G2 lens designation 7-21mm... why so?

Hi,

Just a few questions that's been on the back of my mind!

Does anyone know why the lens is called 7 - 21mm when it's commonly identifed as 34 - 102mm (35mm equivalent) anyway? Why didn't they just call it 34-102mm (35 equiv) in the first place?

Also, what does the aperture designation F/2.0 - 2.5 really mean? I'm guessing, but is this the lowest aperture at which Wide and Telephoto photos can be taken? How come the maximum value isn't shown then?

It's just that I noted that my older Canon A50 had a 4.3 - 10.8mm 1:2.6-4.0 designation and i just wanted to know if i could convert that to a 35mm equivalent value for the lens...

Oh, while i'm on it, looking at lens for SLR cameras, some or most lenses also change the aperture. If the Canon 1.5x teleconverter takes the lens to 153mm (35mm equivalent), does the aperture change accordingly, i.e. will i now be able to take pictures at f/12?

Thanks in advance!

Sam

Sam said:

Hi,

Just a few questions that's been on the back of my mind!

Does anyone know why the lens is called 7 - 21mm when it's commonly
identifed as 34 - 102mm (35mm equivalent) anyway? Why didn't they
just call it 34-102mm (35 equiv) in the first place?

Click to expand...

Well, the actual focal length of the lens is only 7 - 21mm. Digital cameras are built differently to 35mm cameras, but the 35mm equivalents are usually used in marketing because that's what most of us understand.

There's an excellent explanation on the Glossary for this site (see the left menu):

http://www.dpreview.com/learn/Glossary/Optical/Focal_length_01.htm

Sam61309 said:

Also, what does the aperture designation F/2.0 - 2.5 really mean?

Click to expand...

It means that the lowest (widest) aperture is f2.0 at wide angle and f2.5 at full telephoto (which by the way is very fast). The highest (narrowest) aperture is always f8 at both wide angle and full telephoto.

Sam61309 said:

I'm guessing, but is this the lowest aperture at which Wide and
Telephoto photos can be taken? How come the maximum value isn't
shown then?

Click to expand...

It's traditional from 35mm days to not mention the maximum (narrowest) aperture as this is usually of less relevance than the lowest number which designates the maximum amount of light the lens can let in (i.e. how 'fast' the lens is). f2.0 is pretty fast, especially for a consumer digital camera.

Sam61309 said:

It's just that I noted that my older Canon A50 had a 4.3 - 10.8mm
1:2.6-4.0 designation and i just wanted to know if i could convert
that to a 35mm equivalent value for the lens...

Oh, while i'm on it, looking at lens for SLR cameras, some or most
lenses also change the aperture. If the Canon 1.5x teleconverter
takes the lens to 153mm (35mm equivalent), does the aperture change
accordingly, i.e. will i now be able to take pictures at f/12?

Click to expand...

The reason SLR lenses change the aperture is that they are interchangeable lenses and each lens has its own aperture mechanism. When you add a 1.5x teleconverter to the Canon G2 you aren't swapping the lens, you are just extending it. The 1.5x teleconverter doesn't have its own aperture system (its essentially just a magnifying lens) and so you are still using the built-in G2 lens' aperture mechanism, and so f8 will still be the maximum (narrowest) aperture.

Hope this helps

Martin

Depth of field is another consideration that is normally not discussed.

Depth of field is a function of focal length and aperture. To talk about depth of field, let's talk about angle of view first. For the different formats, these are the focal lengths that provide approximately the same angle of view:

G2 Digital: 10mm
35mm: 50mm
6x6cm(medium format): 80mm
4x5 inches (large format): 180mm

The intersting thing is that at any given aperture (ie f8), the depth of field for a given focal length (180mm) is the same. If you've ever used a 180 or 200mm telephoto on a 35mm, you know how narrow the depth of field is. But on a 4x5 inch camera, a 180 takes a "normal" photo, angle of view wise. That's why us large format photographers normally shoot at least f16, with f22, f32 and even f64 routine.

