Lens Specs Basic Question !

[karunanidhis]

Hi Guys !
This is probably a silly question but I tried to find answer in the net but couldn't.

1. For some lenses, the model name provides the f stop. (Ex: EF 50mm f1.8.) Now, the f1.8 is in the model name because that's the widest opening the lens can provide ? I know you are going to be angry with my sub-question: Why can't all the lenses have like f1.8 to all the way upto f32 (afterall, it's opening on the lens diaphram, right, can they not make it big/small?).

2. I have a Digital Rebel XT (8Mpix). In one of the threads, one guy mentioned "With my Rebel, it's 1.6X). What does that mean ? Is Rebel 1.6X bigger than 35mm cameras ?

Also, I would love see any pointers towards a good website to learn about lens basics.

Thanks for your continued help.

[karunanidhis]

Also, my Lens (18-55mm kit lens) reads f3.5-5.8. (Why there are two fstops in the spec) ?

Thanks again.

[Ronald S. Jr.]

I'll explain. On your kit lens, f/3.5 is the maximum aperture the lens can have at 18mm. f/5.6 is the maximum at 55mm. The reason they can't all be f/1.4 or f/1.8 is that the longer a lens is, the bigger the glass has to be to maintain that aperture. I'll get this wrong, I'm sure, but the aperture of a lens is something like the focal length divided by the diameter of the rear element. Ok, so the glass has to be bigger. That means that all of the glass has to be bigger. The bigger you make the glass, the heavier the lens gets. No one wants to carry around a 5lb. kit lens. ;-)

[RossW]

2. I have a Digital Rebel XT (8Mpix). In one of the threads, one guy mentioned "With my Rebel, it's 1.6X). What does that mean ? Is Rebel 1.6X bigger than 35mm cameras ?

And for your second question, the sensor on a dRebel (and most other digital cameras) is smaller than the 35mm film frame. So a lens is said to be equivalent to a lens 1.6 times the stated focal length when used on the digital camera. The lens' actual focal length doesn't change, just the virtual focal length. Therefore, on the digital camera, a 50mm lens "behaves" as if it were an 80mm lens would on a film camera. It really only matters for making comparisons, especially to old timers like me who learned photography with 35mm cameras. Digital cameras with a 35mm sensor, by the way, are referred to as "full-frame" cameras. And small point 'n' shoot cameras have an even smaller sensor, so their lenses will have an even greater multiplier for the 35mm equivalent.

[gkuenning]

Ronald S. Jr. got it almost right. The f stop is the focal length divided by the diameter of the front element, not the rear one. As he said, getting a lower minimum f-stop (more light captured) takes a bigger diameter, which means more glass, which means both money and weight.

Many inexpensive zoom lenses have variable minimum stops because the diameter of the physical glass obviously doesn't change while you zoom, while the focal length does. That's not true for expensive lenses because the diameter limitation is determined by an internal element rather than the front one (the f-stop is specified in terms of the front element but it's not necessarily determined by that piece of glass).

At the other end, getting down to f/32 or f/64 requires more precise machining in the diaphragm that is used to reduce the effective diameter of the front element, so cheap lenses don't go as far. In addition, after roughly f/11-f/22 (depending mostly on the camera) you start to lose sharpness due to diffraction. So f/32 and f/64 are kind of specialized settings.

Finally your 1.6x is usually called "crop factor". If you have a "full-frame" camera (which RossW defined so succinctly), a 50mm lens produces a certain field of view. In other words, if you are standing 10 feet (roughly 3 meters) away from your subject, a 50mm lens will cover a certain fraction of the subject. If you change your sensor size, the amount of the subject covered will change. A smaller sensor means less coverage; a larger one means more.

To a first approximation, this gives the equivalent of zooming in. If you're standing where a 35mm film camera would cover your subject from head to toe, a camera with a really small sensor would cover only her face. In essence, you have the effect of attaching a telephoto lens to your camera (or zooming WAY in). It's much more complicated than that, but thinking of it this way is a good way to start.

So with a 1.6X camera, using a 50mm lens is roughly the same as using an 80mm lens (1.6 times 50) on a 35mm camera. That's a mild telephoto, useful for portraits. If you want to get the effect of a so-called "normal" 50mm lens, you'd need something with a 31.25mm focal length.

The nice thing about the 1.6x sensors is that you get more telephoto for your money. My longest zoom is 300mm, which in film-camera terms is a huge 480, which is great for getting in close from very far away. The drawback is that you don't have as much control over depth of field, you're more limited by diffraction, and your smaller sensor isn't as sensitive in low-light situations.

[Jon]

Ronald S. Jr. got it almost right. The f stop is the focal length divided by the diameter of the front element, not the rear one. As he said, getting a lower minimum f-stop (more light captured) takes a bigger diameter, which means more glass, which means both money and weight.So did you. It's actually not the physical diameter of the front element. It's the apparent diameter of what's called the "entry pupil", which is the aperture opening as viewed from the front of the lens. If it were the diameter of the front element, constant-aperture zoom lenses would be impossible.

[bonneyda]

Did anyone mention that the 1.6 crop takes advantage of the lens's "sweet spot"? I think that is important for everyone to know.

