!!"-case. Who needs that?"

 

Actually more interesting to me then your pronouncements.

[q.g._de_bakker]

Well, Barry, that you are so bored by the subject of this thread that you would prefer to talk about something even more boring makes one wonder what you are doing reading, and responding to, this thread.

Just start a new one, about your discovery that you can produce series of photos that show what you get using different focal lengths.

[arthur_gottschalk]

I showed an environmental portrait I made with a Nikon F100 to a friend who at one time was a filmmaker but otherwise not a photographer. " That was made with a 24mm lens", he said correctly and much to my astonishment.

[Karim Ghantous]

I think some part of this discussion is getting lost in the weeds. Here are two facts:

 

1. What we call compression is a product of the distance between the subject and the camera. Focal length is irrelevant. But telephoto lenses magnify the scene, giving a far better image than a crop, for obvious reasons. Oh, and wide angle lenses don't "stretch" a scene any more than a telephoto would. I'm pretty sure that all participants in this thread understand that. (N.B. There are industrial optics which are 'front telecentric' which are exceptions, but we don't need to concern ourselves with those here).

 

2. You can estimate fairly well the focal length used for any scene, provided that you have sufficient reference points, and that you know the gate size, or image format.

 

Perhaps you spent thousands on MFD equipment because you thought that medium format has more compression. Perhaps you bought a high end cinema camera with a 36mm sensor, on the same premise. Well, you're out of luck, and you're also out of luck if you insist that fSpy can't work.

[glen_h]

I showed an environmental portrait I made with a Nikon F100 to a friend who at one time was a filmmaker but otherwise not a photographer. " That was made with a 24mm lens", he said correctly and much to my astonishment.

 

My thought is that professional filmmakers are the ones that would be able to do it.

I suspect mostly subconscious, though. If you asked what parts of the scene give it away,

I suspect he wouldn't know.

 

I am not at all sure what they teach in film school, but visualizing a scene, and knowing what it

will look like on film, has to be near the top. That includes knowing what distance and lens to

use to get it to look like that.

[q.g._de_bakker]

Second response: "not if the picture is cropped".

 

Gary also stressed the thing about perspective in that post.

 

And that, plus what was said about the difficulty of guessing, is what there is to be said about this topic.

Edited February 1, 2022 by q.g._de_bakker

[Karim Ghantous]

I suspect mostly subconscious, though.

Probably, yes. "Been there, done that" a thousand times, kind of thing. And in cinema, there's a lot of planning and deliberating. And so even if focal length is chosen at the last moment, it's a very conscious decision, kind of like knowing how to tell a B flat from a C. That eventually becomes subconscious, and you just know without thinking.

 

Second response: "not if the picture is cropped".

All photos are crops, because there is no such thing as a universal frame. Practical limits aside.

[q.g._de_bakker]

Karim,

You do not understand. That is a given, yes.

But what about a crop of a photo on format F, taken with a lens of focal length X to the angle of view you would get had you used a lens of focal length Y*X?

What focal length could you guess to have been used on the unseen crop, judging by the crop of a crop that you were presented with (either knowing or not knowing that what you were looking at is a crop of a crop, or not)?

[glen_h]

(snip)

All photos are crops, because there is no such thing as a universal frame. Practical limits aside.

 

I think movies are mostly not cropped, though showing IMAX films on non-IMAX screens does need cropping,

and the other way also might. Also showing movies on TV.

 

But otherwise if it is cropped, one should give the relative focal length to the frame size after cropping.

[Karim Ghantous]

Karim,

You do not understand. That is a given, yes.

But what about a crop of a photo on format F, taken with a lens of focal length X to the angle of view you would get had you used a lens of focal length Y*X?

What focal length could you guess to have been used on the unseen crop, judging by the crop of a crop that you were presented with (either knowing or not knowing that what you were looking at is a crop of a crop, or not)?

I don't mean to go around in circles, but I can solve your problem. You can use fSpy even if you don't know the gate size. Not because you can identify the lens used (you can't in this case), but because you can identify which lens you SHOULD use to replicate the geometry of the source photo.

 

Again, you need sufficient reference points. A bird against a blue sky is going to give you pretty much no reference points at all.

[q.g._de_bakker]

Erm... No, Karim.

For all of the reasons mentioned in this thread. You might want to read it.