snubbr.com

Lenses Vs f.Stops
Hi All this is my first post so please be gentle.....

I've recently purchased a Digital SLR and am haing great fun experimenting with it. I'm starting to understand the basics (i think) i.e apeture, ISO, Shutter speeds etc and am determined not to resort to the auto function to learn how to harmonise them. After all, it's not as if i'm burning film is it!.

I'm now trying to get my head around lenses and what all the numbers mean. I'm most intrigued by a lens and it's relationship to apeture settings. Especially if i'm understanding what i've read so far corecttly....

The lens I have bolted onto my SLR as type is.

17.5 - 45mm - Dictates how wide a lens can see? i.e 10mm would offer a VERY wide angle?.

1: 3.5 - 5.6 - Is this the apeture banding the lens will funtion at?.

0.28m/0.9ft - The distance in which a picture will be in focus (Focal Length?).

As you can probably tell i'm already a little confussed so thanks in advance for your help!..

Comments (5)

Rich_G wrote:.

Hi All this is my first post so please be gentle.....

I've recently purchased a Digital SLR and am haing great funexperimenting with it. I'm starting to understand the basics (ithink) i.e apeture, ISO, Shutter speeds etc and am determined not toresort to the auto function to learn how to harmonise them. Afterall, it's not as if i'm burning film is it!.

I'm now trying to get my head around lenses and what all the numbersmean. I'm most intrigued by a lens and it's relationship to apeturesettings. Especially if i'm understanding what i've read so farcorecttly....

The lens I have bolted onto my SLR as type is.

17.5 - 45mm - Dictates how wide a lens can see? i.e 10mm would offera VERY wide angle?.

1: 3.5 - 5.6 - Is this the apeture banding the lens will funtion at?.

0.28m/0.9ft - The distance in which a picture will be in focus (FocalLength?).

As you can probably tell i'm already a little confussed so thanks inadvance for your help!.

If you want to go with manual settings fine but the camera especially if you are a newbie is going o give you more usuable shots. if on manual you stiull have to use the meter and put the indicator in the center of the range. if not the pics will be overexposed/underexposed..

The lower the mm on the lens the wider the lens can see, the bigger the angle..

Range of fstops in your example means that at different mm on the zoom the WIDEST fstop available is that number. in your case f3.5 is the widest at 17.5mm while f5.6 is the widest at 45mm..

.28m/.9ft is the MINAMUM distance the lens can focus at at any mm..

Try this webpage-http://www.shortcourses.com/in particular the headings on the left. the 3rd through 6th...

Comment #1

Rich_G wrote:.

Hi All this is my first post so please be gentle.....

Oh, all right then, since you asked nicely .

I'm now trying to get my head around lenses and what all the numbersmean. I'm most intrigued by a lens and it's relationship to apeturesettings. Especially if i'm understanding what i've read so farcorecttly....

The lens I have bolted onto my SLR as type is.

17.5 - 45mm - Dictates how wide a lens can see? i.e 10mm would offera VERY wide angle?.

Yes. Are you sure about the numbers? Most common DSLRs have a standard lens which is 18-55mm, apart from Olympus who do 14-42 mm (or something on those lines). Anyway, the smaller number (focal length) is the wide angle end; the longer focal length is the telephoto end. So 10mm would indeed be very wide angle, with a huge field of view. 200 mm would be a powerful telephoto with a very narrow field of view..

1: 3.5 - 5.6 - Is this the apeture banding the lens will funtion at?.

This is the maximum opening (f-stop), which varies a bit as you zoom in. At the wide angle end the widest aperture is f/3.5. As you zoom in to the tele end the widest aperture is f/5.6. As the image gets magnified, a smaller part of the field of view gets expanded, so the light intensity gets dimmer. Fixed focal length lenses (non-zoom) have a fixed maximum aperture..

These numbers only represent the widest-open end of the scale: the lens will probably go down to f/22 (which is a very small aperture, like a pinhole)..

0.28m/0.9ft - The distance in which a picture will be in focus (FocalLength?).

This is the focussing distance, not focal length. Yes, it will be the distance that the lens is focussed on. (The focal length is different and is what determines magnification, see above)..

As you can probably tell i'm already a little confussed so thanks inadvance for your help!.

Best wishesMike..

Comment #2

Rich_G wrote:[snip].

The lens I have bolted onto my SLR as type is.

17.5 - 45mm - Dictates how wide a lens can see? i.e 10mm would offera VERY wide angle?.

This is the focal length of the lens in mm - seehttp://www.dpreview.com/learn/?/Glossary/Optical/Focal_Length_01.htm.

And: http://www.dpreview.com/.../?/Glossary/Optical/Focal_Length_Multiplier_01.htm.

