Focal Length vs Focal Distance: It matters

[sk66]

I see a lot of discussion on primes vs zooms etc where the advocate for a prime (i.e. "nifty fifty") will state they can simply move closer/further to get the composition they want.

This is not entirely true. In fact, it is more "false" than it is "true". You will in fact change the "framing/zoom"...the captured FOV.

But, there are two very important factors that change with focal distance that are not being considered.

Perspective:
A wider lens will capture more "apparent distance" between objects within the FOV, a long lens will compress those distances. This is the same as moving nearer/further and not a function of the lens itself as such. Bt moving closer further will also change the FOV...(can be compensated for to a some extent by cropping when moving further away)

DOF:
A longer lens will have a narrower DOF from a given distance, and a shorter Focal Distance will give a narrower DOF. Sometimes this is a trade...If I move further away but use a longer lens my resulting DOF may remain very similar. However when trying to achieve a narrower DOF being "close" helps immensely. Of course this can be compensated for to some extent by chosen aperture, but when you're wide open (due to desired DOF or available light) all you have left is Focal Distance.

My point is: Lens selection is not simply a matter of getting the desired composition. Most advance to understanding that it is also about DOF and available light (usually choosing wide apertures)...but there is actually quite a bit more to it and it is a balancing act.
To get the desired composition with the desired perspective and the desired DOF, with the situationally available light will generally dictate ONE optimum focal length, aperture, and focal distance....the best most of us can do is "get as close as we can".

[Cypher]

I am going to bump this because it deserves to be read abit more. Nice read TFS

[prince]

Good read. I noticed this behavior when I use my tamron zoom lens but didn't understand how it all works.

Thanks for explaining.

[vandergus]

Yes, the "Zoom with you feet" adage is one that I wish would die. It only confuses the relationship between focal length and perspective, which is already plenty misunderstood.

The way I prefer to think about primes is that they force you to photograph with intention. When you walk out the door with a prime lens, you have to know what types of photographs you're looking for. You've already decided how you want your photographs to look and you've chosen a single lens that will let you achieve that without the distractions of extra focal lengths. That 50mm lens isn't going to get you every shot that you see, but you're not looking for those shots. You're only looking for photographs that are best at 50mm. No zooming with your feet, just move on to the photographs you came to get. (Note: not saying that this is the best way to photograph, it's just the type of process that primes encourage)

An example. Lot's of street photographers, especially the street photographers of yore, liked to use normal focal lengths, somewhere in the 35-50mm range on 35mm film. What these lenses do well is allow the photographer to portray scenes in natural perspectives. Everything looks well proportioned to everything else, just as if you were seeing the scene with your own eyes. They are, in effect, transparent. They put the focus on the subject of the photograph rather than the techniques and equipment behind the photograph, removing any possible distraction for the viewer.

[sk66]

Wow, Only 3 replies and no argument?!!
I've been in the woods at my cabin for the last 7 days (no electricity)...I expected more "discussion"....

[sk66]

Originally Posted by vandergus

Yes, the "Zoom with you feet" adage is one that I wish would die. It only confuses the relationship between focal length and perspective, which is already plenty misunderstood.

The way I prefer to think about primes is that they force you to photograph with intention. When you walk out the door with a prime lens, you have to know what types of photographs you're looking for. You've already decided how you want your photographs to look and you've chosen a single lens that will let you achieve that without the distractions of extra focal lengths. That 50mm lens isn't going to get you every shot that you see,

I agree largely...
One of the best things about a prime is "they force you to photograph with intention"...
This is good because it requires engagement of "creative vision" which requires developing such "vision" and then further teaches how to achieve that vision, thru trial and error if nothing else... The ability to "see" what you want to capture and then chose the tools and methods to reaching those goals; if you can do that consistently you have pretty well mastered the art.

[ravncat]

I think a best practice - is to not use a camera.

(seriously)

Ok - I should say, not use a camera to choose your photograph. Many people walk about with the camera up to the eye - sure, it helps you compose, it helps you get used to the field of view of a prime, but it becomes a crutch and makes it harder to see alternate compositions.

This is in line with the above topic - in that perspective is a key element of any composition. In addendum to what SK66 posts - I'd recommend walking through the world, looking for your photograph and then

First - Choose your perspective. (Controlled by your distance to the subject)
Second - Choose your framing. (Controlled by the field of view of the lens focal length)
Third - Choose your depth of field. (Controlled with the aperture)

You don't really need to bring the camera to your eye until step 2. And if you practice photographing this way - you might find yourself open to seeing more.

In this manner zooms are a really great tool - because they let you control your field of view at a specific position, giving you the perspective and field of view - but they may not have sufficient aperture for narrow depths of field. Zooms also tend to let people forget about how perspective works.

Primes are also a great tool because they force you to think about perspective - and give you lots of depth of field control.

The reality is, unless you have a full bag of primes, you're going to have to give up framing for perspective or perspective for framing (And zoom with your feet). I think the phrase is going to stick around, and there's not much we can do about it. Maybe the best we can do is to make people more aware of perspective.

(SK66, i totally agree with your primary post - hopefully, this works in concert with your post =D )

[sk66]

Works for me.
As in my previous post, being able to "see the picture first" and then choose the tools to achieve that...That is what you are (almost) saying, and that is "the art" in photography.