Lens Focal Lengths? - DPChallenge Forums

Carl Zeiss 35mm f2.4 Jena/Flektogon MC

09/12/2005 11:06:12 AM · #1

I have a Canon Powershot A85. It has an optical zoom of 3X (I keep the digital zoom turned off). When I zoom all the way out and take a photo, then look at the file info in photoshop, it says the photo was taken at a focal length of 5.4mm. Then when I zoom in all the way it says the focal length was 16.2mm. I'm considering upgrading to a DSLR and trying to figure out if this is the same type of focal length as the focal length of a DSLR lens? I guess what I'm trying to ask is: Do I really have that much of a wide angle lens??? If a lens existed on a DSLR that was rated at 5.4-16.2 would it go to 3X zoom? What kind of "X" zoom are the 200mm and 300mm lenses? Thanks for clearing this up.

hopper

Canon EOS-80D

Canon EF 35mm f/2 IS USM

09/12/2005 11:15:57 AM · #2

the 35mm equivilant of your camera is 35-105mm.

A DSLR equivilant would depend on the camera you buy (confusing, I know)

it's a 3x zoom because 105 divided by 35 is 3.

Uusilehto

Canon EOS-1D

09/12/2005 11:16:08 AM · #3

The 5.4-16.2 is the physical focal length of the lens. Converted to "full frame" (35mm film) standards, it's 35 - 105. and 105 divided to 35 is 3X. So the X ratings implicate how many times the longest focal length is compared to the wide end

I would consider anything below 24-28mm to be wider than wide angle (super wideangle)

EDIT: Then there's the crop factor on dSLR cameras. Let's say you had a 70-300mm lens made for a film camera. Because most dSLR cameras have a sensor smaller than 35mm film, the image is cropped. For example, the Canon EOS-300D-Rebel has a crop factor of 1.6, making the shots taken with the 70-300mm look as if they wouold have been taken with a 112-480mm lens

Message edited by author 2005-09-12 11:19:08.

09/12/2005 11:17:09 AM · #4

Your 5.4-16.2mm lens (it probably has that written on the lens as well) is actually equivelant to a 35mm-105mm lens in '35mm' terms. The multiplying factor for your camera is 6.5 - so whenever you see your EXIF focal length, multiply it by 6.5 to get the actual '35mm' focal length.

The multiplying factor changes depending on the size of the sensor. Cameras with larger sensors (like DSLR) have a smaller multiplying factor. (1.5 for some Nikon DSLRs for example)

So, if you put a lens rated at 50mm on a DSLR, it will look like a 75mm lens.

How confusing does that sound? :-)

Message edited by author 2005-09-12 11:19:02.

09/12/2005 11:29:26 AM · #5

I think I actually get it now. I understand that different cameras have different multiplying factors that affect focal lengths. I didn't realize my point and shoot had a 6.5. So it would be basically impossible to get a wide angle shot out of my camera. Now I have one last question... On a Rebel with a 1.6 crop factor, with a 300mm lens at full zoom you would have a 35mm equivalent focal length of 480mm. Does that mean that it will zoom in to over 12X since my 3X point and shoot has a 35mm equivalent of 105mm at full zoom?

09/12/2005 11:31:45 AM · #6

So the rebel with a 300mm lens zoomed in all the way has a magnification of 13.7X. And with a 2X teleconverter it would be 27.4X?

Canon EF 70-300mm f/4.5-5.6 DO IS USM

09/12/2005 11:37:18 AM · #7

SLR zoom lenses don't usually talk about zoom factors, like 12x, or 3x ... the '12x' isn't '12x the size of the image you'd see with your eyes', it's '12x the size of the image through this lens at the wide-angle setting'

So, the '12x' or '3x' is actually zoom *range* ... the difference between the wide angle and the telephoto. For example, a 400mm prime lens doesn't zoom. So you'll never see it quoted as '12x' or '20x' or whatever - People just call it '400mm'. More important for these lenses is the F number... That's what makes them expensive :)

cpanaioti

Canon EOS-40D

09/12/2005 11:40:14 AM · #8

The X factor you are referring to is how much zoom there is between the widest focal length and the narrowest focal length of a particular lens.

