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I'm newbie in the world of lenses ...
I have 24-70mm 2.8. I never got a chance to use 70-200mm. One friend told me that you just have to walk two or three steps forward to get 200mm equal range with your 24-70 lens.

Is that correct explanation and if this is so, would that give me same depth of field as in 70-200mm?

Update: These both lenses are mostly used for Portraits, which doesn't need distances in miles or photographing Moon. And I hope, portraits means within the distance of few meters...

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How much you have to "zoom with your feet" (walk) depends on the distance to your subject. The FoV (field of view) varies with the inverse of the focal lenght (for a given camera), so going from 70 to 200 mm focal length means that your FoV becomes about a third. So your subject will appear about 3x larger in the image. To get the same effect by walking, you'd have to reduce the distance to a third of the original.

But there is a big difference between changing the focal lenght and changing the distance.
Changing the focal lenght doesn't change the perspective, so the relative sizes of items in your image stay the same when you zoom.
When you walk closer to your subject, items that are farther away will appear smaller compared to your subject. So you change the relative sizes of the items in your image.

You can test this easily enough with your 24-70: take 3 pictures: one at 70 mm, then one at 24 mm where you stay in the same spot, and one at 24 mm where you approach your subject to get it the same size as in the 70 mm image. Now, enlarge a copy of the first 24 mm image so that your subject is the same size as in the 70mm image, and crop to the same size as the others. Now compare those 4 images.
If you also want to compare the Depth of Field, make sure you stay pretty close to your subject, and use F2.8 (at 24mm, your depth of field can eadily go to infinity at larger distances, especially when stopped down to F5.6 or higher, see below for a link).

Usig a DoF (Depth of field) calculator for F5.6 gives the following:
70mm @ 10m : DoF ~ 3.5 m,
200mm @ 30m : DoF ~ 3.8 m, @ 10 m : DoF ~ 0.5 m

So with approximately the same subject size, DoF is about the same, at a given focussing distance, the longer focal lenght has a much smaller DoF.

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  • Yeah, my error, thank you :) I mixed up the two, corrected now – remco Apr 18 '18 at 9:39
  • Therefore, in order to photograph something 1/4 mile away, you'd need to step forward 440 feet. It might be a bit difficult at the Grand Canyon. – chili555 Apr 18 '18 at 22:44
  • Here's a link illustrating the effect focla length has n perspective: petapixel.com/2012/08/12/… – Saaru Lindestøkke Apr 19 '18 at 10:40
  • @SaaruLindestøkke No, that's the effect different shooting distances have on perspective. – Michael C Apr 20 '18 at 5:33
  • I'm not sure I understand. The different shooting distances are possible due to the different focal lengths, right? – Saaru Lindestøkke Apr 20 '18 at 6:35
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The focal length of a lens reveals its power. The longer the focal length, the larger the images of objects will be. Your camera is compact digital; I know this because it came with a zoom lens with a focal length range of 24mm thru 70mm. Fist I want you to know that if you take a picture with the zoom set to 24mm, your image will be in the realm of wide-angle. Conversely, if you take a picture at the maximum zoom setting of 70mm, your image will be in telephoto territory. Now the “normal” setting for this camera is 30mm. In other words, 30mm is not wide-angle and not telephoto. What you want to know is: Instead of investing in a telephoto, can I accomplish the same thing by simply moving closer to my subject? The answer is yes. However there is more to the story.

If you mount a 200mm the image size produces is about 3X bigger (200 ÷ 70 = 2.9. Translated: You are imaging a bird from a distance of 100 feet with a 70mm lens. Dissatisfied because you can’t see detail in the feathers, you mount a 200mm telephoto. Now the bird images as if it were 100 ÷ 3 = 33 feet away. Had you merely waked, closing the distance between you and the bird, your image with the 70mm would display the same detail. However, the two shots, one from 100 feet and one from 33 feet display a different perspective. Perspective being the relationship concerning the size of foreground and background objects. These will be different because you changed the camera to subject distance.

Lenses are tools we use when we make pictures. If you are in the bleachers at a sporting event and you want to take pictures of the action, a telephoto is what’s needed because you can’t move in closer. On the other hand, you are imaging your small apartment to show your friends how you live. The rooms are small, so you must mount a wide angle lens to enscope the vista. Again lenses are tools, serious photographers have their gadget bag filled with lenses and camera accessories. If you budget is limited we learn to get by with what we have. Surly closing the camera to subject distance to get more detail is a valid technique.

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One friend told me that you just have to walk two or three steps forward to get 200mm equal range with your 24-70 lens

The other night I took a photo of... the Moon with my 200mm lens. I am pretty sure that I need more than two or three steps forward to get the same framing with a smaller lens.

No. That explanation has no sense.

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The distance for the "same" view is the ratio of the focal lengths.

The field of view from a 600 mm lens is the same as a 200 mm would see if the 200 were at 200/600 or 1/3 the distance of the 600 mm lens. The 600 mm can be back 3x further than the 200 mm lens (for same field of view).

EDIT: I failed to mention that Yes, Depth of Field is considered often the same (same computed span) when focal lengths are adjusted to lens focal length ratios as said, to show same subject size. Assuming of course, both lenses use the same f/stop, and both use the same sensor size. This equal DOF can vary, but it is most often true, and will be true that DOF will be the same if the shorter lens is focused at less than 1/4 of its computed Hyperfocal distance.

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    Using “field of view” in this way is misleading. The fields of view of different focal length lenses on the same camera are never the same. It is correct to say that the field of view of at the plane of focus is the same for a 200mm lens 1/3 the distance to the subject as that of a 600mm, but it won’t produce the same image because of different amounts of foreground and/or background field of view. – scottbb♦ Apr 18 '18 at 15:11
  • I believe it is a very correct term. Angular field of view is independent of distance (and is computed from focal length that is assumed correct, but yes, different lenses can approximate it, not always precise, but we normally don't know a better number). But Dimensional field of view is computed at the specified distance. Yes, background and foreground do have their own distances. Yes, perspective will be different with distance. But subject size is the same when distance is adjusted for lens focal length ratio as said. – WayneF Apr 18 '18 at 17:37
  • Saying "... the 'same' view... " as you do in your opening sentence strongly implies the same shot, including the foreground and background. – Michael C Apr 18 '18 at 18:11
  • my objection is mainly that using "field of view" unqualified is somewhat misleading. Yes, AoV is more precise for the dimensionless sense. But considering OP is asking with little knowledge of photographic principles, using FoV without qualification can be problematic. As an aside, I'm not sure your edit clarifies things any more (by introducing DoF where OP might not even be aware of the issue). Your comment, however, is right on point: the perspective is different (and different background compression), but the subject size is the same. – scottbb♦ Apr 18 '18 at 18:14
  • Scott, I did NOT say Same Picture. Same picture involves standing in the same place. My discussion was about same field of view, as it is commonly used. Dimensional field of view at the specified distance. – WayneF Apr 18 '18 at 18:25

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