[slyone]

Did anyone mention that the 1.6 crop takes advantage of the lens's "sweet spot"? I think that is important for everyone to know. :confused: could you explain?

[Glenn NK]

I'll try - the "sensor" on a 35 mm camera (for which many lenses are designed) is larger than the sensor on many digital cams.

This means that only the central part of the lens's area is being used to project the image onto the sensor.

It's almost axiomatic that lenses are crisper and better in their central area, hence using a smaller (crop) sensor utilizes the "sweet spot" of the lens.

Hope this helps.

[conesweeper]

What do the numbers on the lens such as "18-55mm" mean? I know that's a zoom lens and all but how does "18-55mm, 50mm, 70-200mm" work? Sorry for the noob question! :oops::)

[bonneyda]

:confused: could you explain?

The 1.6X crop factor just cuts off the outside edges of the image, which are usually the area that most lenses fail to do a good job. Therefore, the part of the image that a 1.6X camera uses, is the best part of the image.

Does that make sense, or am I explaining that incorrectly? Let me know, or maybe someone can explain that better.:D

[bonneyda]

I'll try - the "sensor" on a 35 mm camera (for which many lenses are designed) is larger than the sensor on many digital cams.

This means that only the central part of the lens's area is being used to project the image onto the sensor.

It's almost axiomatic that lenses are crisper and better in their central area, hence using a smaller (crop) sensor utilizes the "sweet spot" of the lens.

Hope this helps.

I didn't see your explanation Glenn. Nice job..............I couldn't (and didn't) have said it better.

[slyone]

The 1.6X crop factor just cuts off the outside edges of the image, which are usually the area that most lenses fail to do a good job. Therefore, the part of the image that a 1.6X camera uses, is the best part of the image.

Does that make sense, or am I explaining that incorrectly? Let me know, or maybe someone can explain that better.:D Thamks.

[gkuenning]

So did you. It's actually not the physical diameter of the front element. It's the apparent diameter of what's called the "entry pupil", which is the aperture opening as viewed from the front of the lens. If it were the diameter of the front element, constant-aperture zoom lenses would be impossible.

LOL! Jon is entirely correct, of course. If it were the diameter of the glass, it would be pretty hard to change the f-stop of a lens: you'd need a glass cutter!

That's what I get for being sloppy.

[gkuenning]

What do the numbers on the lens such as "18-55mm" mean? I know that's a zoom lens and all but how does "18-55mm, 50mm, 70-200mm" work? Sorry for the noob question! :oops::)

Hey, we were all newbies once. No need to apologize.

In strict technical terms, the focal length gives the distance from the sensor to the "optical center" of the lens, in millimeters, when the lens is focused at infinity. (I think I got that right--if not, somebody will surely correct me!) But that doesn't really help with understanding.

From an understanding standpoint, a zoom lens gives you a range of focal lengths. For instance, an 18-55 can be set to 18mm, 55mm, or anything in between. A 50mm "prime" is stuck at 50mm forever (but offers compensating advantages).

So what do those numbers mean? On a full-frame 35mm camera, 50mm is considered to be a "normal" focal length. Anything less is wide angle, anything more is telephoto. The word "normal" is tricky; it's really just a convention in that a lot of photographers found that focal length useful in a lot of situations.

"Normal" also depends on sensor (or film) size, so a 50mm lens would be wide-angle on a 2.25-inch camera and extreme telephoto on a pocket-sized point-and-shoot.

In terms of the 1.6x-crop cameras a lot of us use (the Digital Rebel series and the 10/20/30D), the "normal" length is about 31mm. So your 18-55 goes from a moderate wide-angle lens to a mild telephoto. A 50mm lens is also a mild telephoto (unsurprising, since 50mm is close to the 55 top end of your kit zoom). That's useful for portraits. A 70-200 goes from what would usually be considered a medium telephoto to a pretty "long" focal length that's useful for getting close to animals or sports.

[SkipD]

:confused: could you explain?Here's an explanation of the "crop factor" that should make the subject fairly clear to you: Crop Factor Explained (http://photography-on-the.net/forum/showthread.php?t=45388)

[Glenn NK]

Thanks Dave (aka Bonnyda) - we all try don't we. Sometimes it works.

That's a nice collection of lenses you have; so far I just have a 24 - 105L, but I would like something a bit wider and some time in the future, something longer.

Interesting that they are all zooms!! I find them very useful in composing so I don't have to overshoot and then crop. Guess I shot 35 mm slides for so long that I'm looking for the perfect composition (actually it serves me well). God bless zooms - "without them what would little boys do?" (apologies to Alan Jay Lerner).

[keitht]

Did anyone mention that the 1.6 crop takes advantage of the lens's "sweet spot"? I think that is important for everyone to know.
However, since 1.6x crop cameras have pixels that are closer together, the imperfections of the lens are magnified compared to a full frame sensor.

Therefore the same scene taken with a full-frame camera and a cropped camera (same lens, different zoom setting) will generally be sharper on the full frame camera because it does not need such a hight resolution (lines per mm) from the lens. This would be true even if both sensors were to have exactly the same number of total megapixels.

So while I agree that the cropped sensor uses the sweet spot, it may not give a result any better than a full frame camera; indeed it could be worse.