Now, if I'm trying to take a portrait with my G2, even at the widest aperture (f2.5), I still have the depth of field of a 21mm wide angle on my 35mm camera, which is much greater than the depth of field of my 100mm portrait lens for my 35mm. I need to make sure the background is much further away if I want it to be out of focus.

If I haven't confused you yet, let's talk about the minimum (f8) aperture for the G2, which is much larger than the standard 35mm lens minimum aperture (f16-f22). Since the lense of the G2 is much smaller, the size of the iris opening at f8 is very small. If you make the iris opening physically too small, you actually degrade the quality of the image (I'll refrain from discussing why - part of it has to do with light scatter off the edges of the iris and the ratio of the thickness of the iris to the diameter of the opening). That's probably why Canon settled on f8.

Doug

Doug Mac65597 said:

The intersting thing is that at any given aperture (ie f8), the
depth of field for a given focal length (180mm) is the same. If
you've ever used a 180 or 200mm telephoto on a 35mm, you know how
narrow the depth of field is. But on a 4x5 inch camera, a 180
takes a "normal" photo, angle of view wise. That's why us large
format photographers normally shoot at least f16, with f22, f32 and
even f64 routine.

Click to expand...

This is true, however remember that the sensor on the g2 is much smaller than a 35mm film, thus the equivalent circle of confusion diameter on the G2 will be much smaller than on 35mm, so the DOF increase is not that great really.

In 35mm photography, people use a COC diameter of 30um (0.03mm) traditionally. On the G2, you should use a COC diameter of around 6um (0.006mm).

Member said:

This is true, however remember that the sensor on the g2 is much
smaller than a 35mm film, thus the equivalent circle of confusion
diameter on the G2 will be much smaller than on 35mm, so the DOF
increase is not that great really.

Click to expand...

Have you used both? It's drastically greater!

Crunching the numbers from some HTML I borrowed, the depth of field on at f2.8 with a focus distance of 8 feet is:

35mm - 3 feet
digital camera - 44 feet

This is a LOT in my book. These are using your numbers for COC.

Martin Eager56124 said:

The reason SLR lenses change the aperture is that they are
interchangeable lenses and each lens has its own aperture
mechanism. When you add a 1.5x teleconverter to the Canon G2 you
aren't swapping the lens, you are just extending it. The 1.5x
teleconverter doesn't have its own aperture system (its essentially
just a magnifying lens) and so you are still using the built-in G2
lens' aperture mechanism, and so f8 will still be the maximum
(narrowest) aperture.

Click to expand...

If you're still using the same aperature and you're entending the telephoto out 1.5X you're increasing the F number 1.5x. F number is the ratio of the length of the lens to the aperature. Lens increased, aperature didn't. Am I wrong?

I don't know what focal length you were using, but let's consider two extreme cases:

Wide angle comparison (for a landscape):

Using the following for the G2:

• 7mm @ f/8
• Focus at 4 feet (1.22 m)
• COC of 6um
• Result: field of focus from 0.55m to infinity

And the equivalent for 35mm:

• 34mm @ f/8
• Focus at 16 feet (4.88m)
• COC of 30um
• Result: field of focus from 2.44m to infinity

Tele comparison (for a portrait):

Using the following for the G2:

• 21mm @ f/2.5 (full tele, wide open)
• Focus at 6 feet (1.83 m)
• COC of 6um
• Result: field of focus from 1.72m to 1.95m or 9" of DOF

And the equivalent for 35mm:

• 102mm @ f/2.5
• Focus at 6 feet (1.83m)
• COC of 30um
• Result: field of focus from 1.81m to 1.85m or about 1"2/3 DOF

So yes there is a DOF increase on the G2 compared to 35mm (and any other small sensor digicams) I don't deny this but as I was saying it's not that great. The telefocus example above means that you can achieve a portrait with a blurred background by zooming to full zoom and using aperture priority to open wide. The wide angle example means you can take a landscape with an object at 2 feet and yet have everything in focus. It's comparatively harder to achieve this with 35mm and you would have to stop down to f/22 to get closer to this.

