A couple of stupid question!!!

[Saber]

Hi
I am still a bit new to all the tech info for photography so here are a couple of questions.

When lenes are refered to in mm what does this mean and how does it relate to the number of times zoom ie 2x zoom.

They refer to digtial camers as equilivent to ****** in 35 mm. what does this mean

If I was to buy a DC58 @ 1.75x does this mean that the max zoom for the G3 will be 4x1.75. Then of corse again there is the fact that this is refered to as 245mm which again looses me.

If I wanted to get a new telephoto lense for my G3 what would you suggest. I was told that you are realy on suppost to us a Cannon one. But is seem like a lot of money for only a small amount of zoom.
What other good quality options are there.

Lastly what the hell is vignetting. I have seen it posted a bit but have no idea what it is.

Thanks.

[Rich_F]

To start with the easy part - vignetting is where the corners of the picture are darker than the centre.

On a 35mm camera, a 50mm lens gives similar view to the unaided eye. 28mm is a typical wide angle, and 200mm is a reasonable zoom. In all cases, a 2x zoom involves doubling the mm value. So 35-70 is a typical 2x short zoom, while 70-210 is a typical 3x longer zoom.

On your G3, the lens is a 4x zoom, equivalent to 35-140 on a 35mm camera. With a 1.75 teleconvertor, it would become equivalent to 61.25-245 (ie, both values are mutiplied by 1.75). Still a 4x zoom (assuming the full zoom range remains usable), but over a different range.

Digital cameras have smaller sensors than a frame of 35mm film, so the real measurements on the lenses are much smaller than a lens with the same field of view on a 35mm camera. However, we carry on referring to the "35mm equivalent" value because it has meaning to most photographers.

Hope this helps,

Rich

[sdommin]

This is an exaggerated example of vignetting, done with software. Vignetting is sometimes done on purpose for special effects, but is usually considered undesirable.

http://home.att.net/~sdommin/vignette.jpg

[Saber]

At Last an easy straight forward answer.

I have been wondering what that stuff ment for a while now.

Another question. When using DOFmaster, why then, is the lense set to 7.2mm and not say like 35mm ( as is the equilivent for the G3). Does this have to do with the small lenses on digital cameras.

Would I be right in saying that a 2x zoom I would have to set the camera to 14.4 on the DOFmaster. It is hard to use the DOFmaster if you don't know all these conversions

Is there any where you get get a list of all these conversions.

[Rich_F]

Now you're just being awkward!

Although a 7.2mm lens on a digital camera may have an equivalent field of view to a 35mm lens on a film camera, it will have a much wider depth of field at any given exposure.

Therefore, if calculating depth of field, it is important to know the actual lens measurements, not just the 35mm eqivalent.

You're right, though, in assuming that 2x zoom on a 7.2mm lens will put the focal length to 14.4mm.

Rich

[Saber]

Although a 7.2mm lens on a digital camera may have an equivalent field of view to a 35mm lens on a film camera, it will have a much wider depth of field at any given exposure.


You are problably going to hate me after this but can you explain this further or at least possbily point me in the direction inwhich i can read about this.

This is probably pritty basic stuff so I will pick it up pretty easily I think i need to go to lenses 101.

[Rich_F]

If you really want to know about this in more detail, do a google search for "Circle of Confusion", and you'll come up with links like http://www.minoxlab.com/Don_Krehbiel/mpl/dkdof.htm.

Personally, I think that it's easier to just treat it as a fact of life, as increased understanding doesn't really gain you any practical knowledge.

All you need to know is
- The larger the film format, the narrower the depth of field.
- The longer the lens, the narrower the depth of field.
- The larger the aperture, the narrower the depth of field.

The nice thing about digital cameras is that
- You can quickly review a picture, to see if the depth of field was good
- If you need less depth of field, you can add lens blur in Photoshop!

The "nasty" side is that the camera often doesn't tell you exactly what the focal length is, so calculations are a bit hit-and-miss. My advice would be to be more empirical; forget the equations and tables, and learn what works.

Rich