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05-24-2017, 11:41 AM   #1
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focal length and space compression

Not sure if this makes sense, but I thought I'd ask. Telephoto lenses compress space, while wide angle lenses open it up, for lack of a better term. Or at least it seems to me they do that. I've been wondering if I can get the same "compression" on the crop and 35mm shot. So if I use an 85mm on crop sensor, that translates to 128mm on 35mm, right? Does it mean that the "look" of the same subject photographed with an 85mm lens on crop sensor be the same as that photographed with a 135mm on 35mm? Sorry if this is too confusing...

05-24-2017, 11:56 AM   #2
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QuoteOriginally posted by IgorZ Quote
Not sure if this makes sense, but I thought I'd ask. Telephoto lenses compress space, while wide angle lenses open it up, for lack of a better term. Or at least it seems to me they do that. I've been wondering if I can get the same "compression" on the crop and 35mm shot. So if I use an 85mm on crop sensor, that translates to 128mm on 35mm, right? Does it mean that the "look" of the same subject photographed with an 85mm lens on crop sensor be the same as that photographed with a 135mm on 35mm? Sorry if this is too confusing...

A crop sensor APSC camera crops the image in comparison to a 35mm FF camera. If you were to take an image with 35mm and crop it in PP to APSC size, you will get the same image.
Telephoto lens appears to compress subjects at different depth distances while a wide angle appears to open up distances regarding depth.
05-24-2017, 12:01 PM   #3
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For the most part, you are right that an 85 mm on APS-C will deliver the same "compression" as a 135 mm on full-frame 35mm.

Whether a lens is "telephoto" or "wide" is defined by the ratio of focal length to format size (usually the diagonal). A 85mm lens on APS-C is telephoto but a 85 mm lens on a 4x5 view camera is wide angle. It's the width the format projected through the length of the focal length that creates the triangular space imaged by the camera and the tendency for a photo of something at one foreground distance to encompass more or less width in the background distances.

Where things do get a little tricky with the "look" is if you add depth-of-field effects. These tend to imply that that, for example, the background of the 85 mm on APS-C will be a bit sharper than the background of the identical photo taken with 135 mm on full-frame 35mm.
05-24-2017, 12:05 PM - 2 Likes   #4
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QuoteOriginally posted by IgorZ Quote
the "look" of the same subject
Here is some visuals: Bildwirkung und Freistellung | Vergleich Freistellungsvermög? | Flickr

And here is a simulator: DOF simulator - Camera depth of field calculator with visual background blur and bokeh simulation.

I'd be surprised if looking at those things doesn't answer all practise related questions.

05-24-2017, 12:21 PM - 1 Like   #5
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Telephoto lens compress space because you move further away to get the same magnification, and thus relative distance (between nearer and farther away elements) is lessened. Similarly with wide angle lens you move closer and thus relative distances are increased. This effect has nothing to do with sensor size.
05-24-2017, 04:39 PM   #6
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QuoteOriginally posted by dms Quote
Telephoto lens compress space because you move further away to get the same magnification, and thus relative distance (between nearer and farther away elements) is lessened. Similarly with wide angle lens you move closer and thus relative distances are increased. This effect has nothing to do with sensor size.
agreed
this has to do with how far away from your subject you are, you can test this without even using a camera - the closer you get to a subject the larger it becomes and hence the smaller the background detail relative to the subject, different focal length lenses put you at different distances from the subject if you maintain the same size of the main subject in the frame. When they talk about "equivalent" focal lengths on full frame & crop its more to do angle of view I think. The notion of "equivalent" focal lengths IMHO is a nonsense, a lens with the same field of view will give you a different perspective.
05-24-2017, 05:11 PM   #7
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QuoteOriginally posted by dms Quote
Telephoto lens compress space because you move further away to get the same magnification, and thus relative distance (between nearer and farther away elements) is lessened. Similarly with wide angle lens you move closer and thus relative distances are increased. This effect has nothing to do with sensor size.
QuoteOriginally posted by Cee Cee Quote
agreed
this has to do with how far away from your subject you are, you can test this without even using a camera - the closer you get to a subject the larger it becomes and hence the smaller the background detail relative to the subject, different focal length lenses put you at different distances from the subject if you maintain the same size of the main subject in the frame. When they talk about "equivalent" focal lengths on full frame & crop its more to do angle of view I think. The notion of "equivalent" focal lengths IMHO is a nonsense, a lens with the same field of view will give you a different perspective.
To make it absolutely clear, the "compression" is actually the result of the difference in magnification between two subjects of different distances. And the fact that when you aim to achieve the same principle image size, the secondary or further away image has a different magnification ratio when compared to the first, because with a shorter lens you move forward to retain the same magnification and change the perspective.


I will show this with the following 2 examples

1) Consider two people each 1.5 meters tall one standing behind the other by 1 meter shot with a 100mm lens from a distance of 10 meters.

Considering image size = subject size * focal length / distance. The two people will be 15mm and 13.64 mm respectively. Or the person in front appears 10* bigger.

If you use a 50mm lens and shoot from the same distance you will get images 1/2 the size but the ratio of sizes remains the same,

What this shows, is perspective is constant for all focal lengths providing the photo is taken from the same position

2) But now move forward with the 50mm lens so that the person in front is the same size on the image, to do this, you are shooting from 5 meters, but now, when considering the image size of the person 1 meter behind, the person 1 meter behind is now 12.5 mm and the person in front appears to be 20% bigger,

This is because when you move closer with the shorter lens to get a comparable image size,
05-24-2017, 05:45 PM   #8
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There is no "compression", despite what Tony Northrup or Jason Lanier or whatever any vlogger says.

These people fail to control variables ... they change the lens, but they also change the shooting distance.

05-25-2017, 07:44 AM   #9
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It's a complex topic, but the appearance of compression, normal perspective, and expansion are vital components of composition. The appearance of apparent compression from 85mm on crop vs. FF does not change if you are shooting from the same distance in both formats - just the crop changes. The lens is 85mm in either format and the DOF characteristics are unchanged. However, greater magnification of the smaller format can actually intensify the perceived compression (assuming that distance is unchanged).

As you move to change perspective, then the variables change accordingly.

What is generally misunderstood relative to circle of confusion discussions is that you can simulate FOV and depth of focus characteristics transitioning from one format to another. In fact, this is the only valid argument for discussing aperture equivalence.

This article can be helpful: Sensor Size, Perspective and Depth of Field
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