By the way, if the G2 had such a large DOF, then I guess no one would be complaining about out of focus shots, would they ;-)

By your own numbers the depth of field increased by 5.4 times and this isn't that great? You're tough to please.

Unless it's in landscape mode, the G2 tends to open up wide and stick with high shutter speeds. This is part of the reason the weak focusing system isn't cured as you're suggesting by it's large depth of field.

Besides that, the depth of field is never going to mask Exif headers that show the camera is obviously focusing at the wrong distance.

My example was at 7/34mm.

What I wanted to show was that the conception that with digicams, it's impossible to achieve blurred background (that people seem to be craving for) is wrong. Granted you don't get as narrow dof as with 35mm, but with my tele example, you see you can get a 9 inch field of focus which is perfect for sharp headshot and blurred background, if you want that. How deep is your average human's head anyway? ;-)

Now what's funny is that in LF photography, people tilt, swing, pan their camera bodies in whichever way to gain as much DOF as possible, but on digicam's forum people are all worried about the exact opposite...

Jason Hutchinson said:

Besides that, the depth of field is never going to mask Exif
headers that show the camera is obviously focusing at the wrong
distance.

Click to expand...

You lost me here. Does this have to do with my previous post?

Member said:

What I wanted to show was that the conception that with digicams,
it's impossible to achieve blurred background (that people seem to
be craving for) is wrong.

Click to expand...

I see. I just wanted to show that you have more DOF to be creative with digital cameras (for now). I guess the confusion was because we were trying to make different points. I have taken 152mm portrait shots with my G2 and have achieved insanely blurred backgrounds, so I definitely agree with you.

Member said:

Member said:

Besides that, the depth of field is never going to mask Exif
headers that show the camera is obviously focusing at the wrong
distance.

Click to expand...

You lost me here. Does this have to do with my previous post?

Click to expand...

Yes. I was saying that even if the G2 had infinite DOF, people would still complain of a focusing problem because they would still see that it focused to the wrong distance in the headers. This despite the shots would be sharp.

Jason H. said:

I see. I just wanted to show that you have more DOF to be creative
with digital cameras (for now). I guess the confusion was because
we were trying to make different points. I have taken 152mm
portrait shots with my G2 and have achieved insanely blurred
backgrounds, so I definitely agree with you.

Click to expand...

Would you be able to post one or two for us to see? I'm considering the Canon 1.5x telconverter for my G2, particularly for portrait shots and getting better DOF blur, and was interested to see how much better it got at 152mm.

Cheers
Martin

Jason Hutchinson said:

I guess the confusion was because
we were trying to make different points.

Click to expand...

Ah, I see, we were in violent agreement all along, just thinking the other was trying to prove the contrary! ;-)

Yes, large DOF is good for most use. When people want to be 'creative' then they can achieve the beloved blurred backgrounds, even with the built-in 3x lens, and as you point out, even shallower DOF can be achieved with the tele adapter.

Thanks!
Ludo.

Wow! I didn't mean to spark such a spirited discussion. I found the replies interesting, informative and corrective. You can tell my photography degree didn't come from a technical school and my memory has gotten fuzzy in the 25 years since I had photo optics.

When I wrote the original post, I had just gotten my G2. Now that I have had a few weeks to shoot, I don't have any problem with the depth of field. On max tele, it throws the background out just fine. I don't have any problems with the focus, either. I've missed some when I've forgotten to pre-focus, hold the shutter and re-compose, but no more so than manually focusing my 35mm. The optics are so good that I think folks are more critical when the focusing is a little off. I have more bad frames from camera shake or subject movement.

Anyway, thanks for the input. Let's go make photos!

Doug