SL Photography: Focal length

Windlight Contributor Myra Wildmist is back with another SL Photography Tip. This time she explores Focal Length:

We’re back to depth of field, again, picking apart the parameters a piece at a time. This time we’re looking at focal length.

What is focal length?

According to Nikon (Who should know), the focal length of a lens is defined as the “distance between the lens and the image sensor when the subject is in focus”. The sensor takes the light coming through the camera lens can converts it into digital image or, in the case of analog cameras, a film negative. There’s no sensor in Second Life, of course, so the focal length is simulated.

The Firestorm Phototools tooltip says this about the focal length: “This tells the viewer what Focal Length/Lens Length to simulate for the DOF effect. Higher numbers produce a narrower depth of field.”

That’s not what focal length does (I think whoever wrote that tip got it confused with f-number.). In real life photography, a higher focal length produces a narrower angle of view (View angle in Phototools.). And a smaller focal length produces a wider angle of view.

Note: The View Angle in Phototools is measured in radians, a more scientific way of expressing angles. The default value is 1.047 radians, which is 60 degrees.

Focal lengths of common lenses

The Angle of View grows smaller as the Focal Length increases. Image source: wikipedia.org Creator: MRSPYDOS

The above image from wikipedia illustrates how the Angle of View grows smaller as the Focal Length increases. It also divides the focal length of lenses into five categories*:

Ultra wide angle: 14mm – 22mm.
Wide angle: 22mm – 40mm.
Wide angle and ultra wide angle lens are especially good for indoor photography when you’re trying to capture a lot of the scene.
Standard lenses: 40mm – 80mm
Used to capture the scene as your eye sees it. 50mm, the Phototools default, is considered about what the human eye sees.
Telephoto: 80mm – 300mm
A telephoto lens around 80mm-100mm is popular for portrait photography.
Ultra telephoto: 300mm and above.

*There’s no hard and fast rule over where one lens category ends and another begins, so think of these primarily guides.

Style thoughts

You can use this information to simulate various lenses in your SL photography. I’ll show you how in the next post.

References and further reading

Understanding Camera Lens, Cambridge in Color

Using Wide Angle Lenses, Cambridge in Color

DSLR Camera Basics, Nikon

Angles of View, B&H Photography

Angle of View, Wikipedia

Responses

SL Photography: Simulating popular lenses in Phototools – Windlight Magazine

July 25, 2016

[…] categories defined by Cambridge in Color, a digital photography site. In my previous article on focal length, I defined the different categories slightly differently. In the future, I’ll stick with the […]

LikeLike

Reply
Mona Eberhardt

January 29, 2018

You know, your post got me thinking. I’ve often wondered about the view angle and field of view settings, which are literally the same thing, with one being inexplicably expressed in radians and the other in degrees. Setting the focal length and perspective of your virtual lens in SL has always been a pain; in RL, we have it much easier: just mount a different prime lens or turn the zoom ring on a zoom lens, and Bob’s your uncle. So, I suggested a few improvements to Firestorm’s Phototools viewer: https://jira.phoenixviewer.com/browse/FIRE-22207

LikeLiked by 1 person

Reply
1. Myra Wildmist
  
  January 30, 2018
  
  wow. thanks, mona. I’ve nudged the FS team about this, before. I hope your jira will inspire them to do more work in the area.
  
  On fov and angle of view, I figured out the FOV setting is required because monitors have a vertical “stretching” the the eye or a lens doesn’t have. you can see the focus changes subtly when you set the fov.
  
  LikeLiked by 1 person
  
  Reply
  1. Mona Eberhardt
    
    January 31, 2018
    
    I think the view angle and FOV could be set simultaneously as we change focal length. And also, we could also dial arbitrary focal lengths (such as 125mm, as in the Voigtländer Macro APO-Lanthar f/2.5 from the company’s now-discontinued SL series), with the floater performing the necessary calculations (which should’t be too taxing; even a now-obsolete computer shouldn’t take more than a couple of milliseconds to churn out the appropriate view angle and FOV. As it is, the Phototools floater is very powerful, but it could be more intuitive and quicker to use.
    
    LikeLiked by 1 person
  2. Myra Wildmist
    
    February 6, 2018
    
    totally agree. I’d like a floater that uses a slider for the lens length.
    
    LikeLike
  3. Dennis
    
    October 29, 2020
    
    The SL AOV calculation is flat-out wrong and they need to fix it. The value is not radians, it is more like radians/2, but not quite. Try this, like I did. Put yourself at the center of a 1024M**2 parcel, ie at 16,16 and adjust your FOV so that the corners of the parcel (0,0) and (0,32) appear just near the edges of your screen when your camera position is about on the bridge of your nose. In this case your horizontal FOV is actually about 90 degrees, or Pi/2 radians = 1.7, but the FOV in preferences shows it as 1.04. On the mini-map, the angle of view looks approximately 120 degrees which is closer to what is being delivered. In any case, the SL standard FOV is about comparable to a 30mm lens on a 35mm full frame camera, which is on the bordeline between wide and ultra-wide, and much wider than appropriate for general use. The default standard should be more like 0.75 to 0.80 and 9 should reset to this value or whatever is specifically chosen by the client, and this setting should be durable.
    
    LikeLiked by 1 person
Cheeto’s Photography Tips – AVI(L) Community & Fashion Blog

May 26, 2019

[…] SL Photography: Focal Length […]

LikeLike

Reply
Dennis

October 29, 2020

What showed up as ‘9’ I entered as Ctrl-9 but using angle brackets which must have been interpreted as HTML.

LikeLike

Reply
Dennis

October 29, 2020

And I meant to say 20mm FL/35mmFF, which is an ultra-wide with a Horizontal FOV of almost 90 degrees. This is the standard default AOV for SL. Ridiculous. The perspective distortion is painfully obvious to anyone who knows what to look for. (These comments need a preview/edit feature.)

LikeLiked by 1 person

Reply

Kultivate Arts