A Tutorial on Lens Length

Let's have a little fun really comparing lens length. Here's what I did. I mounted a Nikon D100 on a tripod and then took shots at 300 mm, 200 mm, 100 mm, 75 mm (using a 70-300 zoom); 50 mm (using a 24-85 zoom); and 24 mm, 18 mm, and 12 mm (using a 12-24 zoom). I shot every picture at f22, autofocus and auto exposure, and bracketed exposures. I bracketed because the auto exposure will vary the shutter speed according to the field of view of the lens.

Then I picked the best exposure at each lens length--generally either dead on or with a +0.3f bump. Then I reduced each image to 800 pixels wide and saved them as jpgs. The jpg compression varies slightly, as I tried to keep each image at about 150 Kbytes. On a scale of 1 (high compression) to 12 (low compression), then all are 8-11. All are full-frame--no crops (until a surprise at the end.)

These were taken on my N scale layout. Camera-to-subject center was 6 feet.

Below is the 300 mm image. (Film SLR users multiply by 1.5, for 450 mm equivalent) The anchor on the ship is the reference point. Yes, I'm still fooling with the bow--I chose it because of the tonality. You can pretty much see I should really clean that anchor up a bit.



Next is the 200 mm image. The anchor is still centered, but I get a little more of the surroundings.



Now for the 100 mm image. Here the exposure shifted up a bit--I wasn't concerned enough to try to control it, as I'm focusing on lens length.



Here's the last image taken with the 70-300 mm zoom, at 75 mm (below).



I swapped to a 24-85 mm zoom and shot this at 50 mm. Now we are really starting to back off. You can see the whole ship--about 34 inches (450 scale feet). The anchor is still in the center, but doesn't look so bad now.



I swapped again to a 12 - 24 mm zoom, as it is sharper at 24 mm. Now we're really backing off--this becomes more of a snapshot (below).



Since 18 mm is a popular short end of zooms, I took one at that length. I've really got to figure out what to do on that second deck corner! It's about the only pink foam left.



Here's as wide as I can go--12 mm (below). It's a 94 degree field of view. It shows just about 1/4 of the room. The lights are just out of the image to the left, with some rather dramatic fall-off. Man, I've got to finish those fasciae.



That's it for now. Hope you enjoyed! Part II in a while.

 

cool stuff Pete.

Totally off subject but... A big piece of mirror behind those big bridges would make them seem like they are just floating out in space. Of course you'd get two bridges for the price of one too. Just a thought.

 

Part II,

We all believe that telephoto lenses "flatten" an image. Actually, they don't. I'm talking about typical lenses, both fixed length and zooms--not fisheyes nor architectural lenses/cameras.

Let's prove it. Below is a shot from my previous post with a 300 mm lens of a ship anchor, taken from about 6 feet:



And then there's this shot, taken from the same location with an 18 mm lens, cropped, and blown up 450 percent:



Now, I did my best to match the 18 mm cropped image area to that of the whole 300 mm image--I may be off just a little bit. But, is there a difference in perspective? OK, I'll admit there may be a tiny bit, due to lens anomalies, but not a whole lot.

Below is the image that the blow-up came from:



The telephoto, in effect, just squeezes more data in a limited field of view; it doesn't really flatten anything.

The image at 12 mm was a little too blurry (much greater magnification) so I decided the 18 mm image would do.

 

Boxcab:

Hah, ha! You're anticipating Part III! Yes, the D100 is a digital SLR that uses both 35 mm camera lenses, and a new line of digital camera lenses.

Here's the difference: the imaging surface ("the film size") of Nikon digital SLRs is only about two-thirds the area of the film size of a 35 mm camera. This makes a 300 mm lens on the digital the equivalent of a 450 mm lens on the film cameras. And so on down the line. A 12 mm lens on the digital is the equivalent of an 18 mm on a film camera. I'll get into this in Part III.

Here's a way to look at it:

Let's say I locked the tripod securely and mounted a film SLR instead of a digital SLR. It would take about a 450 mm lens on the film camera to produce the same image as a 300 mm lens on a digital SLR. It would take an 18 mm lens on the film camera to produce the same image as a 12 mm on a digital SLR.

Following this 1.5 factor through all the lens lengths, the results would be almost exactly the same, in terms of perspective.

 

Geeky,

I might try it. What I've settled on for now is a prnted backdrop for normal viewing (not finished), and a Chromakey approach for photographs. This involves a magenta card behind the scene which allows me to Photoshop out the door. See below for the test..



As I've confessed before, I got too greedy about running length, so the bridges are too close to the door. They were 18 inches away on the plan, but I moved them closer for about 6 feet of running length (out of 600) and a little easier construction.

Live and learn!

 

Hemi,

Virtually all zoom lenses have a problem with pincushion and barrel distortion. It's barrel at their shortest length, and pincushion at the longest, I think. It drives me nuts, especially when I'm trying to put together a panorama. It's most prevalent in the wider lenses, The 12-24 has no distortion I can detect at 24 mm, but a lot at 12 mm. I think it's about a minus 7 (-7) correction in Photoshop CS. It's been a while . . .

 

Depends on how you look at it. From the same viewpoint, you're right. But if you consider a picture taken with a telephoto lens, then try to recreate the same shot using the full field of view of a wide angle, then there will be some flattening effect since you'll be further away when using the telephoto.

 

Jon (without H) is absolutely right. Take a photo with a 200mm lens and then again with a 18mm lens and move closer so the same amount of the ship fills the frame. The telephoto shot will appear compressed and the wide angle will be seem expanded. That's been a rule of photography since long before any of us walked the face of the earth.

 

Jon and Brian are indeed correct. When you change the distance, you change the perspective, so a telephoto will flatten an image. From the same distance, however, the perspective remains the same.

 

John W.,

Also true. The size of the sensor determines what a "normal" lens will be. A 50 mm is considered a normal lens for a 35 mm film camera. The Nikon digital would thus have a normal lens of 33.3 mm, and the Canon would be 31.25 mm. My wife's Sony point-and-shoot, with a small sensor, has a normal lens of 11.4 mm. Which brings me to Part III:

Because the sensor size is smaller, the optics in the lenses don't have to work so hard to focus an image. This is one reason we are seeing smaller, lighter and less expensive lenses specifically for digital SLRs.

Film lenses have to maintain focus over 864 sq. mm (36 x 24) of film plane, while the Nikon D100 has to maintain focus over 374.4 sq. mm (24 x 15.6) of sensor. The Nikon digital area is 2.3 times less than the film camera. It's actually a little more complicated than that--but the ratio is the same.

It's easiest to build a lens that has a circular focal plane, even though we use a rectangle or square frame. So a film camera lens has a circle that "holds" the 36 x 24 rectangle; a digital lens has a circle that "holds" a 2.3X smaller rectangle. You can use some of the digital lenses on a full frame (36 x 24) camera, but you may get vignetting--the digital circle doesn't cover the whole frame, so you'll see most of a circle, with the corners of the frame darkened or black.

Back in the 60s, my stepfather, a master machinist, and I used to play with mounting different lenses on my eclectic assortment of cheap cameras. (He also had the baddest car in the neighborhood, but that's another story.) While we'd get vignetting, some of the effects were wildly weird.