Zoom from x to y equals walking this dist w/prime

[eric_perlberg]

We've all seen the comment written on zoom questions, "I use primes

and zoom with my feet". As a person who has always used camera zooms

to footzooming it never even occurred to me until last night to try to

figure out the number of steps (I know, not really a scientific

measurement) involved in zooming from say a 28mm lens to a 35mm lens

or a 35 to a 50. I know too that zooming with a zoom lens and

footzooming are not exactly the same but I'm thinking here in terms of

framing a picture in the view finder.

 

Anyway, I gave it whirl and was surprised with my very makeshift test

to see that to go from 28 field of view to 35 field of view was just a

step or two and 35 to 50 maybe double that. Assuming I haven't done

something totally stupid or illogical it then occurred to me that

there must be a way of figuring this out mathematically rather than

just focusing on a few objects and taking a few steps and changing the

focal lengths on my zoom lens. Anyone know a formula?

[antonrussell]

No, but it would also very much depend on the distance to your subject. Standing 100m away and changing from 35 to 28mm you would have to walk further to get the subject back up to the same size in the viewfinder, than if you were photographing a subject 50m away.

[charles_stobbs3]

Zooming doesn't change perspective but walking does. So the two aren't equivalent if perspective is important to the image you want.

[eric_perlberg]

<i>Zooming doesn't change perspective but walking does</i>

<p>

I understand that. I tried to say as much in my post, though inelegantly when I mentioned that the two aren't the same but I was interested in the framing of the image in the viewfinder.

<p>

The way I visualise it at the moment, thanks to Antons point reminding me that distance from the film plane or the lens is a critical factor, is that two different lenses have two different angles of view going out from the camera in a cone like fashion. When object A is x size at y distance from the camera in lens 1, there must be a way of calculating how to maintain size x at z distance with lens 2. And then I could figure out how far one would have to zoom with their feet to go from for example a 135 lens to a 200mm lens with an object that fills 40% of the frame. Unfortunately, maths is one of many weak points.<p>

What puzzles me is how many times people write in on questions about zoom lenses and say somewhat disparigingly something like "get a prime lens, sharper, cheaper, faster, lighter and zoom with your feet" Nobody ever really says, "that doesn't work except in limited situations". All they do is point out the perspective issue which is pointed out above. Whereas my mental reaction till now has been, yeah, stand in the middle of traffic and take your photo, the drivers will understand... or yeah, hike over that river, no sweat, what's your hurry. But then I realised, not having done any prime lens photography, that I didn't know if my quick flippent thought reactions are accurate. Hence my question and its corrolary ...so who are these people who zoom with their feet and what sort of images are they making that little issues like traffic, rivers, fences, etc don't get in their way?

[jbq]

It all depends on the distance for the subject. If the subject is at infinity no amount of walking around will change the field of view.

 

It's all an issue of magnification. A full-body standing portrait shot from mid-height is roughly 1:50 magnification (1800mm subject, 35mm film). That's 1.4m away with a 28mm, 1.75m away with a 35mm, 2.5m away with a 50mm - barely more than a step from 28 to 50).

 

In my opinion, when it's even possible, "zoom with your feet" isn't worth using more than to cover the difference between consecutive primes, i.e. not more than the normal way you'd move around to get the correct framing with a given prime.

[awahlster]

Well actually it si very simple to figure this out based on the horizontal angle of the optic's in question:

 

20mm 84`

 

24mm 74`

 

28mm 65`

 

35mm 54`

 

50mm 40`

 

85mm 24`

 

100mm 20`

 

135mm 15`

 

200mm 10`

 

I don't know the math formula to figure this out but it is very easy to plot out on grid paper with a protractor.

 

Personally I think the whole thing is a City photography conscept. Because it sure doesn't work out in the country. Least not in my experiance. kind of hard to jump the fence, swim the creek, step off the canyon edge, Get the little animal to stand still while you walk up to it, Etc.

[Matthew Currie]

I think it really does depend on the subject as well as the location. If you're taking a picture of, say, a chair or a vase of flowers, it's easy enough to zoom with your feet. For things of a certain scale, a 50 mm. lens is hard to beat anyway. But the inability to sneak up on an eagle's nest with a 50 mm. lens should be pretty obvious. And you also can't take much of the interior of a room from from ten feet down the hall, and if what you want in a landscape is what shows in a 20 mm. lens, chances are you won't be able to duplicate it with anything else even if you can get there with your feet, because other objects will intervene.

 

Sometimes it's just really handy to have a zoom.

[eric_perlberg]

<i>Personally I think the whole thing is a City photography conscept</i> <p>

Even in the city its a mixed bag. I know candid people shooters on city streets use 28, 35 and 50 primes and belittle zoom users, perhaps that's what you're alluding to, but I do a lot of city stuff, urban landscapes and urban details and I just couldn't make primes work for me, at least to this point. I know Jay Maisel (well known gritty NYC advertising photographer) says that his city stuff is always with 70-200 zooms.<p>

Now to find some graph paper...

<p> thanks for the feedback so far.

[tommyinca]

If you stay away from the macro-distant range and Let D1=Object to lens distant, then

 

D1=(Mg+1)Fl

 

Where:

 

Mg = Magnification

 

Fl= Lens Focal Length

 

You can find the details in www.normankoren.com. Get ready for heavy math however.

[socke]

Even in street situations the difference between 50 and 35 may be a parked car between you and your subject :-)

[charles_stobbs3]

I think Mark's table needs a little more explanation. With, say , a 100 mm lens while the subject may be the same size on either a 24 x 36 frame as with a 60 x 90 frame, a lot more area will be included in the the 60 x 90 image.

[craig_gillette]

On quad/graph paper you can lay out a series of angles, matching the lens angles - bisected horizontally (like for 20 degrees, 10 degrees above and 10 degrees below a centered line). Then you can use the parallel horizontal lines to compare where they cross the other lens angles.