A 300mm lens is a 1x lens since it only has one focal length. It is not a zoom lens. It is what is referred to as a prime.

edit: I type too slow.

Message edited by author 2005-09-12 11:40:37.

Canon EF 70-300mm f/4.5-5.6 DO IS USM

09/12/2005 11:48:04 AM · #9

Ok, now I'm becoming lost. Now I get that the X factor isn't magnification as in 12X isn't 12 times as big as the image would appear to your eyes. It's 12 times as big as it would appear if you were zoomed all the way out. So if you had a 70-300mm you'd have a 4.3X lens. And if you put a 2X teleconverter on it, you'd still have a 4.3X lens in "point and shoot terminology". So I guess the most important question is: When trying to figure out what zoom lens I need, how do I go about picking a mm? For example, I do a lot of shots from inside a boat taking pics of people behind the boat wakeboarding that are 60 feet away. How would I determine what lens would give clear focus on the subject? They would be outdoor pics, so F2.8 wouldn't be necessary. But then what If I were taking shots at a football game where the subjects were 100 feet away? How do I determine the zoom lens needed for different subject at different distances? Wow, I'm learning!

cpanaioti

Canon EOS-40D

09/12/2005 11:53:23 AM · #10

Originally posted by chemlabrat:

Use the 35mm equivalent of the lens on your current camera as a starting point and ask yourself a few questions.

1) When zoomed all the way out, do you find yourself wanting to zoom out more in some cases? If so, you'll need a lens, that after applying the crop factor, provides that wider angle of view.

2) When zoomed in all the way, do you find yourself wanting more zoom? Try this experiment with the digital zoom turned on to see how far in you have to zoom to get the closeup you want. Then use the 35mm equivalent of what that focal length would be to look at DSLR lenses.

Nikon AF-S DX NIKKOR 18-200mm f/3.5-5.6G ED VR II

09/12/2005 11:59:20 AM · #11

That makes sense about taking the X factor on my lense and seeing what X I zoom in to and then converting it to the 35mm equivalent. But what if I didn't have a point-and-shoot with digital zoom? I'm curious how people calculate what focal length they need in a lens according to what distance they are from the subject. I never find myself needing less zoom, but very often need more.

BobsterLobster

Fujifilm FinePix S3 Pro

09/12/2005 12:20:58 PM · #12

Any lens will be able to focus on any subject at any distance (unless it's REALLY close, then you need a macro lens).
The focal length tells you how by how much your subject will fill the frame.
For example... you want to take a photo of someone at the far end of a room.
A 12mm lens will show you the entire room, the person will be small in the photo.
A 50mm lens will show more of the person... on a 35mm camera, this lens is roughly equal to what your eye naturally sees, so there is no difference when you look through the viewfinder.
A 200mm lens will show part of the face.
A 400mm lens will show you the nose, or eye.

So you don't choose a lens based on how far something is, you choose it by how much you want to include or discard from your shot.

kirbic

Canon EOS R5

Canon RF 24-70mm f/2.8 L IS

09/12/2005 12:32:48 PM · #13

Originally posted by chemlabrat:

There are several ways you can go about determining what focal length you need... some are a bit technical, some are SWAGs. I'll go over two:

1.) The SWAG route. Your current cam has a 105mm "telephoto" limit in 35mm terms. When you double that, to 210mm, you'll halve the field of view, both horizontally and vertically. In other words, four of the 210mm frames would fit neatly in a 105mm frame (two columns by two rows). Double it again, 420mm, and you'll halve the FoV again... this gives you a way to estimate with your current cam what the FoV of a given tele lens will be. Just take a photo at 105mm and look at what is in one quadrant, that tells youwhat it would look like at double the focal length, then what is in one quadrant of *that* will tell you what quadruple the focal length will look like. You won't be looking at anything more than 400mm for DSLRs anyway, unless your budget is pretty large ;-)