1: 3.5 - 5.6 - Is this the apeture banding the lens will funtion at?.

This is the maximum aperture measured as f-stop numbers:http://www.dpreview.com/learn/?/Glossary/Exposure/Aperture_01.htm.

Your lens is a zoom and the 3.5-5.6 indicates that the maximum aperture varies with the zoom - it is f3.5 at 17.5mm and f5.6 at 45mm..

0.28m/0.9ft - The distance in which a picture will be in focus (FocalLength?).

This is the minimum distance at which the lens will focus..

As you can probably tell i'm already a little confussed so thanks inadvance for your help!.

Chris R..

Comment #3

Thanks for the tips and the reference sites! That makes things a lot clearer ..

Comment #4

I think you have an Olympus ( that's the only 17.5-45 lens I know of )..

Anyhow....

1) Your sensor crop factor..

This will be 1.5 or 1.6 for a typical DSLR, 2x for a 4/3 system ( like an Oly ) or 1x for a full frame system ( a few very high end cameras )..

As I explain below crop factor affects field of view, so although 17.5-45 is wide lens, when it is mounted on an Olympus camera it actually behaves as 25-90 which is not really wide at all..

(2) Aperture.

Aperture is the setting that decides how much light is let into the lens. It is describe as an f-number. large f numbers mean less light. At high apertures ( f16+ ) there is an additional degradation of quality due to diffraction effects ( don't worry about it, just try to avoid very high apertures ). At very wide apertures ( e.g the lowest setting you lens will work ) most ( not all ) lenses are 'soft' at least in the corners. Most lenses are at there peak quality somewhere in the middle ( usually f5.6 - f11 ) but it depends on the lens..

Wide apertures ( small f numbers ) have small depth of filed.Narrow apertures ( large f numbers ) have larger depth of field..

(3) Lenses.

A lens will be quoted with it's widest aperture range and it's zoom range..

The true focal length of the the lens is quoted..

However the crop factor of the sensor actually defines the field of view of the lens, which is always equated to 35mm terms ( full frame is 35mm )..

So an 18-55 on a camera with a 1.5 crop factor has a field of view like a 27-82.5 mm on a 35mm camera..

If you have an Olympus your 17.5-45 will equate to a 35-90mm lens in 35mm terms..

Apertures range on lenses is :.

- first f-number - widest aperture possible at widest zoom setting- second f-number - widest aperture possible at longest zoom setting.

If a lens has a zoom range and a single f-number it means it has a constant maximum ( widest ) aperture..

If a lens has a fixed focal length ( no zoom ) it will have a single maximum aperture rating as well..

Lens aperture is not affected by the crop factor..

(4) Primes and Zooms..

A Zoom lens can have it's field of view altered ( the zoom ring ) and hence it's focal length..

A Prime lens has a fixed focal length and hence a fixed field of view..

Primes are simpler to design and build ( up to a point ) and hence primes can, in theory, give higher quality that zooms because it is easier to deign and build that higher quality..

Zooms are more flexible..

StephenG.

Pentax K100DFuji S5200Fuji E900PCLinuxOS..

Comment #5

Click Here to View All...

Sponsored Amazon Deals:

1. Get big savings on Amazon warehouse deals.
2. Save up to 70% on Amazon Products.


This question was taken from a support group/message board and re-posted here so others can learn from it.

 

Categories: Home | Diet & Weight Management | Vitamins & Supplements | Herbs & Cleansing |

Sexual Health | Medifast Support | Nutrisystem Support | Medifast Questions |

Web Hosting | Web Hosts | Website Hosting | Hosting |

Web Hosting | GoDaddy | Digital Cameras | Best WebHosts |

Web Hosting FAQ | Web Hosts FAQ | Hosting FAQ | Hosting Group |

Hosting Questions | Camera Tips | Best Cameras To Buy | Best Cameras This Year |

Camera Q-A | Digital Cameras Q-A | Camera Forum | Nov 2010 - Cameras |

Oct 2010 - Cameras | Oct 2010 - DSLRs | Oct 2010 - Camera Tips | Sep 2010 - Cameras |

Sep 2010 - DSLRS | Sep 2010 - Camera Tips | Aug 2010 - Cameras | Aug 2010 - DSLR Tips |

Aug 2010 - Camera Tips | July 2010 - Cameras | July 2010 - Nikon Cameras | July 2010 - Canon Cameras |

July 2010 - Pentax Cameras | Medifast Recipes | Medifast Recipes Tips | Medifast Recipes Strategies |

Medifast Recipes Experiences | Medifast Recipes Group | Medifast Recipes Forum | Medifast Support Strategies |

Medifast Support Experiences |

 

(C) Copyright 2010 All rights reserved.