2.) The "techie" method. Since you know your subject size and distance pretty closely, you can calculate the angular size of the subject in degrees. Now look up (or calculate) the FoV in degrees of the lens you want to use. The formula for FoV for rectilinear lenses is FOV = 2 * arctan (frame size/(focal length * 2)) where frame size refers to the size of the sensor in the desired direction (horizontal, vertical or diagonal) and expressed in millimeters. This method can be implemented in a spreadsheet in a snap, but remember that Excel's output from trig functions is in radians (it assumes that input to trig function is in radians as well).

09/12/2005 12:35:01 PM · #14

Originally posted by chemlabrat:

For example, I do a lot of shots from inside a boat taking pics of people behind the boat wakeboarding that are 60 feet away. How would I determine what lens would give clear focus on the subject? They would be outdoor pics, so F2.8 wouldn't be necessary.

Just one thing on this - If sports photography is your thing, then you will still have to consider the F number / shutter speed. Even more so because you'll be moving about and so will they. Fast shutter speeds have to be compensated with either faster lenses, higher ISO, or both.

Also, think about how you would compose your wakeboarding shots (i.e. get the subject in the right position in the frame) using an especially long lens.... Fleeting glimpses of his head through the viewfinder, then you fall off the side of the boat. :-)

Nikon AF-S DX Zoom-Nikkor 17-55mm f/2.8G IF-ED

09/12/2005 12:57:05 PM · #15

Kirbic-
Thanks a million for that explanation. The techie method was what I was looking for. I see how it could be estimated from my digital zoom, but I wasn't sure about how focal lengths related directly to the size of an object and it's distance from the lens. I could estimate it, but being an engineer I'm extremely anal about why I'm doing what I'm doing.

Jhonan-
That's a very good point! I will have to include reasonable framing when calculating what focal length is necessary. That's one reason I turned off the digital zoom on my camera. I would zoom in to get up close to the action, but with me moving on the boat up and down because of waves, and them moving behind the boat back and forth across the wake, all I was able to see was "There goes a face... There they go again..."

peterish

Nikon D90

09/12/2005 12:57:26 PM · #16

I have a similar question about my 12x Panasonic FZ-20. On the camera body it says "36-432 35mm equivalent". Suppose we ripped off my camera's lens and mounted it onto a DSLR with 1.6 crop factor, would this setup be shooting at 57-690mm ?

Uusilehto

Canon EOS-1D

Carl Zeiss 35mm f2.4 Jena/Flektogon MC

09/12/2005 01:06:10 PM · #17

Originally posted by peterish:

The image circle, that the lens in your FZ20 produces, is ALOT smaller than the sensor is in pretty much any dSLR, producing an image with a tiny round image in the middle.

kirbic

Canon EOS R5

Canon RF 24-70mm f/2.8 L IS

09/12/2005 01:13:57 PM · #18

Originally posted by peterish:

Nope. The physical focal length of you camera's lens is very short. If you were able to physically mount it on a DSLR, it would correspond to very wide angle, even at the tele end, on a 1.6-crop camera, even wider on 35mm full frame.
In reality, the image formed would be so small it would never cover the large sensor, so it's a moot point. Also, the lens would have to be located inside the mirror box of the larger camera.

On a more practical note, the "35mm equivalency" numbers on your cam tell you that you have the same telephoto reach as a 432mm lens on a full-frame camera and wide angle equivalency of 35mm (again in reference to full frame). On a 1.6-crop cam, you'd need a 270mm lens (432mm/1.6) to achieve the same telephoto FoV, and a 22mm lens to get the same wide angle view.
In summation, if you were buying a 1.6-crop DSLR and wanted to cover approx. the same range, you might buy a 24-70 or similar zoom lens, along with a 100-400 zoom. There would be a number of other options, but you get the picture...